Compressor shutting down on overtemp?

[Larry2]

I am trying to diagnose an overtemp shutdown problem. My background is EE and three or four summer seasons installing A/C back in the 70's.

My system is a Lennox HP416-3P heat pump with a CB18-51 Coil/air handler. It shuts down late morning on the compressor overtemperature switch on warmer days. The compressor is a Copeland ZK40K1-PFV-230 scroll unit. HCFC22 system. The system has a thermal expansion valve.

It is a pretty bare bones unit. It has no low pressure or high pressure protection swiches. It has a thermal switch on the lines and a reversing valve for the heat pump cycle. I've made a few measurements and need some advice.


Amperage draw 16.5 Amps @ 240VAC
Air return 79.5 F (house warmed up)
Air supply 66.5 F
Wet bulb return 66.3 F
High side 202 PSIG
High side temp after condensor 91.3 F
Low side 84 PSIG
Low side temp 74.5-73.6 F

So I gather the subcooling is around 11. The unit door says should be 12 +/- 2. Superheat seems high. Maybe I'm not calculating right.

The filters are new, coil is clean and not icing over. There is no ice anywhere. The outlet side of the txv is unremarkable to touch with no expecially cold spots. Partially blocking the air return causes the low side to hunt and then settle back at 84 PSIG, suggesting the txv is not stuck. The sight glass is free of bubbles and green, showing no moisture.

It is definately shutting off on the overtemperature switch located at the top of the compressor, confirmed by an ohmmeter check. Pushing the contactor in manually causes the unit to run and if held in for a minute or so it will cool the compressor enough for it to continue running a while.

To me, the gauges seem normal, including amperage draw. Units in the 70's ran higher high side and lower low side, but those were not high efficiency units. Is there something I am overlooking? Is it possible the overtemperature switch is just bad and shutting off early? Is that switch inside the hermetic unit or is it just fastened on top? I have a second Lennox heat pump with this switch shorted to case ground. Is there a workmanship problem with this switch? What is the temperature setting for this switch? How hot is too hot? TIA.

[frank]

Is the system running on cooling or heating with this problem?

With a supply air temperature of 19C it doesn't seem to be doing much cooling or heating?

The suction pressure of 84psig equates to about 9.5C saturated but your pipetemp reading of (74F) or 23.5C means a superheat of 14C (25F). With a lack of cool suction gas the compressor is prone to overheat.

Have you taken any temperature readings on the bends along the end of the indoor coil and outdoor coil to see where the saturation points are?

How does the amp draw relate to the name plate? - I couldn't find the compressor model you gave on the Copeland web site.

My suspicion is either a faulty TEV or worn compressor or both.

[Larry2]

Thanks for your reply Frank. This is in cooling mode. The outdoor unit is rated for around 40,000BTU/hr. My guess would be the air handler pushes 1300 CFM. I was thinking the suction line temperature is too warm and I do understand that the return refrigerant cools the compressor motor. Since my experience is years ago on lower efficiency systems, I wasn't sure what temps and pressures to expect on the low side. The new high efficency systems blow larger volumes of air with less temperature drop. To confound my confusion, the house was pretty warm already with return air at 79.5 F. Regardless, it does seem like superheat is excessive. I'm also suspicious that the coil is too large for the system. I belive an HP18-511 coil would belong in a five ton configuration, but I don't have any data sheets.

My quick check blocking part of the return air caused the TXV to regulate to the same suction line pressure. That led me to think it's not stuck. Maybe it's not stuck but regulating to the wrong setpoint. How would I diagnose if the TXV or the compressor is at fault?

I didn't read temps at various bends on the evaporator core. I did feel the outlet side of the txv and it was not extremely cold. No where on the coil or the loops at the ends had any ice. With 100F+ up in the attic where the air handler is, I have to admit my interest in a more detailed exam was waning quickly.

The amperage draw is on the money for the compressor rating. I don't know the amperage draw spec for this compressor in this exact system.

The contractor who installed this system undersized the orignal spit. Numerous calls for not being able to satify the thermostat resulted in him tinkering the thermostat to regulate at 4 or 5 degrees higher than indicator. Eventually I showed that the contract called for a system 1/2 ton larger than he installed. He swapped the system out for the agreed unit which is still to small for the load. The load calculations were his. I think he calculated by guess and by golly rather than on paper. He installed the unit without the txv. He returned to install that when I pointed out that the manufactures documents require one. That was many years ago and I've later found out he installed the new system without any filter/dryer unit. Perhaps metal bits are in the txv now as a result. It sure isn't good work. Back in the day, I always replaced the filter/dryer if a system was opened up.

I plan to break out the books and calculate how much cooling the system is actually accomplishing. I did that on the previous system before he replaced it and it came surprisingly close to specifications.

[frank]

With the amp draw being OK it doesn't lean toward an undercharge unless of course the comp is nearing failure. The high side of 202psig seems low for such a high suction pressure on R22 but you have an off coil pipe temp of 32C (91F) - sems acceptable.

Now you say that the system maybe mis-matched, I would look at the evap (indoor coil). Take temp reading on the coil at each bend starting from the bottom and find the point where a large temp increase is. My guess now is that you have an over sized coil which is absorbing excessive superheat leading to your problems.
Try blocking part of this coil and let us know what happens

[protect]

What kind of overtemperature switch is located at the top of the compressor. Bi-Metal?

[Larry2]

I believe it is a bimetal thermal switch, but I'm not sure. I guess it could be a polyfuse. It is a contact closure below temp and an open circuit above temp. This connects to the circuit card that operates the reversing valve and contactor.

The unit shut down today again on compressor temperature. When pushing in the contactor manually, the compressor started but the outdoor fan motor didn't. I sprayed the fan motor with cool water and it was successful starting after that. THen I let the unit cool and restarted it. I'm starting to wonder if the fan motor is shutting down first. I've not been there to witness the unit shut off. The unit works for several hours once the house heat load drops after sundown. It's run 3 hours continuously tonight.

I blocked 1/2 the filter panel ahead of the coil. I left it that way for around two hours while things cooled. The low side line temperature was 64.7. I then unblocked the coil and the temp increased to 65.7 F 5 through 30 minutes later.

[Larry2]

On another web site, I'm told my air handler is a 4 ton, 1640 CFM unit. The compressor is a 3-1/2 ton copeland scroll unit.

[wambat]

Larry2, Your last post is correct so you might want to drop the cfm down to 1400. If you have an adjustible shive it would be easyand it may resolve the problem somewhat

[protect]

Hi,

did you check the compressor protection. Is it working? What kind of prtection Bi-metall Thermistors?

I am trying to diagnose an overtemp shutdown problem. My background is EE and three or four summer seasons installing A/C back in the 70's.

My system is a Lennox HP416-3P heat pump with a CB18-51 Coil/air handler. It shuts down late morning on the compressor overtemperature switch on warmer days. The compressor is a Copeland ZK40K1-PFV-230 scroll unit. HCFC22 system. The system has a thermal expansion valve.

It is a pretty bare bones unit. It has no low pressure or high pressure protection swiches. It has a thermal switch on the lines and a reversing valve for the heat pump cycle. I've made a few measurements and need some advice.


Amperage draw 16.5 Amps @ 240VAC
Air return 79.5 F (house warmed up)
Air supply 66.5 F
Wet bulb return 66.3 F
High side 202 PSIG
High side temp after condensor 91.3 F
Low side 84 PSIG
Low side temp 74.5-73.6 F

So I gather the subcooling is around 11. The unit door says should be 12 +/- 2. Superheat seems high. Maybe I'm not calculating right.

The filters are new, coil is clean and not icing over. There is no ice anywhere. The outlet side of the txv is unremarkable to touch with no expecially cold spots. Partially blocking the air return causes the low side to hunt and then settle back at 84 PSIG, suggesting the txv is not stuck. The sight glass is free of bubbles and green, showing no moisture.

It is definately shutting off on the overtemperature switch located at the top of the compressor, confirmed by an ohmmeter check. Pushing the contactor in manually causes the unit to run and if held in for a minute or so it will cool the compressor enough for it to continue running a while.

To me, the gauges seem normal, including amperage draw. Units in the 70's ran higher high side and lower low side, but those were not high efficiency units. Is there something I am overlooking? Is it possible the overtemperature switch is just bad and shutting off early? Is that switch inside the hermetic unit or is it just fastened on top? I have a second Lennox heat pump with this switch shorted to case ground. Is there a workmanship problem with this switch? What is the temperature setting for this switch? How hot is too hot? TIA.

[Larry2]

The protection switch is closed when the compressor runs and is open circuit when the compressor shuts off. I don't know what temperature it is designed to shut off. I tried to measure the temperature of the compressor at shut off, but it was above the range of the digital thermometer I have. Above 169.9 F, my thermometer stops working.

Yes, it's possible the coil is mismatched. The installing contractor may have selected a motor speed tap to suit. The system has run in this configuration since 1994, so I believe we are diagnosing a problem, not the configuration. I believe he consulted Lennox about the equipment selection.

I need to measure the performance again and verify my readings. I agree the superheat seems excessive. If anyone knows what temperture this switch should protect, that will help too. I'll leave the system off today. Tomorrow, I'll start it in the morning and watch gauges, temps and such from start until it cuts off.

Much appreciate all the help from all, keep it coming!

Cheers,

[frank]

the compressor started but the outdoor fan motor didn't. I sprayed the fan motor with cool water and it was successful starting after that.

Quite a big clue here.
Question - does the system have any High Pressure switch/protection?

[Larry2]

In terms of compressor protection, this unit is pretty bare bones. There is no low pressure or high pressure safety switch. If it were empty of refrigerant, it would run anyway, until it burned up.

There is a temperature switch on the top of the compressor. It is configured to open the contactor on overheat. The contactor is wired so neither the fan motor nor compressor run if the temperature switch opens. There is another temperature swtich on the high side where it exits the compressor. A circuit board and relay handle contactor control from there, including the reveral valve for heat pump cycle.

The fan motor failing to start because of internal thermal shutdown surprised me as I had not seen that before. It could be the compressor heat soaked the fan motor. The unit was quite hot inside when I opened the door.

I plan to watch the system one morning soon and see if the fan stops first. If it does, that would explain everything. If not, I guess I am still lost.

[frank]

With no HP safety then the loss of the condenser fan will cause the compressor to operate at high pressure until the klixon kicks in - exactly the symptoms you describe.

[wambat]

The outoor fan motor should never be off while the compressor is running It may be that the OSfan motor if tripping on an internal overload due to heat or hi amperage this of course would rsult in the compressor overheating

[Larry2]

I had never heard a thermal protector called a Klixon. Sure enough, that's what it's called in the industry. I think in my case, I have a Klixoff.

My next step is to fix a cool drink and sit out one hot morning and watch this unit fail. A problem with the outdoor fan motor would explain not only my current symptoms, but burned run cap and compressor wires in the past. I'm hopeful this will prove to be the problem since that's easier to fix than issues elsewhere.

[Larry2]

I've made some more measurements on this system. I've measure wet and dry bulb on the supply and return at a time of day when they system just keeps up. Making the assumptions that the indoor fan is pushing 1400 CFM, I arrive at 23,000 to 26,700 btu/hr cooling. The system should be able to perform at 39,500 btu/hr or so. The system is drawing correct amperage but it's not producing to capacity.

Maybe someone can check my work?

reading 1

return wet bulb 67.3
return dry bulb 77.5
supply wet bulb 62.6
supply dry bulb 65.5

reading 2

return wet bulb 63.9
return dry bulb 76.4
supply wet bulb 60.6
supply dry bulb 65.8

I think this gets back to the original question about excess superheat as my core issue.

How do I diagnose if I have a problem with the txv or the compressor itself?

[wambat]

I've made some more measurements on this system. I've measure wet and dry bulb on the supply and return at a time of day when they system just keeps up. Making the assumptions that the indoor fan is pushing 1400 CFM, I arrive at 23,000 to 26,700 btu/hr cooling. The system should be able to perform at 39,500 btu/hr or so. The system is drawing correct amperage but it's not producing to capacity.

Maybe someone can check my work?

reading 1

return wet bulb 67.3
return dry bulb 77.5
supply wet bulb 62.6
supply dry bulb 65.5

reading 2

return wet bulb 63.9
return dry bulb 76.4
supply wet bulb 60.6
supply dry bulb 65.8

I think this gets back to the original question about excess superheat as my core issue.

How do I diagnose if I have a problem with the txv or the compressor itself?

Your pushing too much air; that is the the by-pass factor is too great. Drop the air flow down until your discharge air temperature( dry bulb) is about 4 degrees lower

[Larry2]

Okay, I checked the fan wiring and the fan was set to the highest setting. I've changed the jumper to the medium speed setting. I'll see what happens tomorrow when it gets hot outside.

[wambat]

Larry, you think we hould keep our fingers crossed? :>)

[Larry2]

Larry, you think we hould keep our fingers crossed? :>)

Golly, I hope so. I can't afford repairs.

I made a meaurement today of supply air at the duct nearest the air handler and another at the furthest outlet. I was astonished to see 2* F temperature gain to the furthest register. These ducts are run in the hot attic. They are insulated, but apparantly not well enough. I shall be wrapping these ducts with more insulation in the near future.

[Larry2]

Gents, I have another warm day and my compressor shutdown again on excessive temps.

My readings today, with the blower throttled back a bit are as follows:

Return drybulb 81F (27.2 C)
Return wetbulb 68.9F (20.5 C)
Supply drybulb 64.8F (18.2 C)
Supply wetbulb 62.6 F (17 C)

guessing at 1200 CFM air flow for medium fan speed on a 1400 CFM blower, this calculates to 24,600 btu/hr cooling. That's close to what I calculated with the fan on high speed a few days back. Several different day's readings at different superheat and outside temperatures have arrived at nominally the same cooling performance. The unit should be capable of 40K btu or so.

The suction line measures 67.1 F (19.5C) directly at the evaporator outlet.

The TXV outlet measures around 76 F (24.4 C)

The top of the compressor near the Klixon measures 200 F (94 C), measured with a non-contact thermometer. That seems a bit high to me. I don't know the cutoff temperature for the klixon. The motor end of the pot is much cooler.

Compressor high side outlet pipe is around 160 F (71 C) before the condensor coils.

Low side 82 PSIG
High side 202 PSIG

I don't see any frosting or unusual temperatures on the reversing valve.

The compressor specs are:

http://americanhvacparts.com/Merchan...&Category_Code=



This is a hermetic scroll compressor with the motor in the bottom section and the scroll pump is at the top just below where the klixon is located. I believe there is another thermal protection device inside with the motor winding and it is not kicking off on that switch.

Does this mean the compressor is failing on excessive internal leakage?

If my compressor needs to be replaced, can I install a 4 ton compressor in place of the 3-1/2 ton designed in by the vendor? My indoor coil is rated for 4 ton. The condensor coil is quite large.

[frank]

It all points to being short of gas. 82psig suction is too warm for compressor cooling and indicates that the evaporating temp is too high leading to a lack of performance (duty).

Check the gas charge.

[Larry2]

Thanks for your reply Frank. Low charge was my initial thought last week when I looked into the problems, but I think the charge is okay. I have no bubbles in the sight glass, so the txv has enough liquid to work with. I've also checked charge with two methods.

The door for the unit describes two charge methods. The first is "Aproach method". It says to measure the liquid line temperature in a thermometer well provided, and to also measure the outdoor ambient.

Ambient temp - liquid line temp = approach

They give a value of 8 degrees F delta as the goal for approach temperature. It goes on to say to reduce "aproach value" add refrigerant.

I measure 6.4 degrees approach, so that test says I'm slightly excess charged.

The second charge method explained on the door is subcooling. It says read the high side gauge, convert to vaporization temp on the R22 table. Subtract this temperature from the liquid line temperature to get subcooling. They give a goal of 12 +/-2 degrees for subcooling. There, it says add refrigerant to increase subcooling. I am right on target there this evening at 195 PSIG and 86 degrees liquid line temperture. Maybe there is something I've overlooked about their steps?

This afternoon I focused my attention on the TXV. I applied ice to the bulb while observing the suction line pressure and the cold air supply temperature. I also measured the TXV line output ahead of the coil.

Applying ice to the txv bulb dropped the suction temperature to 70 PSIG or so from low 80's. It also dropped the temperature of the TXV output line. I guess that no surprise, the liquid turned to vapor earlier in the evaporator (closer to the txv) when it was fooled into throttling back flow. As the these values both dropped, the suction line temperature and the supply air temperature both went up. That says throttling back the flow made the coil less efficient. In other words a larger portion of the coil was vapor.

I'm not sure what to conclude except the valve is not stuck. It does seem that the calculated superheat improved by applying ice by virtue of lowering suction line pressure. However the amount of cooling diminished and the gas return temperature increased.

Is there something I've overlooked in my checks?

[Larry2]

It all points to being short of gas. 82psig suction is too warm for compressor cooling and indicates that the evaporating temp is too high leading to a lack of performance (duty).

Check the gas charge.

I meant to respond to your second statement too. I agree in principle that the low side pressure is too high. My other system of the same vintage, vendor and efficiancy runs 65 to 70 PSIG. The working system also pulls the low side down quickly from a stop. The broken system is lethargic about pulling down the low side.

I also believe the evaporator is too large for the amount of work the compressor is doing. My hunch is the compressor is leaking in the scroll pump thus spending power stirring hot gas around the head instead of through the cooling loop. I don't know how to prove this enough to justify replacing the compressor.

[Andy]

I meant to respond to your second statement too. I agree in principle that the low side pressure is too high. My other system of the same vintage, vendor and efficiancy runs 65 to 70 PSIG. The working system also pulls the low side down quickly from a stop. The broken system is lethargic about pulling down the low side.

I also believe the evaporator is too large for the amount of work the compressor is doing. My hunch is the compressor is leaking in the scroll pump thus spending power stirring hot gas around the head instead of through the cooling loop. I don't know how to prove this enough to justify replacing the compressor.

Discharge temperature, compare the two units, mechanical drama or gas bypass will show up with high discharge superheat

Kind Regards Andy

[Larry2]

Good idea Andy! My 3 ton unit has a compressor discharge temperature of 155 F ( 68.3 C) whilst the unhappy unit measures 178 F (81C). Their suction line temperatures are 60 F (15.5 C) vs 64 F (17.8 C). I know this is a bit different than I reported yesterday, but today it's warmer outside and the indoor temperature has not risen too much yet.

[yhruhteb]

i think your unit are overcharged

[bebad]

Hi Larry, I believe the cut off temp to be 125 C, internally controlled. SUggestion: fit a manual reset HP switch in control circuit, does sound like the cond fan stalling when your not there. Let us know.

[Andy]

Good idea Andy! My 3 ton unit has a compressor discharge temperature of 155 F ( 68.3 C) whilst the unhappy unit measures 178 F (81C). Their suction line temperatures are 60 F (15.5 C) vs 64 F (17.8 C). I know this is a bit different than I reported yesterday, but today it's warmer outside and the indoor temperature has not risen too much yet.

Hi Larry

when you have found the cause of the high discharge temperature you have your problem.

If it was me I would decant 10 to 20% of the charge and measure the pressures or temps again.

Alternatively decant to a cylinder until the system readings in this unit match the other twin unit

Does sound a little over- charged to me, but not being there it is hard to say

Kind Regards Andy

[Larry2]

I have news! It's plenty hot today. I had the wife buy some Lager and we sat outside and baby sat the unit while I surfed the internet.

After around five hours of running, the fan motor finally cut off while the compressor continued to run. I pulled the quick disconnect right away and disconnected my gauges.

A light bulb went off in my mind. In the dead of winter last year I lost heat from this unit. Actually this unit has been a problem child through the years and I think it's well undersized. I traced that failure to a damaged contractor consequent to a ruptured dual capacitor. This dual cap runs both the fan motor and the compressor run windings. This sort of part is impossible to buy locally (see the close shop thread here) unless one pays for a truck roll, service and all that. Mosty service calls result in recommendations to replace entire.

I hastily (Ooops) found a dual cap that was not so close to value online and placed my order for fast deliver which took over a week. Electric back up heat in quite costly too and that $40 cap probably cost me $80 by the time it was installed. I have replaced this cap once before when it was 30% under value. So when my online selection was limited, I accepted a 35/7.5 cap to install where a 35/5.5 that belonged.

It makes sense that the fan motor would draw more power through the excessive sized run capacitor and shut down eventually on thermal shutdown. It also makes sense that this fan motor would be happy all winter and wait for a very hot day to protest. So I think I have found my self-inflicted problem.

It's plenty hot outside today and we need cooling. I've installed my temporary remedy which will help confirm the outdoor fan motor is okay and will keep me cool while I finish a few more cool drinks.



I've connected a pair of series connected 10 uf capacitors from my 2 KW ham transmitter for the benefit of my outdoor fan. The 7.5uf section of the dual capacitor is now disconnected. We shall continue baby sitting the unit today and see if the fan is happier now.

I measured again using the "approach method" and subcooling method several times more in my attempt to acess the refrigerant charge. I even had ice ready for the bottle and was set to remove some charge from the suction side. I'm finding it difficult to believe refrigerant charge is causing any issues. This past week I've gone +/- some on charge and the impact on gauges is negligable. One would think it's a simple measurement, but the ambient temperature reading drifts quickly with breezes. However, I fairly consistently get the specified 12 degrees subcooling comparing the liquid line temperature to the gauge reading. I'm no expert on this so if I'm doing this wrong please let me know. On that subject, when I go on about how I do measurements, it's not to adise experts here how to measure, it's to tell folks how I did the procedure, in case I am lost. Wouldn't a system with a txv be somewhat immune to charge status unless it's quite far off? The high suction pressures can be blamed on a badly sized upstairs unit. This unit has never kept up. Heat rises in this open house design, the attic is quite hot, the ducts inadequately insulated for the 100 degree attic, and the unit has never been large enough to keep up.

Today I measured my downstairs unit performance. It is a 3 ton Lennox system which should produce 36,000 btu cooling. I calculated 31,700 btu/hr cooling with my guess of 1200 cfm air flow for a unit that capacity. I have no way of measuring air flow, so a guess will have to do. For the upstairs unit we've discussed here at length, I keep arriving at 24,000 btu/hr +/- performance for a system that should be capable of 40K btu/hr. I was quite patient reading wet and dry bulb temps the last reading on this system. Unless cooling performance is an impossible thing to quantify, I am wondering why performance is sub-par. So I am wondering if this system is leaking within the compressor pump, making the suction line pressure a bit higher, head temps higher, discharge temps higher and the efficiency poor. The run cap is correct on this side and the amperage draw okay. It just seems like the work input to the compressor is not producing the implied cooling output.

My working system has a destroyed Klixon. I'm trying to locate the correct part now. That klixon is shorted to the case of the compressor. The control transformer blows fuses with that condition, so I have the klixon bypassed (disconnected/ bypassed) in that system. I would like to replace that device to continue compressor protection. I'm having great trouble finding that part online and it's not clear to me how it is fastened to the top of the compressor. Here is a photo of this item.



As you see, this part is atop the compressor head and very difficult to reach. The cabinet is directly above and insulated. Two leads emerge from this klixon and disrupt the power flow to the compressor when it is overtemp. I would like to know how this part is fastened atop the compressor. When I bypassed it, I thought it was integral, so there was little to be lost by abusing it. Looking closer, it seems this part can be replaced. It looks like it clips into some sort of groove or notch (or glued?) in the top but visually it's impossible to see. Are there any other names for this part? I've searched for "limit", Klixon, and "thermal".

I want to thank everyone here for their input, replies and their time on this awesome web site. I've learned loads this week about refrigeration and troubleshooting the last weeks. What a wonderful group!

Larry

[Larry2]

I have a few more questions from the village. The condensor coil is very large on this unit compared with the coils on new units manufactured for R22 today. My fan coil is for a 4 ton system. A copeland compressor (ZR47K3) with 4 ton rating is one inch taller. If I were able to shoe horn this next size larger compressor within this cabinet, would it function okay and give me perhaps 1/2 ton or so more performance and maybe match my coil size better? I am surprised at how little temperature rise there is across the condensor coil, only a few degrees. Decades ago, these used to run 270 PSIG and blow really hot on warm days.

Larry

[wambat]

Larry, The suction line back presure should be about 65-70# or about 37 to 40 degrees SST. The head pressure should be about 225 to 240# at 90* osa. I had a similer problem the past few days on a 4 ton lennox heat pump a few days ago as yours. I found that the TXV inlet temp was higher then the outlet to the coil. (sound familer)
Your quote: "The suction line measures 67.1 F (19.5C) directly at the evaporator outlet. The TXV outlet measures around 76 F (24.4 C)" I found that the insulation in the evaporater section was being pulled into one side of the supply fan inlet and cutting off airflow across the cooling coil, so I used clear tape to keep the insulation flush on the panel aftar a short time I was measuring a 10 * superheat. Make sure your at about 14 to 16 amps on the compresor, and the condenser fan, supply fan motors are drawing the proper amps, do this before you change out the compressor

[Larry2]

My unit continues to run many hours after earlier thermal shutown on the outdoor fan motor. This after my fan run capacitor "upgrade" with the monster cap rigged into the circuit. I think the shutdown on temps question is solved. I'm still concerned that the efficiency is not where it belongs for this unit. I think it is off by 1/3 or more. At the cost of electricity, which is going to be even higher in a few months, this is well worthy of investigation.

I just connected the gauges to my smaller, happy 3 ton system. I get 207 PSIG high side and 62 PSIG low side. Liquid line is at 94 F, for a subcooling of around 13 degrees. The unit door specifies 12 +/-2 subcooling. for my 3 ton unit as well. I really don't see a large difference in charge between the two units here. I do see a great difference in the suction pressure and temperatures. I can't say this difference is due to duct restriction though. Changing air flow has not altered suction line temperatures or pressures to any great degree. I could almost dismiss these differences as due to excess loading of the upstairs unit with high return air temps. I'm just not experienced enough to say this for sure. My recollection for 30 years ago was a unit under similar conditions would run 250-270 PSIG high on a hot summer day and 67 PSIG or less low side, even with a warm home. I'm sort of lost with the improvements in coil sizes and high efficiency, so gauges are not helping me as much. This system is 12-14 years old. The coils seem larger than needed for their ratings to me.

During the past week, I've decreased the blower CFM from high to medium and I've seen no change in cooling performance. I've also blocked half the filter tray with cardboard, again with no change in cooling performance. There is plenty of air flow at the ducts. Dropping air flow helps none. Increasing air beyond 1650 cfm in impossible. I can open up the ducts and look for slipped insulation boards but it's really hot up there and I do get a lot of air flow at the duct outlets.

When gave txv temperatures and suction line temperatures, I measured 19.5 C after the evaporator coil and measured 24.4 C directly after the TXV outlet. So the evaporator was operated with a temperature drop across it. My TXV operated with very little temperature drop between inlet and outlet. The temperature drop was across the coil. Forcing the txv to a different setpoint by cooling the bulb with ice caused less drop across the evaporator even though it created lower temperature at the txv outlet. I took this to mean that the most evaporative cooling was being obtained in the coil without tinkering the refrigerant flow by throttling back using ice.

It is sort of like things are pointing to an excess large coil or inadequate refrigerant flow from a dying compressor.

I'm new to all this, so if you can write between the lines what you're thinking, it would help me lots. At the moment, my notion is a blocked or inadequate evaporator air flow would cause low suction line pressure and low suction line temperature.

If I have an improper setting on the TXV, how would I verify?

I am getting correct amperage draw on the compressor, measured with a 50 milliohm precision shunt at the compressor. I subtracted fan motor estimate of 1 amp. Maybe I am off by 1/2 amp at the most and my estimate would be to the high reading with a total unit draw of 17 amps less my estimate of 1 amp for the fan motor~ a final compressor draw of 16 amps.. I sure wish I owned an amprobe. I don't think I am off enough to explain 1/3 loss of cooling capacity.

Larry

[Larry2]

Larry, The suction line back presure should be about 65-70# or about 37 to 40 degrees SST.

I forgot to say in my earlier reply, when the system finally catches up late evening and heat loading is less, it does measure 65-70 PSIG.

I'm unsure what you mean by SST unless you're referring to subcooling. I seem to have excess subcooling which would imply to me:

1) too much air flow (not insulation fallen in the duct)

2) too little refrigerant flow.

I am leaning towards a weak compressor, causing excess suction pressure and low refrigerant flow. I don't have the experience in this field to prove my notions.

Third possiblity is that I have much more air flow than the blower design implies, due to oversized design on the duct system. This would throw off all my calculations since I am guessing at air flow of 1600 cfm high/1200 cfm low fan speed settings. Still it would be unclear why temps and pressures hardly move on the suction line with blockade of some return air side.

Larry

[bebad]

HI Larrry, SST- saturated suction temperature- inner dials of manifold or refer to comparator, the thermel protection device on this copeland comp slides into a hole in the head of comp. as shown, and the black plastic surround shown wedges it in place, quite simple. The comp. itself seems to be operating OK considering whats it's been through.

[Larry2]

This thermal protection switch is working fine on the system I discussed here and fixed with everyone's help. On my other unit, this switch is shorted to the compressor case. A few years ago, that switch shorted and blew the control tranformer fuse. I disconnected the switch and wired around it so I would have continued service from that second unit. At that time, I thought the protector was inside the hermetic assembly, so I figured on getting whatever use remained from that unit without thermal protection.

Now that I see this part is replaceable, I would like to install a good one on my other unit for continued compressor protection. I'm not having any luck finding this part online. I don't have a part number to go from.

Can anyone provide me a part number for the thermal protection device/klixon?

Thanks,

Larry

[frank]

Try these Larry - they are on your side of the pond with multiple distributors. http://www.sensata.com/about/klixon.htm

[Larry2]

Thanks very much Frank. I browsed their catalog and while I found switches for this application, I didn't find one that resembles the original mounting Lennox used. This one is large at the top, looking almost like an expansion valve diaphram. Maybe that construction makes the part quick to respond when the outdoor fan shuts off. Anyway, I don't find on in their line matching exactly. A Lennox parts dealer says this part costs $105 U$. That seems a bit much for a bistable switch. Still, it's cheaper than compressor service if the fan motor fails. I keep looking for a seller like digikey, but I think I will end up paying the retail price.

Is this sort of high side protection common these days? In the 60's there was a high side and low side switch that actually measured the pressure. This unit uses bistable switches for high, low and the compressor head.

[frank]

HP & LP switches are a necessity as far as I'm concerned. The klixon acts to protect the compressor against prolonged operation at high temperature and is different to HP/LP protection.

You can have a compressor with an undercharge that suffers from a lack of suction gas cooling but won't trip on LP and so operates at a high temp. This is where a klixon comes in to play.

If the direct OEM replacement is expensive you can achieve the same result with alternative bits of kit, all it is in reality is a stat that breaks the control circuit. Wire in a tank stat - it does the same job for a lesser $$$$$ cost

[Larry2]

I located the correct klixon part through a Lennox dealer and ordered the correct run cap and this thermal protection part at the same time. It was excessively priced, but it will put the unit back together correctly. The unit that needs the klixon has it wired in the control ciruit like you describe. It is shorted to the compressor case, so it earths the 24 VAC control circuit. Until I got involved in the second unit, I didn't know the Klixon could be replaced. I thought it was inside the hermetic section, so I wired around it. It's been several years. It's time to fix it right.

I agree that considering the cost of compressor damage, high side, low side and thermal protection devices should all be provided. This was really bargain basement without the bargain price. I'll look into inclusion of these devices for any upgrade system I install in the future. It's not worth opening the sealed system to add it to 12 year old gear.

I'll check Wambat's idea about insulation board blocking the duct. I think air flow is adequate, but I have no way of measuring it. Certainly an inspection is appropriate because I'm simply not getting near the rated work out of this equipment.

Thanks everyone for all the help.

[Larry2]

I've received and installed the correct fan motor capacitor. The system runs all day now without shutting down. However it is still not cooling to capacity. It is actually performing quite poorly compared with how it did prior to these issues. I'm thinking the repeated episodes of the condensor fan shutting off have caused compressor damage. Perhaps it has high internal leakage, causing low high side and high low side.

I wanted to know if I can put a 4 ton compressor in this unit. It is a 3.5 ton system, 4 ton air handler and it was a 13 SEER unit to begin with, so the outdoor condensor has a lot of coil surface area. Would this perform okay with 1/2 ton more compressor installed? The reason I ask is that I have a line on a surplus compressor for 1/2 price.

I also received the klixon replacement for the other unit. It looked quite different than I expected. It is a round cylinder with wires connected that drops down into a well or hole on top of the compressor. Thermal grease assures transfer of heat. The round top I managed to photograph here earlier is just a plastic retainer. That replacement is on my list of things to do.

[umakantvh]

84 psig on the low side in cooling is pretty high. As some one has suspected earlier, the TXV seems to be defective. Even at 84 psig low side pressure, the high side pressure is only 202 psig !!! which can not be the case at all.

[Larry2]

I think I have the wrong txv and it is probably accumulated contamination. I found the blower/coil manual and is shows conversion from a 0.079 orifice device to the txv. It describes a filter/strainer fitting that goes in the liquid line. This part would house the orifice device. The orifice device, called RFCIII, would be captured between the strainer and the distributor.

There is supposed to be an adaptor tube with flare fittings on each end. The filter/strainer is supposed to be removed, the orifice removed, then the strainer assembled. Then an adapter tube, part of a kit, connects the txv to the strainer. The contractor managed to assemble this without the filter/strainer and without the adaptor tube. Probably I have metal bits in the txv from years of operating without the strainer.

The only thing is that I'm having trouble proving the problem is here. I can apply heat to the txv bulb and the exit air temperature increases. I can apply ice to the txv bulb and the outlet air temperature increases also. So it is as though the txv is at a happy setting. However, superheat is quite excessive.

[frank]

I can apply heat to the txv bulb and the exit air temperature increases.

This seems wrong. If you have a warm TEV bulb then the valve should open to allow more liquid into the evaporator causing the air temperature to drop not increase.

[Larry2]

I was thinking that there is an optimum flow rate which the expansion valve seeks. More gas flow or less gas flow will cause less than the full portion of the coil to be used to the greatest advantage. If my idea is correct, the air temperature should be lowest at the flow set automatically by the expansion valve without tinkering it with heat or cold. Maybe I've missed something key?

[frank]

I was thinking that there is an optimum flow rate which the expansion valve seeks.

Yes there is - it is adjusted by setting the correct superheat, From then on the valve, if correctly sized will endeavour to maintain this setting under differing load conditions

[Larry2]

If you take an expansion valve that is set up correctly for a two ton system and installed it in a three ton system, will it give you the same superheat, or would the spring tension require resetting?

[Andy]

If you take an expansion valve that is set up correctly for a two ton system and installed it in a three ton system, will it give you the same superheat, or would the spring tension require resetting?

Hi Larry

no your superheat will be higher as the 2 ton valve will not flow enough refrigerant and about the same amount of tubes will be filled in the larger evaporator leaving the remainder to add even more superheat to the refrigerant.

Kind Regards Andy

[Larry2]

On the expansion valve, where they have brass cover at the adjustment end for setting superheat, can the be field set on all expansion valves or only on some? If the cap is removed to access the adjustment stem, will there be any issue getting a proper seal from the cap again?

[frank]

Hi Larry
If the valve has an adjustment cap then you can adjust the superheat at this point. Re-fitting the cap doesn't normally pose a problem but it is always good practice to spray a little leak detection fluid on the caps/schraeder valves after you have finished.
In the past, I've lost a charge through a badly fitting schraeder cap over a weekend - client not happy so I always leak check now.