Walk-in Freezer problem

[airefresco]

I have a client with a walk-in freezer on R404a. It was installed about 2 years ago and spent the first year being constantly "repaired" by the installers (not us). After about a year of them messing with it I was called and found a sticking solenoid valve and it was a touch short of gas. Since that visit it has worked perfect and held -18ºC. A couple of weeks ago it got really hot here and the temp went up to -12ºc, i got there and found the condenser blocked up with dust. I gave it clean and the next morning the temp was back to normal for a couple of days at which point it started to climb again. Got there and found the evap heavily iced up and blocking the fan from running. Defrosted the evap and ran it up again. It got to -14ºc and wont go lower than that.

I took some readings yesterday,

L.P. 18psi (taken at the condensing unit)
Suction temp 1: 4.2ºC (as above)
Suction temp 2: -20.1ºC (taken inside the room about 1 meter from the expansion valve)
Air On cond: 27.9ºC
Air Off cond: 31.9ºC
Liquid line temp: 32.7ºC (taken just after condenser)
Dischage Temp: 80.2ºC
Air on evap: -6.2ºC
Air off evap: -10.3ºC

The room was down to -14 when I took all the readings outside the box. I had to obviously open the door to get in to take the other readings, judging by the air on, the temp increased in the box to -6(ish) for the inside readings.


There is no port for taking a discharge pressure, or if there is it is well hidden. The condensing unit has brass service valves like you see on small split a/c systems. This is where the suction pressure was taken.

A few other things worth noting. The evap was even frosted all the way along. The expansion valve was completely frosted (quite heavily) and the liquid and suction line feeding it were also frosted. The gas smells burnt, which it didn´t last time I worked on it and the sight glass is full.

I think there is either moisture/blockage in the system or there is a problem with the expansion valve (which is externally equalised). Leaning more towards moisture/blockage as the valve is frozen up on both sides.

Opinions please?

[lma]

Opinion: Most likely problem within your defrost system

Senario: Defrost failure: Expansion Valve closing due to ice-buildup thus low pressure reading at LP port (18psi)

Unlikely: Moisture problem as the system has been operating fine for over two years - no refrigerant added since I presume. It was holding load quite well when defrosted

Probable Reason for frosting: Semi-Closed expansion Valve. This could also be the reason the compressor discharge temp is I feel higher then normal. Oil is probably being sucked from compressor to system under low pressure conditions.

Opinion: Expansion Valve should be fine once defrosted and System defrost corrected. this should balance the system. Oil Problem should be corrected automatically when system balanced - Oil should return to compressor and Compressor temperature should be able to come within normal operating temperature (50-60°C)

Good: Cleaning of Condensor

[airefresco]

Thanks for your reply.

The defrost frequency was set to every 4 hours. I have altered that for now to 10 hours so it runs all day (as the door was being constantly opened and closed the temp wasn´t coming down) with the frequencies longer it is coming down. I was planning on setting it back to down 6 hours tomorrow, as the product should cold enough by then.

I completely defrosted the system last week, if that makes any difference.

[lma]

On an expansion Valve system running at very low temperatures (-18°C) my feeling is that longer then 6hours would create a problem.. especially if at the end of each defrost cycle the ice is not completely removed.. there would be a continuous slow build up of thicker and thicker ice. 6 hour intervals... if 4 does not work.. give it a try. but I would not recommend anything longer then that.

[airefresco]

Thanks, I´ll set back to 4 then, which is what it was originally. Should the fans be on or off during defrost? I have them set to off, but i´m thinking that it may be better to leave them on to help clear the ice. The fans are linked to the door switch, so they do go off when the door is open.

[lma]

At very cold temperatures, below 0°C, its not normally recommended to leave the fans on as the heat from your defrost system (either hot gas or electric) could eventually end up affecting your load especially if it is sensitive to temperatures above 0°C. (I would recommend the method of fan defrost for cold rooms where the temperature required is above 0°C in most cases)

[monkey spanners]

You say the liquid line feeding the valve is frosted also? This sounds like a restriction in the liquid line such as a blocked drier or failed solenoid diaphragm, this later one would also affect the defrost if sysytem is on pump down.

[FEISTY]

Depending on how dirty the condenser coil was and how high the pressure ran, if unit has pressure relief valve just off the compressor you may have vented some *****. Also if you think there is a partial restriction at TXV valve causing it to frost up, try a warm air gun and heat the valve slowly. You might have ice blocking the valve. We use a product called Thawzone which when added to ***** line will help remove the ice problem. It is basically an alcohol based additive that displaces the moisture. Fans off during defrost and right after defrost cycle until coil reaches freezing temp. Otherwise you will get " snow " in the box from fans blowing moisture from thaw cycle. Good luck.

[Gary]

Does the system have a defrost termination switch? If so, the defrost duration must be long enough for the termination switch to end the defrost cycle. The defrost cycle should not be ended by the duration setting.

[mikeref]

Airefresco, the "burnt" gas smell is a concern, what type of compressor is it, sealed/semi/open drive? Refrigerant may be contaminated, same for oil. Pump down system and check drier inlet for grey or copper colour, also do the same for tx. Might have to replace R404a/oil and drier depending on inspection results.. mike.

[airefresco]

Cheers for the replies.

The defrost is controlled by digital controller. The termination is after 30 minutes, I think it may also be set to terminate from the coil sensor. I need to check this I can´t remember, but I am pretty sure it´s 4ºc or 30 minutes, which ever comes first. Another thing which I have actually thought about until now is that the defrost cycle just pumps the system down. There are no electric heaters or hot gas or anything. I would thought that there probably should be on a freezer?

In answer to the other things raised,
The compressor is sealed. The system is fitted with H.P and L.P switches, both of which have not tripped since my repair over a year ago. Don´t think there is a relief valve anywhere.

The expansion valve is hard to get to as you need to lift the fan deck out to get to it. So you can´t run the system, while accessing the valve and have the fans on at the same time.

The burnt gas smell is a concerning me. I think I may replace the gas and drier anyway as a precaution (this would then also eliminate blocked drier), but it would be nice to know how this happened before I do that.

Last thing, if there was a restriction at the at the expansion valve, wouldn´t the ice start at where that restriction was and not cover the whole valve/liquid line. Surely if there is a restriction, it is on the liquid line somewhere?

[nike123]

[quote=airefresco;202777]Cheers for the replies.

The defrost is controlled by digital controller. The termination is after 30 minutes, I think it may also be set to terminate from the coil sensor. I need to check this I can´t remember, but I am pretty sure it´s 4ºc or 30 minutes, which ever comes first.

I would raise from 4°C to 12-15°C. Make sure that fans are not working during defrost. Also check (as Gary already said) that temperature of coil is terminating defrost and not time. If time is terminating defrost , than your defrost function is not performed in full, and that is sign of bad programing or ineffective defrost.



Another thing which I have actually thought about until now is that the defrost cycle just pumps the system down. There are no electric heaters or hot gas or anything. I would thought that there probably should be on a freezer?Check again, you must have either electric heaters or hot gas defrost.


http://contractingbusiness.com/refrigeration/cb_imp_13330/

[coolhibby1875]

you neeed to change the defrost interval time from 4 hours to 8 hours, 3 defrosts are more than enough for a freezer room, also i would increase the termination temperature to 12-14 degrees c, but most importantly move away from 4 defrosts a day.

[airefresco]

Ok just been back and it was at -17ºc this morning, it has now risen to -12, but they have been in and out all morning, so that is not bad. I´ve set the frequency back to 4 hours and according to my drawing there is electric defrost heater. I need to get time and check that the heater is working first I think. I also need to up the termination temperature. Do I adjust the defrost frequency back up a bit as some are saying 6 hours some saying 4 hours? Also, should I change the maximum defrost time from 30 minutes, to allow for the temperature to terminate the defrost?

[lma]

if there is no defrost termination switch.. consider adding one... however 30 min should be just fine. My recommendation: it worked fine for the 1 year at 4hrs defrost interval - so an added two week observation on 4hrs should be fine too. 6hrs - still believe should be the limit as longer then that .. strong chance you will begin to see the problems you were experiencing - on extreme cold temperature applications.
I would recommend the termination temperature of 4°C if you have food products stored .. as safety...provided on inspection no ice is observed at end of defrost interval - otherwise 7°C max - fans not running. Its normally recommended to replace fiter cores after a period of service provided its removal can be safely done without releasing the refrigerant (safe practice) if your system can do it without much problem - no harm in replacing it now.. if not - consider explaining to your client the benefits of installing the required additional components for future service.

[lma]

My mistake - 12-24°C as termination should be fine ... strictly no fan running

[monkey spanners]

If the liquid line is frosty then there is a likely problem with a component in the liquid line, follow the pipe back till you find where it is warm.
Is there a moisture indicator in the system and how does it look?
I think there is a bit of confusion going on between the number of defrosts a day, 4, and the time between them, 6 (hours), a defrost every 4 hours seems excessive, especially if it takes 30 minutes.

One thing to check if there is a lot of ice on the tev and the system is struggling, if it reaches the phial it can make the valve shut down due to giving a wrong superheat reading.

[coolhibby1875]

if there is no defrost termination switch.. consider adding one... however 30 min should be just fine. My recommendation: it worked fine for the 1 year at 4hrs defrost interval - so an added two week observation on 4hrs should be fine too. 6hrs - still believe should be the limit as longer then that .. strong chance you will begin to see the problems you were experiencing - on extreme cold temperature applications.
I would recommend the termination temperature of 4°C if you have food products stored .. as safety...provided on inspection no ice is observed at end of defrost interval - otherwise 7°C max - fans not running. Its normally recommended to replace fiter cores after a period of service provided its removal can be safely done without releasing the refrigerant (safe practice) if your system can do it without much problem - no harm in replacing it now.. if not - consider explaining to your client the benefits of installing the required additional components for future service.

If you defrost the coil every 4 hours all that you do is start to introduce moisture into the room, so when it comes of a defrost the coil frosts really easy trying to rid the room of moisture, if you have a chiller room using off cycle defrosts (fans) then yes every 4 hours are ok, on a freezer application the less defrosts the better, and for freezer rooms that i have commissioned are normally every 8 hours sometimes every 12.

[lma]

if the defrost problem sorts itself out ... Check to see if the compressor temperature at discharge is coming down to the level I indicated earlier... if its not... then recommend: Time to look at refrigerant and oil replacement (check quality of oil when doing so) and system flush as the other members have indicated. If it has ... & system running fine ... just observe and be happy for now

[lma]

Not necessary. A defrost system with good drainage (by the way check drainage for any blockage) should clear any water or moisture away from the room so the issue of moisture in the room should not arise

[lma]

What I am trying to say is ... in a high humidity area at extreme low temperatures frosting is highly possible on the head of the Expansion valve... this destabilizes the expansion valve sensing mechanism (liquid migration to the head) thus making it close when ideally it should be open. A closed expansion valve would create a system imbalance .. an artificiality low pressure system detectable at the lp port .. and a colder then expected cooler .. that will ice up more. A longer period of running is ideal if the humidity in the area is low .. and if using electric defrost .. would lead to less power bills .. but from my experience .. if that balance is not found... you have a problem similar to above..

[Grizzly]

If you defrost the coil every 4 hours all that you do is start to introduce moisture into the room, so when it comes of a defrost the coil frosts really easy trying to rid the room of moisture, if you have a chiller room using off cycle defrosts (fans) then yes every 4 hours are ok, on a freezer application the less defrosts the better, and for freezer rooms that i have commissioned are normally every 8 hours sometimes every 12.

Good sound advise coolhibby.
It does amaze me how many people will insist on automatically defrosting a chamber at regular intervals.
Every 8 or even 6 hrs being common.
Whereas as you state the absolute minimum required is the best option.
After all that is a lot of heat that is being introduced.
Another tip is to increase the number of defrosts during minimum access periods.
Usually during the night and reduce to a minimum those during the day.
Cheers Grizzly

[mcunliffe4]

If the customer is in/out of the coldroom all day advise they install a internal curtain. Also sounds like you have a restriction on the liquid line. Pump the sysyem down if you can, change the liquid line drier, open the liquid inlet on the TEV and check the condition of the orifice. Orifice could be restriced with wax if the condenser has been blocked for a period of time..

[airefresco]

Thanks for all the advice. The reason I set the defrost periods up in the first place was I was thinking that less often would be better, especially since the door is often opened/closed and it´s 30ish ºC here at 80+%RH.

To my knowledge there is only one filter, which is brazed in place inside the condensing unit. Very difficult to get to, but not impossible.

The sight glass has a moisture indicator, and it definitely isn´t green (dry), but it difficult to tell whether it is showing yellow(wet) or nothing at all. I assume it is yellow. I meant to mention that in my first post and forgot.

I want to leave it alone for a couple of days now and let it settle down. After which I think it is best to recover the gas and change the drier and sight glass too (to be sure) and check the orifice. Then take it from there. I also need to check the defrost heater works and where the ice starts to form on the liquid line.

If the orifice is restricted with wax can it be cleaned or is it best to replace?

Thanks again

[lma]

[quote=airefresco;202883]

I want to leave it alone for a couple of days now and let it settle down. /quote]

Good: Would not suggest you open up anything for now... just observe if the system is balancing itself out ... and do keep an eye out on the temperature of the compressor.

[oldesky]

You say the original fault was a sticking solenoid valve. Did you replace this valve and with what? was it a direct acting or servo operated. The problem maybe be in the solenoid valve. If it is a servo type, the solenoid valve may have insufficient pressure drop to lift the servo disc properly. This will cause a restriction in the flow of liquid to the TX valve.

[coolhibby1875]

Good sound advise coolhibby.
It does amaze me how many people will insist on automatically defrosting a chamber at regular intervals.
Every 8 or even 6 hrs being common.
Whereas as you state the absolute minimum required is the best option.
After all that is a lot of heat that is being introduced.
Another tip is to increase the number of defrosts during minimum access periods.
Usually during the night and reduce to a minimum those during the day.
Cheers Grizzly

cheers grizzly, our company put in supermarkets in Finland, Sweden and Denmark, due to the efficincy of the stores air con and lack of humidity, the shopfloor freezer cases only receive 2 defrosts every week, and they maintane -25 degrees easy.
the point i was trying to get across above still has gone over his head as he is talking about better drainage.
i think the answer for the original poster would be to install a simple door swith into the control circut that shuts the liquid solenoid valve and kills the fans when the door opens, this will stop the fans pulling in warm air and humidity, but key is less defrosts.

[airefresco]

It does has a door switch which kills the fans when the door is open, but it doesn´t shut the solenoid. Should it?

[nike123]

It does has a door switch which kills the fans when the door is open, but it doesn´t shut the solenoid. Should it?

If it is pump down than it should. If not, than it should send remote OFF signal to controller.

[airefresco]

Yes it does have pump down. When the thermostat reaches temperature or a defrost is initiated then the unit pumps down and the compressor is switched off from the LP switch, which I assume is correct. If I change it to include the door switch, then wont the compressor constantly be switching on/off due to the door being opened/closed? What is the reason behind having the system pump down when the door is open?

[nike123]

Yes it does have pump down. When the thermostat reaches temperature or a defrost is initiated then the unit pumps down and the compressor is switched off from the LP switch, which I assume is correct. If I change it to include the door switch, then wont the compressor constantly be switching on/off due to the door being opened/closed? What is the reason behind having the system pump down when the door is open?


What would happen with your system if fan is defective?

[airefresco]

There are two fans, but if you mean both stopped, then I suppose it would ice up the coil, eventually tripping on LP. However surely this would take more than the few seconds that the door is open? Surely started/stopping the compressor 30 times a day will do far more damage?

[Gary]

Also, should I change the maximum defrost time from 30 minutes, to allow for the temperature to terminate the defrost?

Have you watched it through a complete defrost cycle to make sure the temperature is terminating the defrost?

The temperature MUST terminate the defrost. If that means adding more time, then add more time.

[airefresco]

No I have not, but I will do. I want to check the defrost heater is actually working. All this could simply be the defrost heater is faulty, hence the icing up and other problems.

[nike123]

There are two fans, but if you mean both stopped, then I suppose it would ice up the coil, eventually tripping on LP. However surely this would take more than the few seconds that the door is open? Surely started/stopping the compressor 30 times a day will do far more damage?

As you could learn from article I linked in my previous post, it is advisable to turn off fans while doors are open.
How that will be executed in specific situation is depending on how good automation of process is executed. Today, with electronic regulators, that is simple task.
If nothing, I would set delay at compressor ON at 6 min. and switch off supply of electronic controller by door switch, or better, light switch (for small walk-in).

[airefresco]

Thanks Nike. I´ve read through the article again and while I can see why the fans should be switched off (they are and have been since day 1) while the door is open, I still don´t understand why the system should be pumped down. Also I don´t understand what you mean by a adding a light switch. Currently the door switch controls the operation of fans, room light and warning light on the control panel. When the door is closed the fans operate and the light is off. When the door opens the fans switch off and room light and warning light come on. Why would I need to add an additional switch? What would this do?

[coolhibby1875]

Thanks Nike. I´ve read through the article again and while I can see why the fans should be switched off (they are and have been since day 1) while the door is open, I still don´t understand why the system should be pumped down. Also I don´t understand what you mean by a adding a light switch. Currently the door switch controls the operation of fans, room light and warning light on the control panel. When the door is closed the fans operate and the light is off. When the door opens the fans switch off and room light and warning light come on. Why would I need to add an additional switch? What would this do?

sometimes on freezer rooms the light switch is included in the control circut, so when light is turned on it kills the feed to the evap fans and compressor contactor and also the solenoid valve, to stop liquid migrating to the evaporator. Once this happens an different controller comes into play this controller is usually called a temperature overide swith, so when the room gets to a desired temperature the overide switch will then supply the control circut again yet leaving the lights still switched on, thus bringing the room back down to temperature and once the diff has been achived the overide switch de energises, killing the feed again for the control circut and the fans and compressor stop. The idea behind this is that on medium sized freezer rooms where staff will be working for prelonged periods with the door closed it kills the equipment making it somewhat more comfortable to work inside, then when the room hits say -14 the overide kicks in and pulls the room back to -18 then cuts out again, also good if someone leaves the lights on over night you know the product will be safe.
most door switches normally just kill the fans when door is opened, leaving the unit running without load, this results in heavy frosting of the evaporator and floodback to the compressor, so if the door is opened all the time for long periods the frost gets that bad that when the fans do start, they dont pull air through the cooler, and getting back to my earlier point when the evap is like this and then it goes into defrost you end up re introducing moisture/humidity back into the room on start up and then your on a hiding to nothing!

[airefresco]

Ah OK I get it now. I don´t think that is really necessary in this application. The room is quite small, probably not even 4m2. It is used as storage so it´s only opened for a few seconds while the staff get something out/put something away. Although they could do this several times a hour during busy periods.

What I have suggested to the client (which he agrees) is that he should have a second small chest freezer as a holding station. So in the mornings he gets what they will need from the main freezer and put it in the smaller chest freezer. This will severely reduce the opening of the door to only a couple of times a day and should help keep the temperatures/defrosts more stable.

Since the weekend, the freezer has been maintaining -17-18ºC constantly and seems to be working normal again now. With that in mind I am going to delay changing the drier and gas, etc until they have there next shutdown which is in November. I still want to check the defrost heater though ASAP to make sure that is OK.

Thanks for all the help.

[lma]

Cheers :) .. Just out of curiosity .. what is your defrost interval ?

[airefresco]

Currently everything is running back as it was originally. So, defrost every 4 hours for 30 minutes OR 4ºc, which ever comes first. I will raise it to 6 hours and 12ºc in the next couple of days.

[lma]

ok :)

Coolhibby.. advise not gone unnoticed. Do note your humidity level when setting your defrost (High humidity ... defrost interval is smaller... Low humidity .. defrost interval bigger). In your case Coolhibby, your defrost intervals appear to have worked due to your low humidity in your area ... Airefresco's is in a high humidity area.

If the drainage is blocked.. moisture condensate is re-introduced back into the room and re-condenses back on the pipe. If its free... it drains out... less moisture in the room .. less RH.... better coil efficiency

[joep]

I would set defrost to 4 per day at 30min with a 8 deg C termination.fans not to run on defrost, set fan temp to -2 deg C. Make sure sensor is not near a heater. I would also check superheat of valve or pump it down and check orifice, if black or burnt at end, replace it.

[Peter_1]

I haven't read all the post but what surprised me in the 1st post: the liquid line was frozen :eek:.

[acb]

thanks peter, ive just read all of this thread, listening to guys like grizzly and coolhibby pat each other on the back for things like defrost duration, door switches (get a room you two) while the whole time im shouting at my computer ITS GOT A FROZEN LIQUID LINE!!! so coolhibby what went over whose head hmmm. this guys freezer ran fine for a long time, ergo, nothing wrong with the initial defrost setup. really, some of you guys need to get back to basics.

rant over

[nike123]

I haven't read all the post but what surprised me in the 1st post: the liquid line was frozen :eek:.
Yep, that still doesn't mean that is frozen now.;)
At least, from his responses I cannot conclude is it frozen now or that was only occurrence while it was frosted all over. I suppose not, if freezer is down on set temperature now.

[coolhibby1875]

thanks peter, ive just read all of this thread, listening to guys like grizzly and coolhibby pat each other on the back for things like defrost duration, door switches (get a room you two) while the whole time im shouting at my computer ITS GOT A FROZEN LIQUID LINE!!! so coolhibby what went over whose head hmmm. this guys freezer ran fine for a long time, ergo, nothing wrong with the initial defrost setup. really, some of you guys need to get back to basics.

rant over

no ones disputing the fact about a frozen liquid lines, but there was more to the post than that, and if your that clever why has it taken some else to point it out rather than you doing it? and to say get back to basics, dont make me laugh, why dont you pm me a response as thats 2 posts you have tried to be smart in dont make it a 3rd,

[Peter_1]

Yep, that still doesn't mean that is frozen now.;)

It can't be frozen under any circumstances, even not temporarily.

[nike123]

It can't be frozen under any circumstances, even not temporarily.

It is not clear how much of liquid line was frozen.
I could imagine one scenario where evaporator is totally blocked with Ice, very little refrigerant is going thru expansion valve, liquid in expansion valve is heavily subcooled and line is lower temperature than freezing point (part of line which is inside of evaporator until TXV). Than some frost could be on that part of line.
Or not?

IMHO, what ever he is seeing while evaporator is heavily frosted is not much worth in diagnostic.

[Peter_1]

Mmm, liquid is coming in the freezer at least at ambient conditions which is warm to very warm (Lanzarote)
To subcool this liquid at perhaps low speeds and cool it finally below freezing point of water...???
If flow is that low, then compressor will be shut off on its LP I guess.
Haven't read all the posts, perhaps the answer was already given, don't have enough time for the moment.

[mikeref]

Airefresco, you may be back to the freezer alot sooner than planned if the compressor gives out because of likely contaminated refrigerant. Ref: burnt smell, unable to read sightglass indicator. If time is a problem, at least changeout drier.. mike.

[Gary]

Although the OP said the liquid line was frosted, none of the other numbers indicate a liquid restriction. If there is a restriction, it is minor.

[airefresco]

Regarding the liquid line freezing - It was frozen inside the evaporator, like Nike said. It was at ambient temperature at the condenser end.

[fridgability]

Something else to watch out for on single phase units with two or more evaporator fan motors, if a motor is faulty or has a faulty capacitor it can run backwards. Initally it does not start but the one or more good ones draw the air backwards over the fan blade and it eventually starts and runs backwards thus cancelling out the air flow of the good motors. The effect is no air flow over the evap with most of the symptoms described. Get someone to switch the unit on while you watch the fan motors start. Good luck.

[Peter_1]

Hey, that's a good one to remember.

[airefresco]

Yeah I have that before and always look out for that now. It must catch a lot of people out as when you look at the fan it looks like it is running and when they pick up a bit of speed, it´s difficult to tell which way they are going.

[Gary]

Yeah I have that before and always look out for that now. It must catch a lot of people out as when you look at the fan it looks like it is running and when they pick up a bit of speed, it´s difficult to tell which way they are going.

This tends to give you a very high evap dT.

[mad fridgie]

The unit is tired, suggest the discharge temp has been very hot, witch has given wear on the valves/rings (not totally rooted), this would give poor performance and also give the smell in the refrigerant,
If the comp is a danfoss, then there could be slight passing under the head (Danfoss have 2 steel bands to hold the internals of the compressor together (in stead of bolts) very difficult to diagnose the fault)

[airefresco]

I like that theory a lot as explains almost everything. Except that it is working fine now and has been for a couple of weeks, so if that was the case would the performance always be poor or would it be more of an intermittent thing?

[mad fridgie]

I like that theory a lot as explains almost everything. Except that it is working fine now and has been for a couple of weeks, so if that was the case would the performance always be poor or would it be more of an intermittent thing?
The ambient would of dropped a couple of degrees lately, so load is slightly reduced, actual discharge pressure would of reduced, net cooling would of increased slight due to this fact, giving the impression all is well (temp is meet) Run time would still be a bit longer than, if this scenerio was not there. If for example the system was down 20% in capacity, then it would struggle, in your hottest periods (which seems to be the case). I suggest you will have a similar problem next summer (getting worse over time)