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Thread: low high-side pressure, help
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06-06-2009, 04:02 AM #1
low high-side pressure, help
i am new to a/c but i am pretty knowledgable about it. i installed a brand new goodman 3ton heatpump split system, with matching down-draft air handler.
the lowest i can get the vent temp is between 59-62 degrease. the condensing unit came with a .073 orface, and the handler came with a .078.
i used the .073 first, but the temp was high, so i tried the .078 and it got better, but the pressures still wasnt right. 70psi on the low side and 155-165 on the high side. so i put in a .061 and the pressures to me were perfect, 70psi(low) and 240psi(high). i then tried a .082 and the pressures were off but the suction line was cold, but not cold enough. the suction line temp is 56 degrease and my vent temp is 57-59. so i know its not an air flow problem. and my hight side pressure is only 150psi. i also have little heat from the condenser and on the highside line. i have replace the suction line all the way, and checked the highside and found no restriction. me and my freind (wich has been doin this for years) are completely stumped. any ideas would be greatly appreciated. thanks
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06-06-2009, 04:18 AM #2
Re: low high-side pressure, help
Initial thought is overfeeding evap., ie ., high suction / low discharge. the compressor current draw would be high. Basically banging its gumbs and going nowhere.
Do a system analysis as in heat in,heat out.
Could be airflow, could be expansion device. Think , system balance and start again.
magoo
ps .., talk SI and you will get better response from forum
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06-06-2009, 05:26 AM #3
Re: low high-side pressure, help
Pressures are primarily a result of the temperature and volume of the air flowing through the coils. Orifice size has very little to do with it.
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06-06-2009, 06:25 AM #4
Re: low high-side pressure, help
Pressures are meaningless if we don't know what refrigerant is in the system.
I'll take a wild guess and say R22?
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06-06-2009, 08:56 PM #5
Re: low high-side pressure, help
yes it is r22 sorry
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06-06-2009, 08:57 PM #6
Re: low high-side pressure, help
Assuming you are charging to a particular suction line superheat, a smaller orifice will result in a higher subcooling. If the subcooling is too high, then liquid will back up into the condenser, driving up the high side pressure and that's a very bad thing. That's what happened with the .061 orifice. It is much too small.
Last edited by Gary; 06-06-2009 at 09:01 PM.
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06-06-2009, 09:15 PM #7
Re: low high-side pressure, help
You are trying to set up the system to match a pre-conceived notion of what you think the pressures and temperatures should be. It doesn't work that way.
All useful information comes from taking two temperatures and subtracting one from the other.
For just one example, the "condenser TD" is the saturated condensing temperature or SCT (from P/T chart) minus the ambient temperature. On a hot day the SCT (high side pressure) is higher. On a cool day it is lower. Whatsmore a high efficiency system will have a lower SCT (and TD) than a standard efficiency system, because the condenser coil has more surface area.
So, you can't tell if the high side pressure is right unless you also know the ambient temperature. And you also need to know the efficiency of the system.
And this is just for starters. There is a lot more to setting up a system than just a couple pressures and temperatures.Last edited by Gary; 06-06-2009 at 09:22 PM.
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06-06-2009, 09:42 PM #8
Re: low high-side pressure, help
As a rough rule of thumb, we would expect the TD to be about 20-35F (SCT 20-35F above ambient). Light load being at the low end and heavy load at the high end. At the same time, higher efficiency would tend towards the low end while lower efficiency would tend towards the high end.
I have no idea what your outdoor temp was at the time you were testing the system, but let's guess about 70F. Your SCT should then be about 90-105F. On a P/T chart for R22, this translates to 168-211psi.
If the ambient was cooler, then this should be lower. If the ambient was warmer, then this should be higher.Last edited by Gary; 06-06-2009 at 09:47 PM.
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06-06-2009, 09:51 PM #9
Re: low high-side pressure, help
I would start with the .078 orifice, and measure the air in/air out temps for both coils (temps close to the coils, not way out at the vents), the low and high side pressures, and the suction line (compressor inlet) and liquid line (condenser outlet) temps.
The temp at the thermostat would also be useful.Last edited by Gary; 06-06-2009 at 09:57 PM.
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07-06-2009, 02:33 AM #10
Re: low high-side pressure, help
ok lets see if i can give u everything u need. let me say thought that it doesnt seem to matter what the temp of the ambient is, the vent temp remains the same (56-58). but i try to keep the house at 70 and it will reach 76 in the heat of the day. the outside temp ranges from 78 in the morning to about 94 in the afternoon, with about 75 percent humidity. the suction line temp at the evaporator and at the condenser is 54-55. the hight side line temp ranges from 82-92, morning to afternoon. the unit is a 13seer. af far as your reference to high pressure, at 93 degrease outside temp, the pressure never gets above 165. and like i said, i know the .082 orface wasnt correct but it made the coldest air of all of the so i left it in there. i just replaced the unit because i originally installed a 3ton 13 seer air handler(new) with a 2 1/2 ton 10seer condensind unit(used). i had the exact same pressures and temps. i figured there was a problem with the used condensing unit so i changed it with this one. well that didnt fix it, so i changed the air handler(wich had a txv). and low and behold...same problem. i hope this info helps.
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07-06-2009, 03:05 AM #11
Re: low high-side pressure, help
Over condensing.
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07-06-2009, 04:48 AM #12
Re: low high-side pressure, help
The temp of the air in the room may not be the same as the temp of the air entering the air handler, and the temp of the air leaving the air handler may not be the same as the temp of the air at the vent. We need more accurate measurements closer to the coil.
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07-06-2009, 05:08 AM #13
Re: low high-side pressure, help
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07-06-2009, 06:10 AM #14
Re: low high-side pressure, help
If we can believe the low side gauge, which is uncertain at this point, then the SST is about 41F. The suction line is 55F, so 55 - 41 = 14F superheat, so we are picking up a heavy heat load with a fully activated coil. So why isn't it cooling your house? Either it isn't large enough to do the job or you are trying to cool the world (leaking and/or uninsulated ducts in hot areas?). Is this an attic unit?
Air in/out temps at the air handler will give us a better idea of what is going on with this.Last edited by Gary; 07-06-2009 at 06:23 AM.
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07-06-2009, 07:32 PM #15
Re: low high-side pressure, help
it would seem the you are basing my sct on line pressure and stating that its impossible. but my guages work fine, ive even used other sets. you sais yourself the you cant base temp on pressure. with a 92 degree ambient temp the high side pressure IS 165psi.
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07-06-2009, 07:35 PM #16
Re: low high-side pressure, help
my low side pressure is 70psi exactly no matter what the ambient temp is. witch is the correct pressure for the r22 to flash at 38 degrees. but its not, its flashing at 54 degrees.
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07-06-2009, 07:51 PM #17
Re: low high-side pressure, help
Where do you have the high side gauge connected to the system?
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07-06-2009, 08:40 PM #18
Re: low high-side pressure, help
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07-06-2009, 08:47 PM #19
Re: low high-side pressure, help
Last edited by Gary; 07-06-2009 at 08:59 PM.
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07-06-2009, 10:08 PM #20
Re: low high-side pressure, help
If there is a restriction in the line between the condenser and the gauge port, then this explains everything. A restriction would drop the pressure and the temperature.
If I am not mistaken, there is a check valve and orifice in this line, which should provide full flow in cooling mode and metered flow in heating mode. Being in cooling mode it should be providing full flow, therefore there should be no temp drop across it. Check the line temps on either side of it.Last edited by Gary; 07-06-2009 at 10:13 PM.
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08-06-2009, 03:09 AM #21
Re: low high-side pressure, help
i connected the guages to the taps at the shutoff valves outside the condensing unit. where is this check valve? and is it possible to have 2 units in a row with the same vale bad? also something else that had me concerned is that i had to install 2 90degree elbows at the condensing unit. they are a sweep 90 in the high side line, and they are about 9 inches apart. would this make a difference? my guages are attached to the condenser before the lines. i would think that if there was a restriction in my lines, it would show high pressure, right? i really believe that is just not compressing the refriderant enough to make it flash at 38 degrees. but i dont know why. judging by the way it act , i would agree with you about there being a restriction or a check valve to be faulty, i also find it hard to believe that 2 completly different units can have the same symptoms. so what could remain the same after everything has been changed?
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08-06-2009, 03:29 AM #22
Re: low high-side pressure, help
Here is a link for a heat pump installation manual:
http://www.alpinehomeair.com/related...ing%20Inst.pdf
Check out the piping diagrams on page 10. Note the liquid line has a check valve/orifice at each end near the bottom of each coil, with the service valve in between.Last edited by Gary; 08-06-2009 at 03:38 AM.
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08-06-2009, 03:55 AM #23
Re: low high-side pressure, help
thank you for the link, that is the exact manual for my unit(lol). i see the check valves you are refering to, and its also the expansion valves, i know for a fact that the valve in the air handler is fine and workin properly, the one in the condensing unit is another story but i really dont have any reason to distrust it being as its new and i have the same issue as the old one. ive even been as far as to check the voltages to make sure it wasnt low and causing it to run slow but thats kinda how it acts, like its just not pumping enough.
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08-06-2009, 03:58 AM #24
Re: low high-side pressure, help
how much differential (between high side and low side) does r22 need to flash at the correct temp?
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08-06-2009, 04:03 AM #25
Re: low high-side pressure, help
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08-06-2009, 04:08 AM #26
Re: low high-side pressure, help
i also understood the the purpose of the condensing coil was to cool the refriderant and tranform it from a gas to a liquid but allow it to stay at a high pressure state. by this understanding, u pressurize the refriderant(causing heat) but then the heat is removed by the coil so u have u high pressure cool liquid, right? but it seems the compressor isnt making enough pressure to cause any heat, like i said before, there is very little heat coming out of the condenser even in the heat of the day. also seeing that u are in port richey u know how hot is been lately, and im only about and hour and a half from u. ive never seen anything like it.
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08-06-2009, 04:11 AM #27
Re: low high-side pressure, help
if u have a low side pressure of 70psi.
witch is the pressure in witch the r22 is supposed to flash at around 38 degrees, is there a certain high side required to make this happen?
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08-06-2009, 04:13 AM #28
Re: low high-side pressure, help
In your original post you said that the indoor unit came with a .073 orifice and the outdoor unit came with a .078 orifice. Perhaps you were supposed to install them?
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08-06-2009, 04:16 AM #29
Re: low high-side pressure, help
say for example, i have a high side pressure of 165 psi, and a low side pressure of 70 psi, that make a differential of 95 psi. so that means there is 95 psi of liquid pressure on the expansion valve and goin through it. is that enough? in my past its usually a diffence of about 170 psi.
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08-06-2009, 04:21 AM #30
Re: low high-side pressure, help
i assumed that the orifice that came with the condenser was for the evaporater, and i assumed this because the unit was factory charged and it would have meant evacuating the system to install it in the condenser. the one that came with the evaporater was installed so i really dont know
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08-06-2009, 04:24 AM #31
Re: low high-side pressure, help
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08-06-2009, 04:28 AM #32
Re: low high-side pressure, help
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08-06-2009, 04:33 AM #33
Re: low high-side pressure, help
ill try it. now you are refering ti the heatpump orifice, right? did u understand what i was aking about the differential pressures?
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08-06-2009, 04:36 AM #34
Re: low high-side pressure, help
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08-06-2009, 04:38 AM #35
Re: low high-side pressure, help
Last edited by Gary; 08-06-2009 at 04:42 AM.
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08-06-2009, 05:25 AM #36
Re: low high-side pressure, help
While you are in there, hook up a gauge to the discharge line access port near the compressor. This will tell you what the condenser pressure really is.
If the discharge pressure is considerably higher than the liquid pressure, this would positively confirm the restriction.Last edited by Gary; 08-06-2009 at 06:12 AM.
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08-06-2009, 06:17 AM #37
Re: low high-side pressure, help
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04-07-2009, 06:05 AM #38
Re: low high-side pressure, help
mr gary, i took the system apart today, i found nothing wrong. the only thing i found is the filter/dryer on the liquid line seemed to have some restriction, so i replaced to- to no avail. i understand that when r22 is compressed through an orfice and enters a 70psi condition, is supposed to flash at 40degrees. if i place my temp probe just beyond the indoor orfice to the lines (with the probe wrapped in insulation tape to the line) i get a temp of 54 degrees. why is this? i dont understand why the refrigerant isnt flashing at the correct temp. what could possibly cause it? like i said before, i am an amature but i have never had this much trouble and i can usually figure it out but im stumped.
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04-07-2009, 08:12 AM #39
Re: low high-side pressure, help
Could it be possibly that the compressor internal suction reed is leaking and giving you wrong pressure readings.Maybe pumpdown the system and see if the reeds hold the pressure
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04-07-2009, 04:35 PM #40
Re: low high-side pressure, help
Did you find the check/orifice in the outdoor unit?
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04-07-2009, 05:25 PM #41
Re: low high-side pressure, help
Did you hook up a gauge to the discharge line access?
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04-07-2009, 05:30 PM #42
Re: low high-side pressure, help
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04-07-2009, 05:42 PM #43
Re: low high-side pressure, help
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04-07-2009, 05:48 PM #44
Re: low high-side pressure, help
ok tell me exactly what temps you want to know and where also keep in mind the temps change by time times if day. i use u mulitimeter with a temp probe. i also found the outside orfice to be in correctly and free (its a .55)
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04-07-2009, 05:52 PM #45
Re: low high-side pressure, help
should the refrigerant passing through the orfice make a constant hissing sound or would it sound erratic?
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04-07-2009, 06:45 PM #46
Re: low high-side pressure, help
Evap air in temp
Evap air out temp
Low side pressure
Suction line temp at outdoor unit
Cond air in temp
Cond air out temp
High side pressure
Liquid line temp at outdoor unit
Discharge line pressure would also be helpful here.
What I am looking for is a snapshot of how the system is performing, so all of the measurements should be taken within a reasonably short period of time.
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04-07-2009, 07:32 PM #47
Re: low high-side pressure, help
High temp liquid exits the metering device and then flashes off to reduce its temp to saturation. The temp in between can be anywhere in between. The correct temp at this point is "whatever it needs to be". So... measuring the flash temp tells you absolutely nothing useful.
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05-07-2009, 05:32 AM #48
Re: low high-side pressure, help
ok the stat says 76 room temp
evap-air in 76
air out 65
condensor- air in 79
air out 88
suction line temp 72
liquid line 89
low side pressure= 66 psi
high side pressure=170 psi
the low side pressure reading was taken from the direct low tap at the compressor(wich is the same pressure as the shutoff taps)
i hope these measurments are the ones you need and thanks for all your help
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05-07-2009, 06:31 AM #49
Re: low high-side pressure, help
Was the high side pressure taken at the direct high side tap at the compressor? I'm wondering if it is the same pressure as the high side shutoff tap.
Last edited by Gary; 05-07-2009 at 06:35 AM.
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05-07-2009, 06:55 AM #50
Re: low high-side pressure, help
these readings were taken at 11 pm which is why the low side pressure is less than 70 psi, the day it reaches 70, which is what i charged it to.
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