NORMAL HEAT PUMP SYSTEM PRESSURES
Someone used a search term referring to “normal heat pump suction and head pressures”. Air-to-air heat pump system pressures depend on the type refrigerant, the mode of operation (heat or cool), SEER value and entering air temperature at the indoor and outdoor coils, just to name a few variables. For R-22 systems operating in “cool”, normal suction pressure would be 70+/-, corresponding to a saturated coil temperature of around 40F. Head pressure will vary significantly with SEER and outdoor ambient. On a 90F day, head pressures 225-250 psi would be reasonably normal for 10-12 SEER equipment.
In the heat cycle, the suction pressure will follow the outdoor temperature. In order for heat transfer to take place, the saturated coil temperature has to remain well below the outdoor air temperature. I usually expect to see a 10-20F difference. If the outdoor temperature is 50F, the suction pressure would probably be in the 50-60 psi range. If the outdoor temperature is 25F, the suction pressure could be in the 30 psi range. Head pressures can be expected to run in ranges similar to the cool cycle. Heat pump system analysis in the heat cycle is usually less than absolute. If the unit has a P-T graph comparing system pressures to indoor and outdoor temperatures, that’s you best tool for evaluating the system. There are no superheat or subcooling values provided by mfg’s, to my knowledge.
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I have a carrier 4ton seer 14 that has a suction of 25psi and head of 175 with outdoor temp of 82. Suction tubing temp at 101deg. I replaced the indoor fan motor this winter and set it at the mid speed which was what the original was set at. I only get 70 deg air out of the indoor registers. Do you think I have an airfow problem or a low charge. The airflow I can do myself (electrician) but the R22 I’ll have to call someone
Mike Johnson - May 10, 2008 at 7:51 am
..the pressures would indicate some kind of refrigerant problem but the 100F suction temperature is near impossible…you sure you measuring the true suction line? and you are talking cool cycle?
wayneshirley - May 10, 2008 at 8:30 am
All I have is a Point thermometer measuring at the tubing at the low side just above schder vlv. Did find Ice on indoor coil this morning (package heat pmp) shut system down pressures at 110 removed ice and yea in the south in cooling mode..ha ha
right now at 9:45am its 78 deg outside inside temp at 70
low side temp 88 lp 18
Hp 140 SH 78 ??
Mike Johnson - May 10, 2008 at 8:49 am
..the most common situation is low charge, due to a leak somewhere in the system. A liquid line restriction or faulty TXV could cause some of the same symptoms. Low airflow would result in a low or normal superheat which you don’t appear to have. A low subcooling value will confirm low charge. Normal to high subcooling would suggest the restriction…that’s about the best I can do via the Net…
wayneshirley - May 10, 2008 at 11:01 am
My heat pump in the heating mode has about a 225psi low side pressure and 250 high side pressure. Is this normal? It s American standard 13 seer 3 ton. The outside air temp was about 67.
JC - September 25, 2008 at 6:27 am
..225 psi low side isn’t normal…if I had to guess, I’d say you’re measuring the pressures at the service valves, in which case both pressures are high side measurements. In the heat cycle, the vapor line (large tube) is the discharge from the compressor and the liquid (small) line is the condensed refrigerant returning from the indoor coil, also at high pressure. There is a constant suction pressure tap located near the reversing valve behind the removable panel on the side of the condenser. There is also likely a high pressure tap. The tap on the larger tube is the suction. At 67 OD temp, a more normal suction pressure would be in the 70-80 psi range for R-22 systems.
wayneshirley - September 25, 2008 at 6:47 am
I did take the measurment on the servicing lines. Would that then be a normal reading for the high pressure side? I am going to retake it in the morning when its cooler right now its 74. It was reading 110 on the suction tap on in the side panel coming directly from the compressor and 350 on the high pressure tap.
JC - September 25, 2008 at 10:05 am
..look at the service literature with the unit, attached to the electrical compartment cover plate…you should find a page of “pressure curves” which are plots of pressures vs outdoor temperatures for both the cooling and heating cycle…
There aren’t any absolutes with heat pump measurements in the heating cycle. All technicians can do is get close and wait till warm weather, where the cooling cycle operation can be adjusted to some design values…and at 74 OD, you should be able to get the charge adjusted in the cool cycle to meet the mfg’s recommendations, with superheat or subcooling.
wayneshirley - September 25, 2008 at 4:25 pm
I used to be an HVAC tech and I’m trying to find the problem with my heat pump in it’s heating cycle. I have a head pressure of around 275 on the suction line and a suction pressure of about 62 on the reversing valve port. Sounds good, right? Well…my problem is I’m getting 60 degree air out the register immediately off the plenum.
I’m getting fairly good air velocity, my evaporator and air filter are clean. Also, I monitored the pressures for 3 start-ups and they’re not hunting nor appear to be surging. The only variation I’m reading is on the discharge pressure upon start-up; I’ll read a 50-60 psi slow fluctuation (250-310) which balances out in about two minutes, then is stable at around 275.
My electrical is 7-8 FLA off the compressor on the C and R windings and the voltage is good (210) on a 208v system.
I don’t understand!! Why is my air cold off the register?
BTW, it’s 50 degrees outside. 1700 sf townhome, 8 YR old 2 1/2 T. 10 SEER.
Any tips?
jeff - February 21, 2009 at 12:19 pm
..where you measuring the 275 psi? vapor line service valve?
wayneshirley - February 21, 2009 at 12:30 pm
i think my indoor txv is stuck closed trane 14 seer heat pump 2 years old already replaced locked up condenser fan motor now my r 410 a system my suction pressures at 55 lbs and liquid is 205 what do you think
kevin - April 8, 2009 at 7:10 pm
..it’s possible…could be low charge, could be a restriction in the liquid line, could be low airflow. How does it do in heat? if the pressures are somewhere near normal in that mode, the indoor TXV would be a good possibility. Pressures alone are not enough to make a diagnosis. Need subcooling and superheat values to know more about what’s going on with the system…
wayneshirley - April 8, 2009 at 7:38 pm
I have a 2 ton 13 SEER heat pump and in cooling cycle, my suction is running 90 PSI and head pressure is 185. I get no heat rejection from condenser and little cooling. My belief is over charge or bad compressor. Could there be something else?
Mike - June 18, 2009 at 2:30 pm
High suction and low head are symptoms of a bad compressor. Something inside the compressor allows the gas to by-pass. Overcharge with fixed orifice metering devices will raise the suction pressure, but also the head pressure…unless the internal pressure relief opens, which would allow the suction to rise, but I’m not sure what the head might run under those conditions. But with that case, you should hear an abnormal compressor noise.
If the reversing valve was by-passing some vapor, the symptoms could be similar.
wayneshirley - June 24, 2009 at 6:08 am
I have a Tempstar 14 Seer R-22 heat pump that is not cooling. Suction pressure is 88 and head pressure is 175. Indoor temp. is 76 degrees and outdoor temp is 85 degrees. Do you think it’s a stuck open indoor TXV? The compressor does not sound abnormal.
Brandon - July 21, 2009 at 4:25 pm
..the suction looks a little high and the head a little low for the indoor/outdoor temps. But a quick measurement of the suction line temperature will confirm or eliminate the TXV as the problem. The saturated temp for 88 psi is 52 or so degrees. The suction line temperature would normally be 10-20 degrees warmer. If it’s closer to 52, you can suspect something with the TXV.
wayneshirley - July 21, 2009 at 7:53 pm
I have a Lennox split system heat pump, the system works good in cooling , however in the heating mode we are not getting warm air, Whats the best way to troubleshoot ? I suspect outdoor unit TXV valve but not 100% certain. What else would cause restriction of refrigerant back to compressor ?
Harvey - November 13, 2009 at 9:12 pm
..you need to confirm: 1) the outdoor unit is running 2) the reversing valve is switching to “heat”…to troubleshoot the TXV, you have to connect gauges/temp sensors to see what the system is actually doing…
wayneshirley - November 14, 2009 at 8:20 am
i am working on a 4 ton ruud heat pump. it works perfect in the cooling cycle, but in heating cycle it runs low suction pressure 10 or below and really high head pressure 400+ after just a little time the compressor kicks out and starts hissing. what could be my problem?
kasey - December 3, 2009 at 3:10 pm
Generally speaking, low suction pressure is due to undercharge, low evaporator airflow or liquid line restrictions. If it “works perfect in the cooling cycle”, undercharge and low airflow would pretty much be eliminated. The heat cycle problem would have to be a liquid line restriction upstream of the outdoor coil metering device or at the metering device itself. If metering is via a TXV, the valve could be faulty. Liquid restrictions don’t usually cause excessive head pressure…unless someone overcharged the unit, trying to get the suction pressure up. Depending on the indoor coil metering device, overcharge symptoms could show up in the cool cycle also as higher than normal head pressure / subcooling values, and higher than normal suction pressure and lower than normal superheat.
wayneshirley - December 3, 2009 at 5:53 pm
I have a Heat Pump that is not heating too well. I had a service tech come out and look at it, he added refrigerant, but its still not heating well. After reading you posts I’m wondering if he overcharged it, is that possible. Last night when it was 38 Deg.F Outside, the pressure on the Suction line was over 200PSI, judging by what is written abe it looks like I should be somewhere in the neigborhood of 40PSIG- Correct? Do I need to have someone come out and recover some refrigerant to get the pressures lower? Thanks
Bob - December 8, 2009 at 6:48 am
..If you’re taking the pressure measurement at the vapor service valve on a split system, the value is discharge pressure in the heat cycle…refrigerant flow is reversed, relative to the cool cycle. Sounds like maybe that’s what has happened. There should be a third schrader connection available to check suction pressure in heat mode.
wayneshirley - December 8, 2009 at 7:55 am
Correct, I was reading pressure on the suction line, The Larger of the 2 Lines. There is a schrader connection between the two lines, is that the pressure I should be concerned with in heat mode? So from the discussions above at an OA Temp of 38 Deg.F, I would want to see pressure of about 40PSI on that third schrader connection, correct?
Bob - December 8, 2009 at 8:49 am
“I would want to see pressure of about 40PSI on that third schrader connection, correct?”
..in the absence of service literature with the unit, I would say yes…keep in mind, the value is a “rule of thumb” I use to estimate correct system charge…I know of no accurate, absolute, manufacturer-approved method for adjusting heat pump charge in the heat cycle, short of removing and weighing in the charge.
wayneshirley - December 8, 2009 at 9:17 am
OK, I appreciate the feedback, at least that is a ballpark # that will let me know if it is grossly over/undercharged after the last service call….
Bob - December 8, 2009 at 9:20 am
I have a Lennox heat pump (6 years old) I don’t know the tonnage, Its too cold out to spend much time looking. I have a pellot stove to help out on cold days.
My question is at what outside temp. does the heat pump start to loose efficiency?
Chuck Metteer - December 12, 2009 at 10:44 am
The heating Btu output of a system is about equal to the rated cooling capacity at 47 F outdoor temp…a “3 ton” system (36,000 Btu’s) will deliver about the same heating Btu’s when the outdoor air temp is 47 F. The output Btu’s progressively decrease as the outdoor temp decreases. At 17 F, the output is approximately 65% of the rated capacity…
wayneshirley - December 12, 2009 at 11:00 am