We have a Rheem Classic X Model # RPKA-036JAZ heat pump.

It is installed on the south-west side of the house where it gets a lot of sun during the winter, when there are no leaves on the trees. (In the Greensboro, NC area.) The indoor unit is located, at the same level, in the crawl space and there is only 16 feet of piping connecting the two.

On moderate spring days, this time of year, where the average early morning temperatures are 40 to 50 degrees and the outside unit is on the shady side of the house everything appears to be working properly. The pressure gauges read 40 to 65 PSI on the low side and the high side reads in the 125 to 250 PSI range (of course based on indoor/outdoor temperatures.)

However, during mid-afternoon when the outside temp is 65 degrees or warmer and the sun is beating down on the outside unit and it warms up to 75 to 90 degrees after sitting idle for an hour or so... and the indoor unit is located in the crawl space where it is 55 to 65 degrees.

Here is the problem, only during the warm afternoons, after the unit has not ran for an hour or more and the thermostat calls for heat, for the first 30 to 60 seconds the high-pressure gauge shows 350 to 400 PSI and this high pressure will cause the over-pressure safety switch to shut down the compressor.

It has a button on the exterior of the cabinet that has to be pushed to reset the safety switch. If I wait for a few minutes and push the reset-button, the compressor will start-up again. This time the pressure doesn't climb high enough to cause it trip the over-pressure switch, but the pressure runs in the 320 to 340 range for about 5 minutes and then settles in between 125 and 250 according to the ambient air temp.

Although, this seems it could be an over-charge problem, during the cooling mode, all pressure/temp measurements indicate an under-charge condition in the unit.

We have checked all the usual trouble-shooting tips like air flow and temps across the coils and blockage in the expansion/check valves, but this one has all of our service guys puzzled because the total system works very well under all of the other conditions except the warm afternoon problem described above.

Thanks to any of you that may provide any ideas as to how we can find what is wrong with this system.

I would want to know the exact indoor, outdoor temps. High and low side pressures. Liquid and suction line temps at the condenser before I can make a more educated decision. After 15 to 20 minutes of operation, indoor humidity can be helpful too, but not so much.

Actually I would like to know if the poster has done there testing and has an approved EPA ticket to tap into a refrigerant system legally. It seems we have a lot of freon jockeys here who operate outside of the laws.

If they had there certification they would not be asking these stupid questions because they would already know the answers.

By reading your question it sounds like you know a little about heat pumps. Do you have a TXV at the heat pump and evaporator coil? If so did you have freon added to the system last summer. Rheem is known to have a bad TXV @ the evaporator coil (for cooling). If freon is added not knowing better you will have this same problem in the spring and or the fall.

NorthenHeat, ran the unit in cooling mode all afternoon today. The outside ambient was between 75 and 81 with the RH at 44% during this period. The inside was 70 to 72, RH in the 35 to 40% range. All indoor air temps taken at the return grills and air at the registers (actually just inside of them 4" into the ducts) averaged 50 degrees. In other words, in this fine spring weather it puts out some really cool air.

Low side pressure was taken between the reversing valve and the accumulator (as the manual states) and was 60 to 63 all afternoon. Low side temps at the compressor inlet 33 to 40 and 45 to 60 upstream of the reversing valve (taken on the plumbing inside the cabinet). The high side ran 130 to 140 and the line temps ran 80 to 85, again taken from the lines inside the cabinet. The super-heat calcs out a little low so I'd say today it appears to be slightly overcharged...but does that cause this right-after-startup-overpressure in heat mode during moderate afternoons when it is 60 to 70 degrees outside?

hvac1000, personally been installing heat pumps and working on automobile A/C units since 1967...way before the tree-huggers and f*%&ing politicians got involved in the business. And yep, two of us here are fully legal with EPA cards and we hold electrical tickets for our county. Does this make you sleep better now?

And another one to make you laugh hvac1000...our distributors rep has been here and can't get the system to work "great" in both heating and cooling modes. He has had us adjust the charge from a little high to a little low with high helping in the heating mode and low helping improve the cooling mode...as would be expected. So, I suppose he must be one of the stupid uncertified ones you are referring to.

T-Top, nope...no TXVs on this one. If so I feel I could fix it. Worked on hundreds of GM A/C's back in the '70s. They were one of the most complicated, but best cooling car A/C's ever built and they all used a type of thermal expansion valve.

On this beast...at both ends are their standard el-cheapo "flow check piston-distributor assembly's" controlling the flow...and yes they are correctly sized for this system, as it is installed (according to the manual).

Again guys, the puzzling thing is that this problem only occurs in the heating mode when the outside unit has been "off" for a few hours in the warm sun. The inside unit is located in the much cooler crawl space. Some days the house is cooler than the outside air during mid afternoon this time of year and the thermostat calls for heat... only then do we see this very high pressure during the first 30 to 40 seconds right after startup. Again, it does NOT do this when the outside unit is cooler than the inside unit located in the crawl space that is in the 55 to 65 degree range year around.

In other words, the unit works OK, in the heat mode, during cool weather and it works OK, in the cooling mode anytime.

This is a 3 ton unit and the house is about 1600 SF (on two floors) with low window to floor space ratios and has decent insulation throughout. If anything it calcs out to be over sized for the house (which, of course, would have nothing to do with this startup over-pressure issue).

Thanks for any thoughts... but no thanks for the political stuff... that may help us solve this one.

On hot startup does the condenser fan start as soon as the compresor starts?

Is the outside condenser coil clean?

Is the inside evaporator clean?

Is the inside fan starting as soon as the compressor starts outside?

I am sure you checked for blocked/dirty air filter?

How long is the line set?

How high or low is the line set from evaporator to condenser?

A liquid flood back on the suction side will cause high innitial head and trip of pressure switch.

Have you recovered the refrigerant and weighed in a new charge? Some units are sensitive to over charge on start up?

Head will be high at 65 degrees F outside in heat mode. Jump head pressure control temporary to see if it exceeds 500 psi before droping down.

Find out the original spec of the head pressure control to see exactly when it should trip. Test the one installed as mentioned above. If there is a difference replace it.

I believe you might have an overcharge condition. I would bet on it. Recover and Vac well then weigh in charge according to the pound/ounce lister for the equiptment.

Everything points to low indoor airflow. Heat pumps need more indoor airflow than does A/C and everyone sets the heating blower speed up to med/low. Even looking at your superheat in cooling mode sugests low airflow, or non-condensibles.

Another problem I have with new installers and service tech's is how to determine if non condensibles could be the cause of super heat issues. Air and non condensibles cause flashing in the liquid line, it inflates the evaporator pressures and retards the coolant evaporation, however your evaporator split doesn't suggest that. Listening for unusual pulsating refigerant flow at the piston is another good indication of non-condensibles.

In short, the vast majority of the time, when the A/C runs very well and the high pressure cut out trips when in heat mode it has been airflow issues.

On hot startup does the condenser fan start as soon as the compressor starts?
ANS: YES, at the exact same time.

Is the outside condenser coil clean?
ANS: YES, very clean. Pulled the fan and gave it a light pressure washing from the inside out.

Is the inside evaporator clean?
ANS: YES, looks almost new considering the sytem is over 10 years old. (It was washed too.)

Is the inside fan starting as soon as the compressor starts outside?
ANS: YES, and it makes no difference if the fan is set to run continiously.

I am sure you checked for blocked/dirty air filter?
ANS: YES, all are new.

How long is the line set?
ANS: 16 ft (as stated in the first post)

How high or low is the line set from evaporator to condenser?
ANS: Almost level with each other. Outside unit/coil about 2" higher than the inside coil and the line-set is blocked up from the crawl space floor with maybe a slight amount of "droop" (less than an inch or two). We are considering rasing the inside unit a few inches to help with the potential flooding that may be occuring. All the ducting is flex, so not that hard to do (for some skinny guys...there is only 20 to 24 inches of head space under there).

A liquid flood back on the suction side will cause high initial head and trip of pressure switch.
ANS: I suppose you mean the small line (suction in heat mode) feeding "toward" the outside coil (that is the evaporator in heat mode). We don't think so because the outside unit is physically higher than the inside coil. Additional info: there is no pressure difference in the lines (where they enter the outside unit) when this over-pressure condition occurs immediately after start-up and then both gauges settle down to a normal reading within 3 to 5 minutes. Another point we have considered; the inside line-set and coil is much cooler than the outside when this startup problem occurs. The physics tells me that the freon should be moving toward and collecting in the inside coil and/or line set during the two to three hours while the unit is not operating. We are "guessing" this may contain a clue. Maybe the line set and to some extent the inside coil may be flooded with just enough liquid freon, so that when the conditions are perfect, the fixed orifice in the "flow check piston" feeding into the outside coil is over whelmed for these first few minutes.

Have you recovered the refrigerant and weighed in a new charge? Some units are sensitive to over charge on start up?
ANS: Yes we did a couple of months ago when it was too cold to run any super-heat test in the cooling mode. 120.5 oz (Calls for 125 oz minus 4.5 oz for a 16 ft line set, instead of the 25 ft line-set use as their standard). We also included some florescent dye and using our infra-red, we found no leaks.

Head will be high at 65 degrees F outside in heat mode. Jump head pressure control temporary to see if it exceeds 500 psi before dropping down.
ANS: Not something I care to do...to chicken to get close enough to watch the gauges. Have already ruined two gauge hoses and they were part of my best quick-connect set. Besides, we can prevent the switch form tripping by starting the unit, letting the pressure build to about 350 and then cut the power, wait a minute or two, then power it back up and it will usually continue without exceeding 400 psi where the pressure switch will cause the disconnect.

Find out the original spec of the head pressure control to see exactly when it should trip. Test the one installed as mentioned above. If there is a difference replace it.
ANS: Like I mentioned in the last paragraph it appears to happen just at the high-side gauge reads 400 psi...but this happens so fast that you can hardly tell when/where the needle stops.

I believe you might have an overcharge condition. As a matter of fact I would bet on it. Recover and Vac well then weigh in charge according to the pound/ounce lister for the equipment.
ANS: I'm betting the same thing...but if we lower the charge any more then the darn thing calls for it doesn't keep the house warm, unless it switches over to the heat strips.

hvac1000, thanks for your excellent questions. I over-simplified my answers in the event some of the "less-informed" happen to read our replies.

Everything points to low indoor airflow. Heat pumps need more indoor airflow than does A/C and everyone sets the heating blower speed up to med/low. (Did you mean lower the speed or raise it during the heating season?) Even looking at your superheat in cooling mode suggests low airflow, or non-condensibles.

Another problem I have with new installers and service tech's is how to determine if non condensibles could be the cause of super heat issues. Air and non condensibles cause flashing in the liquid line, it inflates the evaporator pressures and retards the coolant evaporation, however your evaporator split doesn't suggest that. Listening for unusual pulsating refigerant flow at the piston is another good indication of non-condensibles.

In short, the vast majority of the time, when the A/C runs very well and the high pressure cut out trips when in heat mode it has been airflow issues.

NorthernHeat, I totally agree with your comments. However, you seem to have missed the main point. This almost instant over-pressure is happening only when the conditions are as I previously mentioned and they are happing so quickly that the fans have not ran long enough to make any difference. (See my answer(s) to havc1000). We have tried with the inside fan running continuously.

During the cool-mode testing yesterday it was almost to cool to get decent superheat readings and they indicate a slightly over-charged system... but then a building that is already cooled (and this one was quick to cool during yesterdays weather here in mid NC) will produce similar results. Agree?

I'm guessing there is a puddle of liquid freon somewhere in the interior line set or in the inside unit causing this to happen even when the system is properly charged. And the manufacture is using the most rudimentary components (cheapest) hoping that most customers will not have a similar problem.

I would jump the head pressure switch just to see if it is causing the problem.
At 65 degrees you are at the high output end of the heat pump spectrum. There could be just a flash at 450 lbs and that is triggering the switch and the failure of the unit to keep running.

Remember many units do not come with a high pressure switch at all. There is a secondary safety built into the compressor IE internal relief that will protect it. I think you just have a initial spike of pressure and as you said it settles down there after. Could very well be a defective switch if not an overcharge.

You did a good job of giving us the pressure and temp reading the cooling mode. Wrong mode if the problem is in the heating mode that's the info we need.

T-Top,

I provided the cooling numbers for this unit because this is what the manufacturer's manual tells us... (and it follows the convention of many others that use a "fixed orifice flow-check-valve." As you may already know, there are no TXVs on this unit and most other low-cost systems). Anyway, I'll quote one of the books:

"Final charge adjustments MUST BE MADE IN THE COOLING MODE and then ONLY by superheat if the unit uses fixed orifice flow-check valves and ONLY when outdoor ambient is above 60°F. If outdoor ambient is below 60°F, adjust charge by weight ONLY and recheck later when ambient is above 60°F. Heat pumps should also have performance checked in heating mode. However, the only acceptable method to charge a heat pump, in the heating mode, is by weight."

We have had the gauges on the system for the past two weeks during heating and cooling, and based on the pressure/temps we saw, they are useless to determine the charge during the heating mode... just like the books tell us.

If fact, this time of year, this house cools down so quickly, (it warmed to 75 yesterday with it 80 outside, only after leaving all the windows and doors open from 11:00AM until 3:00PM). So, when trying to fine tune the charge using pressure/temps to add or remove freon, you have be quick and careful or use some electric space heaters inside the building to keep it warm enough for the system to stabilize... which we did. We ended up removing almost 2 oz of freon in order to get a slightly better spread on the superheat. The cooling is working fine, so for now, we'll wait to see if this helps the next time it is cool enough, outside, to operate the system in heat mode.

Thanks everyone for all the help. We are about ready to "throw in the towel" on this one and replace the total system with a smaller more efficient system. You may remember from one of my previous post, the system is an el-cheapo piece of crap and based of my calculations is oversized for this small house. Just another example of a bad contractor trying to get away with the lowest bid.

Oh, did I mention that when we were first called in for poor performance (in both heating and cooling) we found five of the supply side flex connectors "blown" off in the crawl space and two of the return ducts were completely separated from the filter-return-boxes in the attic. Exactly what you'd expect to cause poor performance, blowing most of the air into an unsealed crawl space and drawing in hot or cold attic air at the returns. I can see why the poor souls were complaining about the electric bills and an uncomfortable moldy smelling house. (The crawl spaces around here have a lot of mold.)

The original installer(s) were in such a hurry that they did not even reach in the boxes and bend the flex connector tabs. Hence, they fell off. And the system has a city/county inspection sticker on it... humm...

Initially, late last fall, we fixed the improperly installed duct-work and the occupants were very happy. We left thinking this was the only problem... until the pressure switch tripping problem surfaced when the weather warmed a few weeks ago.

I'll come back to check this system if it cools down enough... otherwise I'll wait till next fall.

Again, thanks to everyone.

Sorry to step on your toes but if the problem is in heating, that's the info that is needed. Rheem has a check valve and a screen that works with the heat-pump.(liquid line right side of the compressor and reversing valve) the freon flows two ways with the heat pump. I would check the filter screen past the " T " in the liquid line. Good luck.

T-Top,

Sorry if I offended.

For what it is worth, I have only had experience with a few Rheem systems and all are from the late 80s early 90s and almost identical except for the scroll compressors. I'm an old Lennox and Carrier guy and think... although most are a bit more complicated... they are better engineered. In other words, the Rheems I've been exposed to have almost identical components as some of the "no-name" over-simplified bargain-basement systems that barely work (efficiently) when the conditions are not ideal.

Yes, we did check the "liquid line tee" that goes to the "compensator-muffler" and found no obstruction (if that is what you are referring to).

This morning it was cool enough to run the unit in the heating mode and it is holding on. (No tripping of the over-pressure switch after removing 2 oz of freon.)

Like I said earlier, we'll most likely replace this relic with a more efficient and smaller system. I only started this post, because all the guys I spoke with around here had never seen this problem either. I guess most of the systems they see are slightly undercharged because that is the only way this one will run consistently.

Thanks again for all the suggestions.

Not at the muffler that's the discharge line on the compressor. Check from the service valve of the unit back to the compressor - about 6 inches away the 3\8 line will have a bulge in it(1/2 inch diameter). Did you check it? It does not take much to restrict the flow of freon.

T-Top,

Yes we checked all the components for restriction, including the part you are pointing out.

Think about it. If there were any restrictions in the liquid line, there would be a sizable temperature difference in proximity to the place where the restriction occurs.

Want to test this yourself? Just crank down on the liquid-line valve, where the line enters the cabinet, until it starts to slow down the flow. Now take some temp measurements on each side of the valve. You will see what I'm trying to tell you. (That is how I've been finding restrictions for almost 50 years, careful temp measurements of the components and their connections. Works for me every time.)

Besides, you seem to have totally missed the point. As mentioned several times above, this system works just fine and all pressures and temps are normal (in the heating mode) when the outside temperature is less the 50 degrees. If there were any abnormal restrictions they would not show up just because the out side temp gets 15 degrees warmer.

Read again hvac1000's answers, he understands the problem.

Thanks again for all the suggestions.

Is the diameter of your line set 3/4 and 3/8?

After going back an reading your question again I see the problem. No heat pump will run right in the heat mode with the ambient temp at 75 to 90 degrees on the condenser coil. Good luck.

T-Top, why are we not understanding each other. I suppose I did not do a very good job of explaining everything.

In the first post I said "However, during mid-afternoon when the outside temp is 65 degrees." (Maybe I should have used the word ambient too). And I should have made it clearer that it is just the cabinet that gets warmer on the outer surface (75 to 95 degrees) not all the components inside. The point I was trying to get across was that when the problem occurs, the outside unit has been sitting idle for a few hours, in a warmer environment than the inside unit located in a very cool crawl space... usually by about 20 degrees cooler. A very normal thing around here this time of year... wouldn't you agree that this is not an unusual situation and that the system should not be tripping the over-pressure switch?

I also tried to make it clear that these were short term conditions, just on these types of days and that the problem only occurs during those exact conditions. I also wanted to get across that the over-pressure problem is almost instantaneous, occurring just after start-up ONLY after the system has been sitting idle for an hour or longer.

For example, on the days when this would happen, before 10:00AM the outside ambient temp was usually in the upper forties to the middle fifties. And only on the days that the sun was out and shinning brightly on the outside unit did we see the problem. Never did I get any reading of the ambient over 65 degrees (always taken in the shade) and it was always warmest only between 2:00PM and 3:30PM. This is normal this time of year in these parts.

It is also normal for the thermostat to call for heat this time of day when the house is well insulated and the shades are drawn. In other words, there was not much thermal gain (inside the house) just because it was sunny outside. The thermal mass of the building materials and especially the much cooler area below the main floor "sucked" enough heat from the air inside the house that the thermostat would occasionally call for heat even though it was comfortable outside in the sun. Again, very normal in these parts.

Yes, I completely agree that heat pumps built with only the most rudimentary controls will not be able to handle outside ambient in the 75 to 90 degree range... although I know of a few dozen installations where the systems are working just fine to warm some offices located below ground level that need occasional heat even on very warm days. However, these systems have more sophisticated controls and TXVs that do a much better job under extreme conditions.

PS; you seem to be very familiar with the Rheem systems. Are you a Rheem representative or dealer?

No not a Rheem rep. or dealer. Tech in the mid-south that's been working with heat pumps for over twenty years. At one time Rheem was big in this area and now I get to try to keep them running.