Heat pump performing way below specification

ZirconGrayMarch 21, 2011

I have a problem with my heat pump and I am hoping that experienced forum members could provide me with 1) confirmation that I do have a problem and 2) what questions and investigation areas I should insist that the service tech address.

I have a 2 year old Carrier hybrid system (ARI# 3261198) that seems to be using twice as much electricity as it should. The 1993 built house is 3 levels and 3,400 sq. ft. and the HP is oversized by the installer's own admission (manual J = 3.5 ton). There are several symptoms that lead me to believe there is a problem: 1) when the outside temperature is 45-50F and running in Hi stage, it typically takes 50-55 minutes for each degree change in the house; 2) when I raise the thermostat one degree (outside=46/47F) to activate the HP, it starts in low stage (half fan speed) and 5-6 minutes the vent temp slowly raises to about 83-84F, then abruptly drops to ambient. At this point the system shifts to Hi stage (full speed blower) and ~5 minute ramp to 84-85F and stays there for about 1 hour. 3) The last indicator is the power bill. The balance point was set at 37F the first winter and 45F for the one just ending. With the 45F balance point, I spent $25 less this winter than last, even though this most recent winter was colder (3677 vs. 3377 heating degree days). So, is this enough to confirm that the system is broken? What should I say to the tech to convince him/her that there is a problem and what should I insist that they investigate and/or correct? I don't want to pay a service call only to get a "no problem found".

Other details that may be helpful: location is the greater Seattle area; winter rates = $0.09/kWh, $1.23/therm; setback to 63F at night, increasing 2F at 5AM, 2PM, and 4PM for afternoon/evening temp = 69F.

Thank you for any assistance,


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you will never get fast temperature increases with a heatpump. Even if outside temperature is 45-50.

Are you saying you have a 3.5ton and 3400 sq. feet and your oversized? I definitely don't think your oversized if you have 3.5 tons for that sq. footage.

I get about a 26 degree temperature rise coming from my vents when its 50 degree's outside. I have a single stage 3-ton heatpump with 3.5 ton coil with txv. This temperature is a achieved after its been running a while. It takes some time to get this temperature rise. It doesn't happen after only running a few minutes.

If you have dual fuel, don't be afraid to let the natural gas be used to bring up temperatures fast. Thats what gas is good for. If you have a good t-stat you should be able to control all of this. What your t-stat controller?

I personally wouldn't bother with the 1st stage of your heatpump when in heating mode unless its 50 degrees outside or above.

    Bookmark   March 22, 2011 at 7:45AM
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Here is the matching AHRI match with performance/eff numbers.

3261198 Discontinued Systems INFINITY 16 PURON HP CARRIER AIR CONDITIONING 25HNA648A31 CNPV*4821A** 58MV(B,C)100-20 45500 11.00 15.00 47500 8.90 27800

A nice system, probably overkill with the 95% eff furnace for a relatively mild climate like Seattle. It is unlikely you are oversized at 4 tons for 3400 sq ft. I assume you have the Infinity controller. Correct?

First, the stop and go between the stages of this mdl heat pump is normal. It is a feature that I don't like but is the way this condenser was engineered.

So to your problem. Two issues stick out. I don't like setbacks like you have. Heat pumps are good at maintaining temperature but can be very slow to recover under certain circumstances. I do not recommend a setback more than three degrees. Extreme setbacks like you have can be false economy. I assume your furnace is locked out at a specific outside temperature. But the low supply temperature at 45+ degrees outside temperature is not normal. It should be in the low 90s at a minimum if not higher. This suggests that your refrigerant charge is not correct. You need a seasoned Carrier Infinity tech with charging charts to check this out. I am assuming ductwork has been looked at for obvious leaks/restrictions. Evap coil also should be inspected for any buildup. Return ductwork should also be inspected. Split levels are notorious for heating and cooling. You have adequate return?

I would get several supply temp readings at different locations in your home at different times of day(early, midday, afternoon, evening). Document this with time, location, and outside temp.

do this over several days, review, and post back. then you have the ammunition to call installing dealer and complain with facts.


    Bookmark   March 22, 2011 at 8:05AM
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Thank you for your responses. Your insights and guidance are exactly what I was hoping for. To answer some of your questions: the controller is an Infinity (rev B); I'm certain the duct work leaks, but since all of it is enclosed in interior space (no crawl space, exterior walls, or attic) the leaks stay within the home. I had the air return in the basement improved and added 3 more vents when the system was installed; it can really breath now. It has a 20x25x4 pleated filter that I change regularly. The unit is 4 tons, but manual J calcs indicate it should be 3.5 tons. So, it's slightly oversized. I don't think I explained the setback properly: at 5AM it goes from 63 to 65F, at 2PM it goes from 65 to 67F, and at 4PM it goes from 67 to 69F. I don't think I'm asking too much of the heat pump. Should I change this? I'm not an HVAC pro, so I've been referring (incorrectly?) to "balance point" what you refer to as "furnace lock-out".

@tigerdunes: I'll inspect the interior air coil, but since the system has always performed poorly, I suspect that a dirty coil would show up as degrading performance over time? When I contact the service tech I will be certain to insist that they verify the refrigerant charge. But most importantly, thank you for the guidance on the vent temperature experiments. With that data I'll know that they can't give me a "no problem found". I'll report back when this is resolved so that other home owners in similar situations will be better armed.

    Bookmark   March 22, 2011 at 1:01PM
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OK, I ran some tests and gathered temperature rise data. I placed a bead thermocouple on the cold side (air filter housing) and the hot side (in the duct, 4" after the coil) and recorded the temperature difference at various run times. The CFM data came from the Infinity "service" menu, so I don't know how accurate that is. I have 20+ data points in a spreadsheet, but here is a smaller representative sample.
Cold side temp/hot side temp/CFM/outdoor temperature (all high stage)
62.6/91.4/1118/45 :: 62.4/92.4/1204/52 :: 65.2/87.2/1400/40 :: 61.8/87.0/997/36
Low stage data
63.4/72.2/882/44 :: 60.2/66.8/627/36 :: 62.4/70.4/627/39
I ran across an HVAC handbook that gave a formula for heat output:
BTU/H=1.08*(TempHot - TempCold)*CFM. If that formula can be believed, I don't think my heat pump is doing too good :(
So, is this enough data to draw any conclusions?

    Bookmark   March 24, 2011 at 1:16AM
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The Infinity heat pumps are only available in whole ton increments. Therefore if you manual J calculation was 3.5 tons, then the 4 ton size is correct. The AC and HP should operate in the low stage during milder periods. You would have had to purchase a single stage condenser in order to get a 3.5 ton unit.

Has your furnace and condenser had maintainence service since it has been installed? Maybe you are low on refrigerant or perhaps you have some other set up problem. I don't own a heat pump so I can't offer specific advice.

What are you gas and electric rates? I am confident you electric rates are lower than mine, but the cost of natural gas continues to drop. You should evaluate your furnace lock out and HP lock out temperatures. The Carrier 58MVC furnace is very efficient. You should be operating the majority of the time during the cold winter months.

    Bookmark   March 25, 2011 at 8:17AM
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UPDATE: the problem has been resolved and I'm providing this update to aid other homeowners who may have similar performance problems. Special thanks to Tigerdunes who instructed me to record the temperature rise across the inside coil. That data made it much easier to convince the techs that there was a real problem. To recap, the HP put out about 70-75% of what it should in high stage and 30-35% in low stage. Initial thoughts were leaky suction valve in low stage part of compressor. It turned out that the piston on the outside unit (heat mode metering device) was never installed. I was surprised that it could put out any heat at all. The tech conferred with his peers and learned that debris in the piston chamber or a wrong size piston would give below spec performance as well. So, if your heat output seems low or your are power bills higher than you expected, then check the piston.

    Bookmark   May 9, 2011 at 11:36PM
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When you say "heat mode metering device" on the outdoor unit, are you referring to the TXV (thermostatic expansion valve)?

The condenser's TXV was not installed at the factory?

    Bookmark   May 10, 2011 at 9:17PM
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@garyg - No TXV. I did some checking on other Carrier/Bryant heat pumps and learned that none of those had heat mode TXVs. They all use the older piston technology as the metering device. In cooling mode, however, it seems that they all have TXVs.

    Bookmark   May 11, 2011 at 10:41AM
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ZG: I'm not sure I fully understand this, but that's ok. Are you saying that the system ran fine in cooling mode but was terrible in heating mode? And have you been running it since repaired and do you notice the difference?

    Bookmark   May 13, 2011 at 8:15AM
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Weedmiester - The behavior was that the heat pump was putting out the heat of a 2.5 ton, but is supposed to be a 4 ton. So, it was sort of working, making it hard to detect the problem, outside of high power bills. As far as the AC mode, it seems to be OK. I don't use the cooling mode much ( 52 hours for the last 2 summers total), so it's hard for me tell. Once the piston was installed, the heat output was much hotter, almost double the temperature rise. It works great now.
Some HPs use TXVs in heat mode. The Carrier (and Bryant) HNA & HPA series use a piston for heat mode. The piston looks sort of like a large acorn nut with a small hole down the center and acts like a venturi. When the refrigerant reverses, the piston slides back allowing the liquid refrigerant to pass around it (cooling mode). In heating mode, it slides forward, and the smooth dome seals against the narrowing throat of the refrigerant line, forcing the fluid through the small center hole, i.e. venturi.
The point I wanted to make is that if a home owner has a heat pump that performs poorly, they should consider inspecting the piston for the right size, if it's there, and even if there is debris preventing the smooth dome of the piston from making a proper seal. It's not always dirty coil or low refrigerant charge.

    Bookmark   May 13, 2011 at 11:55PM
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