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  1. #1
    Join Date
    Dec 2010
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    20

    Temperature at Vents - Carrier Heat Pump

    I have a 25HCB636A300 condenser with a 4 ton coil. It has a Performance Boost 90 gas furnace for auxiliary heat.

    It is 31 degrees outside. Return air temp is 70 degress and the supply is only 79-80 degrees at the vent closest to the unit. Is this reasonable or should the temperature at the vent be higher? The current cycle is 40 minutes and the aux heat has still now kicked in so it must be barely keeping up with demand.

  2. #2
    Join Date
    Aug 2010
    Location
    Monroe County, PA
    Posts
    99
    No one has tackled this lonely little post, so I'll throw in a quick response:

    A 10 degree delta isn't all that unusual for a heat pump - it works by turning over the air in a room multiple times through the heat exchanger instead of just pumping a bunch of hot air into a room (like a furnace) and hoping it mixes with the ambient air. So as you probably already know, it's not designed to be as hot as a furnace would be.

    On the other hand, at 31 degrees outdoor temp, my Heat Pump (different brand) kicks out heat at nearly 90 degrees, depending on where it is in its defrost cycle.

    So a few thoughts:

    1. The heat pump may not be going into 2nd stage properly. If in first stage only, it is going to be generating less heat than it is capable, and especially if it is not a variable or 2-speed fan it will feel colder at the register

    2. The fan speed settings could be too high. In general, the fan should be moving 400 cubic feet of air per minute per ton of heating. So a 3 ton unit should be running 1200 CFM. But if it is operating in only stage one, it is typically at 2/3 of heating capacity and the fan should be pushing only 2/3 of the amount of air as well. If it is pushing through too much air it will feel colder at the register. If your fan is set to push more than 1600 CFM (4 ton heat pump) it will be colder than expected, even when the pump is in 2nd stage.

    3. As the heat pump runs it often builds up frost or even ice on its outside coils - especially in temps near freezing since there is more water in the air as it gets warmer. Heatpumps have different ways of determining when to defrost this ice off of it, but until it goes through the defrost process it continually gets less and less efficient. The 3 or 4 minutes right after a heatpump have been defrosted are when it is at its most efficient. So the 10 degree delta you feel may be more or less 20 minutes before or after you take that measurement.

    4. When the heatpump is in defrost (2-8 minutes usually) it is actually an air conditioner - blowing COLD air into your house. During this period it should turn on its electric coils or fire up the furnace to compensate. During this 2-8 minute period the air at the register may be the same as room temperature or even below depending on how many electric coils or how strong your furnace is.

    5. The return nearest the furnace may not be the best place to measure. Air needs to mix - a duct line taken off the furnace too close to its outlet may be drawing a higher percentage of cool return air depending on the order the coils and furnace elements and flow through air handler. That's part of the reason why good duct design usually has some sort of mixing box before the first duct line is drawn off the system.

    6. There are a myriad of other things that could be wrong - this site is full of posts with questions and answers diagnosing heat pump efficiency issues.

    In any case, it's not the kind of thing that a person can do themselves. Special tools and training are required.

    But to answer your question - Yes, something is probably less than ideal - it should be able to do better than it is right now.

  3. #3
    Join Date
    Dec 2010
    Posts
    20
    Thanks for the very informative post.

    My installer came back out today to double check the wiring. The second stage was not wired up at all. I had him go through the wiring with me and I pointed out that he still had the first call for heat running to the second call terminal in the furnace. Swapped the wires and I finally have the thermidistat running the system (had to point out that a jumper had to be removed from the furnace board).

    After they left I measured the heat out of a vent again when the thermidistat was calling for second stage. It still was only putting out 80 degree air. My furnace is a 1600 cfm fan. I am now wondering if it pushing too much air across the coil when in heat pump mode. Should the cfm be matched to the condenser size or the indoor coil (it is a 4 ton coil)?

  4. #4
    Join Date
    Aug 2010
    Location
    Monroe County, PA
    Posts
    99
    It should be about 400 CFM per ton. There are 2 stages for your heat pump. Stage 1 is typically 2/3 of stage 1 (the rated tonnage) So stage 2 of your 4 ton unit should 4 tons, while stage 1 should be about 2.6 ton equivalent. The indoor coil and outside condenser MUST be matched to work efficiently. Rarely are a mis-matched pair installed together.

    For a heat pump, the ideal air volume is typically about 400 CFM per ton. So the fact that your fan is set for 1600 CFM for stage 2 is correct. Hopefully it slows down to about 1000 CFM when in first stage.

    I can't say whether the 10 degree temp increase in stage 2 at 1600 CFM is normal for your heatpump or not. So many variables to consider. If he didn't check the pressures at the compressor (outdoor unit) there could still be efficiency issues keeping you from getting max performance out of your unit. You can pose the question to your contractor again, although their unfamiliarity with the wiring requirements suggests maybe a different contractor would be worth a try.

  5. #5
    Join Date
    Dec 2010
    Posts
    20
    Quote Originally Posted by Tom Campbell View Post
    For a heat pump, the ideal air volume is typically about 400 CFM per ton. So the fact that your fan is set for 1600 CFM for stage 2 is correct. Hopefully it slows down to about 1000 CFM when in first stage.
    Of course it depends on the static pressure (been reading HVAC documentation way too much ) but according to the manual it does slow down to about 950 cfm in stage 1.

  6. #6
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,089
    Being a 3 ton HP, you should be running 1200 CFM on high. If you are at 1600, that would be a big cause of low TD. Also reading at registers & grilles isn't a good way to see what the equipment is doing. Need to read air entering the return at the furnace and leaving the coil at the outlet. At 30 out with proper airflow I'd expect a 25 degree rise over the coil. Take your reading at the unit and compare to reading at the vents. That will tell you duct heat loss or cold air leaking into the return.

  7. #7
    Join Date
    Jul 2005
    Posts
    5,513
    Quote Originally Posted by Ellbaker View Post
    Thanks for the very informative post.

    My installer came back out today to double check the wiring. The second stage was not wired up at all. I had him go through the wiring with me and I pointed out that he still had the first call for heat running to the second call terminal in the furnace. Swapped the wires and I finally have the thermidistat running the system (had to point out that a jumper had to be removed from the furnace board).

    After they left I measured the heat out of a vent again when the thermidistat was calling for second stage. It still was only putting out 80 degree air. My furnace is a 1600 cfm fan. I am now wondering if it pushing too much air across the coil when in heat pump mode. Should the cfm be matched to the condenser size or the indoor coil (it is a 4 ton coil)?
    The cfm should be matched to the condensing unit. 1200 cfm max for your 3 ton unit. Even at 1600 cfm your temp rise should still be up around 17 deg, so either the system isn't performing well or you're taking inaccurate measurements.

    Since the system is keeping up with the load in low speed at 30 deg, odds are you aren't getting accurate measurements. Not only is the supply grille temperature going to be a bit lower than the temperature leaving the coil, but it's easy to get an inaccurate measurement of the temperature at the grille. You have to insert the thermometer probe through the grille face at least a few inches in order to measure the temperature of the supply air before it begins mixing with the room air and to prevent radiant heat effects from affecting the reading. In this case, if the thermometer sensor is in line of site of the cold floor or ceiling (whichever the case may be), then it'll read low even with 90 degree air blowing past it. If you don't have it inserted through the grill face then you'll also be measuring mixed air instead of supply air. Between these two errors you could easily be getting a reading 10 deg lower than actual supply temp. It's better to measure the leaving air a few feet downstream of the indoor coil in order to eliminate these common sources of measurement error.

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,913
    The cfm should be matched to the condensing unit. 1200 cfm max for your 3 ton unit. Even at 1600 cfm your temp rise should still be up around 17 deg, so either the system isn't performing well or you're taking inaccurate measurements.
    1350 is fine for a heat pump, in heating mode. 1600 a bit too much.

    He is measuring at a register, and no way to tell how much temp drop the run has to that register. Nor what the actual return air temp is at the air handler. At least not from our side of the monitors.
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  9. #9
    Join Date
    Jan 2012
    Location
    WEST COLUMBIA S.C.
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    278
    Will return air at 80 degrees make heat coming out of supply warmer comparedto 73 degrees return air in heat pump in heat mode when its 32 outside

  10. #10
    Join Date
    Jan 2004
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    Lancaster PA
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    Quote Originally Posted by hewitt View Post
    Will return air at 80 degrees make heat coming out of supply warmer comparedto 73 degrees return air in heat pump in heat mode when its 32 outside
    Yes it will.
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