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Hydro Air & Heat Pump

The Coefficient of Performance of a heat pump system (or a straight cooling system) is a unitless figure that relates the amount of "Useful" heat transferred divided by the amount of heat required to perform the compression. In actuality, we are referring to the power usage converted to btu.

If we look at a heat pump system plotted out on a pressure enthalpy chart, we can see that the useful heat transferred by a heat pump system operating int he heating mode takes place between points 1 and 2 on the attached chart. Point 2 represents the outlet of the compressor, while point 1 represents the inlet of the metering device. The line between points 1 and 2 represents the total heat of rejection for the system and is used, along with the mass flow rate of the sytsem and a conversion factor between minutes and hours, to calculate the capacity of the condenser.

The line between points 2 and 3 represents the heat added to the system during the compression process. Between points 2 and 3 we therefore have the heat of work, which can be equated to the power utilized.

Heat of work and total heat of rejection are both expressed in btu/lb of refrigerant circulated through the system. To obtain the coefficient of performance, one must divide the total heat of rejection by the heat of work.

Typically, an air source heat pump will have a COP in the range from 3.5 to 4.

If a heat pump system had a COP of 1, this would indicate that there was absolutely no heat being absorbed by the evaporator and the heat generated during the compression process was the only heat being rejected by the condenser into the occupied space (forgetting, of course, about any heat added to the space by electric strip heaters or other supplementary heat source).


  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,472
    I currently have

    a Munchkin driven Hydro air system with a Hot water coil in my return duct. The water temp is on an outdoor reset curve controlled by my Vision 4 system through a mixxing valve. (The other 3 zones are radiant floor, snow melt and an indirect water heater) With the promise of higher Nat gas prices I have been dreaming of ways to increase heat production efficiency. I am going to install a 14 SEER Heat Pump to replace my A/C system. At current prices it will save me 500 per year. When nat gas prices go up it will save me more, depending on the stability of electric prices. Elec here is driven by coal, prob more stable than other sources such as nat gas. I was thinking, what ifs and an idea came to me. What if... I moved the hot water coil to the supply side of the duct downstream of the evaporator and somehow used my mixing valve to maintain a certain discharge temperature in the supply duct. I could run my heat pump at lower outside temps and still get savings from it below the balance point and still maintain comfort. The next question is how from a controls point of view. Any input is appreciated. WW

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  • Marty
    Marty Member Posts: 109

    With a heatpump the water coil should be after the dx coil anyway. If it was my house I would just use the water in place of the electric heat. There would have to be an atronomical difference in fuel prices to justify running the heat pump below its balance point nevermind the additional wear and tear from more defrost cycles.
  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,472

    How much more wear and tear do you suppose? Does it save more to run below balance point at the shortening of the unit life or cut off at balance point and do dual fuel and perhaps the unit will last longer. My elec costs are 7 cents per KW. My gas is presently 1.74 per therm, and facing an increase of at least 30%. Off the top of my head I say run the sucker into the ground and save more on operating costs. By the time I wear it out there may be even more efficient appliances available. I am also under the impression that the Heat Pump is under increasingly less load as the outside temperature declines. How much stress is it really under? I'm asking. Not sure of much. WW

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  • don_171
    don_171 Member Posts: 3

    only concern I would have doing what you want to do Wayne is
    is when it goes in defrost..that is when the system does require defrost that the hydronic coil does not heatup the
    envelope to quickly and dropout the defrost cycle.

    Also when dealing with heatpump and the defrost mode if you have a wet winter lots of freezing rain your defrost cycle
    can not be maintain..even if its set at 30 mintue cycle rate.

    So yes Mike I would say just keep it simple and not try to
    squeeze all the btus possibe and just run it thru a outdoor stat that takes out the heatpump at 40 degree and bring on the wetstuff.

    Now I pick forty degree because that work fairly well in my
    area...they dont even feel a difference when it goes in to defrost..and that witout the hydo coil or any strip heater to temper the air when it is in defrost mode.

    I guess you can say I've been lucky.

  • Bob Eh?_2
    Bob Eh?_2 Member Posts: 42

    Bad news.... The pump load in heating mode rises as the outdoor temperature falls..... The balance point for most civilized pumps is 25* to 15*.

    Mind you somewhere around there your COP crosses over 1 and keeps falling.

    I assume this is an air source system so the way to determine the effective balance point for your system/structure is to run it until you start to lose ground as the temperature drops and determine what the outdoor temperature is. Assuming you have a decent heat loss you can plot out what that load is in BTU. if it is greater than or equal the rating of the pump (Including the AC power converted to BTU) you are still winning. I generally suggest if the net heat drops below rating + 20% you are likely heading for freezeups and pointless heavy loading..... This is a good point to switch to plan "B".

    Water source pumps with antifreeze in the source often don't pass the COP of 1 untill the loop temp is down as low as 20*

    Does that help?
  • scott75
    scott75 Member Posts: 101

    Does the loop temp ever go down as low as 20? That would have to be a shallow horizontal loop. If budget permitted, I would do water source with a deep vertical loop. You will have a more constant water temp regardless of outdoor temp. My boss put this in his house two years ago and has never turned on the circuit breakers to his electric heat. We are in Central PA.
  • Tom_35
    Tom_35 Member Posts: 265
    Direct exchange heat pumps


    Have you considered a direct exchange heat pump system? Go to and check the systems out.

    We have been using them for the last 3-4 years and they work great. You bury the copper refrigerant lines in the ground as opposed to polyethylene piping. No glycol/water, no freeze ups. We always have back-up heat on our units but have it restricted so that the homeowner has to manually turn it on. Temps in the single digits and the heat pump still takes care of the load.

    Tom Atchley
  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,472
    Interesting, Tom

    My first reaction is how long will the copper last underground before developing leaks due to corrosion. Secondly, how deep does it have to be buried and how long a run?

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  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,472
    Actually the idea was

    to place a sensor in the supply and turn on the hydro air circulator based on a minimum supply air temp. That way I can keep the heat pump going below balance point but keep my comfort with the back up hot water source. This is different from most dual fuel since you usually turn off the heat pump when the oil or gas furnace comes on, so the heat pump is not overloaded with hot incoming air and goe off on high head pressure. Hydro air doesnt care what temp is coming off the heat pump evap coil. I think I may have something here. WW

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  • don_156
    don_156 Member Posts: 87
    If that

    the case Wayne then why not just use your hydro coil as the
    second stage circuit.

    What could go wrong other then issue of the defrost as mention before.

    I have herd some issue of the hydro coil will not heat quick enough but,I have not seen that as being the case.

    That way if you feel you are at balance point you can simply switch to emergency heat.

  • Tom_35
    Tom_35 Member Posts: 265
    Direct exchange heat pumps


    The heat exchanger can be vertical or horizontal. Most of ours have been vertical with the bores being 100' deep. If we go horizontal, we are 5' deep.

    The beauty of the vertical design is that the physical room needed is so small. You actually have a pit that is 4'-5' wide and the bore holes are drilled down at angles from the pit. Our first job had the pit in a flower bed that was next to the home.

    If the ground has any acidic content to it, you can attach a device to the copper that will keep it from being attacked by any corrosion. Contact the ECR factory and they can provide additional info.

    I think I remember Constantin posting that he had looked at this system for his home, but I may be mistaken. Seems like he was having trouble getting a driller or some other info needed.

    We are able to use the units for heating and cooling via ductwork, heating radiant floors, limited domestic on demand water heating (up to 120 degrees), and domestic hot water heating via the de-superheater during the cooling season.

    Tom Atchley
  • Marty
    Marty Member Posts: 109

    Thats pretty much how most air to air heat pumps work anyway 2nd stage is usually electric ,,,actually your idea of a sensor in the supply would be better than a two stage stat.
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