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  1. #1
    Join Date
    Jun 2007
    Posts
    11

    Default water to water heat pump with heat exchanger

    We hired a contractor to install our water to water heat pump. After installation the unit runs for about a minute then shuts off for 5 minutes. It then kicks on again with a surge that dims the lights in the house. When it was first installed the temperature raised to the limit it burned up a transformer within the unit. The contractor replaced it and added an aquastat but it continues to turn off and on every 5 minutes. We installed it to cut down on heating costs not increase our electric bill. My husband has asked him to install a heat exchanger with a domestic hot water heater. Has anyone else used this type of system and have you used the heat exchanger?

  2. #2
    Join Date
    Aug 2007
    Location
    The Great White North
    Posts
    4,045

    Default Re: water to water heat pump with heat exchanger

    Are you using water from a well for the heat pump?

  3. #3
    Join Date
    Jun 2007
    Posts
    11

    Default Re: water to water heat pump with heat exchanger

    We have 3000 feet of ground loops. We could not drill wells in our area because of sandy soil.
    Last edited by ljmcm; 11-25-2007 at 12:08 PM. Reason: error

  4. #4
    Join Date
    Jun 2007
    Posts
    11

    Default Re: water to water heat pump with heat exchanger

    A heating contractor installed the system but we are doing the pex tubing in the floor ourself for radiant heat. He calculated the heat loss factor and determined we needed a 4T unit. The entire house has new insulation with the exception of the walls. They are two walls with an air space between making 22" stone walls that are plastered. All the old flooring had deteriorated in the lower level so we now have new subflooring with 1/2 of the tubing done in the downstairs. The tubing does not have flooring on top yet. For the upstairs, since the ceiling of the lower level is actually the floor of the upper level, we have put the pex tubing in the walls.

    The system is maintaining a nice even temperature but runs way too much.

  5. #5
    Join Date
    Jun 2007
    Posts
    11

    Default Re: water to water heat pump with heat exchanger

    We are still having difficulty with our geothermal system We have now covered the water tubes in the floor so no longer have convection heat. The room temperatures have not gone above 61 degrees. On extremely cold PA days the heat dropped to 56 degrees. The water temperature leaves the furnace at 120 degrees enters the tube at 110 degrees and the return water temperature is 100 degrees. All runs are under 240 feet long. The flow is 3/4GPM. Can anyone tell me if this is a normal function? The tubing is 8" apart on the subfloor. Would additional tubing under the floor help in any way?

  6. #6

    Default Re: water to water heat pump with heat exchanger

    It does sound like a flow issue.There is an apparatus used to fill and purge the system of air and fill with an alcohol mixture (antifreeze), you may have air in the loop or worse a kink in the line.How do you know you have 3-4 gpm of flow.I would need to know the delta pressure and feet of head drop across the loop.The specs for the design should be at hand.These items need to be re-checked and or commisioned to verify performance.Is there a by-pass loop that is open? Over sized or not in cold PA soil there should be greater that a 10 degree drop.

  7. #7
    Join Date
    Jun 2007
    Posts
    11

    Default Re: water to water heat pump with heat exchanger

    Our flow problem is in the radiant tubing within the house. We do have glycol in the outside loops and they have been purdged. The heat pump is producing 120 degree water in the buffering tank. It's approximately 110 degree starting into the floor loops and after circulating the return temperature is 100 to 105 degrees but the room temperature only reaches about 60 degrees with a 30 degree outside temperature. All air has been purdged from the lines and the lines are pumping three-fourths of a GPM. We were wondering if the return temperature should be lower with the possibility that the water is circulating too quickly to actually warm the floor. As you can see we are "grasping for straws" since this has been going on for two months.

  8. #8
    Join Date
    Jul 2010
    Posts
    1

    Default Re: water to water heat pump with heat exchanger

    Circulating the water more slowly would not increase the total heat flow - there is roughly a linear decrease in temperature between the entrance and exit of the tubes under the floor.
    If you know the flow rate, and the inlet and exit temperatures, you can calculate the rate of heat transfer - it is a function of three factors - flow rate (gpm) X temp. difference of water between entrance and exit X heat capacity of water (BTU per degree per gallon)
    1 BTU = amount of heat to raise the temperature of 1 lb of H2O one deg.F; 1 gal = 8 lbs.
    .75 gpm x 10 deg. temp. diff. x 8 BTU = 60 BTU per minute or 3600 BTU/hr.

    The heat transfer from the water into the surrounding material is proportional to the temperature difference between the water and the material, and is not directly sensitive to the flow of the water. The flow replenishes the heat lost. Quick flow, the water is replaced before it gets as cold. Slow rate, the water loses more heat before it is replaced, which results in the heat transfer being decreased.

    Do a mental experiment - slow down the rate so that the temperature of the water at the end is the same as the room. Then at that point there is 0 difference and no heat transfer.
    At the entrance the temperature is highest (110) so heat transfer is highest. At halfway point heat transfer rate is half. So average over the tube length is 1/2 of maximum.

    Now increase the rate to the point where the temperature at the exit is almost as high as at the entrance. Then the heat transfer rate is everywhere close to the max. So the high flow rate gives twice the heat transfer as the slow flow rate.

    In the extreme, at 0 flow, all the water in the tube will slowly cool to the same temp as the room, and the heat transfer will go to zero.


    If your heat pump is only running at 1/5 capacity (1 min of 5) then it's too big relative to the tubing, because you can only lose 1/5 of the heat being produced. You could increase the amount of tubing in the floor (twice the volume of water in the floor would double the heat transfer), or increase the set point of the water.

    Also you are losing 1/2 the heat before it gets to the floor. Can you cut the temperature drop between the furnace and the start of the tubes? Where does that heat go? If the water were 120 in and 100 out, you would double the heat transfer.

  9. #9
    Join Date
    Feb 2011
    Posts
    5

    Default Re: water to water heat pump with heat exchanger

    A heating contractor install the system but we pex radiant heat tubing in the floor are ourself. The heat loss factor calculated and determined we needed a 4T unit. With the exception of the walls of the house has new insulation. They 22 "stone walls that are plastered to create an air space between two walls are all old floors with lower levels had declined. So we now have the tubing under the new 1 to 2 / subflooring is. tubing is not yet up to the top floor.'s, when the low-level roof is really high-level floor, we have put in the walls pex tubing.

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