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Hot water heat with geothermal heat pump

AndyZb | Posted in General Questions on

Hello,

I am in Grand Rapids, MI and just finished building (several months ago) a pretty well insulated large home of 4300 square feet and HERS index 43. We have 5 ton geothermal heat pump and radiant floor in basement floor and master bath on second floor. We have a desuperheater hooked up to two powered electric hot water tanks. Initially the first tank was not to be powered, but at last minute they decided we needed more hot water with the floors and powered both. It’s been not surprisingly a disaster from an energy efficiency standpoint with more than half the electric bill going to hot water. We have also had the system just break for the third time. The HVAC company wants to replace one tank with a standard 75 gallon gas powered tank. We do have natural gas to the house for the fireplace (don’t ask why), but I don’t really want a combustible appliance drawing from the home or another hole to the outside, but they say a hybrid heat pump water heater is not a good option because we have geothermal (although I suspect ours is oversized and can handle it) and they also say high efficiency gas is not a good idea with the floors and desuperheater. I would appreciate any advice.

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Replies

  1. Expert Member
    NICK KEENAN | | #1

    Some clarification would be helpful. There's two ways people talk about "hot water" -- the stuff that comes out of taps that is used for cleaning and bathing, which people in the trade call "domestic hot water" or DHW, and heated fluid that circulates through radiators and radiant floors to warm the house, which people call "hydronic heat" or "space heating." I'm not at all clear when you say "hot water" which one you're talking about.

    That said, I think what might be happening is that your well lacks the capacity to handle both space heating and DHW, and your HVAC guy is suggesting that you take DHW off of the well. If that is indeed the case, your only options are to either drill another well, or introduce another heat source, electric or gas. A heat-pump DHW isn't going to work, because the heat for that comes from the interior and just increases the space heating load, and if your well doesn't have the capacity, it doesn't have the capacity.

    While I'd hate to disparage someone I've never met, from your description it also sounds like your HVAC guy is being a little oblique about the actual root of the problem. Because if the well is undersized, whoever sized it messed up.

    1. AndyZb | | #2

      Sorry, for clarification, I am talking domestic hot water and hydronic. The two are separated by a heat exchanger. The geothermal I think is oversized, not undersized, based on what was previously recommended on this site (HVAC company wasn’t comfortable going any lower) and the fact that it doesn’t seem to run very often even with temperatures down to teens here. The geothermal is not doing much of either the domestic hot water or hydronic except what the desuperheater puts out, which isn’t much in the winter in my understanding so the electric tanks are doing almost everything. My question is about options to replace the electric tanks. Thanks!

      1. charlie_sullivan | | #5

        The two tanks are both DHW? Is there a third tank for the hydronic system, to buffer the output of the heat pump? Or maybe they are just using the thermal mass of the slab for that?

        Assuming it's two DHW tanks, the right thing to so is to have the first tank connected to the desuperheater, and with the heating element in it turned off, and then the heating element in the second tank turned on, to "top off" the preheated water from the first tank. The second tank should not be connected to the desuperheater.

        The first thing to try would be simply turning off the power to that first tank.

        You could also consider using the hydronic output of your huge heat pump to provide heat in more places--you could add panel radiators, or use a hydronic coil in the forced air system. I don't know what your heat is for the rest of the house, but if you start using that 5-ton heat pump for more than it's used for now, you'll get more out of that desuperheater.

        Or you could leave the first tank alone, unpowered, and replace the second tank with a hybrid heat pump one. Whoever told you that's incompatible is simply confused.

        It's possible that the problems are just that stuff is set up wrong. A photo might help us spot how it's plumbed and see whether there are glaring errors.

  2. Expert Member
    NICK KEENAN | | #3

    The heat pump may well be oversized, but that is separate from the well.

    The desuperheater takes heat that is removed from the house by air conditioning and uses that heat to heat water. In the winter, there won't be any air conditioning. The normal configuration is to have DHW on a desuperheater in the summer and something else in the winter. If you have a ground source heat pump that "something else" could be heat pump.

    What makes me suspicious is the statement that "they say a hybrid heat pump water heater is not a good option because we have geothermal." The only way that statement would make sense is if the geothermal is undersized.

    1. charlie_sullivan | | #4

      I actually have a water-water ground coupled heat pump with a desuperheater. It's the other way around. The desuperheater produces all the DHW we need in the winter and doesn't provide useful DHW in the summer.

      Superheat means heat above the temperature of condenser temperature. In the winter, we are heating the hydronic tank to circa 100 F with the main output of the condensor, and the superheat is hot enough that the electric element in the DHW tank never runs. In the summer, the condenser is cooled by 50 F water and so the superheat only gets up to may be 80 or 90 F. If we had a two-tank system, we could use that for useful preheating, but with our single DHW tank, the heat flow can go the wrong direction, out of the DHW tank into the heat pump and into the ground.

      1. Expert Member
        NICK KEENAN | | #6

        In a heat pump system that is used for both heating and cooling the normal configuration is that there are valves or a reversible pump that allow either of two coils to function as the evaporator, and the other as the condenser, depending on whether heating or cooling is desired. With a desuperheater, the other coil is also used simultaneously, and both heating and cooling are provided from the same pump. The most common application for this is providing DHW during cooling season.

        What you describe isn't working as a desuperheater in winter. It's just a heat pump water heater.

        The issue you describe -- and the OP describes -- is the problem with desuperheaters, which is that DHW demand has to be matched to cooling demand. If not, either you don't get enough hot water, or you have to run the heat pump for DHW when it's not calling for cooling and dump the cooled water. If that's the case, you're better off running a standalone heat pump water heater, which provides cooling during the summer and dehumidification any time the dew point rises above the coil temperature.

        1. charlie_sullivan | | #7

          It is possible to rig up a water-to-water heat pump to serve two zones in heating mode, one of them being DHW through an indirect tank. That's not what I have. What I have is a desuperheater. The concept of a desuperheater is different from the concept that you describe. It is not the simultaneous use of the evaporator cooling and the condensor heating. It's the use of the higher temperature superheat in the line between the compressor and the condensor, where the gas is hotter than the boiling point at that temperature. The desuperheater removes some of that heat before it gets to the condensor. In heating mode, that enables simultaneous heating of a little DHW to 140+, higher than the max temperature my heat pump is rated to put out in the main loop. In cooling mode, the reversing valve switches which heat exchanger operatates as evaporator vs. condenser, but the desuperheater still the first thing the hot gas hits coming out of the compressor.

          I can see how you got that misunderstanding of desuperheaters--they are often described as "waste heat recovery" and when used with an air conditioner, they do recover some of the heat going to the condensor, and all of that is waste heat. But the concept of a desuperheater is different from just utilizing rejected heat. In a heating system, a desuperheaters isn't really recovering waste heat--it is taking away some of the heat the would go to heating the house. But it is taking thermodynamic advantage of the small amount of higher temperature heat that is available. The energy use is less than if you had to run the heat pump with the whole condenser at that higher temperature part time, at during which time you'd have higher "lift" and lower COP. This basically allows getting the DHW at a higher temperature than the hydronic heat, but at the COP of the hydronic heat temperature.

          My issue in summer is not the matching of load and demand. I know the load-matching problem well, as that happens to me in the shoulder heating seasons. The problem I am describing is different. It's that the condenser is still cooled by the ground water, and if the condensation temperature is circa 55 F, the superheat isn't enough above that to do more than preheat the DHW no matter how long it runs-- it's a temperature issue, not a quantity of heat issue. My problem doesn't happen in an air conditioner with an outdoor air-cooled condenser, because the condenser is much warmer than mine, and so the superheat is hot enough to get the DHW hot enough to use.

  3. Expert Member
    AKOS TOTH | | #8

    In a typical residential setting the domestic hot water uses on average around 2000 BTU. In a 4000+ sqft house, that load is falls into the squat category. There is absolutely nothing wrong with a heat pump water heater.

    The heat will come indirectly from the geo system but it won't be all that much less efficient than the geo system making hot water plus way more reliable.

    If you find your current hot water tank too small, just add in a heat pump or electric tank heater in series but downstream of the existing one. This way, if the draw is small, the most of the heat would be supplied by the existing unit. For the odd time this happens, the efficiency hit will be so small that it won't matter how you heat the hot water.

    1. charlie_sullivan | | #9

      As I understand it, the current setup already has two DHW tanks, and both of them are heated by resistive electric elements, as well as at least one, hopefully the first, supposedly heated by the desuperheater. So I think the scenario described in your last paragraph could be be achieved simply by switching off the heating element in the first tank.

      But I might be wrong about the current setup--it might be that the two tanks are a DHW tank and a hydronic system buffer tank. And it might be that that there's a resistive heating element in the hydronic buffer tank, which would be a huge energy waste if that was kicking on.

      One other problem might be that the desuperheater loop is air locked and not getting any flow, which would mean 100% of the DHW is being supply by the electric elements. It might be a simple matter of getting that primed, or making sure the setup is correct to make that pump run when it should. Turning off the element in that tank would help verify whether there's any heat coming from the desuperheater.

      1. AndyZb | | #10

        Hey everyone, thanks for the responses. There are two tanks, the desuperheater is only connected to first, but both are powered. They initially were going to not power the first one, but then they thought the amount of hot water needed would be more than the second 50 gallon tank and desuperheater to unpowered first tank could give so they powered both. My understanding is hydronic is pulled off the second and separated from domestic by a heat exchanger. They want to replace the second tank with a gas 75 gallon tank (gas is already stubbed in above) and keep first electric 50 gallon tank, but unpowered for desuperheater. I am leaning that route because cost has been crazy high and I already have gas run, I was just wanting to avoid combustion appliance if possible.

        1. charlie_sullivan | | #11

          Thanks for the added detail and photos. It seems we all misunderstood what the setup was in the first round of discussions. Here's what I think it is:

          You've got a 5-ton water-to-air heat pump, that heats most of the house through forced air ductwork, and additionally has a desuperheater that heats the first of two DHW tanks, which both also have electric heating elements.

          Then the hydronic heat is taken from the second water tank by circulating the water from that second tank through the external heat exchanger shown in the dead center of the first photo.

          The outcome is that not only are you supplying a lot of your DWH with the electric heating elements, but you are also supplying pretty much 100% of the floor heating with straight resistive heat. When you aren't actively using DHW, there's no way for any of the heat that has been provided to the first tank to make its way to the second tank. So even if you turn off the element in the first tank to take better advantage of the desuperheater for DHW, you will still be supplying your hydronic floor heat with the second-tank resistance heat.

          I can see 3 ways you might go from here, and will describe each in a separate comment.

        2. charlie_sullivan | | #12

          **Best solution:

          Make your poor confused contractor who designed this terrible system do it right. That would be to buy the heat pump that is designed to do what you want: supply the forced air heat, the hydronic heat, and some of the DHW. That's the Water Furnace "Synergy 3D". https://www.waterfurnace.com/residential/products/geothermal-heat-pumps/synergy3d

          **Their proposed system:

          Replace the second tank with a gas fired tank. That can work, for example this HTP gas tank water heater is intended to be OK for that type of use. https://htproducts.com/phoenixldairhandler.html. You can then turn off the electric element in the first tank and get the benefit of the desuperheater for DHW although all your floor heat will be from gas. Make sure somebody insulates the hot water pipes, including the desuperheater and the pipe between the two tanks, especially. It's a shame to have to go to combustion just because your contractors are clueless, and there's no guarantee that they can do that right, but it should work.

          **Third option: Replumb to use the desuperheater better for heating, and maybe use a heat pump water heater.

          If the the DHW that circulates through the heat exchanger pulls out of tank 2 and returns to tank 1, you are deposited that water, which is ideally cooled substantially as it goes through the heat exchanger, back into the first tank where it can get heated some by the desuperheater if the main heat for the house is running. So you get some assist from the main heat pump. You could get some energy savings just by making that change, and turning off the electric element in the first tank. You'd also want to make sure the flow rate in the floors was low enough that the water returned to tank 1 was cooled substantially. This will help some, but it's hard to estimate how much it will help without a lot more data.

          You could also replace that second tank with a heat pump tank. It will then pull heat from that utility room. If the utility room is heated by hydronic tubing in the slab, you would then be simply chasing your tail and sending heat around in circles, wearing out equipment with no energy benefit. But if the room is heated by a vent coming off the air handler, that will work well. It won't be the most efficient way to heat the floors but it will be vastly better than what you are doing now--it will at least cut that cost in half. Better if the new plumbing of the heat exchanger loop gets you a significant amount from the desuperheater.

          The first option is best. Neither of the other two is ideal but both will work. If you do the gas tank, you could consider the replumbing as well and maybe use a little less gas.

          1. Expert Member
            AKOS TOTH | | #13

            Typical de-superheater produces between 3000 to 5000 BTU of heat. That just about covers DHW loads and will almost nothing left over for a hydronic floor heat load.

            As Charlie pointed out, the problem is they speced the wrong equipment for the job, using a desuperheater for hydronic heat even with proper plumbing (which is not the case here) won't work for any space much larger than 150 to 200 sqft.

            The only real fix in this case is the gas tank. Make sure to get a unit rated for combi heat (not all are) and usually best to get one with a larger burner (75000 BTU). There are some that come with the heat exchanger built into the tank (Bradford White Combi2 or Laars Combi), but since you already have the plate HX, not sure if it is worth the extra cost.

          2. charlie_sullivan | | #14

            Before anything else, it would be good to check on how much heat is actually needed for the hydronic zones. There might be some calculations that have already been done that could be used for that. It could be that someone did that already and saw that the desuperheater output, which I'd guestimate at 5000 to 7000 for a 5 ton, was sufficient or nearly sufficient for a bathroom and a basement in a well insulated house. You wouldn't get that much all the time, but if the heat pump is sized right, it will run plenty on days when you need that full heat.

            If you don't have those calculations--or even if you do--but you have some kind of electrical monitoring that tells you how much energy the electric elements in those tanks are using now, we should be able to figure out how much you are using total. It might even be worth temporarily rigging it so one tank is only for DWH and the other only for hydronic, and then measuring how much each uses. You could even compare with the desuperheater on vs. off.

  4. AndyZb | | #15

    Great, thanks everybody for your advice!

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