Are there geothermal water heaters? (not desuperheaters)

Discussion in 'General Discussions' started by BlueHawk, Feb 10, 2020.

  1. BlueHawk

    BlueHawk New Member

    Hi all – Are there geothermal water heaters? I mean full-fledged water heaters that supply all the hot water needs of a home, not desuperheaters.

    When I search online, all I get are desuperheaters. It seems like a geothermal loop should be able to power a dedicated, full capacity water heater, so I'm surprised that I'm not finding it. Shouldn't the geothermal liquid medium be able to heat water directly?

    Thanks for your feedback.
  2. ChrisJ

    ChrisJ Active Member Forum Leader

    Yes, it's a water to water heat pump.
  3. wing

    wing Member

    Absolutely such hot water heaters are available. In my opinion, in a heating dominated climate zone it is wise not to rely on the desuperheater for this purpose.

    Here is my hot water setup, a 119 gallon Caleffi SolarCon tank with single heating coil and 4500 watt electric backup.

    System has been configured to turn on a dedicated distribution pump to the hot water tank coil whenever the hot water tank thermistor calls for heat. Further, a hot water call will priority override any of other in floor heat or air handler distribution pumps until the hot water tank call is satisfied.

    And yes, Chris J is correct - a water to water heat pump with properly configured buffer tank and distribution pump system is the front end for a hydronic hot water heater.


    Last edited: Feb 11, 2020
  4. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Waterfurnace has a small 1.5 ton and 2 ton water-water which can be ordered with a double walled and vented coax heat exchanger, so it is permissible for domestic hot water. We use it to make 100% of the domestic hot water via the geo system. It has become defacto the standard for us.

    Attached Files:

  5. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    You are running your tank for your radiant hot water at 130F permanently to pull heat from it via an internal heating coil whenever the DHW tank calls for heat?
  6. wing

    wing Member

    That’s an interesting question.

    I don’t believe I will be able to get the water tank temperature to 130 degrees using load water from the heat pump - the water is just not hot enough. System is set up now to start up the heat pump when the temperature in the buffer tank reaches 95 degrees and shut down at 115.

    So my plan is for the water tank geo thermostat to shut down the DHW circulation pump at 100 degrees and let the 4500 watt electrical heating element in the hot water heater take over heating from 100 degrees to 120 degrees. More properly this will be a ‘geo assisted water heater’. I will miss out on the benefits of my geo system for these last twenty degrees of heat.

    Treated well water coming into the tank will be at 50 degrees.

    Is there a better way using only a single 5 ton water to water heat pump that is also distributing hot water to in floor slabs and air handlers ?
    Last edited: Feb 18, 2020
  7. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    You only heat the water when it drops below 95F and then only heat it up to 100F? If it is 98F it does not get heated?

    Let me ensure you that you can heat water to 130 degrees with a water-water heat pump, It only works with a very large heat exchanger inside the tank, like the Stibel Elton plus.

    Here is an example.
  8. wing

    wing Member

    Thanks for the diagram docjenser.

    An improved control strategy for my configuration would be to have the DHW aqua stat make a separate call to the water to water heat pump during the period the DHW is in in priority override mode. Then revert after the DHW call with the aqua stat connected to the buffer with limits 95 degrees on 115 off.

    What temperature could my existing DHW (Caleffi 119 gallon solarcon with 4500 watt electric backup ) and heat pump (5T single stage Terra Therm THA 060) practically heat water to ? 120 ? 130 ?

    I profess being a little nervous feeding my radiant engineered wood floors over 1 1/2 inches of gypcrete with a burst of 130 degree water following satisfaction of the priority DHW call. I do have slab sensors to minimize any impact to the wood from too high of temperatures and they are set to shut down the heat call at 77 degrees slab temp. Any comments ? I have no ability to reduce water temperature to the slab manifolds as the DHW pump and manifold pumps are on the same manifold.


    Attached Files:

  9. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    The problem is that you run your hot water off your radiant hot water. Thus your radiant has to be at least 120F. Look at the complexity just your radiant manifold has.

    Here is an entire diagram attached, for all the heating, DHW buffer tank and cooling equipment.

    Runs the radiant floors at 90 max, and the DHW at 125F. Do you have the 119 gal tank with the lower and upper coils in the tank, or the model with just the lower? You need both coils to transfer the heat from a 5 ton into the tank for DHW.

    Don't know your Thera Therm, Waterfurnace locks out at 138F leaving load temp, but we never run them higher than 130F. Their high temp model goes up to 145F, we use it for baseboard heat, and large amount of DHW, like big apartment buildings.

    You have a huge amount of complexity in your system, and it does not do what you need it to do, namely simply to heat you radiant to 90F and your hot water to 120F, simply by a single 5 ton heat pump.

    I don't have a slab sensor in my floor, I have an outdoor reset, so it increases the water temp by a few degrees when it gets a bit colder outside. And I don't have engineered wood, I have real wood, which is totally happy with an 84F surface temp on the coldest days of the year.

    Keep it simple. Geo systems need to be lean in order to be mean!

    Attached Files:

  10. wing

    wing Member

    Apologies to Bluehawk for hijacking the thread....

    Excellent critique of my system docjenser, thank you for taking the time to pick holes in what I have.

    The DHW is the single coil model with electric backup. Makes sense that dual coils would increase heat transfer to the DHW. I will see how well what I have works, it may not do the job as you have indicated. Longer term I have a plan to install a dedicated 2T water to water heat pump for the DHW- I have plenty of loop field to add another two tons and the flow center will also accommodate.

    From a control standpoint, I can change things up based on your feedback to functionally mimic your drawing:

    1. Upon a heat call from the DHW aqua stat at 112 degrees the DHW will control the heat pump and DHW zone pump on a priority override basis until the DHW reaches (hopefully) 125 degrees.
    2. Once the DHW call is satisfied at 125 degrees, the heat pump will turn off and the zone pump to the garage slab will operate on a priority basis to bleed excess heat until the buffer tank reaches a suitable lower temperature for the floor slabs - 100 degrees.
    3. From this point, the priority override will cancel, the floor slab pumps will activate upon a call from the thermostats and the heat pump will operate to keep the buffer tank between at 85 to 100 degrees until the next DHW call.

    I never considered using solid wood floors. The flooring contractor was nervous enough about engineered wood over radiant. Right now the floor slab sensor cuts off once the flor slab reaches 77 degrees with the top of floor at 74 degrees. Very conservative.

    Much of the complexity in the distribution panel was due to climatology - a high desert area with large diurnal temperature swings. It is common that you need heat until 10 AM, then cooling until 9 PM and heating again after 9 PM So I operate separate buffer tanks for cool and for heat.

    Thanks again for your very valuable comments.
  11. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    You are still thinking too complex. You flush the floors with 125 after them being heated for a while, your tank will drop quickly tp 100F. 77F slab floor temp is really conservative, not much heat output at that temp. Keep in mind, that is more than 20F below your own body temp.

    And how warm do you think the floors get when the sun hits them through the windows. And what happens with the wood then?....Usually nothing.

    Complex climate is fine, you can use times etc to account for solar etc. High mass radiant then usually becomes the enemy. You want your heating system to respond quickly, and not have a high inertia.
  12. Krush

    Krush New Member

    In a new system layout, has anyone ever used a small Marine water to water heat pump to bump up DWH from a high efficiency low temp hydronic ground source (or solar thermal) set up?

    something like this?
    Thanks for this. To clarify - is it all in the HP controls that we ask it to produce higher temp on DHW calls or do we just let the same function of the HP run longer into the DHW storage tank? And with a dual coil stieblel Eltron would still get the back up electric to top it off to say 140 for safer bacteria levels?
  13. wing

    wing Member

    The ‘second stage’ heat pump would presumably heat the DHW to elevate the tank temperature from say 120 F to 140F and ride piggyback on the primary water to water heat pump. A problem is that the EWT of the piggyback unit is high, efficiency low. Cheaper to use electric vs, a second stage.

    The higher temps in the DHW tank are accomplished by using a separate thermostat control for the DHW tank. When there is a call, the thermostat turns on the heat pump until the tank reaches temperature. In my system there are two thermostats to turn on the heat pump - one is in the buffer tank and the second is in the DHW tank. Isolated relays then functions as an OR gate - the heat pump turns on if either thermostat makes a call.
  14. Krush

    Krush New Member

    Thanks wing. This makes sense. I wonder if the logic would change if i'm scrounging kw for a net zero rating and where and enhanced envelope and distribution system work at closer to 100f. with a delta t of 40f, would it require a significantly fancier HP or controller to ramp up output for a dwh call and take the tank to 120f for dwh when the heat call asks for 100? or would it work to just ask the same settings to run longer?
  15. Krush

    Krush New Member

    i suppose the piggy backing action of an interior air source heat pump water heater would make more sense anyway cause it would do better in the summer.
  16. When it comes down to it, I don't think anything will beat the economic or energy performance of a heat pump water heater. If you get a large enough tank, say 80 gallons, you can run it in heat pump-only mode and not use electric elements. If you use electric elements on anything, you've basically lost the battle on efficiency. For high demand scenarios, an 80 gallon heat pump set to 140F with a mixing valve would be better than engaging electric elements. They also de-humidify in the summer, so they save energy 2 or 3 times over.

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