Desuperheater for DHW
I've been running my DX geothermal system since February and am quite happy with it. Now that the weather has warmed up we don't need it for heat (most of the time). We have an extra loop running through an indirect SuperStore 50 gal tank and the desuperheater looped directly into the same tank with city water filling it as our electric hot water heater draws DHW off it. When the SuperStore drops below 110° F it turns on the geothermal unit until it gets back to 110° F. Does it make sense to use the geothermal unit solely for DHW?
I hope this makes sense and thanks for any advice.
Any way to compare your electric hot water cost to a similar period when you did not have the desuperheater installed and running. say last March and April?
As your tank is a desuper heater, where does the rest of the heat go from the geothermal? Usually the cost of operating a geothermal is more than operating a waterheater. While heating and cooling ok. Over heating or overcooling probably not.
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Just to clarify, we are not using the geothermal for cooling. We don't need to cool in this area.
Also the loop in the SuperStore is more accurately a separate zone.
We don't have any info on heating DHW only with electric. We just finished the house in December and commissioned the geothermal in February.
The heat from the geothermal unit goes into the indirect loop in the SuperStore. The desuperheater water from the geothermal unit goes into the main part of the SuperStore. So effectively all of the heat from the geothermal unit is going into the SuperStore. When we draw DHW from a separate electric hot water heater it refills from the SuperStore. City water replenishes the water drawn off the SuperStore thus dropping it's temperature.
Look at it in "units of energy". If you heat water with gas and say that it is a 90% efficient water heater you will get 90 cents worth of heating for every dollar you spend.
Originally Posted by Kevince52
With a properly installed and functioning geothermal system you get 3 to 4 units of heat for every 1 unit that you pay for. So, when you are going from a 90% efficiency to a 3 to 400% efficiency I think it's GREAT!
What type of DX system did you install?
Thanks for the reply. The COP for this unit is 3.92. But that seems to be under a very specific set of circumstances which makes this difficult to figure out exactly just using that number. What I'm looking at and perhaps I'm wrong in this but here it is anyway. My hot water heater draws 4500W max. In stage 1 my geo unit draws 2460 watts and 3620 watts in stage 2. I'm guessing that this then gets down to a matter of time. If it takes >2 times as much time for the geo to lift the temperature as it does the hot water heater then it's less efficient to run the geothermal. I can time the geothermal unit (by sound) but the hot water heater is a bit more difficult. I'm wondering if anyone else has done these calculations?
The DX system I have is made by Maritime Geothermal or Nordic. So far I'm very please although I must say they could do with more sound/vibration dampening.
I’m going to go out on a limb here and say your HP is either a DX-55 or a DXS-55. You’re probably producing about 10-Igph/hr of hot water.
I don’t think it’s worth running a GSHP for the sole purpose of generating DHW with a desuperheater. That’s a lot of wear & tear on a mechanical system. A DX to (full capacity) hot water would be another story.
Just what model is your HP? DX, DXS, DXSTF or DXTF? 55?
The model is a DXW-55HW. A 4 tonne unit that creates heat and domestic hotwater.
If I did not run the heatpump in the summer what would the ramifications be in terms of letting it sit idle for 6 months?
As I said we did put an extra zone through a 50 gl indirect tank so we are getting all of the heat generated by the HP while it's running.
Thanks for the repy.
There should not be any negative ramifications of letting your HP sit idle all summer. However, having BOTH a desuperheater AND an ‘extra’ full capacity hot water loop running threw your DHW buffer tank will likely result in lower efficiency due to short cycling and perhaps excessive wear – even at stage-1.
The overall efficiency of the heat pump is close to 4 times that of an electric resistance water heater, putting the full heating output of the heat pump into a 50 gallon tank should give a reasonable run time and the controls should be set for single stage heat on that zone. Properly installed and maintained water source units can easily give 25 years of service plus.
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There seems to be a bit of differing on this. To add an extra element to the argument I do have a 50 gallon buffer tank and the 50 gallon DHW storage so when the system does come on it is heating 100 gallons. I'd only be guessing but I'd say that the cycle lasts for about 1 hour when it does come on. In some ways that's the crux of the issue. Does running the HP for an hour beat running the DHW electric tank for however long it takes to heat the same amount of water.
Anyway thanks for all of the input. I guess what I need to do is try it for 2 months with the HP and 2 months without. I'll get back to you with my findings.
Check it all you want, if you don't understand the energy production from the refrigeration cycle verses the heat output from electric resistance I wonder how you ever ended up with a heat pump to begin with. Call it COP, EER, SEER, all just letters to the layman.
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