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Comment by lsllc | Hacker News Reader

If you have a geothermal heat pump, you can install a "desuperheater" [0] to recover the excess heat for hot water during the shoulder seasons. It doesn't work so well during the winter, so you need "backup" heat such as an electric element, or a more traditional fossil fuel hw heater (and you might also want one of these as a backup for the geothermal loop depending on where you live).

[0] http://www.geojerry.com/desuperheater.html (sorry, couldn't find a better site)

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I had a ground heat pump for several years. It could easily warm a 170m2 house plus warm water. Of the temperature went below -15c or more the electric elements kicked in to boost the pump. I could sit in the living room with a t-shirt on and not worry about heating expenses, it was very cheap to run.

Geothermal heating is pretty standard nowadays in North Europe. Heating the house + hot water is totally doable w/o using an auxiliary resistive heater even when the temperature outside is -20C. Geothermal heat pumps would feature the extra heater (like 9kW) just in case but mine has been turned on only when it overheats the water (60C+ once per week) to reduce fauna build up.

Heat recovery ventilators using heat exchangers have proven to be very successful too. Something like over 90% efficient. Couple that with triple glazing and you almost need no heating.

What I do wonder why there aren’t more in-window air conditioners coupled with heat recovery ventilators?

We have geothermal HVAC. When we moved in we had that water heating setup, but then changed the water heater (natural gas tank). When we did, we asked about hooking up the desuperheater again and were told the energy savings weren't worth it on newer water heating units because of the efficiency. This is coming from a well respected company in the area that specializes in geothermal and heat pumps, and generally has bent over backwards to suggest improvements to all sorts of things in other ways (I also am pretty sure they installed the original system).

Not sure if they were right or wrong, but my experience with a lots of these heating and cooling things is that the real world efficiencies are often different from theoretical values.

Geothermal water heating just barely works out in general because efficiency drops with higher temperature differentials. If you would accept you water at noticeably below body temperature it would be great (though I suspect this is ideal for bacteria growth) efficiency. If you want your water as warm as the minimum recommended temperature by efficiency experts you are already in questionable efficiency range. If you go hotter it gets worse.

Yeah, reducing the temperature on water heaters is a great way of causing Legionnaire's disease.

Given that both the desuperheater and the geothermal both need backup heat, the best approach is to install a small high efficiency gas boiler with 2 zones, one as an indirect heater for the desuperheater (so it'll ensure that the hot water stays hot, but won't run if not needed) and the second zone as a hydronic loop in the air handler to provide backup for those cold nights in the winter that are below the design spec.

Of course, the heat input to your house is a function of heat loss, so if the house is very well insulated and sealed against air changes, regardless of how you heat you'll save energy.

TBH, since geothermal is reversible, in that it does both heating and cooling this makes it even more attractive and efficient especially in the summertime. It should be required in new construction where possible and indeed heavily subsidized to encourage it's adoption.

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