Has anyone studied net energy use and efficiency of geo when used intermittently for short periods of time on non consecutive days? Example would be outdoor high temps in 70's one day, then50's the next?. I was wondering if I would be better off using gas until it gets consistently colder. It seems like alot of energy could be wasted bringing the geo buffer tank up to temp, then not using geo for several days, only to have to reheat the tank again. Any thoughts would be appreciated.

If you are referring to DSH what ever energy you put into the buffer tank is that much less energy your water heater will use. It all goes back to fuel costs and type of system used.

No, not the desuperheater tank, the buffer tank. I have water to water heat pump set up which I did not explain. All heat created by the heat pump is dumped into a 65 gallon tank. This is then distributed to in floor heat and a w/w plenum coil for forced air.

That when an outdoor reset control comes in handy. It conditions the buffer tank based on the outdoor temperature and the forecasted heatloss of the house, thus increasing efficiency by not bringing the buffer tank up to full temperature when it is not needed.

I did not get an outdoor reset with my install, the system just kicks out when tank temp reaches approx 120 degrees.

It is someone skilled can easily install afterwards. Keep in mind that most of the BTUs the tank looses still gets into your house and contributes to heating your house. So it is not wasted, unless you use A/C to cool things down now.

I agree outdoor reset is a must for in floor heat.

One would think the tank behaves the same (in terms of heat loss), regardless of the heat source - gas or geo. As long as your relative prices of gas and electricity are the same in fall and winter, and it is more economical to use geo in winter, it should also be more economical to use geo in fall. In fact taking the temp of a tankful of 50˚ water to 120˚ by a closed loop geothermal system, will take fewer kwh of electricity in fall vs in winter, due to higher entering water temps early in the heating season.

Regards,
Masoud

The tank is not in the picture when using gas because it is a gas forced air furnace, which bypasses the geo tank completely--thus the potential temporary advantage of gas. Again, another piece of the puzzle I neglected to explain...sorry. I thought someone with a web energy logger or other power measuring devices may have done some such experiment. Maybe not.

Posted By Dafr on 21 Oct 2010 11:08 PM
The tank is not in the picture when using gas because it is a gas forced air furnace, which bypasses the geo tank completely--thus the potential temporary advantage of gas. Again, another piece of the puzzle I neglected to explain...sorry. I thought someone with a web energy logger or other power measuring devices may have done some such experiment. Maybe not.

Perhaps you could experiment and let us know.
joe

I do not have that equipment, or I would have tried to figure it out myself. I will assume no one knows for sure. Thanks

If the tank is inside conditioned space there are no losses to the geo even if the heat load is non-existent on alternate days. If it's outside of conditioned space an cools off until there's an average heat load, it'll be starting at about 65F. The total amount of heat buffered the abandoned and the amount of power used is readily calculated, and not very much when looking at the COP of geo.

65 gallons of buffer weighs ~540lbs. If you're rasing it from 65F, to say, 110F that's a delta-T of 45F. That's 45 x 540=24300BTUs. Assuming an average COP of 3.0, the amount of electricity used is the kwh equivalent of 24300/3.0= 8100BTUs. At 3413BTU/kwh, you're abandoning/losing

8100/3413=~ 2.4 kwh (multiply by your utility rates, but it's well under a buck except in some oil-fired island utilities. For me it would be about 40 cents.)

And that's only if the tank is outside of conditioned space, and stagnates to the average outdoor temp where a heat load begins (base 65 is used). Most buffers are insulated well enough that it would take several days, maybe even a week of 65F-average weather to drop below 70F from 110F.

And if you're running lower buffer temp than that with outdoor reset control, the losses are proportionally less with the lower delta-T.

If the tank is inside of conditioned space, those losses are heating the space, stored in the thermal mass of the building, and would only be a loss if you had a cooling load big enough to require running the geo in cooling mode then heating on alternate days or something. (Which could occasionally happen, but wouldn't be a typical or usual weather pattern.)

The buffer is there to keep the system from short-cycling and losing efficiency on light loads such as you'll see during the shoulder seasons, so USE it already, eh? Standby losses are pretty small potatoes, even if it sits idle for a couple of days or even a week at a time. And having the thermal mass of the buffer gives it enough a load per cycle that it will always hit a decent efficiency. You're looking at a low single-digit percentage loss in efficiency from standby loss, but gaining double-digit percentages in efficiency over letting it short-cycle during low load conditions (saving on maintenance too.)

That makes total sense. Thank you for such a wonderful, thorough explanation. I learned alot! The geo heat pump is on!!!

Thanks Dana, excellent explanation.
Dafr, you could measure usage with existing electric and gas meters. Heating consumption equals usage minus average daily usage.
j