We have an older house we are renovating and insulating. It has an insulated daylight basement slab and exterior insulated foundation walls. We are doing as much as we can (within reason) to tighten and insulate the envelope. We are now trying to choose between a water to water GSHP and a condensing boiler. I realize that the GSHP is most efficient when it is being used to generate 95 degree water for heating the slab and much less efficient for generating DHW. The model that has been recommended to us by our heating contractor (Geocomfort Compass GWT046), operating at part-load, is (self-)rated with a COP of 5.31 for raising water from 85 to 96.7, 4.87 from 95 to 106.4, and 4.49 from 110 to 120.9. This is for an EST of 50 degrees and 6.8 gpm. (I realize there are other COP values for different gpm pumping.)

1.  Am I correct in my understanding (as it has been explained to me) that this can be set up with two different buffer tanks, one for heating with a setting based on an outside temperature sensor (higher when it's colder), and a second for DHW set for 120 or a bit less, and the GSHP will just run (at a lower COP) until the DHW tank is up to its set temperature, and will then get the heating buffer tank to its setting (at a higher COP).

2.  I have read many postings here that GSHP is not an efficient way to generate DHW. Even used like this?

3.  We would also like to heat our hottub from this. Sensible or not?

We live in Western Washington and it just doesn't get all that cold all that often, so it is hard to justify the ground loop expense unless we can use this to supply our DHW and hot tub heat, preferably at a price lower than natural gas. We are caught in the conflict between trying to first reduce our heating consumption through smart sealing and insulation, but if we do that really well, will we be using so much less energy that we won't be able to justify the expense (and mess) of digging a 400 foot trench through our woods.

Our other option is to just go with a Triangle Prestige, with an indirect DHW tank and a heat exchanger for the hot tub, and save our green for a later installing of a green hot water solar system. Natural gas is currently $1.07 per therm, and electricity is $010.3 per kwh, so at 3 COP, the savings from electricity is only about 10%, and at 4 COP it is about 33%. (I do understand that the COP figures do not include the cost of power for pumping the fluid through the ground loop.)

Thanks for your advice and feedback.

Welcome!

What you are describing is a system similar to this on the WEL server:

http://welserver.com/WEL0383/

You are correct, the higher the temperature between the water coming from the ground and the water to be heated, the lesser the efficiency. In other words the COP goes down with a drop in source temperature (from the loop) and also goes down with an increase in the water temps in your tanks. Thus the lower you can keep the temperature of the water tanks, the higher the efficiency. The rule of thumb is that every 10 degrees bring you 10%. This is when outdoor reset comes in, which lowers the water temperature for heating when it is warmer outside. On the other side, your domestic hot water you want to keep around 120 degrees all year around, so you need 2 tanks conditioned at different temperatures. In addition you do not want to take a shower or a bath in your whirlpool with the water which also circulates through your water pipes for heating, so you need a heatexchanger with what you separate the 2 circuits. Also the chemicals in the hottub are not liked by normal heat exchangers. Thus you need a primary and a secondary circuit and some controls. In the heatexchanger, you loose about 7 degrees of temperature.


So yes, it can be easily done by someone who has done it before. The rest is a number game. And an educated guess where prices will go in the next years. Yes, the best money you can spend is on insulation.

Thanks for the response, and thanks for the link. It is nice to get confirmation that other people out there are doing this.

My heating contractor has installed many, many geothermal systems and many mod/con boilers, and he has done some with the full DHW, and he knows (as do I) that for this system (as in virtually all systems), the heating water never touches the DHW. He's also done a few of the spa heat exchangers.

The mystery is fuel price trends. If we didn't have natural gas, GSHP would be a no-brainer. Instead, it's a tough call. We're not sure which is best for us, and neither is he.

It is the same question whether you buy a SUV or a Prius. As long as fuel is cheap, many people do not find much difference. But long term, the general trend is more towards the Prius like. Not saying that Prius is the right execution here, but I think you get my point. Looking where other countries are going, and how valuable fossil fuel will become, it is hard to imagine that geo is not the trend of the future. Everyone I know who in the past thought the math is not there now regrets it not having gone with geo.

In my opinion, nat gas will be competitively priced for quite awhile and it is the primary alternative to geo.

Given those utility costs it is a tough call. Massive new domestic gas discoveries suggest it will be cheap for years to come.

Depending on the generation mix of the local electrical utility, NG may well be the "greener" choice in terms of greenhouse gas emissions.

In my opinion, nat gas will be competitively priced for quite awhile and it is the primary alternative to geo.

Once the companies that supply natural gas get you "addicted" to this particular fossil fuel, rest assured they'll stick it to you!. LNG will eventually power our cars - and once that happens, home heating with natural gas will go through the roof. Of course, this is solely our opinion, but certainly based in historical fact.

Both Direct Exchange (DX) and water loop Geothermal are exponentially better than any other heating, cooling and hot water system. You don't need to take our word for it. Our clients are constantly sending us their electric usage because they're so pleased with how their systems are performing.

Don't get sucked in with all this talk about natural gas. It's dangerous to extract, transport and use. Not only that, it's bad for the environment. According to the DOE, if one million homes (that's not alot in the scheme of things) were to convert to geothermal, we'd save twenty one million barrels of oil every year. Oil is around $90 a barrel. That's two billion $ a year that stays in the "good ole USA." - EVERY YEAR - FOREVER.

Yeah - we're "geovangelists" and proud of it.

Paul
www.TotalGreenUS.com

From an economic stand point:
As the trend on all energy prices steadily climbs, it makes sense to employ the heating system that uses the least amount of energy unless it costs more to run.
If it costs less to run with gas in an existing home now or not much more one may not be attracted to a 10 year pay back on a geo invest....I mean expense. I have told folks with nat gas after running their numbers, goals and budget, that they may have better vehicles for their money. If it becomes more expensive to burn gas, folks can change then.
With tax credits available there is almost no reason not to install geo in a new home in my area. Even against natural gas, the additional expense may be recouped in less than 5 years.
Joe