Geothermal heating/cooling question

[Z-Michigan]

Wondering if anyone has expertise with geothermal systems to advise on an issue - not sure if it's a feature or a problem.

I have a 1600sf house built in 1999. It is very conventional wood frame 2x4 construction, two stories, nothing unusual. While originally set up with a propane furnace and, as far as I can tell, no AC, around 2002 the prior owner installed a closed-loop geothermal heating/cooling/hot water system. The propane furnace remains as a backup for cold days when the geo can't keep up. There is no separate AC, and the propane water heater is physically in place but not turned on; it serves as a holding tank before the geo water heater tank. We moved into the house last November so we don't have even a full 12 months here yet.

This is a closed-loop system with a ground loop buried (I am told) 8 feet below grade and running a couple hundred feet long. This system does not involve a well or a pond. We had the system inspected about three months ago and no repairs were suggested.

I have a basic understanding of how the geothermal system works, as being basically a fridge type condenser system that can work in either direction to transfer heat either from ground to house (winter) or from house to ground (summer). I am not an expert on the system by any means.

Here's the issue: it's a hot summer here in Michigan. We set our cooling at 70. Most days the house stays at 70; on hotter days if often gets up to 73-75 in the mid to late afternoon and then cools back down. However, there have been two days - today and July 4 - when we had friends over and we were both cooking inside and opening doors frequently because we were grilling outside. On both days, the house temperature got to be over 80, even with the thermostat set at 70. Between July 4 and today the system has been working fine, even on 95+ degree days.

Can anyone tell me if this is a normal feature of a geothermal cooling setup? It is a bit frustrating to have people over on a hot day and have the house heat up!

Related question 1: given this type of ground loop, I am wondering if the capacity to cool erodes by end of summer, due to too much heat being put into the ground surrounding the pipe. We are having an unusual drought in my area, with less than 1" of rain in the last 8 weeks when 5" or more would be normal; I'm also wondering if the drought is both causing the ground temperature to increase and reducing its heat capacity. Also the correlating question: does the heating capacity fade at end of winter as too much heat is taken out of that same area?

Related question 2: since this is also our supply of hot water, would I ever get a benefit on a hot day from running hot water from faucet to drain, presumably pulling heat out of the working fluid and allowing more cooling capacity? Or am I making a bad assumption about how things work?

 

[charlz]

No expert here but a few thoughts:

Likely your system is like any other heating/cooling system, it is rated at a certain output and if your load exceeds that rating it will not be able to keep up. This could be the size of your ground loop, the size of the heat pump unit, your blowers being unable to exchange enough air.

I would get some of those cheap little thermometers and stick them in a couple of vents. (or you could use just one and walk around). Watch the temperatures throughout the day and see if it varies much. If it is putting out 60 degree air in the morning and 70 degree air in the hot part of the day I would say your loop or pump can't keep up. If the temperature stays pretty much the same then you are probably not exchanging enough air.

If it does rise during the hot part of the day you can try draining out all your hot water and see if it comes back down some, this will tell you if that would help or not.


I am curious as to whether or not you think your power bill is 'cheaper' compared to a regular setup. I read somewhere the system pretty much runs constantly so I would think it uses a lot of power.

Charles

 

[Z-Michigan]

Thanks. As far as I can tell the system is significantly cheaper. My prior house was similar sized (1420sf ranch) and had natural gas heat/water and electric for everything else including AC. It was only 25 miles from here so identical climate. That house was built in 2003 and was very well insulated. I probably paid around $200/month average for gas and electric combined, with an occasional cold winter month pushing toward $300. With this house my electric bills run just over $100, half of which is power for the geothermal system - which is metered separately and costs only about half as much since it's treated as electricity for heat which for some reason gets a lower rate. We have a fairly large propane tank - I think it's 1000gal but it might be only 500 - and using propane for backup heat, all cooking, and clothes drying, our propane use is almost imperceptible. The prior owner told us they only had to fill the tank once ever 3 years and I think that's believable (and I realize most people with propane heat would use 3-5 tanks per year). In fact the propane company that supplied the prior owner's tank is mad at us because we haven't bought any more from them. They're a PITA to deal with so once this tank runs dry we're giving it back and having a different, friendlier company supply us a new, smaller tank.

The downside of geothermal is that the initial cost is something like $10-20k, serious money. Glad I didn't pay for it! But once it's installed, it seems to be very economical.

 

[gmsssi]

Well there are a lot of circumstances that could be coming into play. First who designed it and for what load. I'm no expert either but I have had mine for 9 years and I grill the tech every time they come out to check it. They work just like a heat pump, they work on recovery and maintaining. Insulation is a major issue in that respect. They have to respond and MAINTAIN the temperature! Insulation is a major issue in that respect. They DO NOT respond like a conventional system were they fire to set point and stop. Once a geo unit gets the house to temperature it continues to run to maintain temperature in a stage one loading. Do you turn the system off and on?? When it's comforatable do you turn to unit off? Again, a conventional system can be run by turning the system on and off. A geo unit can not be operated this way, it has to stay on and be in operation all the time because it does work on recovery and maintaining!!!

If it's loaded, it's probably running in stage 2 (or emergency in the case of heating). I would guess, if your system is not keeping up, it's insulation or undersized. You should see at least a 25 degree difference in house temperature and out put air. If it's 80F inside your output air should be at least 65F or less!! Another factor, in my case, is a multi-level system. I heat and cool a 3600 ft log home, 3 floors, each with a thermostat and automated dampers. In order for the house to maintain, the thermostats have to be somewhat synchronized. If I want to load the system to work more, each setting has to see a >2 degree difference in order to go into phase 2. If one is within that 2 degree range it will revert back to phase 1 and try to divert the air (with the dampers) to the levels that need the temperature. Could it be the unit is not going into stage 2???

My system tells me when something is wrong (etal.... low freon, low water flow etc...) with warning lights. Water flow is very important also in the system, check your level and for visable water pockets (air bubbles)! For what it's worth, flow should be >10gpm in phase 2. This summer has been a hot one, house is comfortable all the time and I'm averaging about $140 a month, winter is about $80. Keep us up to date, I'd like to know more about whats happening to your system. I may learn something that I need to know about mine! Good Luck!

 

[KYErik]

I have a Water Furnace 3 ton closed loop system for my 1500 square foot 1.5 story house (r30 in ceiling and r19 in the walls). The units are sized for ideal application and for doors that are closed most of the time. From what I understand, the units which are sized just right will be better at dehumidifying than a system that is too large for your home.

I would guess that the cooking and frequent open doors just temporarily over burdened your system.

Once, on a hot 95+ degree day while we had company and my wife was cooking with most of the burners on the stove, our unit couldn't quite keep up. I think that the extra water vapor released in cooking puts an extra strain on the unit which has to remove it as it cools the air. There is a bypass channel on our ductwork so we can close off vents to bedrooms and save money while not over pressurizing the supply ducts. If you have a similar feature, you may be able to shut off a few rooms and avoid the whole house getting so hot.

I am sure that low water in your closed loop could also decrease cooling ability, but the piping joints are usually heat sealed and should last practically forever.

I too have wondered if the soil around the pipes gets warmer by the end of summer or colder by the end of winter. My pipes are only 4 feet deep here. I think that moisture has a big part in helping heat transfer, so if the ground is really dry around the pipes, the system might have to run a little longer for the same effect. We are having a drought here too and I can't say that I have noticed the compressor running for an extra long time (our compressor is under our floor in the crawlspace).

It would seem that running a hot water faucet might help your system out, but you would need to shut off your water heater's power or propane so you were not increasing cost via your hot water heater.

Our house is all electric and our monthly bills are about $90 during the hottest part of the summer and $110 during the coldest part of the winter. Spring/fall bills are about $50. We do keep 2 spare bedrooms and 1 bathroom (the whole upstairs) closed off.

 

[Bill Barrett]

I do install Geo. From what you stated all is ok. The system should be sized for a set of values normally, 72-75 degrees inside and your average outside temps,( inside and outside design temps) and the heat gain and loss of your house. Change any of these and your system will perform as such.
Detroit design temps are 88 summer 6 winter outside and 75 inside as per Manual J ,what most heat loss heat gain programs use. So you see just by the temps how far away from average you were, then add all the people and door use!

Now if you have a two stage system you may have more capacity.

Your loop will start to loose capacity at the end of the seasons.

Yes your water heater circuit is a desuperheater it may help some in summer but will hurt you in winter.

 

[kenmac]

Over heating weather indoors or out doors taxes a cooling system . If you added people or heat (as in cooking ) you added heat load.That said, Units are sized by heat load / heat loss. Everything adds heat load,appliances in operation, people , doors that are opened will also add heat to the home. There's probably nothing wrong with the unit if all is ok now ( after all the heat load is gone) In my opinion, it's easier to heat a home than to cool it

 

[Bill Barrett]

Also soil type and moisture content does affect heat transfer. Dry sandy soil can shrink away from the loop piping cutting transfer significantly. Sometimes in soil of this type a soaker tile is installed with the loop to add moisture if necessary .
There are P/T ports on the loop piping by the Geo unit to check pressure and temperatures . With these readings you can calculate GPM and heat transfer.
P/T ports will be little brass caps on the piping going into and out of the Geo unit.

 

[heehaw]

the systems i have seen have had a way to monitor the loop water temp: ones system i still say was installed wrong, it was a 6 ton unit, with wells, plumbed in parallel: the water temp would go up a lot on hot days and the system never would keep up: i thought the wells should have been in series: the other way, there was no way to tell if all the water flow was going thru just one well? if you can monitor the water temp, it will tell you a lot about the system and how much change there is with seasons.
heehaw

 

[Willl]

as far as I can tell, no AC

No AC, no cooling.
I don't see the problem.