Geothermal HVAC

[walker450]

There was some discussion on geothermal heat pump systems in the "Propane Cost" thread... to avoid derailing that thread I decided to start a new one.

We are building a new home this summer and I plan on using a geothermal heat pump system so this topic is of much interest.

Here are some comments from the propane thread:

and also an electric hot water heater (in summer, hot water is free as part of the geothermal cooling cycle).

Actually you make much more hot water in the winter than in the summer. The hot water generator, known as a desuperheater, removes the "heat of compression" from the freon and transfers it to the hot water loop. In the summer, since the ground water loops are close in temperature to the temperature of the evaporator coil (for AC) there is low pressure and not much compression and therefore not much heat of compression. When heating, the temperature difference between the ground water loop and the heating coil (same coil as AC but a condenser when heating) is typically 70+ degrees. This will result in much higher freon pressures, and thus high compression of the freon. This generates a high freon temperature at the compressor discharge ("heat of compression"), which is removed and used to heat hot water. So this yields much more hot water generation during heating season.

paul

Geothermal desuperheater generates more HOT water in summer, not winter. In the summer when you are taking heat out of your house, you have free heat to use to make hot water, before dumping the heat into the ground. In the winter, you want all the heat you can get to heat your house, it only heats water if it's not supplying heat to your house.

Sooooooo... please elaborate techman. I had the same understanding as buckeyefarmer: In the summer, hot water is basically "free", because that heat would have been rejected to the ground loop. In the winter, it is not "free", but is still available, because the heat's first purpose is to heat the air inside the home.

I am confused on the 70+ degree differential you describe. Can you elaborate? I'm just not seeing the difference between seasons on a geothermal unit.

Is the desuperheater between the compressor and reversing valve?

 

[Coyote machine]

Try: heatinghelp.com. I had a question about my Buderus gas boiler and someone here sent me there and there are very knowledgeable techs over there. Not that there aren't here, just makes sense to go where the people in the know hang out.

 

[eth555]

I am not sure about the exact answer to your question, I had geo put in over the summer and I can tell you I do get hot water in the winter time. I do remember the installer saying something about the hot water is opposite of what you think as far as getting more hot water in the winter time. Maybe it depends on brands too?

 

[eth555]

Also check out GEOexchangeｮ | Geothermal Exchange Organization they are primarily Geo related forums there.

 

[techman]

Walker450:

The domestic water heating (DHW) is a separate, small heat exchanger that is connected to the outlet of the compressor. It is NOT the same heat exchanger that is used for the ground water loop.

When you compress a gas, the gas heats as part of the process. This warm, or hot gas is not the energy used to extract heat to warm the home. While the hot gas has some heat value, it is not very large since it is only the mass of the gas and the temperature change due to the rise of temperature. Usually only between 1000-1500 BTUs for an average system. The house heating is a result of the freon condensing from a gas to a liquid in the coil that heats the house (enthalpy). Any additional heating from the fact that the gas is hot is minimal compared to the heat energy given off in condensing.

So given those facts, the DHW heating is a function of how hot the freon is. In the summer, assume that you have 60 deg water from the ground loop. The compressor output pressure will be around 170 PSI. A simple evaluation of energy is that you are going from 60 deg water to 40 deg air. The refrigeration loop does not need to do much work to achieve this temperature differential, so less work means less freon heating. The DHW would extract heat of compressing to 170 PSI. In the winter, if we assume a water temp of 30 deg and an air discharge of 90 deg, then the compressor output pressure will be around 275 PSI. The much higher compressor pressures are a result of going from 30 deg to 90 deg. In this case you are compressing a gas to 275 PSI so it will be hotter than compressing to 170 PSI (PV=NRT). So you can get more heat for the DHW due to the higher pressure.

Cooling the hot gas does have a slight impact on the heat produced, but it is a small percentage compared to the heat resulting from phase-change in the freon. In the example I cited, a 38,000 BTU unit will generate, on average, 1500 BTUs for hot water. That is less than 4% of the capacity.

I hope you followed this and see why heating will generate more DHW than cooling. It does all depend on the water and air temps, and there are cases where cooling might produce more DHW, but not in a normal set of conditions. You can feel or measure the DHW loop temps summer and winter and feel the difference as well.

Paul

 

[japody]

We installed a Bosch system with a gound loop 2 years ago. I cannot say enough about how pleased we are with it. The house is total electric, over 2200 sq ft on the 1st floor, with additional 1100 sq ft on 2nd. We keep the thermostat at 71 - 72 degrees. Bill for this last month with record low temps was $140.

 

[BitChin1]

One thing to think about is when the unit will run? I find at my house (SC Kansas - zone 4) that my heating demand is > than my cooling demand. Also during a sunny day all of my heat demand is covered by the sun. given this I get plenty of DHW from the desuperheater when the GSHP is operating in either mode but only during extreme cold or hot days/nights.
I'll quickly admit I don't know the details related to the base question about available heat for DHW in cooling vs heating mode.

 

[doxford jim]

My comments would be that a backup heatins system would be a good idea. It depends a lot on the type of weather that your area experiences during the winter, but if the new geothermal unit went down you could be in trouble.
In my case I have had two or three occasions when our "Waterfurnace" unit failed for different reasons - in an area where -40C/F temperatures are possible and -20C very common. I am lucky that I have a propane backup heating system that saved my butt.
I also have a 12kW emergancy backup diesel generator that can be used in the event of power failures. This unit will supply 100 amp service to allow the geothermal unit to run, as well as supply electricty for all other functions.
Othwerise I am happy with the geothermal system and how it operates - it was already installed when I bought my property .
HTH

 

[DKCDKC]

I have a 2000 Waterfurnace geothermal unit. I don't know the technology, but the installers said that the unit would produce more hot water in the summer than in the winter, and my experience seems to bear that out. However, my evidence is not scientific, just seat-of-the-pants. Either way, the unit works beautifully. After the initial warranty period, I have it checked every other year and have no complaints. I think I should have had another half-ton of size on it, but otherwise all is well. I don't like the Skuttle whole house humidifier that they installed with it, my water is too hard. So I use floor humidifiers in the house. A small woodstove seems to provide enough heat to avoid use of aux heat on days above 5 outside.

 

[s219]

Here's a schematic of my Climatemaster geothermal unit if that helps the conversation:



The tech manual does mention hot water in all seasons.

Note the HWG is part of the reversible compressor refrigerant loop.

Also note that in cooling mode, the air coil is the evaporator and the ground loop is the condenser. In heating mode, the air coil is the condenser and the ground loop is the evaporator. That was a simple observation that cast the operation into agreement with my understanding of regular AC / heat pump systems (where the exterior fan and coil replaces the ground loop of a geothermal system).