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How do you properly size a geothermal unit?
- Mike Roberts
It's essential to properly size a geothermal unit. If it is a new construction or an existing home we do a site visit to see the home and to determine the best location for the loop field (vertical bore, horizontal bore, horizontal dig, open loop or pond loop.) Then we talk to the homeowner so we can gather information on the home they intend to build or on the current one. We need to know what type and how much insulation is to be used or is already in place (ceiling, walls and basement), windows sizes and e-value. A copy of the blueprint is helpful if available — new homes usually have an extra, but existing homeowners may not have them so the houses need to be manually measured. Then a heat loss and heat gain calculation can be determined for the home.
Let me insert one thing here — the better insulated the home is, the smaller the geothermal needs to be. The initial cost to install the unit and the monthly utility bills will be less. Translation — you save more. So if you have an existing home think about more insulation, especially in the attic and basement if you can, as these are usually easily accessed and have a significant heat loss.
Once the necessary information is gathered, we enter the data room by room into a data base to find the heat loss and heat gain, and the air flow needed for each room. This helps to size the duct properly.
Once we know what the loads are, then we look at sizing our unit. Many geothermal companies have software programs that help us determine unit and loop size needed.
Now this is where a true geothermal contractor can really help the home owner. Do you remember when I talked about a sight visit and how best the loops can be designed? The loops are the heartbeat of the system and where the energy for your geothermal comes from.
Here is where education comes in to play. The ground temperature is always a constant 52-56°F yearround once you get down to the depth of eight feet. The loops have a fluid in them water and some type of antifreeze (methanol, ethanol or propylene glycol). (Do you follow me so far?) In the world of geothermal there is a thing called loop temperature at a certain balance point — the coldest length of time in the winter and the warmest length of time in the summer — where the loop temperatures can be as high as 100 degrees and in the winter the loop temperature can be as low as 27 degrees. These are temperatures that the units can safely run. There's a loss of efficiency at these temperatures. A properly designed geothermal loop should have balance point loop temperature of 35 degrees in the winter and 85 degrees in the summer. When the loops are designed with this in mind your geothermal will be geothermal all winter long and should have no need for any back-up heat source
I hope I have shed a little light on the steps involved in properly sizing a geothermal. There are a lot of things to think about to get a geothermal unit to preform to the highest levels of efficiency.
Make sure the contractor talks to you and shows you accurate reports on your home so you'll get the best performance from your future geothermal.
These expert insights are provided by Mike Roberts, an ACE (Approved Contributing Expert) with ProudGreenHome, owner of First Geothermal Energy and an Enertech Global LLC geothermal dealer.
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