Sizing a Geothermal Unit

The first step in properly sizing a Geothermal system (or any system for that matter) is to generate a BTU heat loss load calculation for the structure based on the heating degree days of the area the building is being constructed.

With our Geothermal installations I am not concerned with the ‘cooling’ degree days due to the dominant heating load in Wisconsin. (unless unusual circumstances exist – One example would be a commercial building that has an abnormal amount of internal heat gains or solar gain.) There are many acceptable programs to accomplish this.

I have generated a block load for both the viewing area, apartments above and the home. The only ‘unknown’ entry while generating loads is the normalized ACH (air change per hour). Also referred to as infiltration. The tighter the structure the lower the ACH. I have assumed a .3 ACH (or 1/3rd of an air change per hour). This is quite high for today’s construction practices but, never the less, a practical number if we decide to utilize the quoted air to air heat exchangers for controlled ventilation. Once the load calculations are complete I input that number into our geothermal sizing calculator. The sizing calculator will then let me know what my balance points are and if auxiliary heating will be required.

All systems quoted (viewing area, apartments and home) are sized to handle 100% of the heating and cooling loads. Meaning they will heat the appropriate areas to a minimum of 72 degrees when it is -10 degrees outside with NO required auxiliary. Not to confuse the issue, but, both systems are technically ‘oversized’ based on my load calculations. It is a common industry practice set by IGSHPA (International ground source heat pump association) to size a geothermal system to handle 95% to 98% of the heating degree days. In Walworth that is approximately 5 degree outdoor temperature. Because geothermal systems are manufactured by nominal tons only (1/4 & 1/3 ton not available) I choose to move up a size versus dropping a size and requiring auxiliary for certain times of the year. I would definitely not recommend any systems larger than the ones designed. Some of the reasons are as follows:

(1) – An over-sized geothermal system will actually run LESS efficiently due to starting & stopping losses from motor startup surges and refrigerant equalization. Much like city driving with a vehicle. The longer a geo unit runs a cycle the more efficient it operates, running at higher COP’s causing the unit to operate at the advertized COP’s means you get more heat for your money.

(2) – Over-sized units will ‘short-cycle’. (Even the 2 stage units) Causing comfort issues, noise issues and unneeded wear and tear on parts. Essentially, short cycling itself to death.

(3) – Over-sized units make for poor dehumidification during summer as well as reduced efficiencies due to the short run times as it takes approximately 10 minutes of operation before the system will start removing moisture from the structure. So in search of dryer air you may find yourself lowering thermostat settings. Over-sized cooling systems have (2) set points – hot and muggy or cold and clammy, much like a basement.

So, rather than allowing any of these things to happen (and there are more), it is imperative to match the installed system to the structures heating / cooling loads derived from some acceptable load calculation program (NOT square footage or another guess) so your geothermal system will live up to the low energy bills, high comfort and long life they are noted for.

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