How to Size a Geothermal System

How to Size a Geothermal SystemHow to avoid the pitfalls in sizing a Geothermal heating and cooling system for your home. The accepted method of sizing any heating or cooling system, especially when installing a geothermal heating system for your home, is to have a competent person do a heat loss for heating requirements and heat gain for cooling requirements.

There are a number of good heat loss software products on the market and most competent heating and air conditioning contractors, engineers, and energy professionals know how to use them.

The first step is to access the building’s living areas, orientation in terms of North, East, South or West of each wall area, the composition of materials and insulation systems for the floors, walls, knee walls, and ceiling or roof areas, sizes and types of windows and doors then any other criteria that might affect how many BTUs it will take to heat or cool the home.  These dimensions and systems information items are entered into the heat loss.

There are two additional items of critical importance that can’t be determined by a cursory inspection but need to be tested.  One is infiltration and the other is the efficiency of the heating delivery system.

Infiltration can be accurately measured using a “Blower Door” test.  The blower door equipment is affixed to one of the entry doors then turned on.  The resulting amount of air coming into the home is measured by the machine so the operator can determine the amount of infiltration.  In an existing home where a retrofit geothermal system is being considered, it is a good idea to fix leaks, weather-stripping , leaky fireplace dampers, along with holes in partitions that leak air to bring the home as close as possible to the requirements currently in force in your area for new construction.

The second test being a “Blower Duct Test” is done on the duct work where a hot air system is specified or exists to determine how much leakage there may be in an existing duct system and to determine if the system will deliver the required amount of CFM (Cubic Feet per Minute) of air necessary to heat the home at the design temperature specified for your area.  Deficiencies in either CFM or ductwork quality and insulation may result in added expense to bring the system up to par or using a back up heating system to supplement the geothermal system when temperatures get below the threshold where the geo systems are capable of heating the home.

For instance where Central Air Conditioning has been added to homes after they were built, you will typically find that the AC system was designed to cool the home 20 degrees from the outside air temperature.  This meets the comfort test most of the time except when temperatures get to 100 as they do occasionally then you can expect the system to cool the air down to about 80 degrees.  This is fine for cooling but if that same system can only deliver 20 degrees of heating when the temperature is zero out, then the temperature in the house would only be 20 and that is way too cold.

After the heating and cooling requirements have been defined it is fairly easy to recommend a geothermal system and size that will do the job.   For instance if your home requires 120,000 BTU’s of heating at a design temperature of 4 degrees outside and 68 degrees inside and 50,000 BTU’s of cooling then by using the rule that there are 12,000 BTU’s per ton you can see you will need 10 Tons of heating and 2.25 tons of cooling.

If a closed loop well system is being specified at 150 feet of well per ton, you can see that you will need 1500 feet of wells (perhaps 4 – 375 foot wells) to get the design BTU’s that you will need.  Most contractors will add a reasonable safety margin on top of that keeping in mind that every extra foot you drill costs money but it is better to be safe than sorry later.

Keep in mind that with Geothermal, the energy available is determined by the footage of the properly grouted pipe buried in the ground.   With traditional systems you can buy more electricity, oil or gas but with Geo you have a fixed potential amount of energy and when you get to 100% of that potential there is no more unless you are willing to drill again.

Now you can size the equipment.  If this is a 2 story house, it might work out that you will need 6 tons for the first floor and 4 tons for the 2nd floor and an appropriate manufacturer with equipment that match those capability needs is selected.

Beware of contractors that do not go through all of the steps outlined above unless they are specifying a back up system because there is really no other way to be accurate.

For new construction the materials and building systems are defined by the plans and specifications and so long as the workmanship is reasonable, there should be no problems.  Again you can test the completed job by doing a blower door test and duct blower test to verify the design performance standards were in fact met.

Note:  You can take your heat loss BTU estimate and calculate by using the FUEL COST chart on this web site to see how much money you will save during the year using the Earth’s energy instead expensive and potentially unreliable fossil fuels.