How to size a geothermal heating system.

Geothermal heating systems and not much different than standard air source heat pump system. The main difference is geothermal uses the earth to add or remove heat from your home instead of the outside air. With geo the ground will vary from 30-75 degrees typically depending on season. Air source heat pumps need to deal with air temps ranging from -30 to 110 degrees depending if heating or cooling your home.

The big difference with air source heat pump is they can't provide enough heat from the home once outdoor temp gets below 20 degrees. So you need some sort of extra heat whether it be natural gas, oil, LP, or electric heat. Geothermal systems can be designed to heat your home to what ever outdoor temp you want.

Here is the catch to be careful of. If you are converting your current heating system to geothermal the duct work was designed for something else. For example purpose I am going to compare an average gas or oil heat. Now electric heat system won't be much different.

EXAMPLES BELOW ARE FOR A MINNESOTA HOME

Gas heat systems can provide higher temperature heat in the duct system with less air flow. So when a home is designed for gas the average home needs 1000-1400 CFM. (CFM= cubic feet per minute of air) A home that has a duct system for 1000 CFM with gas heat may now need a 4 ton geothermal heating system. Average design is 400 CFM/ton of geo. So a 4ton geo will need 1600 CFM. The problem is over 50% of home duct systems are undersized in the first place. An educated HVAC company will be able to test your home and see what you can do. Below is a typical solution that way to many heating companies do wrong.

Home needs a 4ton geo which is 1600 CFM but home has duct system only good enough for 1000 CFM. A common answer is to select a 3 ton geo system with gas back up heat. The 3 ton geo will need 1200 CFm. Usually some minor modification can be made to the duct system to handle 1200 CFM. Now when you get to the dead of winter in January when it is -10 and colder for a week straight the geo may not be able to keep up so you will need to use the gas furnace a little. But by going from a 4ton to a 3ton geo you will save about $2,500 just in well drilling. Plus you don't have to tear your home apart making the duct system large enough for a 4ton system. So by going to a 3ton you may save $3500, 4000, maybe 5000. Yes the 3ton may cost an extra $200 per year to heat your home, but it could take you 20 years to equal the $4000 savings.

So in short you maybe able to install a 3ton geo and have a 10 year pay back or a 4ton geo and have a 30 year pay back.

Now these examples above are very general and will vary by area you live or type of home. Point to be aware of bigger definitely is not better when it comes to geo. Now with the 30% federal tax credit geo is cheaper than ever to install and pay back is a lot faster. But don't let a poorly educated installation company steer you the wrong way.