I’ve been asked by several people to do an article on the pros and cons of a geothermal furnace, and how should we choose between a geothermal furnace and a geothermal heat pump. After asking multiple questions back to them, I thought it would be best to write an article defining everything.
The short answer is not what you’re expecting. A “geothermal furnace” doesn’t actually exist. Most geothermal units are heat pumps, period. Thus, there really is no decision to be made. I’ll elaborate.
What is a Furnace?
A furnace by definition is a heating-only device that uses combustion of a fuel to produce heat. This fuel can be anything, coal, wood, or natural gas. One of the most common misconceptions people have about their own heating and cooling system is that it is in fact a furnace, when it is typically a heat pump. What’s even worse is that this isn’t really their fault, as most manufacturers have ambiguously labeled their equipment as such for long enough that it’s confusing.
If you have an HVAC system that has an outdoor condensing unit, you have the capability for cooling mode operation. It is the indoor unit that causes confusion. The indoor unit is typically sold as a “furnace” since it is that specific piece of equipment that provides heating operation. The tricky part is that today most indoor units also have a cooling coil, which eliminates the “heating-only” part of the equation. It is now an air handler, not just a furnace. Tricky, huh? It’s been this way for over a decade.
Fast forward to today, and people are used to calling what they see in their basement (the indoor unit) a “furnace.” Since geothermal systems don’t have an outdoor unit (they use a ground loop), people think all they need is a “furnace.” They are correct in idea, wrong in semantics. Geothermal indoor units are heat pumps, as they have both heating and cooling modes. I say this because people are often asking if the heat pump or furnace is better, when technically a furnace doesn’t exist.
But Heating it is Critical to My Application
Now we have the lingo correct, we’ll move on to the other commonly mentioned aspect. A few people I’ve talked to asking about furnaces knew the differences, but wanted to know that heating was mission critical in their specific application. This is the reason they were seeking out a furnace, as opposed to a heat pump. Whether it be for your house, animal shelter, or storage application, there are heat pumps that provide a high level of heating through auxiliary coils. In fact, most manufacturers offer supplemental heating in two ways (electrical resistance and natural gas) on EVERY heat pump size they make. That should give you peace of mind.
This works because most heat pumps can be configured to have airflow in multiple directions. The modularity of these units also facilitates an add-on module that is simply a heating only coil. Here are the pros and cons of adding an auxiliary heating coil:
Pros:
- It adds a supplementary heating element to operate as a second stage heat producer.
- Supplementary heat capacity via natural gas is cost effective to get a lot of heat.
- Redundancy – if the heat pump fails, you still have heating ability.
Cons:
- Pressure Drop – You will lose a little fan energy by blowing across a supplemental heating coil.
- Overheating the airstream causes stratification and not comfortable air mixing.
If more heat is your goal, you’re looking for a heat pump with an auxiliary (supplemental) heat coil, not a furnace. Like I said earlier, manufacturers and contractors have ambiguously labeled home HVAC systems as furnaces for a long time. Don’t feel bad if you’ve made this same mistake. However, with geothermal we want you to know the pros and cons, and sometimes that includes what’s right and wrong.