On paper, geothermal, or more accurately, Ground Source Heat Pumps (GSHP), are amazing. They pump out endless heat during the winter and cold during the summer at costs that are often much lower than conventional heat pumps. They don’t pollute (directly). They’re quiet, comfortable and, when properly maintained, should last decades.
But what’s the reality? Are geothermal heat pumps really worth it? Keep reading to learn the answers!
Before diving into to answering these questions, I need to explain more about ground-source heat pumps. How they work. How they’re installed and maintained. Only then will the answers make sense so you can determine if a GSHP is right for you.
What’s a heat pump and how does it work?
If you’ve seen a refrigerator or an air conditioner, then you’ve seen a heat pump in action! It’s really a marvelous application of some basic physics. A heat pump works by circulating a refrigerant, you probably know it by its trade name, “Freon.” A compressor circulates the refrigerant around the system and through the coils which provide the heating or cooling then around to another set of coils which eliminates excess cold or heat, then back to the compressor.
Let’s focus on a standard refrigerator first to make it clearer how a heat pump works.
In a refrigerator, the compressor takes the refrigerant, which is in a gaseous form, (think of steam as the gaseous form of water) and compresses it. Under pressure, the gas changes phase and turns into a hot liquid (think of liquid water). This hot liquid passes through the cooling coils in a way that causes the hot liquid to turn into a vapor. This process requires a lot of energy which it extracts from its surroundings, cooling them down.
That causes the cooling action in a refrigerator or air conditioner. Changing from a liquid to a gas takes energy. Taking energy is another way of saying cooling.
After cooling and becoming gaseous, the refrigerant flows out of the cooling coils and moves to another set of coils where the gas is converted into a liquid, releasing heat. If you’ve ever burned yourself on steam, you know that steam contains a load of heat! The physics of it is that when a gas changes phase into a liquid, it releases energy, heating its surroundings. In a refrigerator, this is done through the external coils. There’s a small fan that blows over the coils. That’s the warm air you feel coming from under or behind your fridge.
So there you have it! Your fridge is a perfect example of a heat pump, on a small scale. The cold goes into chilling your food, the warm is pumped into your room.
Now picture a giant refrigerator. The components are exactly the same. A compressor, refrigerant and two coils – one for heating, one for cooling. Let’s consider it operating during the summer, in “air conditioning mode.”
In a ground-source heat pump (GSHP) when air conditioning, the cooling coil is in your house and the heating coil is releasing the “waste heat” into the ground. The question you’re probably asking – “why go through the trouble to put the heat into the ground instead of the air, like a normal air conditioner?”
The answer to this is that converting the gas to a liquid occurs more efficiently if the surroundings are colder. In a normal air conditioner, a huge fan blows outdoor summer air through the coils to cool the gas. That air might be at 90 degrees F in the summer. But the ground is much cooler year round, typically between 40F and 60F depending on where you live. That makes a GSHP much more efficient during the summer.
Here’s the first potential problem with a GSHP. When you run the air conditioner, you’re heating the ground. The longer you run it, the hotter the ground gets. Ideally, the ground dissipates the heat to its surrounding ground, so that the ground temperature around the coils in the earth only rises a tiny amount. But the longer you run it, the less chance the ground has to dissipate the heat, and the ground can actually get hotter than the outside air. When this occurs, the GSHP is LESS EFFICIENT than a conventional air conditioner under many conditions.
Industry pros will immediately jump on me for this. They’ll say “you idiot, in a proper installation, this should never occur! You’re misrepresenting ground-source heat pumps!” and they’d be right. But guess what? Many, many installations of GSHP system, even by experienced, highly reputable companies, are inadequate, leading to poor performance under stressful conditions (long periods of hot or cold).
So what happens in this situation? The ground gets hotter and hotter through the summer and the air conditioner gets less and less efficient. Instead of being much more efficient than a normal air conditioner, the GSHP might only be as efficient or less efficient. And the system gets less able to cool your home. It might even trip a safety control in the heat pump.
Again, in an ideal situation, this should never occur. A proper system is designed by a trained engineer who knows the thermal characteristics of the ground and its ability to dissipate excess heat. They’ve measured ground water flow which greatly impacts the ability to dissipate heat. They do engineering studies and predict the usage of the system and run simulations to model long term system performance over many years. That’s what happens in multi-million dollar industrial ground-source heat pump installations. But guess what? How many residential HVAC installers do this? Probably zero, because these engineering studies cost more than the entire residential system! If you’re lucky, they’ll run the sizing software provided by the manufacturer, but even this only gives an approximation because nobody is taking the measurements needed to design the system properly.
The second problem that plagues many installations is cost shaving, leading to inadequately sized installations. Let me explain…
One of the popular types of GSHP system uses vertical “wells.” The depth of the well dictates the amount of heat (or cold) that can be put into the ground and properly dissipated. Typical estimates are that you need a minimum of 150 feet of well and preferably 200 feet of well, per 12,000 BTUs of system capacity. Of course, these are estimates based on typical types of ground etc. but they are generally accepted.
The problem is, it is very expensive to drill and prepare the wells. The industry estimate is about $5,000 per well. Most homes should use four or five wells, at a cost of $20-25,000 just for the wells! This is for a proper installation. Unfortunately, few people want to pay for a proper installation, and the salespeople know that. So they’ll say something like: “200ft per ton is just a theoretical estimate. Those engineers have never dug a well. I’ve been installing these systems for 20 years and I know that’s overkill. I can use half as many wells and save you $10-$15,000!” – How many consumers will argue with that?
The third issue is related to the second – total system cost. Even an inadequately sized GSHP system costs roughly twice as much as a high-quality air-source heat pump.
Fourth is long term maintenance. Most GSHP systems require regular maintenance in order to function efficiently and properly over time. Without this, the system will fail or cease to work effectively over time. If you’re a homeowner that doesn’t mind paying for (at least) yearly maintenance, this is not a problem. But most people will complain about spending another $1,000 per year for proper system maintenance.
All of this sounds very negative, but let me state unequivocally – a properly designed and installed ground-source heat pump system greatly outperforms even the best air-source (conventional) heat pump. There are many GSHP systems installed around the world that work amazingly well, providing cost effective heating and cooling. I heartily recommend GSHP systems if you are willing to hire a qualified engineer to design and oversee the installation of the system and are willing to pay for an HVAC company with ample experience at installing and maintaining GSHP systems.
On the other hand, if you’re a typical homeowner, and are not willing to spend an additional of $15-$25,000 over a normal system plus an additional $500-$1000 per year in maintenance costs, then I cannot recommend investing in a ground-source heat pump. It is very unlikely that you could justify the purchase financially and the chances are very high that you would be unhappy with the system’s performance over time.
So, are ground-source heat pump systems worth it?
For most homeowners, the answer is no.