Geothermal Heat Pump, Is It Worth It?

When researching what our heat and hot water system for the house would be I took a look at geothermal heat pumps ("GHPs") also known as ground source heat pumps (there are also air source heat pumps). Often just called "geothermal" this is very different from what's being leveraged in Iceland for power and can confuse people. However, it shares its name due to the principle of leveraging the earth as an energy source (in the case of a heat pump as the heat source or sync). The idea is simple: take source energy (be it air, water, or a refrigerant) and convert it to be hotter or cooler through condensation or evaporation. It works just like your conventional refrigerator or window air conditioner. This is due to condensing certain refrigerants which give off energy.

GHPs are 200-400% efficient vs. even the best oil or natural gas furnaces which are about 95% efficient. This is because they source from the ground. Twenty four inches below the earth's surface, the temperature all year round in most livable climates is 55-60 degrees. Thus, think of how, when you turn on your window AC when its 90+ degrees outside, it takes its input from that hot air and must produce liquid that is around 40 degrees to blow air across the cold coils. A GHP takes its input from 55. That only requires a 15 degree change vs. a 40 degree delta. Heating is just the reverse. I was convinced this was the way to go.

I had received quotes from a local GHP and solar installer, Hudson Valley Clean Energy. They quoted a water-to-water system of nearly 10 tons which was over $110k. That was well above our budget of around $40k for heat and DHW systems. I put the idea of geothermal on the back-burner. Just to sanity check, I asked some other HVAC installers about the sizing HVCE had quoted. I came to find that 10 tons is more like a commercial installation and that there was no way I needed that size. Four different people told me that I'd most likely be in the 3-4 ton range but certainly under 5 esp because of the tight envelope and other energy efficiency I was building in. I went back to HVCE and they affirmed their quote after their engineers supposedly looked again at my plans. I tell this part of the story both for others to learn the lesson of doing the extra work to get multiple quotes and getting someone else who isn't biased to sanity check them. I also tell it because I believe Hudson Valley Clean Energy is either dishonest or incompetent (not uncommon in the building industry as I've posted about before).

After I received multiple other quotes for different systems all sizing around 3.5 ton, I went back to HVCE just because I felt they were doing a disservice to geothermal by keeping the cost artificially high (an example of this is my pretty-knowledgeable-about-sustainability architect advised against geo because he said it was too expensive mostly based off quotes he'd seen from HVCE). The president of the company actually told me that they oversize their systems just to be safe. Well, 10 ton vs. 3.5 ton is a bit more than conservative sizing. In addition, my energy star rater actually told me that oversizing an HVAC system in a tight house is actually worse then undersizing. This is because the system will short cycle. All types of HVAC systems are better when they run for few but longer cycles. This is for energy efficiency reasons (the biggest cost to heating water is the initial heat up vs. maintaining a temp) and maintenance as short cycling where there are far more starts and stops is harder on the mechanics and probably will break down sooner. Thus, do not work with Hudson Valley Clean Energy as I believe they are exploiting people's lack of knowledge in these complex systems for their own financial gain. I had a similar experience in the quote they gave me for solar PV which I fortunately knew more about.

Back to my GHP decision. I really did not want any fossil fuels on site at the house. This was both for dependency and cost reasons as well as my belief that we're at or beyond peak oil and we all know what that's doing for geopolitics and wars. In addition, fossil fuels are unhealthy and I didn't want them around. I wanted to be future safe being based on electricity especially since we were installing solar PV as well. I can always source electricity from either my sources or green sources from the utility (NYS has been installing a lot of wind farms and we opt in to it being our source from the grid).

In one final attempt, I took another look at GHPs and came across what's called direct exchange systems. Direct exchange differs from water to water in one important way. In water to water, the pipes you run into the earth are filled with water to extract and/or sync the energy with the earth. That then has to exchange the temp with refrigerant for the condenser which then exchanges it back to water again. Those two exchanges have loss no matter how much efficiency you build in. In a direct exchange system, you actually run the refrigerant through the pipes in the ground to the compressor and there's then only one exchange to water for your heating/cooling. There are important benefits here:

1. Its much more efficient due to the single vs. double exchange
2. It requires smaller wells (both in diameter and depth) meaning less cost for installation and less impact on the environment.

I came across a company named Earthlinked who had just received the highest efficiency rating, 400%, by the EPA. Thus, for every unit of energy (electricity) you put in, you get four out (for heating/cooling your water). They had been around since the early 80s and continually improving this technology now using more environmentally friendly refrigerant materials and gaining efficiencies. Their GHP can be used to heat water for radiant, for domestic hot water, and for air conditioning. We hadn't planned AC as its rarely hot enough up in the Hudson Valley to require it but, as we found out this summer, whatever is causing global warming is making it necessary at times and are thankful we did it.

I got a quote from an Earthlinked installer (NOTE: Earthlinked's reps and installers have proven to be difficult to work with -- and that's putting it nicely which I'll detail in a future post -- but I still am supportive of their technology). The total cost was only about $65k. While still above our budget it was the most energy efficient way for heat and DHW that I saw and required no fossil fuels. Seeing that we were under budget at the time (which was a farce obscured by bad project accounting) we went for it. This was coupled with me finding out about induction cookers which was the last missing piece of not needing fossil fuels (more on that in another post). We had found a way to not use propane (the most common heating/cooking fuel in the area).

The installation called for the GHP to feed into an 80 gallon tank for the radiant system. GHPs are a great match with radiant because radiant is very efficient at heating. Modern radiant installations in concrete slabs only need to heat the water to around 100 degrees. GHPs easily produce up to about 110 degrees. Baseboard systems need to be hotter at around 125 and forced air upwards of 140 degrees. DHW is also typically around 110 degrees and we have two 40 gallon buffer tanks it heats for that. All of the buffer tanks also have electric elements in them. In the radiant case, this is on a control which only allows that to turn on if the outdoor temp drops below 20 degrees to help out the GHP. We actually found that wasn't necessary and I think I'm going to set that to 10 degrees next winter. For the DHW tank, the elements help heat the water quickly (along with the GHP) when we come up. They also run periodically to maintain the temp in tank. One of the main things I've been playing with is how to optimally use the electric elements in the different seasons. If this were a full-time house, at least one tank would always stay on since we'd regularly be using DHW. These tanks themselves have very little heat loss (less than 1 degree per hour) and once they're at temp they only run about 30 mins every six hours @ 4kw. However, when we leave on a Sunday I turn them off and when we arrive back they are turned on. They are aligned in a series and the tank directly connected to the GHP heats up in about an hour (more in the dead of winter) @ 8-10kw (both GHP and element run). The problem is tank 2 which is hooked to tank 1 and only has the element to heat it up. It takes upwards of 3-4 hours @ 4kw. NOTE: I have a TED whole house energy monitor installed that tells me second to second energy use.

What I've found is, as a three person family with water saving showerheads, we don't need 80 gallons of hot water even in the dead of winter. When we have guests over and there are 4+ showers happening in the morning then it is needed (I think). Thus, when we come up now I only turn on tank 1 and the GHP gets it up to temp quickly and efficiently. Tank 2 stays cool until it is fed by hot water from tank 1. The only downside is that you have to run through about 20 of the gallons of cool water in tank 2 (which is what is connected to the plumbing) until it starts feeling warm at the tap. That's generally fine because we typically get to the house on a Friday night and don't need showers. What I do is put the faucet in hot position no matter what I'm doing (even getting a drink of water) so that it starts drawing from tank 2 and pulls hot from tank 1. I find that a round of dishes, watering plants, and a few drinks pulls it through. I'm still looking for ways to make this better so there's no inconvenience for the family. In addition, the benefit of having the electric elements is that they provide redundancy in the case the GHP is out of service. We've had to use this more than I'd hoped. Most people report that GHPs are generally maintenance free. Think about it, how often do you have to have your AC or frig serviced? I believe they are low maint but we've struggled with it being out of service a few times. I don't believe this is due as much to the GHP as it has been to a bad setup by the incompetent installer and many control system problems (we had 3x w/o heat due to the knucklehead not turning a valve open after he worked on the system). We're hopefully on the path to having that resolved with a new installer but still an open question.

Now about 12 months and one cold winter after installation, how has the GHP been doing? Pretty well I think. Winter was about playing with the thermostat temperature to find the right balance between comfort, away time setbacks, and energy use. When our GHP runs, it uses about 4.5-4.8kw of power (this includes associated pumps) or 19-20 amps @ 240 volts (the manual says it should run around 19 amps). For each degree of temp that it needs to increase room temp (for the whole house) it has to run for an hour. Thus, each warm up degree costs me about 4.5kwh. One downside of radiant is that it takes a long time to change temp in the house. I find that its about 1 degree per hour. That means if we're coming up in winter and the setback temp is 53 it takes about 15 hours to get it to 68 with the GHP running pretty much constantly. That's a lot of energy and I have a few days last winter with 75-100kwh of electricity use. This, in fact, was our greatest single use of energy in the house: the 10 or so Fridays we came up in winter getting the house to temp.

Maintaining temp is far easier and what radiant does well. I haven't nailed the exact figure but from my observations it takes about 1/4 the energy to maintain temp vs. heat up. Thus, 15 mins of GHP runtime to maintain 68 in winter per hour or 1.1-1.3kwh. There are many other factors affecting this like outdoor temp, how much sun there is, etc. so its not an easy figure to nail down. For how we should use the system, I did a back of the envelop calculation and determined that its actually less energy use to keep the house at 68 even when we're not there in winter assuming we'll be up again within two weeks. More than two weeks its better to allow it to setback to 53 and then re-raise. The irony is that we generally go up to the house every two weeks thus not clear on which is best.

Why 53 degrees for setback? Its generally held that you shouldn't let your house go below or too near 50 degrees as it will start affecting woodwork, glues, and other sealers. At first I kept the house at 58 during the fall so there was no chance. I didn't have the energy monitor at the time so I had little insight into usage when we weren't there except for our bi-monthly bill where it was too late to do anything and I still didn't know which component was using what. After I installed the monitor in Dec, I saw how the GHP ran and what appliance used what. I tested every degree in the 50s and ultimately settled on 53 in our main room and 55 in our bedrooms. I found that at this temp, the GHP barely ran during the transitional month of March. In fact, when set at 53 degrees, the GHP barely ran at all from late Feb into the spring. This meant that, with our solar PV, we were staring to be net positive on electricity in Feb/Mar which is crazy. We used more energy last fall that I had projected. I believe that much of this is due to having a higher setback and this fall I'm guessing that we won't even use heat @ 53 in Sept, Oct, and the first part of Nov. The rule I've found is that if the mean outdoor temp is above freezing, we don't need heat to maintain 53 as the house is so tight, even things like the exhaust hot air from the fridge maintain its temp (an air source heat pump itself) and warming from the sun is sufficient. We typically don't start seeing mean temps below 32 until mid-Nov.

This winter, I'm considering employing a different technique to raise the house temp when we go up. I plan on starting the GHP 4 hours or so before we arrive (I control my thermostats remotely via an Internet connected Z-wave system) to start getting the slab warmed (during the transitional fall and spring I might not put it on until we actually get there). That will only put the temp in the upper 50s. We do have a wood burning fireplace which was mostly installed for the fun of a fireplace but it has shown that it can be quite a furnace in its own right even though the FireOrb isn't the most efficient burner out there (but she sure is pretty). I find that one good burn of 3-4 logs actually saves at least one if not two cycles of the GHP. My plan is to light a fire when we get up there so that the fireplace helps quickly warm the place which its actually good at. From the trees we cleared we have about 10 years of firewood so no expense there either. I also have a couple of efficient Vornado space heaters we'll use for bedrooms that are away from the fireplace. This way the GHP just focuses on warming the slab and then maintaining. We'll see how that goes.

That brings me back to the original question: was the GHP worth it? I have to say that I'm glad we did it but I don't think I have enough info for a final conclusion. Part of this is driven by reading a lot about Passive Houses which I wish I had heard about before we built. Our house incorporates the key design element: south facing glazing to pick up sun heat in the winter. I've found that a bright sunny winter day can heat the house by itself about 10-15 degrees. It warms our concrete slab so it stays into the evening. In fact, all but the coldest (below 20 degrees) sunny days in winter use the sun as the primary heat source during the day. This saves a ton of energy: 6 hours * 15-30 mins GHP runtime per hour * 4.5kw = 7-13.5kwh per day.

Passive Houses tend to use minimal heat sources like air source heat pumps that require little energy. They are highly insulated tight houses like ours that benefit from the sun and other ambient heat sources to heat the house like body heat. Conceived of in the US but refined and implemented in Germany they are starting to make noise here. However, given that the northern part of NA is much colder in winter than Germany, there has been mixed results on these shores to date. However, in investigating them, I came across one article which basically said that radiant heating was overkill for a well insulated tight house. The idea being that the heat and temp stays constant for a long time even in the coldest winters and thus the slab cools down. Having to re-heat the slab is expensive and radiant works best when the slab stays somewhat warm.

This was eye opening to me as that's what we had experienced last winter (esp. when it wasn't below freezing out). The slab would cool and, as pointed out above, even in 70 degrees, if your feet are cool you feel cool. Yes, we put socks and slippers on but I still want to solve this problem. Ironically, I suspect that one way I can solve it is actually raising the thermostat temps. When I asked our moron HVAC installer what temp I'd have to keep the house at with radiant (I had no experience with it before) he said 64-65. I thought that was amazing given 68 is generally considered the min and our steam heat building in NYC is rarely below 72 and often at 75 in dead of winter. Well, 65 was fine with me but not with the Brit partner nor a small child. We tried 68 and settled on 69 with her wearing a cardigan most times. There were a few times I tried 70+ and it was quite comfortable and warm. This is one fun experiment with a super tight house, a 1 or 2 degree difference really can be felt. I think this winter I'm going to keep it at 70 or 71 which will certainly please some members of the household. I think that'll have the GHP keep the slab warmer and net out to less than 4kwh per degree. That's my hypothesis at least.

I'm hoping that the combination of warming up with the fireplace but asking the GHP to maintain temp will be more efficient energy-wise. After all, heating and DHW make up nearly 2/3rds of our energy use given that everything else is so efficient so it'll have a real impact. I'm also going to experiment with keeping it around 65 when we're not there to affirm my calculations. More as the weather turns.