Heat Pump, Schmeat Pump - Fine Homebuilding
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Taking Issue

Taking Issue

Heat Pump, Schmeat Pump

comments (13) July 15th, 2010 in Blogs


If homebuilders had a magic appliance that could provide an endless supply of inexpensive and environmentally friendly energy, then we would have an excuse to keep building homes without paying attention to how much energy they will use. People who installed the technology could use it as their excuse to consume in excess, and people who didn’t could point to the cost of the appliance as their excuse. (You didn’t think it would be cheap, did you?)

Ideally, the appliance would be produced in a factory and be packaged in a reasonably sized box that could be trucked to the construction site and connected to the building with just a few wires and pipes. It would not be noisy, smelly, or dirty, nor would it use any fossil fuel. If it took some heat from the earth, it would seem like it was using an inexhaustible resource. If most people didn’t understand what it was or how it worked, it would be hard to argue that it wasn’t a good idea. The modern ground-source heat pump fills all these requirements except one: It doesn’t necessarily save energy.

Let the electric company do the dirty work

Before I explain why ground-source heat pumps are not the magic appliance that some people would like to make them out to be, let me explain how they work.

Imagine digging a well in your yard to fill buckets of water. If you dig the well deep enough, the water will probably be at around 55ºF. Next, you put the bucket into your refrigerator to cool it down to 50ºF. The heat removed from the water will be released through the coils on the back of the refrigerator and into the air in your home. With enough buckets, refrigerators, and electricity, you can cool all the water needed to heat your home. You’ll eventually get tired of carrying buckets, and then you’ll install a pump to do the work. You just invented a geothermal heat pump.

One of the main selling points of ground-source heat pumps is that they supposedly use no fossil fuel, which is only partially true. It’s true that the heat pump doesn’t burn any fossil fuel in your home, but it consumes electricity. In other words, the electric company is burning the fossil fuel for you. So it is also partially true to say that heat-pump users heat their houses with coal, because about 50% of the electricity in the United States is made by burning coal. Much of the rest of the electricity in the United States is made by burning oil or gas. For those of us who think that burning coal and fossil fuels is an environmental concern, it’s important to take an honest look at which appliance is responsible for more fuel consumption: a gas furnace or a heat pump.

Heat pumps depend on electrical-grid efficiency

The electrical production and distribution system in the United States is about 32% efficient according to the Department of Energy. In other words, about one-third of the available energy in the fuel burned at a power plant makes it to the customer’s electric meter. For a heat pump to be fossil-fuel neutral compared to a (90% or more) high-efficiency furnace, it would have to extract about 2w of heat from the ground for every watt of electricity it uses, sending a total of 3w of heat to the building. In the language of heat-pump ratings, this 3-to-1 ratio is called a coefficient of performance (COP) of 3. COP is the standard measure of a heat pump’s efficiency. (For more on heat pumps, see “Is a Heat Pump Right for You.”)

Manufacturers’ COP ratings for ground-source heat pumps vary with the temperature of the groundwater and the temperature of the air in the building, but are usually between 3 and 4 for groundwater temperatures in the 50s. But those ratings do not include the electricity used by the water pump, and do not reflect real-world groundwater temperatures, which get colder after a heat pump has been extracting heat for a while.

Where’s the real-world data?

Getting an accurate view of the whole picture requires turning to actual measured results. Amazingly, despite the installation of thousands of ground-source heat pumps in the United States, nobody has apparently ever gone around and measured the COP of randomly selected installed systems and published the data.

The closest thing available to a real-world study of installed heat-pump efficiency is a small study done by a heat-pump trade association in cooperation with an electric utility company—surely not neutral parties.

For the study, they installed new systems that included such energy-efficient features as high-efficiency fan motors and PVC ducts, and still managed to squeeze out a COP of only just over 3 when the systems were new. (The trade association and utility will not share the actual measured data for this study.) As the groundwater temperature drops, this COP can be expected to drop as well.

I have done some measuring, and people I know have done some measuring. Based on these measurements, I think a whole-system COP of about 2 is a realistic estimate of the performance of a typical, real-world, installed system. Unfortunately this means that the geothermal heat pump actually will burn significantly more fossil fuel than a high-efficiency gas or oil boiler or furnace.

Almost nobody knows this, though, because almost nobody is measuring. And no measuring means nobody is learning from mistakes that are wasting energy. More important, it means that many people are blindly installing what they are led to believe is a magic heating and cooling appliance. What we should be doing is rewarding manufacturers and installers who are trying to improve on these shortcomings. Instead, we just keep renaming the heat pump to make it sound less technical, and avoid the implication that the systems actually use any energy at all. Heat pump has become geothermal, geoexchange, and sometimes even geocomfort.

A more honest approach would be to admit that heat pumps use electricity, which in most cases means burning more fuel than would be consumed by an appliance that burns gas right in the home. Admitting this could lead to finding out what, if anything, can be done to reduce the amount of electricity heat pumps use. Meanwhile, with heat pumps no longer seen as magic, attention can shift to making buildings energy efficient. Anyone interested in reducing the amount of energy that buildings use can focus on using good insulation and air-tightening, well-designed ventilation systems, shading windows from summer sun, sealed combustion appliances, and thermostats in every room.

From Fine Homebuilding 211, pp. 10-14
Henry Gifford designs mechanical systems for energy efficient buildings including a 24-family apartment expected to be the first new-construction apartment building in the US to conform to the strict European PassivHaus energy standard. His website is www.energysavingscience.com.

Henry will be happy to answer questions and comments to this article.

posted in: Blogs, energy efficiency, green building, hvac

Comments (13)

VancouverHolly VancouverHolly writes: The author alludes to the fact that energy is a very complicated topic. But he, unfortunately, stops too soon and makes simplistic conclusions. He states that North America energy conversion efficiency is 32%. This is true of coal fired Rankine cycle plants. But the 1GW hydroelectric generator about 40 miles from me delivers about 92% efficent power to my meter. The author also fails to mention the true cost of natural gas. Maybe the Albertan gas that is pumped 1,500 km to my house is OK, but is its ecological footprint the same as the fracked gas coming from Pennsyvania and elsewhere?
Posted: 5:27 pm on July 26th

siobhanws siobhanws writes: Oh dear! The point is well made elsewhere ~ this is a very (italics) silly article.

For instance, the author complains that, after a while of extracting heat in a ground source heat pump installation, the ground source cools! This would have to involve (because, as he states, he and his confederates have done the measurements) that he is actually using a thermal collection coil that is some thousands of miles deep moreover covering the whole of the planet... The planet continuously generates heat!

Other respondents appear totally ignorant of the Laws of Thermodynamics and how subsequent losses in high tension power transmission effectively limit the maximum efficiency that can be obtained from electricity generation.
Posted: 4:46 am on November 21st

jcur88 jcur88 writes: I don't have gas available where I live, what's the cost and impact of adding tanks, deliveries etc. What I can say is putting in a geo system after my oil tank began to leak was the best single improvement I have made to my home. I had read the pro's and con's of environmental impact, but what I didn't know was that the system would make the house so much more comfortable, summer and winter. No more dramatic temperature shifts that I had with the oil forced air, and it assist the water heater as well. And yes I am saving a lot of money, guilt free.
This discussion proves only that there is no magic wand to solve the energy problems.
Posted: 6:39 am on March 17th

sunnyh sunnyh writes: This is a silly article. I wish I had seen it earlier so I could have responded before now. Everybody knows it takes some energy to run heap pumps, the fact of the matter is that geothermal heat pumps are far more efficient than any other type of heat source - gas, electric or oil. I put one in last year and saved more than $4000 in winter heating costs between January and April alone. While my savings were terrific what really made me feel good was that I wasn't contributing any longer to our county's reliance on foreign oil. No soldiers died to keep my house warm.

I added solar panels in the spring and the cost to air condition my house last summer was negligible. I also have a desuperheater which sends excess geothermal heat into a water storage tank which then sends preheated water to my domestic hot water heater, saving me even more money. The desuperheater works both summer and winter, though not so much during the spring and fall seasons. I can actually feel the temperature difference in my hot water when it's running.

The point of a well designed geothermal system is efficiency and comfort, and believe me, it delivers.
Posted: 10:08 pm on March 5th

ModernYankee ModernYankee writes: I would love to remove emotional arguments and cost to the consumer out of the equation and simply have a look at the true overall efficiency of the energy usage. Actual cradle to grave energy usage.
By that, I mean...let's look at the initial energy used in geo vs gas-burning appliances (to produce the equipment, to transport & install the equipment, etc...). then look at the ongoing energy used in operation and the efficiency of that energy usage for both types of systems...but not just the systems themselves, we have to look at the energy used to 'make' & transport the fossil fuel just as we have to look at the energy lost to make & 'transport' electricity. And as one person noted...let's also look at the cost of COOLING the house. And finally, let's look at the life expectancy of the different systems (from an energy usage standpoint...not a cost standpoint).
When all of that is done, THEN we can have a discussion about costs and why costs are what they are (subsidies, incentives, infrastructure, etc...). But lets have the scientific discussion first so we can all agree on what the best thing is from an energy usage standpoint. Only then can we can attempt to tackle the political/financial issues. This is not the place go into that kind of detail, but it would make a great article, don't you agree?
Posted: 9:35 am on February 27th

nowlan nowlan writes: The author is quick to point out that the water pump is not included in the COP rating. He didn't bother to mention how many circulating pumps are utilized in the average oil/gas fired boiler zone distribution system for the home. My GSHP with forced air of two years now, has much less water pumping capacity than the water base board heat system I removed. Anual heat cost went from $3500 to $1000, and my home is far more comfortable (all year long).

If installing solar or wind generated electricity on my propperty is my next step, would that have helped my old oil burner use less oil?
Posted: 7:59 am on February 9th

wavehp wavehp writes: Don't know well on GSHP, but for air water heat pumps, they do save energy a lot. I have done test with this brand: "wave heat pump". The COP in 20C ambient temp can reach 4.58. http://www.waveheatpump.com
Posted: 2:37 am on November 30th

Posted: 12:15 am on November 16th

droodman droodman writes: Thanks for this very interesting post. Why do geothermal heat pumps have COP's that are so much lower than those of air-based heat pumps, whose SEERs can run to 15 or higher?
Posted: 4:47 pm on November 3rd

8nailsplease 8nailsplease writes: When I bought an air based heat pump / air conditioner with gas furnace forced air system I never gave a thought to the cost of replacement of heat pumps. No matter the assumed savings of this kind of system and no matter the warranty of the pump itself the cost of the site visit and work on the unit negated any assumed savings of this kind of system. And when the warranty lapses the assumed savings turns instead into a real costly event for the owner. Forget heat pump technology and think about life time warranties of the pump and the associated cost to replace it. If for no other reason the heat pump is a bum deal over all....unless there is a life time warranty covering all future cost of pump replacement. But that is a pipe dream. The dealers count on this replacement business as their bread and butter. No different than burying a man in a rented suit. But at least I understand that issue now.
Posted: 11:43 am on July 19th

HenryGifford HenryGifford writes: Yes, heating with a heat pump can cost less money than heating with fossil fuel, while also using more enrgy. Yes, using more energy can save money, the explanation being political. Nothing I wrote mentioned money, because it is impossible to talk about energy cost without talking about politics, and about people forced to pay for things for others, which most geothermal fans don't want to discuss, because most geothermal systems are largely paid for by other people.

I don't advocate forcing anyone to do anything or pay for anything for anyone else. I do advocate looking at how much energy different heating and cooling options use.

As for the environmental impact of natural gas drilling, it is higher with a heat pump, as almost all new electric generating capacity added in the US recently burns gas. The impact can be lowered by burning gas directly in a house, because less will be burned.

Gas burning appliances, especially the sealed combustion type I advocated, stay almost as efficient when old as they were when they were new. I base this on having measured the efficiency of thousands of gas and oil burning appliances over the years. The decline in efficiency is slight, and can be corrected, yet with geothermal it is significant, with no solution avaialable.

As for summer cooling and hot water production, yes, water cools refrigerant to a lower temperature than air, but at the offsetting energy use associated with running a water pump. If data was available it would be possible to know if energy is saved or not, but despite many reports of data, none has been published.

Saving money is great, but if the cost is merely shifted to other people, none is saved overall, just like shifting energy use to the power plant saves no energy overall. Enough people advocating an approach that has someone else paying money to save them money, and eventually there is no more someone else to pay the extra cost.
Posted: 2:15 am on July 17th

khakinurse khakinurse writes: Who said a ground source heat pump didn't use electricity? I've never heard that and would have never imagined it to be true. The efficiency of them is the fact that the units condenser/evaporator (heat pump cycle reverses) is located in a relatively constant temperature medium. As opposed to located in hot summer and cold winter air. This makes for access to a persistent absorptive medium for heating the evaporator in the winter and for cooling the condenser during the summer. Much more efficient than the changing temp of air. And, water is a better more efficient transfer medium that air. Two wins for ground source heat pumps.
They are more expensive to install but that should be recouped via saved electricity = saved $$.
Posted: 12:29 am on July 16th

cussnu2 cussnu2 writes: But the point the author is making is that the consumer should sacrifice their own hard earn dollars to pay for 90% efficiency when they can get greater economic value out of a geothermal heat pump. In short, in terms of dollars out of pocket, a geothermal is cheaper to use for the consumer than a 90% natural gas unit

The author assumes everyone is (or wants to force everyone to be) a green weenie. Why is it the consumers fault (or even their concern) that the electric distribution system is inefficient? Additionally, the author never takes into account ANY amount of inefficiency or environmental impact in the exploration, drilling, production and distribution of Natural gas. To read this article, one would have to assume that 90% equals 90% and the natural gas system in the US is 100% efficient. Furthermore, while he goes into explanation of how the geothermal system will become less efficient over time, he never concedes that a gas appliance will do the same. In the end, he compares an adjusted geothermal system to a perfect world gas furnace.

and here I thought the only ones that were perfect were Mary, Jesus and my wife.

Additionally, the author never once accounts for the use of geothermal in the summer to produce air conditioning which his 90% efficient furnace can't do AND the geothermal can be set up to provide you hot water during the summer using the heat extracted from the house to heat your water thus boosting efficiency greatly especially over an electric water heater. People who own geothermal systems actually can pay lower utility bills in the summer by RUNNING their air conditioner.
Posted: 5:31 pm on July 15th

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