A couple of weeks ago, I had the distinct pleasure of defining a heat pump, explaining how it works, and illustrating the meaning of coefficient of performance (COP). Read that article here.
Today, let’s continue down the path of developing a stronger working knowledge of these concepts as well as their application.
A COP of 2.7, or an efficiency of 270%, may be typical in in the previously-referenced conditions (outdoor air temperature of 40-50 degrees F), but let’s consider outdoor air at 0 degrees F. (Yes, we all know there is heat present in zero degree air. If not, re-read my “You can’t make pizza in a freezer” one more time here!)
For this set of conditions, even though there is heat present in 0 degree air, there isn’t as much. So, your COP will be significantly lower. It may be close to 1.3, for example, but you’re still getting out more than you put in.
Q: If I can pump heat from 0 degree F outdoor air into my home and keep it heated to 70 degrees F, why oh why don’t we use heat pumps exclusively and simply stop using combustion appliances?
A: Even though the COP is attractive with heat pumps, the aggregate number of BTUs available for consumption is greater with traditional combustion systems. Older, leaky homes with poor insulation simply have higher energy demands. Newer homes, especially those built to a net zero or passive standard, thrive on heat pumps and don’t need any combustion appliances. For example, I’m building a house so tight and well-insulated that it will have zero duct work. None. I’ll be able to have higher ceilings with no space consumed by large ducts. And my utility bills will be minimal.
Q: What is a BTU?
A: BTU stands for British Thermal Unit. It is defined as the amount of heat (or energy) needed to raise the temperature of one pound of water by 1 degree F.
(Don’t shoot the messenger.)
The majority of the housing stock in this country is old. And leaky. And not superbly well insulated. There will be a market for combustion appliances that burn fossil fuels for quite some time. Unless, miraculously, all homeowners simultaneously decide to jump on my bandwagon. But even then, we have to consider existing commercial and industrial real estate and the fact that the retro-fitting of heat pumps in those instances would not generate a good return without substantive upgrades to the thermal envelopes.
Q: Aren’t heat pumps called different names? Doesn’t that irritate you?
A: Don’t get me started!
Not too long ago, “geothermal” was a sexy term. Instead of pumping heat from the air, you are pumping it from the ground. Mother earth stays at a constant temperature of around 55 F beyond a certain depth.
But forget ‘geothermal’ – let’s call it what it really is. It’s a ground-source heat pump.
I also get irritated with the term “mini-split.” (It’s really an air-source heat pump. Or, “air-to-air heat pump.”)
A heat pump water heater moves heat from the surrounding air in your mechanical room (that’s why you need to have a mechanical room of 1,000 cubic feet or larger which is 11×12 with an 8 foot ceiling) to the water it stores inside. It also has the ability to use electric resistance heating to make up for any shortfall in energy available versus energy called for to meet your hot water needs.
An air-to-water heat pump extracts heat from the air and transfers it to water, which can circulate in tubing under a slab and provide ample heat for the thermal comfort of your entire home or building. If there’s excess heat available compared to the demand, it can be stored in a buffer tank of water (water has much more thermal mass or thermal storage ability than air), or it can be used to heat your domestic hot water.
Now that you know what the correct terms are and what they really mean…
You can use heat pumps as part of your thermal comfort design for your home or building. Build it tight enough, incorporate enough insulation, and you’ll never need another combustion furnace. The occupants will be comfortable year round. Your bills will be low. Your carbon footprint will lighten.
That’s what I’m doing right now for a friend’s (soon to be) new office building. The building is new to him but was probably built 50 years ago. This building has FOUR rooftop furnaces and THREE regular furnaces. AND (since the building once housed a commercial kitchen) there is heating ability within the make-up air unit…for a total of EIGHT gas-powered heating devices in an 8,000 square foot building! (By comparison, my 15,000 sq ft building is happy with only five furnaces.)
In my friend’s example, we’re only going to occupy a little over half of the space, but out of those EIGHT existing combustion appliances, how many do you think I am aiming to use?
The answer is two. The facility is still in the design phase, but I feel that two existing furnaces and one or two strategically placed heat pumps will provide for excellent thermal comfort in the robust thermal envelope we are planning. (“Robust.” I love that word.)
And it will keep all his cold-prone female staff (no offense, ladies) comfortable all year and in any season.
Maybe we should call it the YETI office! (But the term is probably trademarked).
This brings me back to the title I chose for this article series, “Robbing Peter without Paying Paul.” I feel that an efficiency of over 100% (corresponding to a COP of > 1.0 which really is on the low side of reality) supports the claim in the title. I hope that you can appreciate the design alternatives, energy efficiency, and distancing from fossil fuels that are not only possible, but probable when you employ heat pumps in your energy modeling.
If you have any questions about building design, thermal envelope, air sealing, insulation, indoor air quality, energy efficiency, or value engineering then please reach out to me at Lee@DrLeeNewton.com I am happy to have successfully helped several other doctors and health care professionals design and build their facilities and have been more than pleased to pass on savings gleaned from the fact that I don’t allow abdicating these ideas, studies, and considerations to the architects, engineers, and general contractors.
I’d also love to incorporate your ideas and questions into a future article.
I know you’re waiting with anxious anticipation for next week’s article: a deep dive into the bold claim that “Savers are Losers!”
Until next time,
Dr. Lee Newton
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References:
- https://basc.pnnl.gov/images/iecc-climate-zone-map
- https://atlas.eia.gov/datasets/eia::climate-zones-doe-building-america-program/about
- https://truenorthenergyservices.com/heat-pumps-101-faq/#:~:text=To%20provide%20heat%2C%20a%20heat,air%20over%20the%20hot%20coolant%2C
- https://home.howstuffworks.com/home-improvement/heating-and-cooling/heat-pump.htm
