How the Heck Does a Heat Pump Get Heat from Cold?!

It's been cold out this winter, so I've been talking a lot about heat. Invariably when I talk to people about how heat pumps can keep your home warm in winter, they get a look of bewilderment on their faces and ask how that's possible. No one ever tells me that they're confused about how a refrigerator or air conditioner works, even though it's the same, exact process - moving heat from a cooler area to a warmer area.

My guess is that we have blinders here because of our personal experience with cold outdoor temperatures. When we go outside in winter, our bodies have a much higher temperature, being at about 98.6°F, and we experience cooling. We personally always are cooled off by cold outdoor air, so it's hard to imagine that that same air could ever heat anything else up.

It really does happen, though, so to understand heat pump operation, let's start with the basics. I discussed heat flow, thermal energy, and temperature previously, and in that article I said heat flows when you have a temperature difference (ΔT). So if you're trying to get heat out of 40°F air, what do you have to do? Put it in contact with something that's at a temperature lower than 40°F! That's the job of the refrigerant in a heat pump.

If you want a good, short description of the refrigeration cycle, see this article called Air Conditioner Basics by Martin Holladay, the Energy Nerd at Green Building Advisor. I'm going to focus on just one part of it here - the expansion valve.

There are four basic processes in the refrigeration cycle. All are important, but in my opinion, the expansion valve is where the magic happens. Whether you're using it for a refrigerator, air conditioner, or heat pump, achieving a low temperature is the key, and that's what the expansion valve does for you.

Here's an example for you that you may have experience with, especially if you're a serious bicyclist (as is my fellow Energy Vanguarder, Chris Laumer-Giddens). CO2 cartridges contain carbon dioxide under high pressure. (Does this count as carbon sequestration?) When you use them to inflate a bicycle tube, for example, the cartridge gets very cold. Try it! It also works with aerosol cans like hairspray. This is a thermodynamic property of gases. When they're allowed to expand freely, their temperature drops.

Same thing happens in a fridge, AC, or heat pump. The refrigerant is pushed through the expansion valve, and the temperature of the refrigerant drops -- a lot! So, that cold outdoor air is actually the warmer object then, when it comes in contact with the outdoor coil of your heat pump. And, as we know, heat likes to move from warmer objects to cooler objects. Once we get that heat from the air into the refrigerant, it's just a matter of bringing it into the house and then transferring it into your home's air.

So now the mystery is solved! It's our old friend, the Second Law of Thermodynamics again (i.e., heat flowing from warmer to cooler).

 

Dog photo by Tobyotter from flickr.com, used under Creative Commons license.

Views: 363

Comment

You need to be a member of Home Energy Pros to add comments!

Join Home Energy Pros

Home Energy Pros

Home Energy Pros was founded by the developers of Home Energy Saver Pro (sponsored by the U.S. Department of Energy,) and brought to you in partnership with Home Energy magazine.

Latest Activity

Jim Gunshinan commented on Jim Gunshinan's blog post My Energy Upgrade California—The Numbers Are In
"To all, one thing I don't lack is advice from the experts! Now if only you all could agree on…"
13 minutes ago
tedkidd commented on Jim Gunshinan's blog post My Energy Upgrade California—The Numbers Are In
"Jim, I'm glad you are open, that's great!  We all learn best when we are open! The…"
15 minutes ago
David Eakin commented on Jim Gunshinan's blog post My Energy Upgrade California—The Numbers Are In
"Jim, Well, as the editor of Home Energy you should already know that the order of remediation is…"
16 minutes ago
Greg Labbe posted a blog post

Technical Tape Desecration

Lets face it – building science is pushing the performance of adhesive tapes to a new levels and…See More
2 hours ago
Stacy Hunt posted an event

High Performance Enclosure Strategies: Part II, New Construction at Online

August 13, 2014 from 1pm to 2:30pm
Please join the Building America Program for our free webinar: High Performance Enclosure…See More
3 hours ago
Richard Beyer replied to Howard Katzman's discussion UV lights on filters
"Howard, There are numerous manufacturer's who swear these systems work and then you have the…"
12 hours ago
Christopher Morin posted a blog post

How do You Test a TXV?

  Thermostatic Expansion Valves (TEV or TXV), one of the most popular metering devices for…See More
16 hours ago
Howard Katzman posted a discussion

UV lights on filters

I recently saw UV bulb installations in 2 HVAC systems in a home. Each system had the Lennox…See More
18 hours ago
Don Fitchett joined Michael Stuart's group
Thumbnail

INFRARED THERMOGRAPHY USERS

This group is dedicated to knowledge sharing and discussion of infrared thermography for building…See More
18 hours ago
Don Fitchett commented on Diane Chojnowski's group Pinterest
"While most of our (BIN95.com) energy post and boards are industrial related, there are crossovers…"
18 hours ago
Don Fitchett joined Diane Chojnowski's group
Thumbnail

Pinterest

Do you have a Pinterest board for energy topics?Post the link to your Pinterest energy board in the…See More
19 hours ago
Profile IconJosh Guenther, Deborah Green, Kenneth Brooks and 3 more joined Home Energy Pros
19 hours ago

© 2014   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service