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: 775

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.  Home Energy Pros is sponsored by the Better Buildings Residential Network.

Latest Activity

Richard C. MacCrea replied to Damien Greenfield's discussion Is there is cost saving for you in having a tankless water heater?
"Most of the posts here are far too narrow in scope. To make the best decision you need to…"
1 hour ago
Tom Mallard replied to Damien Greenfield's discussion Is there is cost saving for you in having a tankless water heater?
"Thx, Jose, add in u"
3 hours ago
tedkidd replied to Damien Greenfield's discussion Is there is cost saving for you in having a tankless water heater?
"Thankless water heaters are a GREAT deal.... For the installers. They are usually very…"
4 hours ago
Jose Carreno replied to Damien Greenfield's discussion Is there is cost saving for you in having a tankless water heater?
"I have been working with RUUD/RHEEM tank less water heaters (0.94 energy factor) with excellent…"
6 hours ago
Profile IconColleen Neel, Sarah Mostafa, Michael Kelemen and 4 more joined Home Energy Pros
7 hours ago
David I. Shepherd-Gaw replied to Ryan Miller's discussion Wouldn't it make more sense to post jobs on a single board nationally? in the group Job Board
"I have managed and overseen several efforts with www.energyexpert.org and other similar sites. It…"
8 hours ago
David Baerg replied to Bachi Brunato's discussion Modeling a Gas Fireplace in Wrightsoft
"I would not include it.  Given that it is likely much less efficient than the gas furnace, you…"
10 hours ago
Ryan Miller posted a discussion

First NC and now GA - Building Performance Associations taking shape

Hi All - Just a quick "this is what we're up to" for the community.The NC Building Performance…See More
10 hours ago

© 2015   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service