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: 315

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 Energy Upgrade California—Up Close and Personal
"I had a revelation while attending Bruce Manclark's session of duct leak testing at the Energy…"
10 hours ago
George J. Nesbitt commented on Jim Gunshinan's blog post Energy Upgrade California—Up Close and Personal
"Blower Door; the 2007 test was a depressurization test, and the 2014 a pressurization test, which…"
10 hours ago
George J. Nesbitt replied to Kaushal Bharath Raju's discussion Affordability & Deep Energy Upgrade/Passive House Retrofit in Berkeley, California.
"Plan, plan, plan, plan. The 1st step to is to understand the house, how it's built, the…"
10 hours ago
George J. Nesbitt posted an event

High Performance Windows - A Panel of Experts at Pyramid Alehouse`

April 26, 2014 from 3pm to 5pm
Join a lively panel discussion on high performance windows. We'll cover some basics, as well as…See More
12 hours ago
Kaushal Bharath Raju replied to Kaushal Bharath Raju's discussion Affordability & Deep Energy Upgrade/Passive House Retrofit in Berkeley, California.
"Hi David, Thanks for pointing out Martin Holiday's article. I do not wish to engage in the…"
12 hours ago
Profile IconAdam Penberthy, Scot Davidson and j jarvella joined Home Energy Pros
12 hours ago
David Eakin replied to Kaushal Bharath Raju's discussion Affordability & Deep Energy Upgrade/Passive House Retrofit in Berkeley, California.
"More food for thought on this subject - read this (fairly) recent blog by one of the most respected…"
17 hours ago
Curt Kinder commented on Christopher Morin's blog post 5 Things New Energy Efficiency HVAC Contractors Need to Know
"You left out air distribution...Without at least a passing glance at how much air moves through the…"
23 hours ago
David Eakin replied to Kaushal Bharath Raju's discussion Affordability & Deep Energy Upgrade/Passive House Retrofit in Berkeley, California.
"I mis-spoke - I was referring to the Midori House in Santa Cruz. Have an in-depth discussion with…"
yesterday
Kaushal Bharath Raju replied to Kaushal Bharath Raju's discussion Affordability & Deep Energy Upgrade/Passive House Retrofit in Berkeley, California.
"Hi David & Bud, $400/sqf > $300 sqf. The latter is the minimum cost of new construction in…"
yesterday
David Eakin replied to Kaushal Bharath Raju's discussion Affordability & Deep Energy Upgrade/Passive House Retrofit in Berkeley, California.
"Kaushal, There was a deep energy retrofit upgrade project (to Passive House standards) in San Jose…"
yesterday
Kim Tanner updated an event

Beyond Residential Testing at The Energy Conservatory

May 14, 2014 to May 16, 2014
The Energy Conservatory (TEC) is hosting a Beyond Residential Testing event. In addition we are…See More
yesterday

© 2014   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service