My previous post was about our Mini-Split A/C unit, purchased to get us through the few weeks of >100F weather we had this summer in the Twin Cities. But the post also alluded to the ability of these units to heat. Now that it’s cooling off, let’s take a look at that.
What these units do is move heat. Amazingly, they can move it in either direction! In the summer, they move heat out of the house, obviously enough, just like any A/C. But in the colder months, they can move heat into the house as well. Even when it’s cold outisde, you ask? Yep! How is that possible?
There is heat outside even when it’s “cold.”
Even when it’s freezing outside, there is still heat. Really! Until we get to absolute zero, there is still heat present which can be moved around. Don’t believe me? Consider your freezer. Let’s say you keep it at 0F. How does it stay at 0F? Well, if it ever gets to 1F or 2F, it extracts heat from the inside of the freezer, and moves it to the outside (i.e. your kitchen). So yes – it’s moving heat from a very very “cold” place and warming up a warmer place as a result. The mini split heat pump works this same way.
My Fujitsu ASU12RLS2 / AOU12RLS2 indoor/outdoor units can actually extract heat from the outdoors even when it’s -5F, although the efficiency diminishes as the outdoor temperature drops. Air source heat pump efficiency is expressed by the HSPF (Heating Seasonal Performance Factor), which, via Wikipedia, “is a ratio of BTU heat output over the heating season to watt-hours of electricity used.” HSPF is expressed in BTU/Watt-hour, and my unit has a rating of “12” meaning that for every Watt-hour of electrical energy it uses, it moves 12 BTUs into the house, on average, over the heating season.
Over 100% efficiency
BTUs? Watt-hours? Ok that might not mean much. But get this: Watt-hours (Wh) and BTUs are both expressions of energy, just expressed in different units. There are 3.413 BTUs in a watt-hour of electricity. So for every 3.413 BTUs of electrical energy we input, we get 12 BTUs of heat into the house, for a multiplier of about 3.5x. How’s that for efficiency! By comparison, a simple resistance space heater is 1:1, providing 1 unit of heat for every 1 unit of electrical energy input. The difference is that a heat pump moves heat rather than creating it directly, and is therefore able to do so with more than 100% efficiency.
So it’s 350% efficient, that’s great and all, but natural gas is cheap, and electricity usually comes from coal – does it make sense from either an environmental or a cost point of view? I created a spreadsheet on Google Docs to take a look. An abridged version is here:
|Natural Gas||Mini-Split Heat Pump|
|47,560,000||BTUs Utilized||47,560,000||BTUs Utilized|
|96%||Cost vs Nat. Gas|
|6,786||lbs CO2||4,601||lbs CO2|
|68%||CO2 vs. Nat. Gas|
Looks like a win
With the following assumpti0ns (take a look at the spreadheet to alter them): 82 AFUE boiler, $0.71/therm natural gas, $0.10/kWh electricity, a 12 HSPF heat pump, and 1161 lbs CO2 per MWh, it looks about break-even on operating costs, but about a 30% reduction in CO2. Of course things like energy costs and carbon intensity vary by region; for carbon intensity numbers for your grid region you can look here (I actually took numbers from my utility’s annual report).
So that looks pretty good, and in fact if we may have even a bit better outcome, because:
- We have solar on the roof and buy wind energy, so our CO2 intensity should be lower
- We’d probably use the mini-split in the shoulder seasons, when it is more efficient and the boiler is less efficient
The one downside, right now, is that we have just one of these things, at the top of the stairs. It’s a point source of conditioning so distribution is something of an issue. But it’s really impressive how much heat it puts out on a chilly night, with not much of a spike in the daily electrical energy graph.
Edit Nov 15 2012: There’s a very cool calculator at buildinggreen.com which lets you easily compare any two heating fuels. Give it a shot!