Heating with a Mini-Split? Looking at costs & emissions.

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

0.82 AFUE 12 HSPF (BTU/Wh)
580 Therms Purchased

475.6 Therms Utilized

47,560,000 BTUs Utilized 47,560,000 BTUs Utilized

3,963 KWh Input

$412 Cost $396 Cost

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!

30 thoughts on “Heating with a Mini-Split? Looking at costs & emissions.

  1. Hi Eric!
    Great work on the mini-heatpump. As I think you mentioned in one of your posts, the only way to be “more efficient” would be a ground source heat pump. Even so, it is hard to argue with 350 percent efficiency.

    Our heat pump here is a Trane 16 SEER single unit. I could have gotten up close to 20 with a split unit, but the house was already setup for a single (package) unit. About 3 years ago we moved from a propane gas pack to the heat pump. The A/C part of the old unit was about 10 SEER so we saw a big reduction in electricity during the summer. I kept working putting in a double radiant barrier and bring the insulation up.

    Here in NC the A/C gets run a lot during the summer due to the humidity. We rarely use the “heat” part of the new heat pump. Back in 2006 I installed a biomass furnace, heating with whatever the farms were going to throw away: aflatoxin corn, barley, wood pellets, etc.

    The 100K btu ducted furnace became too much as the house was insulated and sealed better. The last two winters we have heated 3,000 sq ft with a Quadrafire MT Vernon AE insert. I only run wood pellets now that I make from saw dust graciously delevered by wood workers, or purchased from hardwood floor companies. I guess according to the DOE we should be carbon – neutral with the biomass. Obviously we have some energy input to either compact the saw dust into pellets or to go pick up the pellets. I would guess that it is maybe a little better than using a heat pump, unless the heat pump ran completely from solar.

    Ken Clifton

    • Thanks Ken –

      Yup, whether due to an old-school boiler guy, home improvements, or what, we probably have an oversized boiler here too. 105k BTU, delivers 88k BTU (only 83AFUE, ugh) and the heat-loss analysis we did this summer came at something closer to 30kBTU required (a little hard to believe in this old house). But given that low heat load, I think a couple mini-split heads might just cover us for all but the coldest weather.

      Aflatoxin corn? That sounds scary, I’ll have to look it up.

  2. It would be if in large amounts. However we are talking about parts per million. The number is really minuscule — and it is being burned — not eaten. If you eat corn or peanuts you are getting some regardless.

    By the way, the 1.6 PV addition had one issue last week. It’s Envoy failed, the network interface died. The link light stayed on solid even when not connected. Enphase did a good job with the warranty. They air expressed me a replacement. The new one was rev3. It must have a faster CPU, the local screens are super fast now, even running the latest FW.

    Ken Clifton

  3. eric, ran across your blog while looking for ans to why my mr slim 4 zone hp seemed to consume as many kwh’s as resistance heat when temp drops below 32. Put in 05/12 and loved the cooling side. i too was attracted to big “efficiency numbers”.
    my research took me to a web site (eia.gov) and their “heating fuel comparison calculator”. the hspf ratings for all these systems are baselined for the far south, ie. gulf coast area. for example my listed 9.3 hspf must be “adjusted” for the actual location of use. in pittsburgh area climate it would factored to an actual 6.5 hspf. your 12 hspf would equate to 7.5 in pit and only produce a rated efficiency of 12 in new orleans area (acording to conversion chart). devil is always in detail and fine print.

    • Thanks – do you happen to have a link to that chart? I would not use mine in the depths of winter, but the shoulder monthts should have a fair bit of efficiency (and those shoulders seem to be getting broader all the time – 50s for highs this late-November week in the Twin Cities…)

      How are you measuring energy use of the unit? Mine has defrost cycles when it’s very cold out which do apparently sap efficiency.

      • eric, http://www.eia.gov/neic/experts/heatcalc.xls this is an INTERACTIVE spreadsheet that is great for comparing true energy costs of all fuels. also links to lots of real world (no hype) info for self education. If you scroll down, the “adjust hspf for location” chart comes up. follow the chart and enter your hspf rating in proper column. a 12. rated hspf hp would factor down to 5.7 in MSP but factor up to 12.1 in Biloxi MS.
        (if above address doesn’t work send me your dirct email for link)

        • From my reading of that, it applies to air source heat pumps which fall back to resistance heat for aux heat below a certain temp, something which, as far as I know, mini-split heat pumps don’t do. The heating capacity certainly drops with outdoor temp, but I’m not sure how that should translate to HSPF. There was a mini-split study I saw recently which might look at this, I’ll have to find it.

          In any case, I agree with you that using the mini-split for heat only in the shoulder months, and falling back to the boiler in the dead of winter. I’m just not quite sure how to quantify it.

  4. Eric,

    I’ve been asked to determine whether the claims for savings of installing an air sourced heat pump would hold true in a colder climate. Could you send me an email so we could discuss? I’d love to hear how things are going now that you have data from the winter months.

    • Sure thing, will do. But to be honest I’ve not been using it (much) this winter. Its location is not such that we can heat the whole house, so it’s been a mix of gas boiler + little boosts from the mini-split now and then when it’s not super cold out. So, I really don’t have any good data from this year. You might also check out Marc Rosenbaum’s blog Thriving on Low Carbon – he’s done a bit better real-world analysis (though in a different climate).

  5. I have a direct air unit. Installed in aug 2012. First winter heat bill came in jan 2013. It was about $400. We did use the pump a lot. But we also didn’t use any less oil. Used the thing less over the next two months and still up $200. Any help?

    • Not quite clear on your situation. A “direct air unit” is a mini-split? And your $400 / $200 heat bills, is that for electric? Gas? Oil? All of the above? And how does that compare to previous years, how did the weather differ, etc?

      From my quick calculations, for my utility costs and my appliances, the cost per BTU delivered was about the same for the mini-split vs. my natural gas furnace. But your situation may well be different…

      • Yep it is Min Split, electric heat from the pump. I have forced hot water also. Weather was the norm compared to the year before it was up for that bill about $440. I have run it in the summer and noticed very little difference in the power bill. burned the same amount of oil as the year before.

        I think when on heat it is going into auto mode. Why? Because the louver is always moving from one direction to another as it suggests it will on auto mode. On AC mode it will stay where I set it. On heat it will point down and then just switch when it wants. I can’t set it. Have you seen or heard of this happening. Direct Air Multi Zone. Two 18000 heads. Anything?

        • I have never used my mini-split extensively for heat, so I don’t have a real-world comparison.

          To really get to the bottom of it, you’d probably need to look at heating degree days for the season, and make sure it was really your mini-split and not something else that bumped up your electric bill (I don’t know how $440 compares to your “normal” winter bills…)

          Is this a Fujitsu mini split? Mine keeps the louvers pointed down until the heat has reached steady state so that it doesn’t blow a cold draft across the room…

        • I m not happy either! installed 16 mini split units in large home–was told to expect huge decrease in elec/utility bills has only increased over the past 2 and half years from appx 800 to over $1,000 a month contractor put 9k btu units in each 250 sq ft room now have mold issues that will require exhaust fan to run 24/7 and further increase elec cost

          • I’m not unhappy with mine, actually I’m quite happy.
            But you should be unhappy with your contractor, I think.
            The combined cooling capacity of all those heads is 12 tons for what sounds like a 4,000 square foot home; I’m not sure what the outdoor units total to, but in any case that is well beyond any rule of thumb of tons per 1,000 square feet. And that rule of thumb is usually too much AC already! This all depends on the details of your house, but it sounds grossly over-sized to me.
            What is probably happening is that with all that cooling power, you’re reaching your cooling setpoint well before the units have any chance to dehumidify the air.
            I can’t help you with the money wasted on (IMHO) too many heads, but you may have better luck running no more than 4 or 5 of them at a time, and give them a chance to actually remove some moisture.

  6. Not quite clear on your situation. A “direct air unit” is a mini-split? And your $400 / $200 heat bills, is that for electric? Gas? Oil? All of the above? And how does that compare to previous years, how did the weather differ, etc?

    Mini Split: Electric and that is $400 more than our budget of $170. We also have forced hot water (Oil). I figured it to be about the same. Weather was cold but it is Nova Scotia and always cold winters

    • You can use the fuel cost calculator at http://www.buildinggreen.com/calc/fuel_cost.cfm to plug in your actual numbers for cost of electricity, oil, efficiencies, etc and see if it looks out of whack.

      Without knowing all that (and whether some other electrical appliance could be soaking up some of that bill) I’m afraid I can’t really speak to your situation at all.

      And to be honest, until I get separate monitoring for my mini-split, I can’t say for sure what mine is costing either. I’m not using it in the deep winter months for heating, mostly just on the shoulder months when the boiler seems like overkill. My calculations in this little article are just theoretical, based on the published specs for the unit.

  7. I bought 3 heads, mitsubishi 2 ton condensor mini split. Live in ductless house (of course!) and need to find an installer. A side guy type deal. I bought the unit from a HVAC shop my cousin owns in Iowa. If you have any advice about installers please contact me. One other conceptual question is that I am thinking about hanging the bracket on an “interior” facing wall rather than an exterior facing wall. Then running about 6 feet of the condensate tube and wiring thru a closet to, then out, the exterior facing wall. Reason for doing this is so that I can hook up the housings interior (in a closet) then drill thru my exterior wall and run about twenty feet down to condensor box. Otherwise I have to get a cherry picker or scaffolding if I go directly with the exterior facing wall. Hope you get the drift of what I’m saying. I can describe more if you want later. Cool website! Live near Macalester.


    • Dunno about an installer – I had a place called Team Mechanical do mine, but I have no idea how they’d feel about 3rd party equipment.

      I don’t think the line run makes too much difference; length and/or elevation change affects capacity and/or efficiency but only a little bit. I have a pretty crazy line run – through the foundation, across a wall in the basement, through a suspended ceiling, back out the other side, up the outside wall, and finally back in on the 2nd floor. All to get the head on the wall we wanted it,and the outdoor unit on the unused side of the house. :)

  8. Question re Trane 12 blowing cool air most of time during winter. I have a 2 bedroom with loft and the Trane 12 is 90% of time blowing cool air during winter. Had one furnace person over and he said it was normal. This can’t be normal. There are no returns in the 2 bedroom (at opposite ends of home). The outside 2 units were installed 10 inches from house wall and fence with less than 8 inches between them. Frustrated with Dealer who installed this and rationalized why they did not put returns in. Tired of cool air blowing and being cold in home. PLEASE HELP

  9. Hi! Really informative blog you have here, it helped me understand how my unit works :). However, I have an issue. I try to find the lowest temperature when my mini split becomes inefficient for heating compared to electrical heating but there seems to be no answer on the web. I live in Canada and we have some pretty cold winters going as low as -20/-30 Celsius (-4/-22 Fahrenheit). My unit has 11 HSPF and 21.5 SEER and it heats 2 floors of my house.

    Thanks in Advance!

    • I’m sure it depends on the unit, but I don’t think I’ve seen temperature-to-HSPF tables; still, electric resistance is 1:1, and I imagine even an inefficient heat pump will be better than that.

  10. Love the numerical analysis (on this post and others on your blog). I am in Wash DC, evaluating Mini split systems. Based on my situation, they suit me well (multiple unused rooms, not home during daytime etc).

    This winter I switched gas heat (very old furnace + radiator) to electric space heaters. That dropped my heating bill from $1,700 (avg of last few winters) to $450 (expected usage 4,000 kwh this winter).

    As I evaluate mini splits, I am trying to gather data about whether they will draw less or more kWh than space heaters. I will greatly appreciate if you can share your winter time kwh usage since upgrading to multiple indoor units.

    • Sorry for the late reply –
      I really don’t have a good comparison of electric before & after, because my other heat source was/is natural gas, not electricity.
      We did heat the basement with an electric radiator for a month years ago, and heated our house with several of them when the boiler was off and work was being done a couple years ago, and those are the two highest electric bills we’ve ever had.
      I’m actually a little surprised that swapping gas heat for space heaters saved you that much – that must have been a terrible boiler!
      In general, though, replacing electric resistance heat with a heat pump should be an overwhelming improvement, on the order of at least a 2x improvement, I’d wager. Simple resistance heating is 1:1 for watts in vs. watts out; with an HSPF of 12, a heat pump should be closer to a 3.5 to 1 ratio.

  11. You also have to consider pretty big losses of having the furnace in the attic. The ducts are usually not sealed to the connections very well and leak air. Then the insulation jackets are usually not connected well and radiate heat. Then the insulation on the flexiducts is usually R-6 to R-8 at best, which leaks some heat on longer runs. And then the ducts lose heat when bent too much. Also the vents themself get hot 120 degrees and radiate heat into the attic. And the unit is in the attic, if it is not balanced properly, it sucks air into the ductwork from the attic, which is dusty and dirty, and blows into the rooms. And also if negatively balanced, it sucks air from outside the house into the house, which is basically like cooling outside air. I am curious to know how efficient the central gas is in an old home with leaky ducts and systems in the attic. I have a feeling that it can’t compete with ductless.

    • My home doesn’t have any ductwork at all, so I can’t speak to this first hand, but certainly any ducted system with ducts in unconditioned spaces needs careful attention to insulation and sealing for efficiency’s sake.

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