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Backup heat for small building?

I believe you may have answered this question in an article but darn I can't find it.
So, need to ask.

I have a new small <800 sq. ft. structure, one story, built to Energy Star 2012 standards in zone 6. Mostly open design.

Building has an insulated crawlspace with rat slab (as of this week the insulators have not come, so exposed studs to work with, but i need to decide).

I plan to use a small wood stove for primary heat (it is piped for outside air intake).

I need a backup heat source for shoulder seasons/ unable to use wood for some illness etc.. I thought a vented propane wall heater without a fan (so a winter power loss wouldn't faze it) would work fine BUT they don't seem to meet the energy star requirements of 80% efficicency. Empire appears to come in around 70% for direct vented . In my state we can not use non-vented propane heaters.

Any advice without going to great expense to meet my needs? I would like to keep it simple and open to rethinking the backup plan, just short on time to make more holes in the envelope.
One propane dealer suggested drilling a hole through the poured foundation to vent a furnace installed in the crawlspace. Others suggested electric type space heater (that won't work during power outages).

All seem puzzled by just how much or...how little heat will be needed to heat this small building.

Seems like the suggestion to go small still has this area to address for help and advise for owners of moderate means. I am retiring this coming year and a suggestion of geothermal is out of my price range plus I would have a difficult time making that cost effective in my remaining years..

thank, and great site!

Asked by brenda field
Posted Nov 12, 2012 5:10 AM ET
Edited Nov 12, 2012 8:41 AM ET


6 Answers

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Option 1 (the best option): Install the Empire propane space heater, and forget about the Energy Star label.

Option 2: Install a ductless minisplit unit.

If you really insist on a heating system that will work during power outages, your only real option is the propane space heater with through-the-wall venting.

Answered by Martin Holladay
Posted Nov 12, 2012 8:48 AM ET


The more combustion vents you poke in the wall, the bigger the gaps in the insulation and the more air leakage you end up with. The installed cost of a 3/4-ton mini-split heat pump is likely to be less than the cost of a propane install if you consider the cost of the tank.

Almost any Energy Star 3/4 ton mini-split heat pump puts out at least 11,000BTU/hr at 17F, and would still be over 10KBTU @ -4F. The Mitsubishi H2i series would even put out 8000BTU/hr @ -13F.

Any of them could handle most of the load even in mid-winter for most zone 6 locations, even if they don't have a rated output (or not enough output) at your 99% outside design temp. For shoulder seasons it's a slam-dunk, and you'd have air conditioning/dehumidification to boot! If the place is so tight that you need to dehumidify even in winter the smallest Daikin Quaternity (RXG09HVJU using the FTXG09HVJU interior head) would be your only option, but if that's not an issue any of these high efficiency 3/4 ton mini-splits would work:

LG LSN090HYV / LSU090HYV rated 28SEER / 12.0 HSPF

Daikin RXS09LVJU / FTXG09HVJU rated 24 SEER/ 12.5 HSPF

Fujitsu Halcyon 9RLFW / AOU9RLS2 , rated 26 SEER/12 HSPF

Mitsubishi MSZFE-09NA / MUZFE-09NA rated 26 SEER/10 HSPF

At internet pricing hardware cost on any of them is under $2.5KUSD, some under $1.5K, figure $2.5-3 if you let a pro install it (recommended, even though DIY installs are possible if you read up on how to do it right and are comfortable handling it.)

During the shoulder seasons they will meet or beat geothermal efficiencies, but not so much during January. Though they're not terrible at +15F, they're sucking pond water at -5F, but that's what you have the woodstove for, eh? In some MN electricity markets heating primarily with a mini-split would be cheaper than heating with wood, except during extended cold snaps, assuming you are buying cordwood and don't own a woodlot that will supply your need. If you think of the mini-split as the primary heat and the woodstove for backup, you have a solution that works even when the grid goes down.

A mini split put's one 3" hole in the envelope that has no heat or drafting issues. You can foam it in place to air seal around it if you like.

Answered by Dana Dorsett
Posted Nov 13, 2012 5:53 PM ET


That was a very thoughtful answer with quite a bit for me to ponder! I had just read that mini-spiits struggle with the real cold stuff and it gave me pause regarding the mini-split.
Looking at it in the manner you discribe just maybe a good answer as I am not young and enjoy putting my firewood up BUT who knows if next year would allow that same enjoyment. So the idea of a future using less wood or as my backup for really cold temps is worth considering.

Going to such a small building space with good insulation does take me and most of the contractors/ dealers out of our comfort zone of knowlege of how to heat this space.
So finding this site and using it is something I recommend to all to come to see my "cottage" and wonder if they should be building something like my cottage for their retirement. I can tell you that those in the north country do worry about those -20 degree for a week or more that we are prone to some winters.

Answered by brenda field
Posted Nov 15, 2012 7:14 AM ET


People are starting to use mini-splits in places with outside design temps of -10F, but not much lower. If your 99% outside design temps are -20F you'll definitely NEED the wood stove, but a mini-split can still carry most of the mid-winter load. See if any of the locations listed here are near you:


The Mitsubishi H2i (aka "Hyper Heating") series all have a rated output at -13F, and if you want to limit the amount of wood burning you'd want to up-size the mini-split so that it's -13F output covers the heat load at that temperature. For the Hyper Heating series that's about 70% of the nominal heating output. The popular 1.5 ton (MUZFE18NA / MSZFE18NA ) has a nominal heating output around 22,000BTU/hr and delivers ~15,000 BTU/hr @ -13F, which is probably more than the heat load at that temp given you're going with an EnergyStar level of construction.

While I'll bet there are many examples of weeks where the nightly low got to -20F, I sincerely doubt there's an example in any US zone 6 location where that was ever a weekly average temp (zone 8, central AK, sure). Even in International Falls MN (zone 7) the daily highs are reliably warmer than -10F, even on days when it dropped under -30F overnight. Plug your zip code into WeatherSpark.com and take a look at what happened in the 24 hours before and after those -20F days, and you'll find that most of the time it was above -13F, even on the coldest days of the decade.

Key to success in any heating strategy is to do a careful Manual-J or similar heat load calculation using the REAL R/U values for the construction type. If your architect/designer didn't provide one, you can buy a $50 homeowner version of HVAC-calc and run your own- it'll be way better than a wild guess, and better than a random heating contractors 10 minute pencil-scratch load calc:


I can't imagine even the -20F heat load exceeding 18-20KBTU/hr with an 800' Energy Star cottage, and it's probably under 15K @ -13F. If it calculates out to be under 10KBTU/hr @ -13F the -FE12NA one-ton version would be a better choice.

These are starting to show up at ski-condos and vacation cottages all over zone-6 parts of northern New England, where the only other reasonable option is propane wall furnaces since the operating cost is lower and comfort higher, and they definitely see -20F temps every winter at 2000' + base-area altitudes at ski areas.

BTW: What wood stove? Most non-catalytic epa-rated cast iron wood stove put out more than 30KBTU/hr at the minimum "secondary-burn" firing rates, which could turn the place into an unintended sauna pretty quickly. A tiny higher thermal mass ceramic or soapstone version with a max output under 40,000BTU/hr w/throttled back sustainable firing rate under 15K would probably be more comfortable (if more expensive up front) due to the lower surface temps and thermal mass. The only model that jumps to mind immediately is the Hearthstone Tribute. It's a real cutie, but it only takes 16" (max) wood, which can be frustratingly short when going with purchased cordwood in markets geared toward 18-20" wood. But there are surely others (?).

Answered by Dana Dorsett
Posted Nov 15, 2012 6:16 PM ET


So finding this site and using it is something I recommend to all to come to see my "cottage" and wonder if they should be building something like my cottage for their retirement. I can tell you that those in the north country do worry about those -20 degree for a week or more that we are prone to some winters.

Brenda, do you have information about your cottage on the Web?

Answered by Jack Woolfe
Posted Nov 16, 2012 4:51 PM ET


Rais and Morso make smaller wood stoves. For example, the Rais Gabo wood stove has a min max output of 7-20 kBTU. The max wood size is quite small, 12" I believe. Morso has 7 models that the max output is 30,000 btu/hr.

Answered by Rebecca Surprenant
Posted Nov 17, 2012 10:46 AM ET
Edited Nov 17, 2012 10:55 AM ET.

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