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Q&A Spotlight

Designing an HVAC System for a Cold Climate

An owner-builder weighs two options for heating with an air-source heat pump

A heating system for snow country. There's more than one way to design a heating system with a high-performance air-source heat pump.
Image Credit: Scott Gibson

Randy Bunney is building a new house in a challenging environment — north central Minnesota, where overnight temperatures plunge well below zero and heating-degree days over the last three years have averaged more than 8,600 annually.

The high-performance, passive-solar home will be a relatively small 1,100-square feet with two bedrooms, an open living-kitchen-dining area, 1 1/2 baths, a mudroom and a mechanical room. Bunney is planning on exterior walls insulated to R-40, the roof to R-60, and “near airtight” construction.

The main source of heat will be an air-source heat pump able to function at low temperatures. The question is exactly how the technology will be put to work. Bunney and his advisors are considering two possible alternatives.

The first is to pair a Mitsubishi air-source heat pump with an electric furnace that would kick in when the heat pump hits its cold-weather limit of 13 degrees below zero.

“Our design team concludes the cabin’s small footprint does not warrant a heat pump with multi-indoor units for zone heating,” Bunney writes in a post at GreenBuildingAdvisor’s Q&A forum.

“Instead, the design calls for running a single heat pump coil into [a] plenum for [an] electric resistance coil. The plan offers initial cost savings over multi-splits. Duct work will transport heat to individual rooms. We will use an ultra small marine wood stove for backup in case of power failure.”

Plan B seems to be installing a number of minisplit heads in the house, their number and location to be determined. An electric furnace would still provide supplemental heat when outside temperatures were too cold for the heat pump.

“We have a good design team open to suggestions,” Bunney says as he invites questions and comments. That’s the focus for this Q&A Spotlight.

Keep it simple

Jin Kazama…

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  1. jinmtvt | | #1

    Mini defrost
    i would not even consider a mini unit that does not use "reverse" to defrost during heating operation. Think of it, it uses the heat of the house at probably a pretty high COP to defrost the exterior unit...this energy loss is included in their HSPF testing, i believe.

    Again, simplicity.
    Baseboards/convectair/radiant electrical panels/underfloor electrical heating...
    All those are simple, effective and cheap solutions that kick in only when required, maintaining a stable ,comfortable temperature in each desired location around the house and are all less intrusive than a forced air system. I don't see how one can save money on initial cost going with a central forced air heating compare to baseboards setup with independent thermostats.

    Just to add to the point, i installed ~11 individual thermostats , one in each room around my house, which gives me a reading of the current temperature and operate the in-floor electrical heating system ( via relays etc.. ) . I can tell you that being able to adjust set points in each room down to 0.5c is very convenient. Paid ~50$ for each thermostat which are Z-wave enabled ( not used yet but it's there ) and it would've been very hard to adjust the 4 mini splits without the help of secondary readings.

    Dana suggested recently in a Q&A that efficiency of minis during cold snaps could be improved greatly by using an auxilliary heat source such as a wood stove, to reduce the work load, which in turn raises the COP of the heat pumps.

  2. Expert Member
    Dana Dorsett | | #2

    A few reactions....
    "The first is to pair a Mitsubishi air-source heat pump with an electric furnace that would kick in when the heat pump hits its cold-weather limit of 13 degrees below zero."

    The -13F/-25C temp is not an operational limit for Mitsubishi cold weather heat pumps, rather, it's the coldest temp at which the heat output is fully specified & guaranteed. They keep running and putting out substantial heat at temperatures well below -13F.

    Many of their cold weather mini-splits with a -13F spec have an auto-shutdown feature that occurs at temps as warm as -18F or colder spelled out in the fine print on their submittal sheets. See the the text at the bottom of page 2, where it reads:

    " ** System cuts out at -18º F (-28º C) to avoid thermistor error, but recovers from cutout operation and automatically restarts at -13º F (-25ºC)"

    But according to internet scuttlebutt (the ultimate source of real-world information :-) ), the -18F number is an upper bound, and that the actual shut-down temp seems to be well into the -20s F.

    From a heating design specification point of view it's fine to assume that it will still have substantial capacity at -18F, more than half the full-on capacity that it has at +5F, but not a a lot more than half.

    Bottom line, the BTU./hour capacity at -13F or even the -18F cutout is not a show-stopper in any MN location, the way it might be in Fairbanks Alaska or Churchill Manitoba. There are extremely few days over a decade in MN where the daily high doesn't recover to above -13F. Grid reliability is likely to account for more annual hours of the heat being out than the outdoor temperature capacity/cutout issues would be.

    Not mentioned in the discussion, for operating a Mitsubishi for extended cold snaps when the daily high is still under +10F, it's probably be worth installing the base heater option, to avoid ice buildup from defrost cycles. Running the base heater will cut into absolute efficiency numbers, but it may be necessary to keep it running without damage if it's a climate that can go weeks with outdoor temps low enough that the defrost water re-freezes before it can drain out of the bottom pan of the outdoor unit.


    "Don’t undersize your wood stove. Since the grid seems to go down often when it is really cold, it seems wiser to have that extra capacity in your stove than not."

    Good advice from a general advice point of view, but for the case under consideration it's actually sort of difficult (but not impossible) to undersize a wood stove for an 1100' high-R house. Without heat load calculations on the house or stove specs it's hard to say for sure if they've actually crossed that threshold with the "...11 inches by 13 inches by 18 inches...." wood stove.

  3. rjparker | | #3

    Sometimes people underplay the defrost requirements of a heat pump regardless of type. Humid and wet weather will increase the frequency of defrosts and may require supplemental space heat particularly with conventional heat pumps. An earlier article and comments covers the bases.

  4. kyeser | | #4

    MInisplit Placement
    I would pay attention to the outdoor placement of your minisplits condenser. I have found that when the temp gets below 10F the condenser located about 6ft from bedroom window makes a lot more noise. Hi-speed fan noise, defrost cycling what have you, but it gets pretty annoying in the middle of the night when everything else is so quiet. We have had a lot of sub zero whether this winter in Northern NY and although my Fujutsu is still keeping the house nice and cozy, I have been turning of the unit when the whether is below 10F because it wakes me up at night.

  5. krom | | #5

    I am going through the same
    I am going through the same decisions. My HVAC guy said that mitsubishi will be releasing zuba central for sale in the US, this spring:

  6. rjparker | | #6

    Mitsubishi Zuba Central for Traditional Central Ducting
    I would sell your stock in some of the unreliable HVAC companies before Mitsubishi Zuba Central ducted systems become available in the US.

  7. brp_nh | | #7

    base heater option for Mitsubishi Hyper Heat
    We had a Mitsubishi FH12NA installed in 2014 at our house in NH, zone 6. The installer (a Mitsubishi "Diamond Dealer") consulted either a rep or literature about the base heater, he said the base heater is mandatory in very cold climates or the warranty may not be honored. Something like that.

  8. user-1005777 | | #8

    Mitsubishi hyper heat
    Our mini home was heated by electric baseboards with thermostats in each room. We had a Mitsubishi FH12NA with base heater installed in October of 2011. We had programmable thermostats that are now pretty useless, but they had already paid for themselves before we retired. I just set the baseboards to come on at 16 C. The baseboards in the end rooms only come on a couple times a year.
    If I was building a new home I would install convection baseboards with built in proportional thermostats, with the heat pump in the main rooms of the house. Because of increased insurance costs and installation costs, I would avoid wood heat and go with a backup generator.

  9. loracekim | | #9

    I live in a old farm house in
    I live in a old farm house in Minnesota. Only half remodeled, but even with limited insulation upgrades So far our minisplit is handing most the heating load we do run a propane forced air furnace in that am and evening when we come home but have it set back to 50 at night and during the workday. So far our minisplit keeps it above 50 in between. During the summer it has no problem cooling the entire house. It was still running and producing heat at-28 F, but at a reduced rate. As never shut down that I know of. So overall I'm very happy. The only issue is I agree at colder temps it can be very noisy, so much so that I thought there was something wrong when I first heard it, Since everything I read said they are so quite. So I suggest mounting the outside unit on a raised platform above snow depth that is NOT attached to the house, mine is, I also covered mine with a small roof. Seems to help with ice buildup on rainy humid or near freezing snow days.

  10. Expert Member
    KOHTA UENO | | #10

    BSC Mini Split in Cold Climate Building America Report
    Just to keep folks in the loop, BSC's Building America report on monitoring the Massachusetts (Zone 5) houses built by Transformations (Carter Scott) is currently posted on BSC's website--Carter conditioned most of his houses with wall-hung mini splits, typically two in a 2 story house; more on spread-out floor plans. Most of the lessons learned echo what has already been written above (especially by Martin and Peter Y).

    BA-1407: Long-Term Monitoring of Mini-Split Ductless Heat Pumps in the Northeast

    But a few more items to bring up:

    One head/2 floors vs. two heads/2 floors: the builder had a few small 2-story houses that could easily be heated by a single 12,000 Btu/hour head. He had also noticed that the second floor head barely turned on in the winter. He tried conditioning a few houses with a single head on the first floor: worked fine in winter, but had problems with warm conditions on the second floor in the summer. Just straight up thermal stratification--you can't get cold air to rise. You can probably add all sorts of anti-stratification fans, but you will need to push a lot of CFM (e.g., 40 CFM didn't have an effect when it was tried). See "6.4 Thermal Buoyancy Effects"

    Rooms over garages: if you have a room with very different exposure conditions than a typical room (one or two walls to outside), you'll have more problems maintaining temperature--worst example we saw was a room over a garage--basically five of the six sides of the cube were outside. If the homeowners kept the door closed in the worst of winter, temperatures could drop to ~50 F. (See 6.6 Bonus Room Comfort Issues)

    Square footage limitations: This is a very unscientific estimate, but in the anecdotes where there were space conditioning complaints, the layout was pushing greater than 1100 sf per MSHP head. Not saying that there's any physics reason behind it... it's more of a warning, if you're pushing over 1100 sf/head, be a bit careful (see 5.3.1 Design Loads and Equipment Sizing).

    Snow loads: in a cold climate with snow, be sure to keep the outdoor MSHP unit out of snow cover. (see 5.5 Snow Blockage of Mini-Split Heat Pumps). Marc Rosenbaum has done a nice job talking about this in various presentations and papers--at the very least, using risers; place it so it doesn't get snow dumped on it from the roof; consider a wall mount (I believe Marc also did that to avoid equipment damage from students).

    On off operation: let the MSHP modulate at a constant (or near constant) setpoint--using big setbacks (or worse yet, turning the unit on and off) not only gives you worse comfort, but gives you substantially worse energy performance (see 6.7 Temperature Setbacks (On/Off Operation).

    Electric resistance heating: another option for electric resistance heating are radiant wall or ceiling panels--again, Marc Rosenbaum used them (see 2.2.6 Rosenbaum: Eliakim’s Way (Massachusetts) Energy Monitoring).

  11. GBA Editor
    Martin Holladay | | #11

    Response to Kohta Ueno
    Thanks for the link, and for your extremely useful list of rules of thumb.

  12. Expert Member
    Dana Dorsett | | #12

    When using radiant resistance backup heat...
    ... in addition to a thermostat, occupancy/vacancy sensors are useful for saving power. With radiant panels the comfort comes on long before the air or wall temps reach the setpoint since it increases the average radiant temperature of the room. Just as it's possible to comfortable outdoors at 40F or lower on a windless day when standing in direct sun, being next to or under a radiant panel can be comfortable at fairly low room temperatures. Having the controls automatically turn off the radiant panel when no one is in the room has little to no comfort downside, and avoids far more power use than set-back thermostats, etc. Pretty good 1.5-2kw sensor switches can be had for $30 or less.

    The occupancy / vacancy sensor approach doesn't work nearly as well with electric baseboards, since they heat primarily via convection, not radiation.

  13. user-1119494 | | #13

    I agree on getting a decent stove....
    I've seen some marine stoves that only put out 7k BTU and figure you will need at least 20k if the power goes out at 30 below for a couple days... and I'd go for 30k BTU...

    Of course, this ignores solar and other gains...

  14. Robert Opaluch | | #14

    Upgrade insulation?
    Agree keep it simple. Although your insulation levels R-40 walls R-60 ceilings are good, it gets pretty nippy up there in northern Minnesota. I'd suggest considering higher levels, (especially insulation protecting your passive solar thermal mass slab!). If the power goes out, insulation still works fine. Doesn't require maintenance, not affected by snowfall, doesn't need replacement after 20-30 years. And its mostly pretty cheap compared to heating systems. If additional insulation saves you from adding anything to your heating system, be sure to consider those savings in figuring out the rate of return or years to break-even by adding insulation.

    Same for windows, you don't mention R-value, type of window, or percent of glazing facing south.

    Dana's idea about occupancy sensors is great! I've used radiant heat in the ceiling in a passive solar home I built, and it is very comfortable compared to any hot air system. Warm objects including you. Putting a heat lamp in your bathroom can also provide quick comfortable heat, and its cheap.

    Would suggest not putting any electric heating in your passive solar thermal mass. That mass not only soaks up the sun's rays, it will be quite difficult and expensive to drive up the slab temperature. Radiant heat from the ceiling or cove seems more practical.

    Solar tempered certainly makes sense, but can you engineer passive solar to work well in your area? Can you provide enough south-facing glass to overcome your BTUs lost? How many cloudy days can you get in a row?? I feared that if the slab in my house wasn't insulated or charged with enough sunlight, it would be a heat sink, tough to keep the house feeling warm. Radiant heat is nice, a cold slab floor is not. Worked out well though, all the heat loss-solar gain calculations and extra slab edge insulation were worth doing. One advantage in your case is that if you are heating with a wood stove, the slab also can store that excess heat and give it back after the stove is not operating.

    Best of luck and congratulations in designing and building your home!

  15. wyobunney | | #15

    Knowledge sharing helps warm the heart at 46 degrees North
    Thanks to all of you who have weighed in on best HVAC for our cabin / home up north in Minnesota. I appreciate the work you did, Scott Gibson, in pulling all the content into an article and to you too, Peter Yost, for reaching out to your network of green building pros.

    At times high performance HVAC systems may seem unknowable for a consumer. And then along comes people like greenbuilding advisor pro Martin Holladay who cuts to the chase, as he dips deep into his well of sage advice and experience.

    Still. HVAC is dang complicated. And building a successful high performance home is fraught with potential dangers and mistakes that will dramatically impact the performance and comfort of our homes. Even though we refer to building science, I would argue that we are practicing the Art and Science of performance building sprinkled in with some old time bias that we all have for or against a particular solution.

    Finally, at times it feels like my wife and I are on a rite of passage, a vision quest, if you will to build a home that will be our last where we will grow older together. We hope to do so in great comfort while also leaving the smallest carbon tracks that our budget will allow.

    Best to all. Randy and Julie

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