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Community and Q&A

Looking for advice on a new build heating plan

Tim Burke | Posted in Mechanicals on

I am trying to figure out the best solution for heating our new home and I would greatly appreciate any suggestion from the wise folks here.

The house is in Northern New York, climate zone 6. We have great Southern exposure on the site and went with more of a passive solar design. The house is basically a simple rectangle with two stories and is built into the hill with a walkout first floor that will also be used for living space (attached are the floor plans). The house has a shed roof with 14ft ceilings on the South side and 8ft on the North. For our envelope we have R-20 under slab, ICF first floor, 8″ SIP second floor, and 12″ sip roof and triple pane Zola windows. The energy audit came in with a max heat load of 30,000 btu.

We will have a “hybrid” masonry wood stove from eco firebox on the second floor that we plan on doing the majority of our heating with.

My concerns/questions are:

I’m concerned that the heat from the wood stove will not travel downstairs…
We would really like to stay all electric in the house with plans to add solar in the future. Perhaps a mini split would work with one head downstairs and one upstairs? My concern with the mini split is that it is not uncommon to get multiple days in a row of -20F in our area, and we also travel a fair amount in the winter.

I could easily add some electric baseboard heaters downstairs as well but I am not crazy about the efficiency/looks. I’m also not sure if every room downstairs will need some type of heat or if something in the family room, such as mini split head, could work. Then again, with the mini split, what would happen downstairs on those days of of -20f?

I would greatly appreciate any thoughts/ideas/suggestions! Thanks in advance.

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Tim. How are you planning to insulate? To what level? And what are the specs on your windows?

  2. Bob Irving | | #2

    Minisplits! We converted from oil/propane/woodstove three years ago, partly to see if our (somewhat upgraded) old farmhouse could be as comfortable as the new double wall houses we are building. It's more comfortable now than ever, and with our new solar array, will be close to net zero. I've seen no downsides and have no complaints; these things are great!

  3. Jonathan Lawrence CZ 4A New Jersey | | #3

    Tim,

    Were the folks at Eco Firebox able to provide you with the BTU rating? My concern would be that the unit will provide too much heat on the 2nd floor. Your max heat load is 30,000 BTU's and it is not uncommon for high-efficiency fireplaces to put out more than 65,000 BTU's. Plus, this is a hybrid unit with a masonry heat-sink so it is not as if you can just damper down the fire if it gets too hot. In fact their website says it will continue to radiate heat for 12-36 hours after the fire has burned out.

  4. Tim Burke | | #4

    Steve,

    The house is insulated with the icf's and sip's, so there should be very little air leakage and bridging. I believe the icf's for the walkout basement are R-25, sip walls R-35, sip roof R-45 and windows U .11. Thanks.

  5. Tim Burke | | #5

    Hi Jonathan,

    I have not heard an exact btu rating for the Eco Firebox, but I would imagine that this also can be somewhat controlled by the size of the fire you burn. I have read many discussions about having unit like this is a well insulated, tight house and there seems to be many different opinions.

    Of course one of the reasons we are going for this unit is that it will be the "centerpiece" of our home and we wanted something that we could customize the looks of. We also wanted to avoid a traditional fireplace because we at least wanted the unit to have some function. Ultimately, we decided for the unit after talking with another home owner who has one of these units and also has a very similar house to ours. He originally had a normal wood stove and claimed that this would quickly overheat his house. After switching to the Firebox he was much happier with the results. He said the thermal mass of the unit allowed for a lower btu output over a longer period of time, making for a more comfortable home.

  6. Brian P | | #6

    We're entering our third winter in our all electric house, zone 6 in NH. Small, two story, primary heat is one mini-split on lower floor. For backup heat, we have Stiebel Eltron wall mount heaters in bathrooms, bedrooms, and a couple in open downstairs area. We only activate those if temps are expected to be -20f or below. A few thoughts...

    What do you mean by "Northern", are you zone 6 close to NYC....or way up there, Adirondacks?

    I can understand the appeal of some type of wood burning stove, would also be nice backup during a power outage. With a tight and efficient house, I would also be concerned about overheating and it seems ideal to have that heat source on the lowest living level.

    I would recommend the mini-split strategy, would be a great system with solar PV. For backup, you could plan for the lowest wattage Convectair wall mount heaters in a couple/few select locations. Get the 120v plugin versions, no special wiring and you could even take them down from spring through fall:
    https://www.convectair.ca/en/products/120v-plugin/apero

    They are more expensive than the Stiebel Eltron, but I wish I had gotten the Convectair because of the 120v plug in option and they have a better thermostat.

  7. D Dorsett | | #7

    You may be at greater risk of overheating with all the south facing glass than with the eco firebox. The BTU/hr firing rate hardly matters when dealing with a high thermal mass masonry wood burner- the BTU/hr rate emitted at the surface of the masonry does, and that changes only slowly. A single 1 hour fire at 100,000 BTU/hr will not overheat the place, and the stored heat in the thermal mass gets destributed over many hours. There's a bit of a learning curve of WHEN to run the fire for optimum comfort to deal with the thermal time lag through the masonry but it's not a continuous fire.

    You don't have the same thermal-mass forgiveness with solar gain. The shed roof being a north facing pitch makes it useless for solar PV, and was probably done to increase the total amount of south facing glazing. Unless you simulated what happens on a sunny 15F winter day, you may not realize just how much temperature overshoot you might experience.

    If you re-orient the roof pitch so that it's a southerly slope and shade it with PV panels, with U0.11 windows you can probably hit Net Zero with mini-splits, and have much better control over interior temperatures.

    You may also be better off minimizing or eliminating the west facing windows, since late in the day solar gains in summer lead dramatically increases the peak cooling load. North facing windows are ideal for daylighting- it's shadow-free, and since there is never solar glare from northerly windows, visual acuity is improved even at lower absolute lux intensity. It doesn't take a lot of north facing window to provide excellent daylighting, and note, unless the windows are explicitly designed to reject solar gains, most high performance windows are net energy gainers in winter, even when facing north. Size the south facing windows for optimum rather than maximum winter gains, and design the overhangs to limit summertime gains.

    Do the room by room heat loss math at -20F (which is probably more than 10F colder than your 99% outside design condition). The dynamic thermal lag of an ICF will reduce the peak wall losses relative to the outdoor temperature compared to a steady state analysis, but that can be modeled if you really care to. With a 70F interior temp and -20F exterior temp (a 90F delta) a U0.11 window loses about 10BTU/hr per square foot- it takes about 20-25 square feet of window to equal the amount of heat being emitted by one sleeping adult human. An R35 SIP loses about 2.5 BTU/hr per square foot under those conditions (provided they didn't lie too much on the performance spec), and an R25 ICF loses about 3.5 BTU/hr per square foot steady state, but dynamically it's lower than that, since it doesn't dwell at -20F for 10s of hours at a time- call it 3 BTU/hr per square foot for quick & dirty estimation reasons.

  8. Tim Burke | | #8

    Brian,

    Thanks for your input! I really like hearing input from someone who is in a similar situation and I'm glad to hear that the mini-split is working well for you.

    We are in the High Peaks region of the Adirondacks, so we definitely experience some pretty low temps in the winter. Thanks again for your thoughts.

  9. Tim Burke | | #9

    Dana,

    Thanks so much for your detailed response and suggestions. I hear you loud and clear with the concern of overheating on a clear winter day and I will definitely keep that in mind going forward.

    Most of the windows on the lower level will be completely shaded all year because of the large deck off of the second floor. Some of the windows on the upper level will also be completely shaded all year because of the covered porch. The remaining Southern windows will have large overhangs, but I realize this won't be any help in the winter months when the sun is low.

    I have been a longtime lurker on this site and I always enjoy your responses. Thanks for all of your time helping us amateurs, our homes, and the planet, are better for it.

  10. Jonathan Lawrence CZ 4A New Jersey | | #10

    Tim,

    Glad to hear that you have been able to get the input of someone who has experience with the same firebox. I am in a similar situation in that I am trying to figure out how to transition from a high BTU, high-efficiency fireplace in a drafty house to a fireplace in a high performance home that I am building next spring. In my current house, the fireplace is properly sized for the first floor family room with a cathedral ceiling. However, I have another unit in the living room and it is unfortunately oversized for that room. Even with the blower turned off, just the radiant heat is too much unless I burn a dampened down fire which defeats the purpose of being an EPA certified fireplace. I have been contemplating using a ducted fireplace in the new house which would allow me to transfer some of the warm air to the second floor. However, I am going to take a serious look at the Eco Firebox and see if I can make it work. Thanks for sharing.

  11. Joel Cheely | | #11

    I'm in central New York, zone 5. I switched from oil-fired furnace to geothermal, and have since added PV to the mix. I'm very happy with the geothermal system and love having summer cooling on the few times we need it. I'm getting prepared for an addition and am also considering some kind of wood-burning heat and am having a hard time justifying it other than for the appeal of the fire.

  12. User avatar
    Stephen Sheehy | | #12

    Is overheating really ever a problem in zone 6? Our pretty good house in zone 6 has a lot of south facing glass, shgc is.49. On sunny days in December or January, the house warms up to about 75-77°F. I don't consider that uncomfortable, but opening a window for a few minutes would cool it right down. The design anticipated a significant amount of heat gain when calculating what we'd need for heat.

  13. Malcolm Taylor | | #13

    Stephen,
    Yes - surely overheating is only a problem if it occurs when the outdoor temperature is so high you can't dissipate it, or if you paid for the energy to overheat.

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