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Community and Q&A

Will this plan keep us warm?

Jim Sweazey | Posted in General Questions on

Zone 6A New construction 7500 HDD.
We are questioning if our heating plan will cover our needs. The energy modeling, I did with Remdesign (.5 ACH) shows Peak heating load at 22 kbtu/hr. The manual j that was done shows heat load at 29,000.
The current plan is to have a 30K BTU Mitsubishi hyper heat Mini Split condenser with a 12K ceiling cassette (main living area), 12K Ducted (two bedrooms from a common upstairs hall), and a 9K wall unit (master bedroom). We have a large Tulikivi “fireplace” that would provide heat when the outside air is too cold for the Mini Split. We also put hydronic tubing in the basement floor and plan on the same for three tiled areas on the main level. We did not buy the boiler and manifold and only put the pex in for resale, and as a backup plan if needed.
So far, we are disappointed with the heat given off by the wood. The Tulikivi, by design, holds a lot of heat and radiates it out for a long time. My research says it should give off 10,000 BTU with one load and 20,000 with two loads per day. We haven’t given it an accurate test. The house starts out very cold and doesn’t have a ceiling, I need to get better at building the fires, and we should have more continuous fires. That said, the best we have done is radiant heat from the fire while its burning (an hour or two) and 160-degree surface temperature, on the stove, that only is felt if touched.
My question/concern is, does it sound like this plan will handle the cold Minnesota weather? I have found charts that show the BTU percentage efficiency as the ambient air temperature drops but am unable to find a source for this information for the models I will install. Does anyone know where this can be found? The additional heat I’m considering is to run pex to some areas of the house without connecting. If the current plan fails, I could install a toe-kick unit for the open kitchen/ living room and baseboard in a few other places. I think I could provide the hot water with an additional propane water heater. The current plan is to be all electric.
There are so many variables. I feel the most difficult to predict is how air tight the house will end up. My contractor keeps saying, that the HVAC is getting too complicated and thinks we should just install a conventional forced air system. My goal was to have the house so efficient that this wouldn’t be necessary and could rely on the benefit of zoned conditioning and wood burning.
Sorry this is so long but it’s hard to explain in a few words.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Jim,
    I'm somewhat confused concerning the stage of construction. Evidently you still have a contractor on site, and you mention that the house "doesn't have a ceiling," whatever that means. (Does that mean that your air barrier is incomplete?)

    I have never owned or operated a Tulikivi, which is an expensive accessory. But I'm sure it's true that the BTU/h output of a wood-burning fireplace depends on many factors, including (a) the species of firewood, (b) how dry the firewood is, (c) the average diameter of the firewood, and (d) the skill of the fire builder.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Jim,
    Unless Mitsubishi has changed its rating methods, Mitsubishi heat pumps have a heating capacity equal to 100% of the rated output at an outdoor temperature of 5°F. At -4°F, the heating capacity drops to 82% of the rated capacity. At -13°F, the heating capacity drops to 62% of the rated heating capacity.

    So if you have equipment rated at 30,000 BTU/h at an outdoor temperature of 5°F, then the equipment will have an output of 18,600 BTU/h at -13°F.

  3. Brendan Albano | | #3

    My parents have a Tulikivi, but they live in a milder climate (4C), and a well insulated single level home. One thing is that if you want the heat from the Tulikivi to warm the bedrooms, you definitely have to leave your doors open all day.

    The surface of the Tulikivi never gets super hot like a woodstove does. If your house is not yet finished, and not yet airsealed, I feel like that probably is at least part of why you are not impressed with the heat given off by the Tulikivi.

    My parents' experience is that it can take a few days of building fires every day to get the mass of the Tulikivi really "charged up" so to speak. Not sure on the details here, but that's the impression I get from them. So once your house is air-sealed, you might try doing 2 fires per day for 3 or 4 days and seeing how the Tulikivi feels once its fully heated up.

    You might also ask the person who installed your Tulikivi if there are other Tulikivi owners in your area you could talk to about how they build their fires and all that.

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Jim,
    You wrote, "My contractor keeps saying that the HVAC is getting too complicated."

    Your contractor may have a point.

    You also wrote, "The current plan is to be all electric." If that's the plan, it makes sense to size your ductless minisplits to handle the load at -13°F. Then you could add a few electric resistance space heaters (wall-mounted heaters or electric baseboard units) to handle the occasional night when the temperature drops to -30°F.

    Anyway, that's what I would do.

    The Tulikivi can't hurt, although it's a particularly expensive way to provide wood heat. And I don't see any reason to install in-floor hydronic tubing.

  5. Jim Sweazey | | #5

    Thanks to all for your help,
    Martin, my contractor is actually my son who became licensed to help me build the house. I would be lost without him and we try to share responsibilities. The house is s double stud with all wall insulation done except the inside cavity. ( 4" Rock Wool, 3" of polyiso in the space between the walls). The windows and doors are in. we have a poly cover over the ceiling and the stairs to the second floor bedrooms is sealed. We have an old propane furnace hanging in the basement so we can warm it enough to work. We can't put the ceiling in till the wiring gets finished. The wiring can't be done until I get the Mini Splits, Bath fans, and ERV installed.
    I'll attach the cold weather chart I found for a model different than my plan. If I'm reading it right, it is a 36K BTU compressor that puts out 45K BTUs until it gets cold. At -13 it goes down to 33.8K BTU. From what I have seen documented, these units put out more heat than the rated BTU. Assuming this information is correct, it would be a great help getting it for the models I'm looking at. Or if your % numbers are applicable across the product line, that would work.
    All the variables you list about burning wood certainly are true. The Tulikivi is a want, not a need and something I think we will enjoy. It is also part of the cold weather plan and still might fill that need.
    We are considering the eclectic baseboard option. That was part of the original plan. Code won't allow me to run the wires without an appliance connected but I can run pex and leave it in the wall if not needed. If we do need additional heat, I think that warm water would be both more economical and comfortable than electric resistance.
    Brendan/Andrew. Thanks, everything you are saying about Masonry heaters is true. My first fires were a mess. Small, dry wood, start the fire at the top (seems so wrong) but works.
    The real question is- do I give up on the mini- splits and go with a furnace, or gamble and deal with the few cold times when I would need to rely on a back up system?

  6. Andrew Bater | | #6

    Jim, while I don't personally own a Tulikivi proper, I do run a different style masonry heater.

    My impression is your home is still under construction and that you may be using the Tulikivi to bring it back up to temperature during work periods.

    As you and Brendan mention, it may take multiple fires each day to keep the unit "continuously" warm. More importantly, if the Tulikivi is like my masonry heater, you won't get peak heat out of the unit until hours after your burn. You may not experience benefit when you expect it.

    As Martin pointed out, the type of fire and how it's managed are very important.

    I burn my masonry heater twice a day.

    * One fire between 4 - 5 AM each morning, all dampers closed by around 7 or 8 AM.

    * One fire between 4 - 5 PM each evening, all dampers closed by around 7 or 8 PM.

    I split my wood quite small so it burns very fast. It's been seasoned for at least a year, sometimes two.

    I close the input air dampers on my masonry heater as soon as any visible flames disappear, and close the output damper shortly after even though there are still considerable red hot coals left. (My dampers not being perfectly sealed facilitate that, the dampers on the Tulikivi may not.) This traps a considerable amount of heat in the masonry heater core.

    Did I do the above fast damper closing the first year I owned the unit? No, I had to gain experience with the unit (and my smoke alarms) before I felt comfortable operating it this way. There is a learning curve.

    Late last winter I re-engineered the floor of the masonry heater to pre-heat combustion air and improve the cleaning process. This has led to faster and hotter burns, and even less ash. Only took me three winters of operation before I came to this redesign! Speaking of learning curve...

  7. Andrew Bater | | #7

    Jim, two more quick masonry heater tips for you:

    1. I use "Fatwood" firestarter sticks when lighting my MH each time. They jump start the fire. https://www.homedepot.com/p/Fatwood-All-Natural-Firestarter-4-lb-Bag-201274/100335836

    2. I use a Bernzomatic torch to light those fire starter sticks. I get things going fast!

    Picture from this morning's fire: Top down burn charge stack with the Fatwood fire starter sticks nestled between an upper layer, and the fire roughly 40 minutes later. Note the absence of soot on the firebox wall, that is what you are going for.

    Enjoy!

  8. Brian P | | #8

    Many people in climates as cold as yours successfully heat their homes with mini splits, so I'd stick with the mini split plan, just make sure the system is designed/installed well (which others can hopefully help with).

    With an energy efficient house like yours, even if the mini splits shut down on the coldest nights, you should be able to coast through until the morning, especially with the wood. So, I don't think you need to worry about installing a complete separate electric resistance system.

    If you do want some electric backup, check out the 120v plug in wall units from Convectair (https://www.convectair.ca/us/products/120v-plugin/apero). You could just put some extra plugs in a few spots where they would make sense and not worry about the code issues. They are more expensive than cheap baseboard, but you could live in the house for a bit and evaluate before buying them.

    In regards to the other wiring issues, couldn't you run the mini-split/fan/erv wiring and leave it stubbed out in the locations, allowing you to finish the house envelope?

  9. Jim Sweazey | | #9

    Andrew, I'll check out the "fatwood'. I have a huge pile of old cedar shingles that I think will work to start the fires. I do use a propane torch. I know what you mean about the clean firebox walls. If it burns hot, the walls are clean. My infrared temperature gun maxes out at 600 F. I read that the fire can reach 1500-1800.
    Brian, thanks for the words of encouragement. That plug in heater looks to be a good quality product. The "as seen on TV" version is a less expensive option.
    I too, think the plan will work but with so many variables it's almost impossible to predict. The building inspector said he would do exactly what my plan is. One other thing in my favor is the house has a lot of south windows and usually when it gets extremely cold, the skies are clear.
    We have talked about stubbing out the wiring so we can get a ceiling in. I'll revisit that. Thanks for the reminder.

  10. Jim Sweazey | | #10

    Part of my question was if anyone knows where I can find cold weather BTU output for the model Mitsubishi I'm considering (MXZ-3C30NAHZ2).
    When I originally posted the question, I thought I attached an example of BTU output for a different unit as an example. But now I can't see it the attachment. So I tried attaching it again. Sorry, I don't mean to be "beating a dead horse".

  11. User avatar GBA Editor
    Martin Holladay | | #11

    Jim,
    Mitsubishi provides some information here: https://nonul.mylinkdrive.com/files/MXZ_Multi-Zone_Efficiencies.pdf

    The MXZ-3C30NAHZ2 is rated at 28,600 BTU/h at an outdoor temperature of 5°F. If my understanding is correct concerning the de-rating required for -13°F, the capacity at -13°F should be 62% of 28,600 BTU/h -- in other words, 17,732 BTU/h.

    More information here: How To Buy a Ductless Minisplit.

  12. User avatar GBA Editor
    Martin Holladay | | #12

    I found more info. At -13°F, the heating capacity of the MXZ-3C30NAHZ2 is actually 88% of its rated capacity. So make that 25,168 BTU/h at -13°F.

    The information comes from here: https://nonul.mylinkdrive.com/files/M-Series_Engineering_Manual.pdf

    See also the graph below from that engineering manual.

    .

  13. Jim Sweazey | | #13

    WOW- that looks like exactly what I need. Thanks Martin

  14. User avatar
    Dana Dorsett | | #14

    Most Manual-Js will overestimate measured reality by a bit, even those that done "by the book" with aggressive assumptions.

    The H2i Mitsubishis will definitely handle zone 6A coolth, with the caveat that at some point when its in the -20s F most will automatically turn off, and won't re-start until it warms up to -15F or so. The shut down temp in the spec is -18F, but treat that as a maximum- there are many field observations of them continuing to work at temps below -20F. (I haven't dug that detail out of the MXZ-3C30NAHZ2 though- perhaps they have changed that "feature" with the -NAHZ2 s. Consult with Mitsubishi.)

    Note, those capacity numbers in the table are...

    ... "Heating capacity without any flost"...

    ...which in 'merican-English really means "...without any defrost".

    When there is rime-icing high humidity conditions outside defrost cycles will take a sizable bite out of the capacity cookie, but most of the time it won't.

    With the Tulikivi backing it up (and you remember fire it and keep it stoked several hours in advance when the temps are predicted to go that low) there shouldn't be any issues with it keeping the place warm enough, but a load of 10,000 BTUs of wood needs two burns per HOUR to average 20,000 BTU per HOUR.

    Your house needs way more than 20,000 BTU per DAY. If your firebox is really that small you'll have to run nearly continuous burns for it to carry the whole load when it's in negative double-digits, and you may want to invest in additional backup if the ductless system has the auto-shutdown / auto-restart feature.

  15. Jim Sweazey | | #15

    Thanks Dana X2

    It's my understanding that the Tulikivi has a maximum heat storage capacity. I can burn 54 lbs of wood in 24 hr. Normally that would be 4 loads once a day. The soapstone holds the heat for long periods of time and it's not good to over heat it. I read that you can have two fires a day and it would then put out 20,000 BTU. (reliable source?) All these numbers are BTU per hour.

    We also think it makes sense to try to predict the cost of operation for this option. I haven't researched how to calculate this yet. I'm sure that I have read posts where Dana did the math, I can start a new thread for this if I find it necessary or if anyone has a link to useful information it would be greatly appreciated.
    Thanks all

  16. User avatar
    Dana Dorsett | | #16

    The HSPF efficiency numbers published for heat pumps are for a zone IV heating climate based on the number of heating hours (independent of temperature), not to be confused with DOE climate zone 4 which is based on heating degree-days. (See this map: http://www.fsec.ucf.edu/en/publications/html/FSEC-PF-413-04/images/Figure5_lg.gif )

    Most of MN is in zone V, not IV so the MXZ-3C30NAHZ2 is not going to quite hit it's 11 BTUs per watt-hour HSPF number, but if properly sized it can still be pretty close, averaged over a season. For a first rough cut assume it'll average 10 BTU/watt hour = 10,000 BTU/kwh, but during but during sub-zero weather it'll be more like 6 BTU per watt-hour (6000 BTU/kwh), and when it's 40F outside it'll be more like 12-13 BTU/watt-hour.

    You didn't specify the design temperatures at which the load is 22,000 BTU/hr, but for the sake of quick calculatin' let's call it -10F outdoors, 70F indoors, an 80F temperature difference. That/s 22,000/80F= 275F per degree-hour, which is (x 24=) 6600 BTU per degree-day.

    In a climate with 7500 HDD it would take 7500 x 6600= 49,500,000 BTU per heating season, which at 10,000 BTU/kwh (the presumed as-used efficiency, below the nameplate zone IV efficiency) means 4950 kwh of power use.

    The average residential retail rate for electricity in MN is 13.5 cents/kwh (https://www.eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_5_6_a ), so without burning any wood heating with just the Mitsubishi would be costing about 4950 kwh x $0.135 = $643 per year.

    At 13.5 cents/kwh and an average efficiency of 10,000 BTU/kwh, a MMBTU (million BTU) costs $13.50

    When it's wicked cold out and running an as-used short-term HSPF of 6, it's closer to $22-23/MMBTU.

    Depending on the hardwood species a typical cord of wood has ~25,000,000 BTUs. Burned at 90% efficiency (possible if it's really dry, in a high mass wood burner) that delivers 22,500,000 BTU/cord. Don't know what seasoned hardwood cord wood cost in your neighborhood, but in mine (central MA) it's about $250-300/cord, which would make it about $11-13/MMBTU, somewhat cheaper than the ductless averaged over the season, but roughly half the cost when it's below zero outside.

    Note, for a Tulikivi operating at 85-90% efficiency to deliver 20,000 BTU/hr for 10 hours over a cold winter's night (= 200,000 BTU) requires burning about (200,000 / 22,500,000 =) ~1% of a cord per cold night or about 1.3 cubic feet of cord wood (stacked cord wood density, not solid wood.) So yes, averaging 20,000 BTU/hr number for hours out of a couple of burns is credible.

  17. Jim Sweazey | | #17

    Thank you Dana, this is very helpful information.

    Just to be clear, I wasn't questioning the math concerning two loads per day being 20K BTU. The soapstone can only hold so much energy. After that, a lot of heat goes up the chimney and you risk damaging the stove. I am just not sure about who the source of the information was that I was quoting.

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