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Community and Q&A

Wood stoves and thermal mass

Rick Dee | Posted in Energy Efficiency and Durability on

Hello, I’m building an off-grid super-insulated home this summer in Central Ontario (Climate Zone 6). We are self-building on a tight budget. 2 storeys + basement.

We have wood on the property and want to use it for heating. The house is also designed with passive solar principles in mind. We planned to install a masonry chimney run through the centre of the house to take advantage of thermal mass, but now considering whether the cost is justified against the added costs. Our woodstove advisor is recommending we install a regular double-wall stainless chimney instead.

I’ve seen the posts on this site expressing skepticism about the benefits of thermal mass and calling into question some of the passive solar dogmas, but I’m wondering if thermal mass is actually more useful in the context of a woodstove, because it could “even out” the fluctuating curve of a woodstove. Moreover, the masonry chimney would be exposed to lots of winter sunlight coming in through our south-facing windows.

Any thoughts you have would be much appreciated!

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Replies

  1. Steve Grinwis | | #1

    As a controls engineer, I can tell you that systems with a lot of inductance are really hard to control well.

    Think of this for a second:

    Your house is cold, so you want to build a fire and warm things up, so you start a small fire. The small fire takes a long time to heat up your chimney, and takes a long time to warm up your house. It's eventually comfortable, but won't be for a long time.

    Knowing that this didn't work well last time, the next time, you build a large fire. This quickly heats up the masonry, and the room, but after the room is up to temp the masonry still gives off lots of heat, and the room overheats. It's eventually comfortable, but won't be for a long time.

    Obviously, the above is contrived, but you will definitely run into these scenarios. Everyone I know who heats primarily with wood has hit this. Those giant cast iron stoves with huge stone chimneys are impressive looking things, but very hard to operate just so. This is why we recommend mini-splits or air-source heat pumps that modulate in the background You can probably get a minisplit that will cover most of your heat load for less than the cost of your fireplace.

    1. Rick Dee | | #2

      Thanks Steve, yes certainly there will be a learning curve with the woodstove. If we had a grid electrical connection I would agree that minisplits are the way to go, but our PV system won't be able to handle large loads in the winter. So I think we're going to stick with the woodstove but do our best to be smart about how to incorporate it into our house.

      1. Steve Grinwis | | #8

        I missed the off-grid part. Wood stove isn't a terrible idea, I would personally just avoid the large hot thermal mass. A small efficient wood stove with secondary combustion will do quite well in your home.

    2. Tyler Keniston | | #5

      Steve, I'm interested in your analogy to inductance. Did you mean capacitance? I've never thought of thermal mass as a form of inductance.

      1. Steve Grinwis | | #7

        Whoops, yes. Capacitance.

        I was thinking about how thermal systems are unique in that there is no thermal inductance, and I must have typed the wrong thing.

    3. Brian Carter | | #15

      What you are explaining is a problem that can also be addressed by a better practiced way of managing the stove. Rather than waiting until the house is too cold, and considering the thermal inertia of the mass to be heated, the stove is run in anticipation of the need for heat. In typical masonry stove use this comes down to a regular schedule in a consistently cold climate.
      It's true that this is not a way to hit a constant temperature the way a thermostat connected to an instant heat source would. In a super insulated home that is probably the best choice. It's possible with experience to manage wood heat without extremes. But sizing the thermal mass is an important consideration, and the chimney is a poor place to try to do that. Better is to size a masonry stove and use the stainless pipe to vent it.

    4. AlexPoi | | #19

      Finally, someone else saying it. I've studied control engineering as well and people don't realize it but having too much thermal mass is not a benefit. You need the right amount to filter out the daily temperature fluctuations while still allowing your system to be controllable. Too much thermal mass means a lagging system that is hard to control. Just like in a electrical circuit, you don't just put the biggest capacitor you can find, you actually need to calculate the value needed to filter the fluctuations you want to get rid of.

      Like Jon R wrote, a boiler + a water tank is a much better system design if you want thermal mass.

  2. User avatar
    Jon R | | #3

    If you want controllable, get a wood boiler, a large tank of water and thermostatic controls. Power to circulate water can be quite low (a few watts).

    Design the house to allow Winter vacations without damage.

    1. AlexPoi | | #18

      Totally agree with that. You want a quick responsive system and a controllable amount of thermal mass. A lagging system is very hard to control.

    2. Burninate | | #24

      This is the right answer. Keep the boiler in a shed out back, connected with a buried closed loop to an insulated thousand gallon tank that fluctuates between 160F and 180F. That delta represents 176MJ, or perhaps 15-30kg of wood in a 60% efficient boiler; Alternately, it equates to 49kwh of solar energy storage with resistive heaters (compare to $15k in lithium ion batteries!). Then run another closed loop through that tank and into some type of hydronic heating in the house, as well as another separate loop for your 120F household hot water (on a thermostatic mixing valve I guess?).

      Concrete and brick make for expensive, inflexible, obtrusive thermal masses. Water is cheap and can be pumped at a rate you decide, which immediately makes it far superior.

      There is an argument for water supplemented by paraffin phase-change material with a melting point inside your working temperature range, but I think this is a slightly exotic setup, and I have not done the cost tradeoff against a bigger tank of water. Possibly this is a better solution for cold-water thermal storage tanks that use day-night temperature change and forced-air radiators.

  3. Tom May | | #4

    If your stove is going to be in the middle of the house I would use the cheaper stainless steel flue. The key to even heating with a wood stove is good circulation to move the heat about the house to heat up all the mass within the house. I used to pour water directly on to the stove to add some humidity and also to see how the air (water vapor) flowed about the house depending on the location of where I placed a fan in order to ensure heat is traveling into every part of the house or those parts that needed the heat. Installing floor vents to upper rooms is also an option and fans can be used in upper floor doorways to pull the heat through the vents.
    The same principle applies using passive solar especially if sun rooms are used which gain and contain the heat. Finding ways to duct or distribute the heat evenly throughout the house.

  4. Tyler Keniston | | #6

    What other heating systems—whether 'back-up' or coincident— will you be using, if any?

    1. Rick Dee | | #9

      We are going to rough in PEX tubing into the basement slab for a future backup propane hydronic system - but that would mainly be for keeping pipes from freezing if we went away during the winter. There are also electric baseboards for shoulder seasons and to meet code but not reliable for winter heating during low sun periods.

      1. Akos | | #10

        Martin has a number of good articles on off grid living:
        https://www.greenbuildingadvisor.com/article/how-to-design-an-off-grid-house

        Generally the theme is, never rely on electricity to keep your house from freezing.

        So any setup you come up with, needs to work when the power is out. No matter how big of solar, battery or generator, eventually the power will go out and you don't want your place to freeze.

        So a hydronic propane system as a backup is a non-starter. Just the boiler itself generally consumes 20W to 70W, which quickly adds up over a week of no sun.

        Same with lot of the wood boilers that need power for the combustion blower.

        An efficient correctly sized wood stove is the way to go. Some can have a water coil and heat your DHW through thermosyphon. If you have the height and the space, it is possible to design a larger thermal storage that works the same way.

        It does mean that your tank would have to be above your stove and your rads with TRVs above the tank. Great way to heat the 2nd floor. Except for some larger pipes, you won't need any pumps or controls.

  5. Malcolm Taylor | | #11

    Rick,

    If you want to include thermal mass as part of your wood heating, the best place for it is surrounding the stove, or integrated into it. That's what soapstone and brick stoves like Kachelofen do. Extending the thermal mass as a large chimney which penetrates the roof doesn't make much sense. It just creates a huge thermal bridge.

    1. Rick Dee | | #13

      Thanks for your insight, Malcolm. We've been considering transitioning on the second floor to Class A chimney pipe to mitigate the thermal bridging problem, but factoring in the adapters and complexity to accomplish that, it's getting expensive...

  6. Nathan Bean | | #12

    Rick,

    A couple of alternatives you might consider if you wanted the visual impact of a masonry chimney and also wanted to eke out the best efficiency from it.

    First, a masonry heater is a highly-efficient fireplace that takes the thermal mass idea farther than a normal chimney by either providing a convoluted path for smoke to follow, or a "bell" the smoke collects into with a low chimney exit that encourages stratification of the hot exhaust. They normally have an insulated secondary burn chamber where combustion gases mix and burn further at higher temperatures in order to combust completely. You burn two or three fires a day, letting the masonry soak up the heat, and it then radiates into the living space. Built correctly they are safe and have lower particulate emissions and creosote deposits than wood stoves. A good resource to learn more is https://www.mha-net.org/

    Traditionally they are built by skilled masons - at least the "inner heart" as the inner part gets very hot - the intent is to completely combust the fuel to eliminate particulates and extract all the heat possible. There are also companies that now manufacture the inner heart from cast refactory cement, allowing the outer skin to be built by most masons.

    Second, a more DIY-friendly option is the batch box rocket mass heater (batch rocket) - an evolution of the rocket stove that uses a bell to capture heat from stratifying exhaust gas before it escapes the chimney and a combustion chamber with a door much like a stove or masonry heater. When well-built they also do a great job at completely combusting the fuel and reducing particulates. The site http://batchrocket.eu/en/ explains the theory and provides guidance for sizing and building.

    As with any heat source, it is important to size it appropriately to your needs. There is building code guidance for both kinds of heater, but adoption varies.

    1. Rick Dee | | #14

      Thanks Nathan. We started out interested in the masonry and rocket mass heaters, but were put off after speaking to people in our jurisdiction who had a lot of problems with building inspectors and insurance companies, which is how we ended up at this "hybrid" idea. Things change slow in the regulatory world, unfortunately... but for someone who doesn't mind either negotiating with building authorities or flying under the radar, it can be a good option.

    2. Brian Carter | | #16

      thanks for the batchrocket link!

  7. Joe Norm | | #17

    Have you looked at Blaze King stoves? They can easily have 20 hour burn times and can be operated at a very low heat, almost a smolder for lack of better description. I will be buying one for my house as I enjoy heating with wood. It's abundant and fun to split.

  8. Doug McEvers | | #20

    Superinsulation in a cold climate is US Heating Degree Days divided by 120 for ceiling R-value and 180 for walls. Couple this with an ACH50 of 1 or less and you will need about 1 Btu per square foot of living space per Degree Day to keep this house comfortable. This may help with sizing the supplemental heat.

    I recommended a masonry heater for a home SW of Denver and it does perform beautifully. This is a log home so there is some heat loss through the walls but the remainder of the house would meet the definition of superinsulation.

  9. Daniel F. Vellone | | #21

    I'm also in zone 6, Northern New York and heat my off grid home with a wood stove. Once you get familiar with the unit, and particularly if your house is super insulated, you'll find that keeping your home comfortably warm throughout the cold season will be easy. Unless you have advanced masonry skills a masonry heater most likely will not fit in your tight budget. I'd avoid building a masonry chimney with the expectations that the mass will act as a thermal reservoir. Your stove will likely require a 6 inch flue, so the masonry chimney won't be required to be very large, and even you oversized it a bit it's not likely you'll realize much benefit (particularly if you have to hire a mason). The design of your home and how well it's insulated is what will make keeping it comfortable with a woodstove easy.

  10. Josh Durston | | #22

    I'm also in Ontario and happy with my Blaze King catalytic stove (Ashford 30). It has way more turn down than the stoves my friends and family have. It also means most of the winter I only have to load it once a day. If I keep it on low I can keep the catalyst active for over the advertised 30hours on a load. It also avoids the massive blast of heat most wood stoves have after you load them. It really as nice even output over the whole burn cycle. I've had it for 3 winters now and love it more than ever.
    If it's it cooler out, I can run the stove constantly without overheating the house, and if it's warmer I'll do a short hot fire (finely split dry wood), in the morning or evening to take the edge off and then coast.
    Thermal mass seems to matter more in the shoulder seasons so you can coast thru the cool of the night or heat of the day. But once you get into serious cold or heat, then it's about managing your heat and cool loads efficiently.

    I would do a stainless flue with an outdoor air kit, and really be careful about air-sealing (while maintaining required clearances to non-combustibles). A masonry flue is hard to air seal, and is a long term maintenance liability. It's costs a fortune to get your brick chimney redone on an older house.

    1. Rick Dee | | #23

      Thanks everyone for your useful feedback! I have some reflecting to do before we decide on the best system. Definitely going to research the Blaze King catalytics some more ... I am a little concerned about keeping a good draft in the chimney in a new, airtight house with a woodstove designed to run on low. How has your experience been with that?

      1. Josh Durston | | #25

        With a low output stove you should follow some guidelines:
        -Minimum height (13-15' is what BK recommends)
        -Double wall for the black pipe section (inside exposed)
        -Insulated chimney, you want to keep the flue gases hot to ensure a good draft and to keep things clean.
        -I crack a window during startup or if I open the door when it's not hot to ensure neutral space pressure, but once things are hot it seems pretty tolerant of the various fans in my house. If you open the door with a lot of fans running you might get smoke rollout.

        Check out hearth.com (specifically the giant BK threads), but a warning Blaze Kings are a religion over there which seems to polarize people's attitudes. There are lots of good stoves, but few that run as low as a Blaze king or as efficient.

        If your house is tight I'd recommend an outdoor air kit. You want to avoid having a negatively pressure house but for me it has been an issue so far. I'm working towards switching from a powervent NG hot water tank to a heatpump, and a heatpump drier instead of NG, and will install a balanced HRV/ERV instead of exhaust only bath fans.

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