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Log home heat loss

Northernbuilt | Posted in Mechanicals on

I am currently working on a project that the homeowner has asked me to help size a replacement furnace for a log home. Currently the home is heated by an outdoor woodstove, which used 15 cord last year. The outdoor stove heats water which moves to a water to air exchanger in an LP gas forced air furnace. The gas is used only as emergency heat. I’m planning on conducting a blower door test. (Last log home blower door test I conducted was 19 ACH50!) There is some roof repair that needs to be completed, should be able to figure out the R-value in the roof at that time. I believe the roof is insulated with 4 inches of polyiso, 1980’s vintage. My understanding is that older polyiso looses it’s R-value over time. The walls are 10″ to 16″ hand scribed logs.

Question is in calculating R-value of mass log wall. Should I use an average log diameter or is there some other way in calculating log R-value?

Home is located in zone 7. Homeowner is hoping to use a storage electric furnace on an off-peak electric program but furnace will only store 800,000 BTU’s. I have a feeling that’s not enough for this home. May suggest a cold weather heat pump with modulating LP gas forced air furnace. No natural gas available.

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  1. GBA Editor
    Martin Holladay | | #1

    My advice to the homeowner: "Sell the home as soon as you can, and buy a different home."

    Seasoned hardwood has a fuel value of 21 to 26 million Btus/cord. If we call it 21 million, then 15 cords = 315,000,000 BTU.

    If we assume that the outdoor wood-fired boiler has an efficiency of 50%, this is equivalent to 1,512 gallons of fuel oil burned in a furnace or boiler with an efficiency of 75%.

    Q. "In calculating R-value of mass log wall, should I use an average log diameter or is there some other way in calculating log R-value?"

    A. Using an R-value based on the average log diameter is as good a method as any. If the average diameter is 13 inches, the R-value is about R-16. But as you already know, the heat loss in this type of home is dominated by air leakage.

  2. Expert Member
    Dana Dorsett | | #2

    HVAC terminology: If it's heating water with wood it's a wood BOILER, not a wood STOVE. Similarly if it's heating water with gas or electricity, it's a gas or electric BOILER, not FURNACE. If it's heating ducted air directly with an exhaust gas to air heat exchanger (and not through a hydronic coil in an air handler) it's a furnace.

    Without knowing the real numbers for this location, let's assume the 99% outside design temperature is -15F, or 80F degrees below a presumptive 65F heating/cooling balance point), and the season was 10,000 heating degree days (base 65F). Martin's calculated 315,000,000 BTU over 10,000 HDD is then 31,500 BTU per HDD.

    In a 24 hour day that becomes 31,500 BTU/24= 1,312 BTU per degree-hour.

    With a design temperature difference of 80F from the balance point that's an implied load of 80F x 1,312= ~105,000 BTU/hr at -15F outdoors. So 800,000 BTU stored in an electrically heated buffer tank/boiler would get you through nearly 8 hours of -15F weather, and probably enough.

    ASHRAE recommends no more than 1.4x oversizing, so that would call for a furnace or boiler with an output between 105- 147,000 BTU/hr.

    The 75% combustion efficiency estimated may also be quite a bit higher than the as-used system efficiency, since the boiler is outside of conditioned space. It's unlikely that an outdoor wood boiler would have a system efficiency as high as 75%, and most would not be below 50% (which would mean 33% standby & distribution losses from a 75% combustion efficiency boiler.) The real number is somewhere in the middle. If one assumed a 50% system efficiency after standby & distribution losses the load would be more like 70,000 BTU/hr, not 105K, and the backup furnace of no more than ~100,000 BTU/hr out.

    Look up the actual 99% outside design temp and use nearby weather station data on to determine the actual heating season HDD over which the 15 cords were burned, and run the same napkin-math on the better numbers, but use 75% and 50% efficiency to bracket the likely load. With these stakes in the ground you can sanity-check any other calculation methods.

    For other clues, what is the BTU output of the existing propane gas furnace?

    What are the BTU ratings of the hydronic coil in the air handler at 180F or 140F (or both) at a few gpm?

    The 1980s vintage polyiso blown with ozone-depleting CFCs does NOT lose R-value over time or temperature in the same way that HFC or pentane-blown foams do. Figure on R6-R7/inch, but in some instance it can be even better than that.

    R-value isn't everything, especially when factoring in the substantial thermal mass of 10-16" logs. The particulars vary by species, but there is even a phase-change "apparent thermal mass" in logs that can more than double the more simple thermal mass calculations with heavy log structures, and give it a peak-loading performance substantially better than would be implied by the steady-state R values. While the thermal mass effects aren't going to affect the average energy use by very much in a zone 7 climate, it can substantially reduce the peak load that the furnace or boiler has to cover. Using the simple R-value will overstate the conducted wall losses.

  3. GBA Editor
    Martin Holladay | | #3

    315,000,000 BTU is the amount of fuel burned, not the delivered heat.

    I guessed that the wood boiler system had a system efficiency of 50%, but it might have been higher.

    Delivered heat might be in the range of 157,000,000 to 189,000,000 BTU.

    You misread my efficiency assumptions. I assumed that the outdoor wood-fired boiler had a system efficiency of 50%, while a hypothetical oil-fired boiler (brought in for comparison purposes) might have a system efficiency of 75%.

  4. Jon_R | | #4

    With a hydronic system, consider Chiltrix heat pump(s) for some or most of the load. You can add thermal storage with water tanks.

  5. Expert Member
    Dana Dorsett | | #5

    DOH! Martin, that's right, it's the source fuel heat. (I should be more careful!)

    So the highest possible implied load example (75% combustion efficiency wood-burner) would be 0.75 x 105,000 (source fuel BTU/hr) = ~79 KBTU/hr .


    ....and at the low end (50% system efficiency) 0.50 x 105,000 = ~53 KBTU/hr

    My use of 75% combustion efficiency for the wood boiler might confusing, since it was the same number Martin used for an oil-burner's SYSTEM efficiency.

    Using the raw combustion efficiency of the wood boiler (estimated by me to be ~75%, manufacturers' claims of something a bit higher notwithstanding) establishes a fairly firm upper bound- there is no way the system could be MORE efficient than that even if the wood boiler was installed inside of conditioned space.

    Using 50% system efficiency for the lower bound is a bit squishier. It could perform quite a bit worse than that in a worst-case, but if installed with a bit of forethought I would expect most systems to beat that. The IBR capacity specs on oil or gas fired high temp cast iron boilers installed outside of conditioned space imply only ~15% system & distribution losses, not the 33% losses that would be necessary for system efficiency to be as low as 50%. Lots of factors will affect those system loss numbers for a wood boiler (or indeed a cast iron fossil burner), but a 33% standby & distribution loss seems like it would be lossier than most.

    Using just a presumptive 50% efficiency and upsizing by 1.4x for this exercise risks under-sizing the furnace by a bit, since the implied load is lower. (It's a small risk if the upsizing is actually 1.4x, but real if only upsizing by 1.2x)

    Using the implied load derived 75% and upsizing by the full 1.4x could risk oversizing the furnace by as much as ~2x, (more, if the wood burner's system efficiency is less than 50%). But for hot-air furnace, if the backup is truly a furnace, that's not a disaster the way it would be for a high mass boiler. The term " electric furnace ..." makes me think it's something else though.

    Having calculated both numbers puts some stakes in the ground to establish the range of the heat loss, but if the " electric furnace ..." is going to be installed outside of conditioned space there are some size adjustments to be made. (There could be some dialect differences adding to the confusion here.)

  6. Northernbuilt | | #6

    Ok, got back to the log home to do some testing and gather a little more information. The home is 1767 square feet plus a full basement with walk-out. 8/12 vaulted roof. The blower door test results are 4.39 ACH50, 1.24 CFM50/ft2. By far the best log home blower door test I've conducted. I'm sure the blower door number would be worse in the winter, we've had a couple months of humidity to swell the logs a little. The walk-out basement has an un-insulated wood 7 x 7 garage door (needs to be changed). Lots of air leaking around this door. The home also has a masonry fireplace, damper was closed but still was moving air down the chimney. The rim joist is un-insulated, going to suggest closed cell spray foam for this space. The roof insulation is not as good as I was hoping. From the inside to the out, 1 1/2 inch T & G paneling, tar paper attached to the bottom of the 2 x 6 roof framing, (I can see the tar paper from an outside vent I removed. Not sure how far the tar paper extends into the home. Common for the ceiling to be polyed in my area.) 2 inch poly iso and R-11 fiberglass. No air vent chutes present. There are 4 gable ends in the roof with about 20 feet of vented eave. Very poor air movement, if any through most of the roof. No water damage present in ceiling. (Either the ceiling is well sealed or that tar paper is helping move liquid moisture out the eaves.) Plywood and asphalt shingles finish the roof. Window are mostly 1980's double pane casements with some fixed glass. Probably U-.32 at best.

    Existing LP gas forced air furnace is rated at 110,500 BTU and is 95% effecient. Not sure if the rating is for Natural gas or LP. I also found out the approximately 15 cords of firewood that was burned last winter was Tamarack. I could only find a BTU listing for western Larch (Tamarack), 28.7 million BTU's per cord. The outdoor wood boiler is a Central Boiler Classic CL-6048 manufactured in 2013. I can't find an efficiency rating for the unit, but would estimate the efficiency at around 60% .

    My heat loss software kicked out 103,000 BTU heat loss. Design temp is -21 and heating degree days are often over 10,000/year, we did have 186 cooling degree days last year!

    Dana- the owner wanted a Steffes (manufactured in North Dakota) electric storage forced air furnace. Typically, these units are controlled by the local rural electric coop, and are only "charged" for a few hours over night. Current electric rate is $.049 per kWh. The 800,000 BTU's need to last 18 to 20 hours. There is no water with these units, heat is stored in bricks within the unit and the furnace is installed within the thermal envelope.

    I'm thinking of steering her towards a modulating LP gas forced air system with a cold weather ASHP with smart controls to set-back the temperature remotely. The home is often vacant for weeks at a time. Any other suggestions?

  7. Jon_R | | #7

    I'd calculate up-front cost and operating cost for each option. Include the 250A feed needed for the Steffes furnace.

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