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Impact of Wind on Heat Loss

paul_wiedefeld | Posted in General Questions on

Dana Dorsett’s heat loss calculation using fuel usage and heating degree days seems simple and highly accurate for existing homes. However, I’ve seen comments that posit particularly windy days cause high levels of infiltration and high heat loss, which seems intuitive to me and I’d like to further understand.  I’ve attached regressions of a 1930’s house in Zone 4: Daily therms over 2 years are extremely well (R^2 = .94) predicted by knowing the heating degree days. Adding average wind speed adds a little of accuracy, but ~3,000 BTUs per wind MPH over the course of a whole day seems minor. My questions are: are there data or studies that show that wind speed matters to heat loss? If so, under what conditions does it matter? What impacts the magnitude?

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  1. tommay | | #1

    The faster a fluid moves, the lower the pressure. It's this difference in pressure that creates airflow as the two pressures want to balance out and be equal. Same principle as temperature differences.

  2. Expert Member
    BILL WICHERS | | #2

    Air flow also carries heat away faster. This is why fans are used to cool heatsinks in electronic components. You WILL see more heat loss on windy days, exactly how much depends on your levels of air sealing and insulation. I would expect large expanses of glass to show more change with different wind speeds too, since the air film insulating layer becomes larger as a total percentage of the R value of something like a window that is relatively low R value to begin with (compared to a typical wall).


  3. joshdurston | | #3

    In my house the windows are the most affected by wind speed, specifically for some of the sliding ones that have significant leakage along the tracks. The rooms with good windows don't seem to be affected nearly as much by the wind and my bedrooms where you can feel the cold air pouring in when it's windy.

    I think Dana has recommended 1.2-1.4x sizing from the consumption calcs (for high wind/recovery). The base heat load isn't the magic number for sizing.
    The commercial heat loss calc tools have this built in and is part of why they always seem to be 25%+ high.
    The key thing is to not keep stacking sizing factors where your sizing 1.4x 1.4x etc. If you multiply by 1.4x and give that number to your HVAC guy and he'll likely go up to the next size "to be safe", and you'll quickly go from 38kbtu to 80kbtu sizing if you're not careful.

    With modulating equipment sizing can sometimes by approached differently, 1:1 mini splits may benefit from a bit more oversizing.
    With multi-splits I wouldn't be surprised if the opposite is true, you may be better off to size tightly (or even assume some diversity factor and undersize the outdoor unit, you can often put 120%+ indoor capacity on a outdoor knowing your max totaly output will be limited to the outdoor units capability) and strategically install 25% electric heat (bathroom floors/baseboard/duct heater) in to get your safety factor. I say this because short cycling appears to be a real problem with poorly selected multi-splits.

    With modcon boilers you want to make sure your min fire rate is low enough to not excessively short cycle, sometimes the next size bigger boiler has the same min fire (Lochinvar 50 and 80kbtu boilers). So going up to the 80,000btu/hr boiler isn't really oversizing if the min fire is the same 8000btu/hr as the 50,000btu/hr boiler. And you can often limit the max fire rate.

    I'm also a big advocate of a heterogenous heating system for robustness and resiliency. I would rather have tightly sized equipment with some supplemental heat from a difference fuel source. You just need to be careful with your control strategy to ensure you don't accidentally rely on expense electrical heat for the heavy lifting. (I went a little overboard on this, hot water tank boiler + modern BlazeKing wood stove + cold climate minisplit)

    1. paul_wiedefeld | | #13

      You're right Josh, it's a bit academic considering how equipment is sized and how low some can modulate. I think the benefit is exactly what you say: keeping it 1.2-1.4x oversized not 2x to 3x (my old furnace was 4x), and if you want to spend more on capacity, consider using a different source entirely and give yourself redundancy.
      I think you should fit in a wood gasification boiler for the next round of heating!

  4. Jon_R | | #4

    Here is my best linear fit that includes wind speed:

    btu used = deltaT * 675.00 - 100 (internal) + deltaT * 6.00 * wind
    (degrees F and mph)

    Not accounting for wind could cause a 20% error.

    Solar gain causes a much larger error (so I only look a nighttime data). Switching on the kitchen exhaust fan causes an even larger error. Internal gains vary a little.

    Keep in mind that sustained periods of less than design temperature are common - so don't size equipment for design day (but Manual J accounts for this). And account for other unusual events. +25% might be reasonable.

    1. paul_wiedefeld | | #6

      This helps! I'll give that a shot. 6 is just from experience? Understood that design day means coldest 1%, there will be some oversize here.

      1. Jon_R | | #7

        6 is calculated from a large amount of my hourly furnace burner on-time data. Every house will be different.

        1. paul_wiedefeld | | #11

          Was this a regression? How's the fit?

  5. paul_wiedefeld | | #5

    Thanks, Tom and Bill. Agreed that wind causes heat loss, it makes sense that windows would be most impacted. I'm more interested in if it matters in this quick heat loss calculation application that slots in between a sqft-based guess and a manual J. Is there a study showing wind speed makes a significant impact based on existing houses? This is a leaky, uninsulated house and the room with windows on 3 sides has a thermostat (it's the only room not on the main zone) yet a day with 20 MPH wind seems to add maybe 3% onto design day heat loss.

    1. Expert Member
      BILL WICHERS | | #8

      For an accurate analysis, you need some measure of wind speeds. For temperatures, you'd use "degree days", but you need something like "windspeed days" here, and I'm not sure such data exists, at least in a form suitable for simulations like this. The best you might be able to do is to get a long-term average wind speed for your area. I know, for example, that many of the suburbs around Denver have a lot of windy days, areas in the woods of Michigan are very calm by comparison.

      Some historical weather data archives might be a good starting point for some basic info, but I'm not sure how you'd go about normalizing your heat loss to allow useful predictions based on wind data. You would probably need to take some actual measurements to see how much wind speed affects the numbers, then work out some scale factors from that.


      1. paul_wiedefeld | | #10

        Yeah NOAA gives daily average (highest daily average speed was 20 MPH) and hourly speeds, but it's so far been a bit of a bust - everything points to wind having an impact except the actual meter. I think another hurdle comes from gas usage not exactly equaling heat loss if the main thermostat room isn't feeling the heat loss as much as the second floor.

  6. jackofalltrades777 | | #9

    Casements and Tilt & Turns work better for air sealing than sliders or hungs. My old house had sliders & hungs and they would whistle in the high winds :(

    Triple pane windows would provide better results than double pane?

    1. Expert Member
      BILL WICHERS | | #12

      >"Triple pane windows would provide better results than double pane?"

      Yes, they would -- but they still won't perform as well as the walls.


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