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Thermal bridging of dryer duct?

calum_wilde | Posted in General Questions on

I’ve had my heat pump dryer for almost a year now.  It’s working really well and I’m satisfied with it.  Last year when I installed it I stuffed the vent hood with fiberglass insulation to slow the air flow and heat loss. Now I’m curious, after reading articles here about the thermal bridging of steel studs, how much heat am I losing through the 4″ steel duct.

For that matter, has anyone considered how much heat is lost through frost free hose valves?

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  1. Expert Member


    Dryer outlets, vent stacks, door knobs, hose-bibs. My intuition is that they all add up to less than the price of a coffee at Tims once a year, but I don't have any evidence to back that up.

    1. calum_wilde | | #2

      In a game where it's not about finding one item that's costing you 200/month, it's about finding 200 items that are costing you $1/month, my intuition tells me they a little less trivial than that. Maybe not a lot more, and certainly not worth paying a pro to change them, but as a diyer with time on my hands, it might be worth it. I've reduced my homes power consumption from ~25000kWh/year to ~9000kWh/year with this approach.

      1. Expert Member
        MALCOLM TAYLOR | | #6

        I see the appeal in a broader sense. Reducing standby loads, air-sealing and going after the myriad of small savings makes some difference. Where I'm not sure it is worth while is in well-built new construction, where the difference between a house with insulation under several interior footings, a plastic hose-bib and some attention to the vent-outlets doesn't represent anything you would notice.

        I'm also not sure I can buy your math on this one. Extrapolating from your calculations, a house with four hose-bibs would be burning through an extra 600 kWh/ a year? That just doesn't make sense.

  2. calum_wilde | | #3

    Well, ok. I used some google Fu and I think I came up with a reasonably accurate answer for the hose bib. With a wall thickness of 35cm (ICF), brass hose bib with a thermal conductivity of 144 W/k, wall area of 0.29cm^2, and a temperature difference of 30°C, it looks like it's about 35W. Including the air inside the pipe at 25W/k, that brings it to about 46W.

    Does that math seem right?

    Taking things a few steps further that's in the realm of 150kWh/year here, or about half a panels worth of PV. At roughly $900/panel installed I'm gonna go ahead and say replacing the hose bib with a non frost free unit with pex pipe - which I expect to have vastly less thermal conductivity - would be worth the effort.

    1. Jon_R | | #4

      You need to include air film R value (which makes a big difference). Even so, you are way off. For example:

      2.9E-5*144*30 = .13W-m (maybe .35W), not 35W.

    2. Expert Member
      BILL WICHERS | | #5

      I suspect your units are off, probably by at least a decimal point in the “too big” direction. Your number sounds awfully high. I just calculated this for nails through rigid foam fir another Q and A here.

      You also need to account for the length of the pipe, longer pipe shows less conducted energy.

      As an example, my nail conducted heat calculation was somewhere around 20 BTU/hr for 32 x1/8” diameter nails. That’s about 0.4 square inches, about 2.5 square centimeters. 20 BTU/Hr is about 5.9 Watt*hr, or 4.25kwh per month. Not much.

      Insulate a few feet of the indoor portion of the pipe and you’ll drop the heat loss more.


  3. canada_deck | | #7

    Do you live somewhere where it ever freezes?

  4. Deleted | | #8


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