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Thread: Separation between cold water line and heating ductwork

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    DIY Senior Member DavidTu's Avatar
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    Default Separation between cold water line and heating ductwork

    The heating supply trunk line on a 38000 btu furnace runs down center of house. How close can we run cold water trunk (1" pex) as it must run parallel to it? Obviously we don't want to heat our cold supply.

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    Moderator & Master Plumber hj's Avatar
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    If the heating duct is going to heat the cold water supply at any distance away, then someone forgot to insulate it. But, then, a 38,000 btu furnace is awfully small anyway.
    Licensed residential and commercial plumber

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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by hj View Post
    If the heating duct is going to heat the cold water supply at any distance away, then someone forgot to insulate it.
    I wish water lines and ducts were insulated as they should be...but in the past insulation on such things has been surprisingly rare. It's not uncommon on new home builds to find whole sections of the attic, etc. that were supposed to be insulated, but weren't.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by DavidTu View Post
    The heating supply trunk line on a 38000 btu furnace runs down center of house. How close can we run cold water trunk (1" pex) as it must run parallel to it? Obviously we don't want to heat our cold supply.
    If you have access to it now, if it isn't insulated, insulate the ductwork and make sure you've sealed all of the seams so it doesn't leak. you can do the same with the water lines. Generally, that is enough. But, insulation just slows heat transfer, won't stop it.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the Trades liquidplumber's Avatar
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    I dont think anyone actually answered your question. you could run that waterline INSIDE the ductwork if you wanted to. theres no code stopping you from running it next to it, inside it, 2, 10, or 20 feet away from it. As far as heat transfer from the duct work to the cold water, I cant imagine that unless you actually did run the line inside the duct that much if any heat would transfer to the water. And if it did transfer some small amount of heat.... what problem would that be anyway? I think perhaps you are over thinking on this. I cant see your specific situation, but i just cant even imagine how this could be a problem worth worrying about.

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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by liquidplumber View Post
    As far as heat transfer from the duct work to the cold water, I cant imagine that unless you actually did run the line inside the duct that much if any heat would transfer to the water.
    You need to expand your imagination then. Some of my water lines sit alongside the ductwork and the temperature impact on that water from the ductwork is obvious. If I could reach them without tearing out the sheet rock they would all be insulated...and sealed. The whole cavity is warmed or cooled and that includes the water in the lines. Makes for a toasty floor in the rooms above in winter...which is nice...but it also means that the air reaching wall registers is colder than it should be and the core of the house is warmer.

    What is far more problematic is to have supply and return air running side-by-side touching one another with no insulation between. We call that a heat exchanger, and yes, my system has that problem as well. Of course the idiots that ran the ducts also didn't seal the seams. Although the build was for the previous owner I talked with the co-owner of the business once, feeling out their knowledge: idiocy confirmed. When I replaced the AC unit, the new installers were in stitches over the humidifier configuration.

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    Moderator & Master Plumber hj's Avatar
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    quote; the new installers were in stitches over the humidifier configuration.

    That is not unusual. Often when a worker sees something that is not done the way he would do it, he will make some comment about how it was done.
    Licensed residential and commercial plumber

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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by hj View Post
    quote; the new installers were in stitches over the humidifier configuration.

    That is not unusual. Often when a worker sees something that is not done the way he would do it, he will make some comment about how it was done.
    True, but it was both ugly/overly convoluted/bulky and performing poorly. The new install re-oriented the existing humidifier and the humidistat. The new orientation makes fuller use of the pad (an astute observation on the installer's part) got rid of at least six feet of contorted metal ductwork (mostly swivel elbows) and a stupid looking hand fabricated metal box hanger on the wall. The difference in performance is night and day. An added bonus is that it opened up several square feet of floor space in the utility room.

    But the real icing on the cake is that it didn't cost me a dime. The rework was part of the new HVAC install and made use of the existing humidifier and humidistat.

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