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Community and Q&A

Thermosiphon to Prevent Freezing

Nick_NH | Posted in Mechanicals on

I plan to build a detached garage with a guest apartment above. The apartment will be occupied ~90 days per year. I live in CZ 6 with a 99% heating temp of negative-18 F. 

I plan to piggyback the plumbing supply lines from the main house. When the guest apartment is occupied in winter, the supply lines will be exposed to outdoor air temps when passing vertically through the (unheated) first floor of the garage. 


My hope is to (1) prevent frozen pipes with as little complexity/maintenance as possible and (2) reduce hot water wait times in the guest apartment. The run from the electric tank-type water heater to the guest apartment will be about 50 feet (35′ horizontal, 15′ vertical). 

Does it make any sense to create a passive thermosiphon loop in the hot supply line? Where the hot and cold supply lines pass vertically though the unheated garage, I will bundle the lines together within a well-insulated chase.

When the apartment is occupied, the siphon of hot water will warm the inside of the chase. When the apartment is unoccupied, I will drain all of the lines. During warm weather, I could turn on the supply lines but close the return hot water line, reducing wasted energy from the siphon (though increasing wait times for hot water).

Workable? Useless? Just use heat tape? Thoughts appreciated.


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  1. Expert Member
    BILL WICHERS | | #1

    Don't do this.

    You don't want to run plumbing lines in unconditioned spaces. Not only can you have freeze/burst issues with the supply lines, but you can have icing form within the drain line until the drain line is eventually blocked. You should really plan things so that the the pipes can all be run through an interior wall, or at least a condioned wall that you can box over. By "box over", I mean bump out the insulation so that the building envelope "bumps out" around the pipes. You would remove a bit of existing insulation, then put new insulation in the bump out so that the pipes are effectively in the interior "side" of the wall.

    If you have no way to do any of this, you'd need to run heat tapes on everything. This is not an ideal solution though, and you risk problems if the heat tapes fail, which they almost inevitably will. If you have to use heat tapes, the only ones I trust are the self regulating ones made by Raychem, which are pretty expensive.


    1. Nick_NH | | #2

      Thanks Bill! I’m still early in the design phase so your idea is definitely doable.

  2. onslow | | #3

    Zephyr mentioned drain lines but don't forget the toilet or sink traps. Toilets crack apart quite readily if left to freeze. Same for sink traps, some tenants taught me this. Heat tapes can fail spectacularly and might very well not please any insurers. Do you not face any codes where you live? Seems like the whole idea would get you flagged at the permit level.

  3. Expert Member


    I'd suggest one of the following approaches:

    - Design a conditioned entry stair to the apartment above, and maybe include a vestibule at grade too. Architecturally it will make the space a lot more appealing, and provide somewhere to get services up to the second level. Here is a link to a much larger version just to give you some ideas:

    - Build an insulated core, much like they did with this house on piers. because the distance is longer, I'd make the inner sleeve larger to allow more air around the waterline:

    In neither case would I try and run off the water heater in the house. The losses will be huge, and the wait times lengthy.

  4. Expert Member
    NICK KEENAN | | #5

    Can you heat the garage when the plumbing is in use? Even to slightly above freezing would work. And then insulate it enough to it doesn't take a lot of energy? Or can you divide the garage in half and have half of it heated and insulated and half of if not?

    Also, you say the garage is detached. How does the plumbing get from the main house to the garage?

  5. tommay | | #6

    If you can, forget about running a hot water line. Get yourself a small 120 V electric hot water heater instead. Then you only have one water line to deal with that can be "insulated". Make all your water line runs within the apartment walls and keep them all close together eg, back to back. Insulate traps in the floor (shower/tub).
    Since it's a new build and detached, use the normal black, plastic, water service pipe to run a line underground to the garage. That pipe can be brought straight up into the apartment. It's much more durable and easily fixed and tapped into should you want some water in the garage, as well as draining/winterizing.
    What are you using for heat in the apartment? Could you use part of that to keep the garage above freezing when necessary or send heat to the pipe? Bring the pipe up in a sunny spot?.....

  6. Nick_NH | | #7

    Thank you all for the thoughtful replies. I'll try to answer a couple of questions.

    I had been planning to trench both hot and cold lines from the main house to the garage/apartment. A separate water heater in the apartment seems smart since my thermosiphon idea is sounding like a no-go.

    Unfortunately there is no inspection of residential construction in my town. This led to interesting defects (since remedied) when we bought our house.

    The garage apartment will be about 500sf of conditioned space. I plan to insulate the 12" TJI floor to R40, walls to R40, ceiling to R60, and use electric resistance heat. I was not planning to heat the first floor garage.

    The only plumbing fixtures will be in the bathroom. I hoped to keep the bathroom heated to, say, 50F all winter, and bring the whole space up to 68F for a few days at a time when guests visit.

    The idea of a heated entry and stairway to the apartment seems like an elegant solution. My other thought was a small utility closet on the 1st floor that would be heated and allow for supply and drain lines to stay warm.

    Thanks again for the insights!

    1. tommay | | #10

      If there is no inspection, then you are all set....think about putting in a kitchen sink , back to back to the bathroom won't regret it...drain lines have no water in them...
      They sell small propane, non vented heaters that may work for what you need. Some look like fireplaces mounted on a wall like a tv....

      1. Expert Member
        BILL WICHERS | | #11

        What happens with "frozen" drain lines is that the water freezes as it goes down the drain. Water in a drain travels as a sheet of water on the inner surface of the pipe, and that thin sheet freezes more easily. Once it freezes, the inner diameter of the pipe is slightly released. If the pipe stays cold enough, successive use of the drain will gradually build up the ice on the inner wall of the pipe until the pipe is eventually completely obstructed by ice -- then you have a problem! It's a different problem from the bursting issue you have with the supply lines, but it's still a Bad Thing if it happens.


  7. Expert Member
    NICK KEENAN | | #8

    I like the utility closet. Trench the cold water and sewer and have them come up in the utility closet. Put the water heater in there as well.

    Be very careful of any traps in the floor of the second floor. If they freeze and burst they can make a big mess. They can also just crack and leak and make a tiny mess that goes undetected until it becomes a big mess. With the shower in particular it's hard to position it between the i-joists and get meaningful insulation below it. You can protect a trap when it's not in use just by pouring a pint of RV antifreeze into it but you're going to need protection even when it's in use.

  8. johngfc | | #9

    I suggest you consider the wait for hot water in that bathroom. Assuming you can get away with 1/2" pipe, it'll likely take more than 30 seconds to clear the line. You might consider an on-demand recirc pump or a small (1 or 2 gal) undersink electric heater. The recirc will return house-temperature water back to your HWH, rather than dumping it down the drain and replacing it with (cold) ground-temperature water in your heater. The small heater would provide instant hot water and temper that cold burst before delivering it at the facet.

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