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Community and Q&A

Carapace Heat-Line freeze protection for off-grid water lines

Scott Wilson | Posted in Energy Efficiency and Durability on

To answer Malcolm’s question (in my other thread about multipoint foundation systems) about how to go about freeze protecting the water and sewer pipes that will be servicing my off grid cabin I think the solution is the “Carapace Heating Water Pipe”.

The cabin will be off the ground by about 3 feet and the potable water supply pipe (as well as the greywater pipe) will be exposed to freezing temperatures from the underside of the cabin to the ground (and possibly below). The pipe uses 3 watts per foot to operate and be used with a thermostat so that the heat tape is not on continuously.

From their website:

To further increase the energy efficiency of your CARAPACE system, it is recommended to add thermal insulation and a thermostat. The additional thermal insulation works to reduce heat loss, and the thermostat allows you to duty cycle your CARAPACE system based on external pipe temperature. Both accessories combined can increase energy efficiency by as much as 80%. In applications where pipes will be exposed to atmospheric (free) air, closed-cell or other approved weatherproof insulation is required.

Heat-Line recommends to use insulation that is best suited to the environment in which it is going to be installed. A common type of insulation employed with CARAPACE heating cables is water proof pipe foam insulation sleeves. You are also able to implement fiberglass insulation provided it is properly weatherproofed based on the environment.

Electrical Specifications
•Direct earth burial and wet location certified
•120V systems – 3 watt/ft (9w/m) at 50°F (10°C) maximum length of 240 ft. (73m)
•120V systems – 5 watt/ft (15w/m) at 50°F (10°C) maximum length of 240 ft. (73m)
•240V systems – 3 watt/ft (9w/m) at 50°F (10°C) maximum length 660 ft. (201m)
•240V systems – 5 watt/ft (15w/m) at 50°F (10°C) maximum length 540 ft. (165m)

I expect that I will also build an insulated chase around the pipe from the underside of the cabin to below the ground surface.

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  1. Jon R | | #1

    Figure R20 for the insulation, But this depends greatly on the chase size and the weather.

  2. GBA Editor
    Martin Holladay | | #2

    I don't understand the specifications. What does "at 50°F" mean? Is that the ambient outdoor temperature (air temperature or soil temperature)? If so, it's confusing. No one needs freeze protection at 50°F.

    You need freeze protection at 0°F. What's the watt draw under those circumstances?

    If you need to protect 10 feet of pipe, it will obviously draw 30 watts if you assume 3 watts per foot, or it will draw 50 watts if you assume 5 watts per foot.

    For a 24-hour day, that's either 720 watt-hours or 1200 watt-hours -- a significant load for an off-grid house, but one that can be planned for. The tricky part happens if you are on vacation in January, and no one is home to start the gasoline-powered generator to charge the batteries.

    -- Martin Holladay

  3. Scott Wilson | | #3

    Martin, the temperature inside the pipe is kept at 50 degrees F. Sensors.measure the temperature of the fluid inside the pipe and adjust the amount of heat needed. The heating tape is also designed so that it "micro-adjusts" along it's length and sends the precise amount of heat to a specific point rather than keeping the entire length of pipe at a set temperature.

    As well, by using a thermostat and pipe insulation you only have the heat on when it's needed. Their tests show that you can reduce the heat required by as much as 80% over a system that uses a conventional heat tape that's on all the time. I think this system is probably the most practical and energy efficient means of ensuring that your pipes don't freeze.

    I suppose you could add a device to your generator that starts it automatically when the charge drops too low.

  4. Pierre Donner | | #4

    For several years I had a situation similar to the one you describe.

    -Off grid home
    -House above grade with no basement
    -Self regulating electric heat line to keep the water line from freezing

    The previous owner had built a very substantial insulated chase for the pipe using foam insulation board and spray foam. In my case, the heat line was inserted in a standard PE water pipe instead of being a part of the pipe construction.

    The "self regulating" heat line technology is not very self regulating. The power does vary with temperature; however the min to max variation is not very impressive. Even at 70 degrees, power consumption is still very high. A thermostat is recommended but they gloss over the difficulty of placing a thermostat such that you can accurately monitor the coldest point of the pipe. If you have good insulation on the pipe, outdoor temperature is not a very good indicator of the pipe temperature.

    If you are off grid, resistance heat is a problem. When we were off grid, we were not comfortable leaving the heat line on while we were away for any length of time. If you supersize your system, you might be OK but otherwise I would be wary.

    There is a basic problem with most commercial heat lines that I have seen. They are way overpowered for any situation where there is substantial pipe insulation. A heat line that can keep a pipe from freezing with R2 insulation on a really cold day is a fire hazard with R20 insulation. Self regulating lines help with this situation but not enough.

    If the insulation is good (say R20 or more), then even moderate water usage is enough to keep the pipes from freezing. If we draw some water once every 8 hours, we do not need the heat line to prevent freezing even when it is -20F outside.

  5. GBA Editor
    Martin Holladay | | #5

    Pierre gave you good advice. If your house is off-grid, you really don't want a solution that requires electric-resistance cables. (I've lived in an off-grid house for 42 years.)

    Remember, when the weather is cold, the sun doesn't shine very much, so you won't have much electricity.

    You really need a crawlspace or a basement for an off-grid house in a cold climate.

    -- Martin Holladay

  6. Jon R | | #6

    If you want minimal power draw, create a loop in the water supply line, put a small radiator at the top and add a very small pump (say 2 watts). Ie, move building heat down into the chase. As a backup, use PEX for it's ability to tolerate freezing.

  7. Charlie Sullivan | | #7

    Are you keeping the building heated continuously? You could make a conditioned chase, sort of like a conditioned crawl space, but only 1 square foot, to bring the water lines from the house to down below frost line. Jon's idea is the make a hyronic system to bring building heat into the pipe. In my scheme, you could use a 2 W fan to bring building heat into the chase, although it would probably work to simply have heat flow down through an uninsulated floor and up from the insulated earth below frost line to keep the chase above freezing.

  8. Scott Wilson | | #8

    All the utilities will be in a workshop/barn building that is about 10 feet away from the cabin (solar, hot water, electrical, etc). The issue is getting the water from the barn to the cabin (which could be done underground ) so the only problem will be the pipe rising 3 feet out of the ground to the underside of the cabin. The floor of the cabin is made out of 12 1/4 " thick SIP panels (with an 8" false floor on top of that to run the pipes) resting on top of a multi-point space frame foundation (an open web of structural metal beams).

    I can't get a concrete truck in there and I don't want to hand dig and pour concrete piers so this foundation system seems to be the best. I suppose I can build a SIP chase from the underside of the floor panels to below the frost line and then run a pex water pipe inside a larger plastic pipe and then spray foam the interior of the chase as well.

    I do plan on having a small Hobbit wood stove inside the cabin and a solar hot water heater on the barn roof. There will be a backup Empire propane heater in the cabin and a propane cook stove. For backup hot water in the winter I suppose I'll have to use propane as well although I'd like to find a small wood fired hot water heater.

    Martin, what's the best way to dispose of greywater? There's a composting toilet so there will be no blackwater waste.

  9. Expert Member
    Malcolm Taylor | | #9

    I've used chases (or mini-crawlspaces) like Charlie describes, but a bit larger at around 3'x3'. The problem you may find with your limited floorspace is where you can afford to leave an open area to keep the chase continuous with the heated area above. I situated it under a water heater with a metal grill as the floor. You don't want to fill the chase with insulation. That defeats the purpose of using the cabin's heat to keep it warm. An insulated chase will eventually freeze.

    I'm not clear on what the advantages of having the utilities in a separate building are. Wouldn't it make more sense to increase the area of the cabin enough to accommodate them when you have to build the same amount of space anyway in a separate structure? And that structure now needs to be heated to prevent freezing too.

    Before worrying about the best way to treat grey water, it's probably a good idea to see what the municipal authorities in Nelson allow.

  10. GBA Editor
    Martin Holladay | | #10

    I've built mini crawl spaces (and mini basements) with CMUs and mortar. You can move those materials to your site with a wheelbarrow. Or you can build everything with pressure-treated lumber and pressure-treated plywood. Just make sure that everything is absolutely perfectly airtight, and well insulated with rigid foam.

    I agree with Malcolm that trying to keep two separate off-grid buildings from freezing is asking for trouble. Something's going to freeze eventually. It's much easier to try to keep one off-grid building from freezing than two off-grid buildings from freezing. Even with one off-grid building, a pipe is eventually going to freeze. It happens.

    You can dispose of graywater in a drywell. A drywell has to be located downhill from your water supply. Dig a big hole. Make a mini-basement (maybe 30" x 30" or 36" x 36", and at least 4 feet deep) with CMUs. Some of the CMUs can be installed sideways, or you can create some deliberate holes in the walls. The CMU walls are sitting on footings, but the bottom of the drywell is dirt.

    Make a cap for the mini-basement out of poured concrete (reinforced with rebar and welded wire). Leave a loop of rebar sticking out so you can lift the cap after it's poured.

    When it's time to backfill your drywell, use rocks or crushed stone on the outside of the CMU walls. The inside of the drywell should be empty (filled with air). Put at least 6 inches of soil over the concrete cap.

    Your PVC drain pipe leads from your house to the drywell. The pipe trench is at least 3 feet deep so that your drain line doesn't freeze.

    -- Martin Holladay

  11. Scott Wilson | | #11

    Thanks Martin. I just found this website listing the most common errors in designing a greywater system. I was wondering why you hadn't suggested gravel in the bottom of the pit but on this site they discuss the problems with that and suggest wood chips instead (under Error: Gravel")

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