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I have a hot water tank with a concentric PVC vent that's both an intake and exhaust for the tank.

Whenever it's -30 celcius and below (I'm from Canada), the air intake on the concentric vent clogs up with frost, causing the hot water tank to stop working. When this happens, I need to go outside to clear out the frost.

Does anyone have any tips with this problem? I've heard some posts that people recommend insulating the pipe. What kind of material would you recommend to insulate the vent to ensure it doesn't frost?

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  • This can be a tough one as a heat pump is going to make more cold if a sealed combustion when pulling in air on the outer chamber will cool it quickly. Adding a heat tape may be your best option with insulation on the outside of that. I like neoprene insulation it is usually the most expensive though.
    – Ed Beal
    Feb 9 at 16:57
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Foam plastic pipe insulation for the size of the outer pipe.

You want something closed-cell / waterproof, since it's out in the weather. Possibly an overwrap of aluminum tape to prevent sun damage.

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  • Definitely closed cell. I use neoprene wrap. The aluminum tape would extend the life but not used with most neoprene jobs but at this extreme temp a good idea.
    – Ed Beal
    Feb 9 at 17:07
  • This does very little, short of marginally improving the extent to which your exterior portion of ducting radiates the heat from the exhaust through the material itself and any heat from a conditioned/heated space directly inboard of the wall. The fact that cold air is surrounding and entering the exterior portion of the duct (that portion exterior of any baffle, such as the cap) will render any insulation outside the building envelope what we call "short circuited" - think of cold beer with beer cozy that just wraps the sides, that you set on the hot sand at the beach. Not worth much. Feb 9 at 18:09
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Edit: Be wary that I am in no way suggesting putting fiberglass insulation in a position to be exposed to weather, nor am I in any way suggesting there is no value to insulating your existing PVC. Fiberglass insulation should not be exposed to weather, and by all means start by insulating your existing ductwork if you want to start that way because even if that doesn't solve your problem you might be able to reuse it if you move on to another approach such as may be outlined below or something similar. You have a lot of input here, between the various Answers and Comments in response to your question, that are the equivalent of throwing parts at car trouble. To carry that analogy, just because you get an emissions fault, doesn't mean it's your evap canister, your gas cap seal might be compromised. That is, just because you're getting ice doesn't mean it's a lack of insulation. Try looking at all parts of the problem; ice needs moisture and low temps to form. In laymens terms, are you sure this ice is forming from natural precipitation? If you build a little roof over your wall cap (or if you had a roof vent, above your roof cap) to keep snow and rain from falling on it, would you still have a problem? Then it's the hot air from your exhaust, carrying moisture from inside and absorbing humidity from outside, then condensing and ultimately freezing on the cold surface of your intake cap.

Here are some resources that will help you understand how to approach your problem, and for others who don't understand my answer please read these so you become an informed used:

  1. Exhaust ducting that carries moisture, same principle as your concentric exhaust duct
  2. You might want to find your specific exhaust model's installation instructions something like the document this links to and check that it was properly installed, especially that it is far enough away from other moisture sources
  3. Another, Canadian specific, incredible resource to help you understand weatherization and how insulation functions; where is is effective, and how to use it, which no reputable resource I know ever recommends applying insulation outside the thermal envelope of a building for what I thought were obvious reasons.
  4. Figure 2-7 from this resource demonstrates the concept of thermal "short circuiting" where the insulation plane is interrupted at the top of the foundation wall over the plate and the boldest arrow shows the greatest transmittance (although in the context of the article, it is demonstrating moisture, the same is true with heat). This is the same principle that renders insulation outside the building envelope practically useless. That is, there's no sense in insulating an open pipe, without a cap that heat is just going to radiate away out the end. And when the exhaust blows - that is already a hot envelope around the intake, there's not heat lost to the inner pipe and neither are you going to insulate the cap itself for obvious reasons, so a uninsulated exterior portion is getting as much warming as it can from the exhaust regardless of whether it's insulated. Best practice, and building science, puts insulation inside the building except in the case of conveyance components of Mechanical systems (like refrigerant supply lines for example).

If you can give us your make/model information for the ducting and some photos, folks might be able to give you an exact solution. Not always, but it usually helps a lot, in your situation there are quite a number of concentric exhaust designs and figuring the best way to deal with your problem heavily depends on the specifics of the design.

Without specifics, some general notes then in order from what you should do first:

I think you may be thinking about the problem wrong if you're thinking insulation is your answer. The first line should be mitigating the moisture that is causing the problem, not preventing the moisture from freezing... you need to find a way to direct the moisture away from your intake. Once you get a cap aka concentric exhaust "head" (the part that projects out of the structure) or find a means to install a deflector to keep the moist exhaust air from getting pulled back to the intake, then your next step should be to insulate the ducting on the interior of your building, starting at the exterior wall and working your way back to conditioned space. Keep combustable materials an appropriate distance from any ignition (including exposed exhaust and/or heat) sources.

As to your specific problem, I'm not sure, but your post might be a duplicate. I'm fairly certain you'll find your problem along with a solution here: Someone with what sounds like a nearly identical problem in a nearly identical climate posted here on DIY.SE before your post. Maybe you saw this and it's not entirely what you're dealing with, but maybe you didn't.

If you did read that, and that's not your huckleberry, before you insulate, try replacing your PVC with typical metal duct and exhaust cap. That way the exhaust heat will easily transfer to the intake parts. PVC is not a great conductor, so the heat from your exhaust is not transferring as much as it could to the intake. Then insulate. Insulating your PVC duct might help, but the extreme cold might be overwhelming the lower conductivity of the PVC... that is, you might be dealing with a material limitation: Even with infinite insulation on the PVC ducting, the material properties itself may not be able to transmit enough heat through itself to overwhelm the extreme cold to the point of melting. Metal ducting and wall cap will transmit drastically more (that goes both ways), so when your exhaust is running, the material itself will be heating up and radiating that out to the fins on your wall cap far better than PVC ever could.

If neither of those steps are where you want to start, go ahead and insulate the duct with whatever you want. Generally fiberglass unfaced rolls are an easy wrap-it-yourself solution because you can build it up as much as you like. This goes inside the building. No insulation should go on the wall cap (the part outside the building). Just take care to note any specified offsets required to stay fire safe between insulation and heat source, as mentioned above - think fire safety.


Sure, my post might be frustrating to everyone out there that says "well, I've always insulated my PVC concentric exhausts - dual, single, and otherwise - and that's always taken care of the problem." Go ahead, try insulation first before you rip your existing exhaust out and replace with metal. But I want to give you insight in case insulation alone isn't enough, and I want to help the next person searching the internet saying "I live at the north pole, and no matter how much insulation I put on this thing, it still freezes up." Downvote me to oblivion if you want to, but my answer and the order of recommended operations still stands as best practice.

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