Hello All,

I have installed few Zehnder systems and I am not sure if my rep is giving me the info I need or the info that helps me buy more foam piping from him. The large ducting (7" Comfopipe) coming off the unit  at the 4 ports for supply (fresh and to house), exhaust to exterior and from interior). Which are most critical to be insulated piping. If my supply from the ERV is running through a conditioned space, would a sheet metal duct without insulation gain condensation? Would the exhaust from my interior space require insulation? Since the comfotube is not insulated it seems like the additional cost for 39" runs at $77 is not the best cost option.

There are a lot of situations where I have extended internal runs from the ERV to the silencers/manifolds in order to keep my Comfotube runs under the recommended 35'. In a current install I have about 25' horizontal to a 12' vertical of 2 runs of Comfotube. That is not cheap and if I can do these types of runs in sheet metal with sealed seams and no insulation, there is a big cost savings to my client.

The supply and exhaust from the exterior I am more comfortable using the Comfopipe to ensure I don't get condensation.

Any thoughts?



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Good questions. I would say that on the "house" side, which is connected to the inlets and outlets in the living space, there should be no need for insulated ducting, if the duct runs are in conditioned space. But the ducts connected to the outside could certainly have condensation during the heating season. A house at 70F and 40% RH has a dewpoint temperature around 45F. I'm guessing your outside temperatures are often below that?Check out a psych chart for other temp/RH combinations. The duct surface will be cold, likely below the house air dew point temperature, voila, condensation. I would consider insulated ducting for the runs connected to outside inlets/outlets, but none for the others. As for using the expensive comfotube product, it is a choice, with pro's and con's. I think that well-executed rigid sheet metal ducts would perform very well, if you insulate and seal them appropriately. Cheers.

Also, if it's exceptionally cold outside, you could have a supply air temp into the living space that is below the dew point. For example, if it were 70F inside and -30F outside and you have a sensible efficiency of 70% (Zehnder is higher, I think), then your supply air would be 40F (70-100*(1-0.7)), also below the house air dew point. Also notable, some condensation is NOT death, as long as it dries out reasonably quickly, etc. If it happens for 6 hours of the year in 1 of 5 years, don't sweat it.   

It's a good question and certainly saving ComfoPipe is a good thing. The ComfoPipe is Insulated R-7 and non-conductive. I wish it were available somewhere else. Flexible Insulated spiral duct sadly will sweat like tin on some days unless installed perfectly. Which is nearly impossible.

A double wall tin pipe with insulation between is available, and more expensive than ComfoPipe. Non conductive R-6+ is what you want to penetrate the walls of a performance building. The ComfoPipe attaches to the AirBarrier. Use this pipe from unit through wall. It handles the Cold Side of the ERV/HRV. Cold Pipes in Warm Spaces-> Insulate it.  Connections from the ERV/HRV should be 8' +/- to the exterior air to aid efficiency of ERVs/HRVs.

Also note the ComfoTube; which is the 3" white flexible rigid duct that fits in 2x4 walls is also non-conductive. It's like fiber-optics for the internet, but for precise fresh air and exhaust. Very worth the money especially to deliver fresh air to the far rooms in buildings with precision.

ComfoTube is insulated, but like R-8 flex duct insulation it may not have enough R-value for some situations. The other big problem with ComfoTube is it isn't large enough for the airflows in a lot of systems. Based on Lifebreath duct design software runs, I use 8" steel for the outside connections and keep them close to 8', and use 8" for the inside duct runs for the first few Y's or long runs. I have never used ComfoTube on any Zehnder install, just putting a 7x8 3-piece increaser right on the connections and going steel as far as necessary to 6" American-style diffusers. These systems commissioned nicely.

In one house with some very restrictive structural limitations I had to use 3" Zehnder flex to go through steel I-beams a long way from the CA-350. So I ran 8" steel up to the beam, put on a 8x10 increaser and 10" cap with 4) 3" holes cut in it, then cannibalized a Zehnder flex manifold for the four flex connectors I put on the end of the cap. This gave me an inline 4-tube manifold that fit inside the joist bay for almost no cost. I had to do this with a 6" cap for a two-flex run to a high flow register. At the duct boots I belled back up to 6" round for standard HRV diffusers like the rest of the house. This commissioned properly.

By the way, the only thing I use American flex duct for is insulating steel duct; You can tape the inner liner onto the steel duct and pull the insulation and outer liner over the steel. Discard the inner liner. But we probably want more than R-8 unless the ducts are inside the thermal and pressure envelope, and the outer liner will deteriorate from UV, even in an attic with roof vent light. American flex duct is a temporary duct system in most locations.

@Alan, I see you're located coastal California so there's no chance for any condensation on any supply runs inside the thermal envelope, so uninsulated metal pipe is fine for all house side ducts -- supply and exhaust. Brennan's example is spot on for cold climates.

The fresh air duct from outside to ERV needs to be insulated to prevent condensation in cold weather, but only the section exposed to house humidity, i.e., from the thermal boundary to the unit.

I'm probably the odd man out when it comes to insulating the exhaust line from the ERV to the outside. I don't see the need for insulating that duct run, especially in your climate.  With an enthalpy core, the outside exhaust line could potentially sweat on the INSIDE surface in cold weather as it passes through unconditioned space (depends on inside-outside dew point differential and outside temperature), which is why installation instructions typically specify a slight slope to the outside.

In summary, the only ERV duct that must be insulated in your relatively mild climate is the fresh air intake duct, but only the section that passes through conditioned space.

Thanks for your feedback and information. I will be making adjustments in the future when I am running in conditioned space.


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