Sheetmetal HRV ducting vs Zehnder
(I did a search and did not find anything, so here we go. If this has been discussed here, pls provide a link. Thanks.) I was talking with an HRV guy today, and asked if a brand of HRVs he sells has gone to the Zehnder-type plastic ducting; I showed him my Zehnder setup last year. He said “There is no substitute for sheetmetal”, and said the reasons why were not a 5 minute conversation so we should discuss it some other day. I’m having a hard time reasoning why Zehnder tubing is not as good at moving air as sheetmetal, and it sure is a lot easier to install. Granted, the smaller diameter may be a problem for large houses or very long runs, but for a modest sized house I question his statement. However, he is an engineer and I’m not. Does anyone have any links to unbiased studies comparing smooth (inside) plastic ducts vs sheetmetal? Or, opinions from your education on the matter? Thanks. john
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Replies
John,
The Zehnder ducting system is a good system. A few points:
1. Your HRV guy may be confusing smooth Zehnder ducts with corrugated flex duct.
2. People who have been in the trades for decades are always going to prefer galvanized ductwork, because it's always been consider the most durable, fireproof duct -- the gold standard. But sometimes, times change.
3. Some green builders are suspicious of plastic ducts because of off-gassing worries. However, I'm pretty sure that Zehnder ducts are polyethylene, not PVC, and most experts don't worry about polyethylene. That said, some people are going to say, "I don't want plastic ducts."
4. There is a possibility (I haven't done the calculations) that there is more static pressure (resistance) from a Zehnder "home-run" system with a bunch of little ducts running to 8 destinations than from a well designed galvanized duct system with a trunk and branches. Anyone want to comment on this possibility?
On Martin's numbers 4: You can design for as low a pressure drop as you want with either system, by using a bigger sheet metal duct, or by splitting the aiflow among many Zehnder tubes. The total space required for equal pressure drop and total flow will be more with the many parallel tubes. But they are easier to tuck into available spaces, so the net result on fitting them in without needing extra chases and soffits is likely better.
I'd add: 5. The fire safety of metal ducts is surely at least a little better. I would consider extra drywall protection over a group of Zehnder ducts, and some jurisdictions require UL listed ducts, which they are not.
6. More skill is required with metal ducts, especially to properly air seal them. The Zehnder system is pretty foolproof.
And yes on 3, polyethylene. Probably less outgassing than whatever you seal your metal ducts with.
John,
If your HRV includes resistance heating, (which I think the Zender does?) you may want to talk to your building inspector before deciding. Our code precludes using combustible ducting for any appliance that supplies heat.
Many thanks for the informative replies. Yes, the possible confusion between Zehnder tubes and the heavily corrugated ones may be the problem; I plan on asking said engineer about that. No question about the fire safety; that is on the back of my head w/ PEX tubing, too, though those tubes are generally full of water. I am not aware of any resistance heating in the Zehnder, but I will check. I have only a small electrical cord to it; #14 or so. The pre-warming box is geothermal.