I have new construction and am about to install two small HRV's. Most of the ductwork is 4". Are there options to installing 4" galvanized duct? Typical "flex" is not acceptable (except for the really short (< 12") section for sound isolation) since the corrugated interior generates huge static pressure. Is there any long section plastic duct out there with a smooth interior. I even thought about using 4" lightweight HDPE drain pipe - is there a code issue with that? Since these are HRV's there is no heat source - only ambient temp air. Of course I would not consider this for other applications (range hood, wood stove make up air, heating/cooling ducts or dryer vents). Seems this stuff would be cheap, low friction, easy to clean, no corrosion..... Am I crazy?

You can get smoothwall galvanized 4" ducting at the orange supply store. Pretty much anywhere, really. It comes in flat sheets and you need to roll it and lock the seams together. It gets joined with a water based mastic at the joints and a quality metal repair tape around that and down the seams. There is some nice insulation sleeves that go around it, too.

While I wouldn't hesitate to embed PVC in concrete for air duct, if you put it anywhere else, it could be exposed to fire and become a highway for poisonous PVC combustion fumes to travel directly to your bedroom.

I'm very familiar with snap lock round duct and the various fittings and I'll use that in at least some of the ducting (if not all). I was actually asking about HDPE pipe (thin wall used for drainage) not PVC Schedule 40 DWV pipe. As to the fire issue I already have PVC DWV piping all over and Romex with PVC sheathing so I'll get the toxic fumes in a fire anyway I suppose. I think the fumes from HDPE are better but wouldn't want to breath either. I'm not sure what plastic flex duct is made of but if I went that way that wold have a smoke issue too.

Some PEX companies (Uponor) now have sprinkler systems that should put out the fire before plastic piping ducts, etc. burned.  I would think that installing piping that did not require a lot of connections would make the systems more affordable.

Plastic ducts should be easier to assemble and make air tight than seams and joints in galvanized steel.

I have used smooth 4" single wall PVC sewer and drain pipe. Advantages are A) it is very smooth to facilitate air movement; B) it glues together so there is virtually NO air loss and C) it goes in pretty fast. Disadvanages: A) it is PVC, B) this pipe is very hard to find since most sewer and drain distributers have transitioned to a double wall pipe that does not glue together and C) there is some concern about fire in PVC ductwork related to static electricity and dust. I doubt that there would be enough dust in an airtight house; I think this concern is more relevant to PVC used for workshop dust collection, but...?

I would never, ever use PVC for air supply ducting except where it is adequately protected by a fire barrier, such as being embedded in concrete. Other plastics follow that prohibition closely. I might entertain using metal for the supply duct and plastic for the exhaust, however.

You'd never use it due to the fire danger? then why is it okay for the exhaust?

Bob, just about everything in a home is combustible, ergo "fire danger". The difference is that PVC (and other plastics) generate tremendously toxic fumes and if you use it in a supply duct, the fumes are headed directly towards your sleeping areas, even if the fire hasn't yet reached those areas. If you think it is worth it to do exhaust ducts in plastic, at least the fumes are headed outside.

good point. I think combustion is an excellent reason to avoid the use of flamable gasses and liquids in the house also, which we can do easily in a high performance home.

On the same token, the use of cellulose or rock wool insulation is much better from a fire standpoint than foams which also give off toxic fumes. Lets agree to elminate foams and plastics and make our home safer.

Lets agree to elminate foams and plastics and make our home safer.

I'm not sure that's necessary, but not using plastic in your air supply duct is probably a good idea. I'd be surprised if a fire could get a foothold in mine. I tried lighting the ICF block on fire and it wouldn't burn. Just about everything else is concrete, stone and tile, and 5/8 gypsum. I guess there are some sticks upstairs......

Going up an inch in size more than compensates for corrugated ducts.