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Lots of buildings have rigid metal ducts. And lots of buildings have (much-maligned) plastic flex ducts. But why do rigid plastic ducts not seem to be a thing in North American construction? Such ducting would seem to solve a lot of problems: Like flex duct, it would be cheap and fast and easy to install, but give better static pressure due to its smooth walls like metal ducting. Whatever plastic they make flex duct out of seems temperature-resistant enough; why not make rigid plastic duct segments out of the same material, with it being field-assembled like PVC or ABS pipes?

Is there any reason why you couldn't use sufficiently large diameter rigid ABS or polyethylene pipes for ductwork if you wanted? These are both relatively temperature-resistant plastics (176f and 221f, respectively) and would seem to be especially well-suited for carrying the output from heat pumps where the maximum output temperature is much lower than what a gas furnace can do.

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    Price large diameter rigid plastic tube and the same diameter rigid metal duct. Bet the metal is way cheaper.... – Ecnerwal May 6 '16 at 1:17
  • way cheaper big pipe cost big bucks. – Ed Beal May 6 '16 at 13:42
  • It isn't just about existing products. Polystyrene is stable up to 212f. You could easily make very large diameter high-density EPS pipes that would be lightweight and inexpensive. Is this a case where code is holding back progress, or is there a good reason why such a thing doesn't exist? – iLikeDirt May 6 '16 at 16:22
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1) cost. plastic is way more than steel

2) weight. so much heavier per unit volume

3) thermal expansion and contraction, deformation, etc.

4) it is used in lab environments for fume extraction, and in exhaust for high efficiency gas appliance exhaust (for its resistance to corrosion from the combustion gases)

5) you also cannot use rigid plastic ducting for interior air distribution for fire spread reasons. the fire code forbids it. imagine a flamethrower coming out of your vents as a small spark is fanned to superheated air as the forced air fans the flames from the burning plastic.

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  • Ah, fire code. That makes sense. – iLikeDirt May 6 '16 at 16:24
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Actually I think if you research high velocity A/C systems they use schedule 20 or 30 PVC for supply to 2 or 3 inch ports.

The expense is probably much higher than regular HVAC methods. Every site I found wanted my information to provide a quote which means it is expensive. They advertise they install these systems in historic homes and buildings since they are not as noticeable as normal HVAC.

Happy day!

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Rigid PVC or ABS ducting is actually becoming popular is Europe although it's used for smaller diameter ducting jobs, matched to 150mm or 200mm inline mixed flow fans usually. Because or the narrow spaces between apartment and townhouse floors this square PVC duct is commonly used.

Google search shows plenty of options: https://www.google.com.au/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#safe=off&q=rigid+flat+ventilation+duct

Couldn't find much in the way of an actual installation, but this shows how it's assembled: https://www.youtube.com/watch?v=Fx3zTf7CFck

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pvc is self extinguishing but can release toxic fumes if burning,ignition temp would be fairly high compared to many plastics.For mainly AC use here in fla I would use it if it was not expensive,most electrical insulation is pvc

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