A question came up on exhaust duct connections. I have always considered screws necessary for mechanical fastening of one duct to another. I have been reminded recently that screws impede airflow and catch particles and fibers, all of which is probably true. My source then said that, in their opinion, all ducts should be joined solely by either tape or mastic, or a combination of the two. Is this common practice? Does this increase the time of installation? Does it hold up over time, especially in horizontal runs?

I would appreciate any advice you have.

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Hi Don. We have never considered screws as impeding airflow, but I can see how in some ducts (e.g. dryer exhaust) they would catch lint etc. We always look for am minimum of two screws per joint in round duct. In cases where there are high solids etc we would use draw-bands to provide mechanical fastening with no duct penetrations. My experience tells me not to trust tape. Not being installers, I have no idea about the installation time etc.

Various studies have indicated very little air flow along surface of duct (for ventilation - have not studied forced air).  So if this is for venting, the screws would not be an issue at all as the pressures are to low. 

This document shows very little difference between smooth wall elbows (die stamped) and gored elbows. http://www.spiralmfg.com/wp-content/uploads/2015/09/engineering_dat...  

There is however a huge difference when comparing the centre line radius of elbows (1.5 CL compared to 2.0 CL). 

I have also reviewed multiple sources that advise to never rely on mastic to secure ducts.  It will crack when dry as soon as duct is banged or expands and contracts with energy changes. 

I was advised by the gurus in my region to first tape the joints, then secure with three screws, and then coat with mastic.  all but eliminated air leakage allowing the system to operate as designed and put air where wanted and needed. This must generally be done before the duct and fittings are installed into place to be most effective, this way you just end up with just small amounts of sealing to do once installed.

Great post, Sean. I agree that screws are wise when joining metal ducts, but its also true that screws can be problematic in dryer exhaust ducts. For this reason, I used thin-walled drain & sewer PVC for my dryer exhaust in my previous home, which I built. Otherwise, you can minimize lint build-up by using the shorted sheet metal screws available.

Thanks David - I was going to look at PVC for the HRV but was cautioned against it, not because of the offgassing (which would not be relevant for dryer), but because the static build up leading to a high resistance to air flow.

Why would you think PVC has high resistance? If anything, I would think PVC would have a slightly lower friction than metal (although I'm not sure about impact of the tiny gaps where the spigot end meets the bell shoulder).

Apparently the air passing over the PVC creates a strong static field that in turn impedes air flow.

Static pressure drop through a straight length of pipe is a function of friction rate, and friction is largely a function of surface smoothness. Like metal ducts, PVC pipes have a relatively smooth surface.

Here's a friction loss calculator for your perusal: http://www.freecalc.com/ductloss.htm

It looks overly complex, but for a simple comparison of a straight length of pipe, you only need to fill out four fields: Select Duct Type (round in this case), and Duct Material (PVC vs. Galvanized Metal), then enter flow rate and length. Scroll to the bottom and click the Calculate button.

Hi Don,

Summary:

DRYER VENT: screws bad idea, possibly prohibited by code.

METAL DUCT: must be screwed together or other mechanical connection S/Drive etc

FLEX for conditioned air or ventilation: Panduit strap is preferred mechanical connection.

I had been told by some of my HVAC contractor clients that in their areas at least, screws are PROHIBITED by code in dryer vent for fire safety reasons.  Makes sense to me, but I have never verified this. 

David Butler: i bet you have every code book out there ... have you ever heard this?

As far as not using a mechanical attachment for H/AC or V ducts, I would not recommend that as a best practice.  Mastic, pretty much any tape and even Aeroseal sealant are sealants only and mechanical connections that prohibit movement are needed to ensure the ducts dont come apart, or open up.  Fabglass embedded in mastic is considered by many an acceptable mechanical connecion, I'd still like to see screws first.

With flex, a Panduit strap properly tensioned is best practice, not screws.  So if an exhaust duct was all flex (in itself usually not a best practice), one could install that without any screw tips in the airstream.

Brendan,

Thanks for that response. Because of your warning, I just went down to the basement and disassembled my 4 inch diameter metal dryer duct, joined with screws (and taped), and peered down it with a flashlight. It has probably been up for 10 years, but I do not use the dryer as much as many families would. There was a bit of accumulation on the bends and a bit on the screws, but the ducts themselves were clean and shiny. That was reassuring.

I never checked codes on this question. Checking now...

Section M1502.4.2 of the International Residential Code prohibits screws or other fasteners that protrude more the 1/8" into a clothes dryer duct. M1502 also requires manufacturer instructions to be followed. I checked two sets of instructions and both prohibit sheet metal screws carte blanche. The International Mechanical Code also prohibits sheet metal screws without regard to protruding length (Section 504.4).

In my experience, foil tape covered in mastic does a good job of securing metal duct joints as long as there's no potential for the pipe to be disturbed. I don't recommend this in spec homes since there's always potential for pipes to be bumped during construction. But in a one-off situation, it's easy enough to re-check the joints just prior to covering them up.

BTW, both IMC & IRC specify dryer ducts must be metal. So my PVC duct would be illegal. That said, although dryer exhaust could conceivably exceed PVC melt temperature (~!80F), there's a big temperature drop in the connector duct. InspectAPedia has an interesting page on clothes dryer and dryer vent temperatures: https://inspectapedia.com/Appliances/Clothes-Dryer-Temperatures.php

David,

If the IMC mandates metal dryer duct and prohibits metal screws, they must be accepting other fastening methods for metal ducts (unless the duct is continuous metal flex).

Good to hear you have clean dryer ducts Don.

The dryer duct issues that we run into are predominantly upper story dryers that are vented out through the roof. Installers use a roof-cap with a screen and the lint packs up inside the screen. This can usually be cleared using a shop vac. Most of the work is getting access to the duct behind the dryer. Fantech makes a nice box that sits in the wall with a lint filter, but the real value is quick access to the duct to clean it. http://www.fantech.net/enca/home/Products/fans--accessories/accesso... Most installers use screws in the joints.

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