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Community and Q&A

Pressure Switch for Dryer Duct

jadziedzic | Posted in General Questions on

I’d like to install a low-voltage pressure switch on a section of 4-inch (galvanized) clothes dryer duct – or better yet, buy such an assembly pre-made – to control a make-up air fan.  I found a couple of pressure switch units available (Broan, Tjernlund) online, but none of my hits explicitly called out use in a dryer duct.  Is anyone aware of a pressure switch or switch/duct section assembly that is suitable for use with a clothes dryer?  (Most of these units have an inline air tube that sticks into the duct; seems like that would have the potential to clog due to lint build-up.)

While I intend to use a heat pump dryer in this house, I’d like a backup method in place that could be used with a conventional electric clothes dryer in case it becomes difficult to source a heat pump dryer.  Goal is to air seal the house to achieve 1.0ACH50 or less value, so a conventional dryer will likely need a source of make-up air.

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  1. silkwj | | #1

    The two makeup air systems I'm aware of - Electro-Industries and Fantech - both use a current transducer (CT) on the circuit of the exhaust appliance. Any sensor in a dryer exhaust duct is going to fail for the reasons you mention - and be torn out should anyone ever actually clean the duct at some point. So just wire the dryer with a j-box in the line to get the current sensor installed and then you have reliable control for the MUA.

    For the specific case of an electric dryer, where the heating elements draw much more power than the motor, you may need to use a current-sensing relay that can trigger at the lower power draw of just the blower motor, but not overload its controller when the heating elements are also on - a drawback of the CTs noted above. Fantech sells the ACCS40 relay, which is really just a Greystone CS-425-HC-0, which triggers at 1 amp but can handle 50 amps. That would give you a simple, reliable on-off signal for controlling the makeup air system.

    Closing thought - in a big enough house, 1.0 ach50 isn't necessarily tight enough for a dryer to need dedicated makeup air. It's really the cfm50 of the house and where on the building leakage curve the flow of the dryer lands. In a big house it may only be a few Pascals of depressurization, and as long as you don't have fireplaces/woodstoves or other atmospherically vented appliances a few Pa is fine.

  2. PBP1 | | #2

    Even though I was going with a heat pump dryer, I was told that a dryer vent was required by code for each W/D station (I wanted to avoid installing them). Thus it may be good for other reason(s) that you're installing a dryer vent. In some places in Europe, vented dryers are banned (fire hazards and perhaps backdraft on gas appliances). As CS notes, you don't want depressurization if you have certain appliances. Knew a couple found dead due to CO from an appliance.

  3. Expert Member
    BILL WICHERS | | #3

    +1 for current sensors (usually called "current relays"). I actually just designed some of those into a system for a current project at work, where the current sensors are on the power leads running to the dryers in a few large print lines (BIG inkject printers that print invoices for people, similar to newspaper printers). The current relay closes when the dryer draws power, and we use the relay to operate an exhaust fan. There are some extra time delay relays too, but the basic goal of the system is to automate the exhaust fan while ensuring it doesn't cycle too frequently. A system like this is simple and robust.

    You can get air pressure switches, but they're much less reliable than current relays, and they are likely to get clogged with lint if used on a dryer duct. Current relays are available from many HVAC supply houses and aren't very expensive.


  4. jadziedzic | | #4

    Thanks for the replies, appreciate the suggestions, particularly the Greystone current-sensing relay. That certainly beats having to deal with a clogged air pressure switch.

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