How to seal up a noisy downdraft vent when not in use?
September 3, 2006 5:36 PM   Subscribe

Our cooktop has a downdraft vent to the outside. We live in a windy area and the flap on the vent bangs often. It's also a huge heat loss. How can I keep it closed when not in use?

The cooktop is a JennAir four burner with grill. When cooking, you can manually turn on the downdraft fan by an on/off knob on the front of the unit. The cooktop also automatically starts the downdraft fan when you light up the grill.

Ideally, I'd like the vent to seal up when the switch is off and allow for normal use when the fan switch is on. (i.e. I'd like to use the existing wiring.)

I've considered an electromagnet switch, but am not certain what to purchase or how to wire it up. Would it need to be always on in order to make a seal? Is this a problem with electricity consumption or heat buildup in the magnet?

Is there a different solution that I'm not seeing? (Hopefully one that can be controlled from indoors - we're in a snow belt.)

posted by TauLepton to Science & Nature (8 answers total) 1 user marked this as a favorite
If I understand this right, it vents to the outside through a nearby wall, with a flapper-type vent. The flap is pushed open by the air pressure created by the exhaust fan but is supposed to stay shut otherwise. You might look at a home center for a similar vent that has a heavier flap that is less likely to open unncessarily.
posted by drmarcj at 6:16 PM on September 3, 2006

Figure out a way to put a longer piece of duct outside beyond the flap. That should reduce the wind effect. Also, be sure there are not other leaks or openings in the house. Really, the only way you get significant inflows or outflows past this flap is if an equal amount of air can get in or out someplace else. For example, do you have a fireplace damper that doesn't close tightly? If the rest of the house is pretty tight, even if you hear the thing banging from time to time, you're not losing a lot of heat that way. Opening your back door a couple of times a day probably loses you more warm air than the duct does.
posted by beagle at 6:39 PM on September 3, 2006

Best answer: I'm not sure I follow, but let me see if I have this-

You have a flap which is, as drmarcj suggests, supposed to open from the outpressure of the fan but be closed otherwise. If that's correct, I'd be very concerned that drmarcj's solution would not work well, as vent fans are often not very strong. I would think that the fan might easily be weaker than the wind.

What I think you're suggesting would be an electromagnet which would prevent it from opening when activated and allow it to open when closed. In that case, the one issue is that you'd want the magnet on when the switch is off. In that case, you'd need to use a relay:

The electromagnet would be attached to its source of power through the normally-closed switch of the relay. (That is, it will be active when the relay's coil is not activated) The switch for the fan probably runs on 110VAC (if you're in the US). If so, you would need a relay with a 110VAC coil. (Definitely use a multimeter and check this, though.) The tiny additional resistance of the relay coil should not affect the operation of the fan at all.

Power consumption should not be much of an issue, though this would, of course, depend on the strength of the electromagnet. I'm thinking you probably want a weak version of the kind that is often used to hold doors open. Heat buildup in the magnet shouldn't be an issue as long as you use the power supply that is specificed by the magnet. I hope this helps, but feel free to ask for followups. I'd say a magnet is the easiest option, though for an indoor controlled solution.
posted by JMOZ at 6:40 PM on September 3, 2006

You shouldn't need an always-on electromagnet. An electromagnet that comes on with the fan and undoes a spring-loaded catch should do the job.

If that were my house, I'd invent something based on a solenoid ratted from a discarded washing machine.
posted by flabdablet at 6:55 PM on September 3, 2006

And a cupboard latch.
posted by flabdablet at 6:56 PM on September 3, 2006

What flabdablet suggests would work, but a more mechanical system is MUCH more likely to break. And then, you have to go out in the snow to fix it.

I'd look up the power draws on electromagnets before discarding the always-on approach. I'd guess you might be pleasantly surprised by how little power they draw.
posted by JMOZ at 10:26 PM on September 3, 2006

Also, the key thing to search for is "electromagnetic lock" or "electromagnetic door holder lock." Doing a froogle search, there are a lot of expensive options, but also several options < $>
Your other option is to look for an "electric door strike." These seem to all operate on DC, so you might still need a power supply, but you can probably operate it directly off the 110VAC in the stove. (assuming that's what you have)
posted by JMOZ at 10:36 PM on September 3, 2006

Response by poster: drmarcj - Yes, that is a perfect description. We've asked if it is possible to have a heavier flap, but all plumbing stores have said no. Like JMOZ, we were concerned about the exhaust fan being strong enough to open it. Homeowners Fire Insurance and all that...

JMOZ - You are heaven-sent! Even I understand what I'm looking for now. Thank you for filling in my lacking terminology. I don't mind the always-on approach as long as the draw is minimal. I'll be doing a search this evening & post our full fix for others as it occurs.

Flabdablet - I appreciate your idea as well. I'll be investigating JMOZ's first, but will keep yours in mind.
posted by TauLepton at 4:20 PM on September 4, 2006

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