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Question DetailsAsked on 6/20/2011

Is the door on my roof vent frozen shut?

We had a roof vent installed to our over-the-stove microwave. Now when we start the micro, the vent motor runs on high for 20-30 seconds before switching to low, when freezing temps outdoors. Is the vent door frozen shut? Will that damage the motor? Do we need an adjustment?

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2 Answers


I don't know anything about the microwave, and would suggest calling the manufacturer if what youa re experiencing is normal.

However the only ways I know to check the vent would be to climb on the roof or in the attic and remove the vent pipe/hose.

If the door, what we call a baffle, is stuck closed then the fan is useless. Where is it pushing the air? This could burn out the fan motor premature. If the door is stuck open, this too is bad because cold air can be blown in constantly. That is the purpose of having the baffle.

It is also possible some debris fell intot he vent pipe. I would inspect this if I were you.


Answered 8 years ago by ReliableAmericanRoof


This is very common in cold climates - more with bathroom fans, but kitchen vents too at times, as the moisture freezes. My theory is that kitchen vents tend not to do it as often because the flapper (damper) is greasy, so does not stick shut so easy.

It sounds like it is frozen, then after some warm household air blows by it (the flapper gates are far from airtight) it thaws enough to open up. The two fan speeds you are hearing is more likely just one fan speed, but it sounds higher pitched and louder when stalling out without air moving, then when the stuck flapper opens the air starts moving and the fan is no longer stalling out, so get quieter and lower pitched. Probably not great for the fan, but as long as it does not go for more than 30 seconds or so probably not going to kill it either, because while air is not flowing up the duct, there is still a lot of air moving around to cool the motor.

If you can see into the vent hood on the roof, either by climbing up there or by looking from below with binoculars or telephoto camera, have someone watch while the fan is turned on for the first time that day in cold weather to see if you can see the flapper opening. Typically a flat sheet metal flap, usually near flat when closed, then tilts up upon opening, though some are hidden inside the hood. That would tell you if that is the issue - likely is.

If not, then there may be a flapper valve in the microhood itself also, which is sticking due to grease buildup. You should be able to see if this is the issue by removing the grease trap screens, and being careful not to get hands or hair tied up with the fan, watch the valve when you turn on fan - usually only an inch or two inside the start of the duct to the roof.

Partial solution for sticking roof flapper - coat the mating surfaces and the hinges for the flapper with spray teflon lubricant. For flapper in the vent hood - clean off grease with paper towel with good dose of Dawn or equal - turn off breaker first to cut electrocution or fan cut risks.

Answered 6 years ago by LCD

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