Microwave turns on the Furnace Ventilation Fan

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Posted by: from Okotoks
2/8/2011 at 2:14:35 PM

Weird question here and I see it has been asked before but no final conclusions made. Whne I turn on my microwave my furnace ventilation fan kicks on. Here is my scenerio.

Built in Microwave / Hood Fan

Plug in recepticle not hardwired

Thermostat is about 20ft away

Curently Winter so the furnace is set to heat

Is this normal? Any sugestions?

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Date/Time2/9/2011 at 8:38:04 AM

The two should be totally independent - however it's possible that the activation of the fan create a draft of cold air to pass the area of the thermostat causing it to activate the thermostat.

Suggestions: Set your thermostat to cool and turn on the hood fan and see what happens

Have an electrician or a heating and ventilation contractor have a look at the problem.

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Date/Time2/15/2011 at 3:41:16 PM

Hi there. Let us asume that the ventilation and drafts created are not playing a part here. If your house is an older house with a bit of character to it, I'd say twenty years or older, you may have a reverse electrical feed. Even if your house is not that old, and recent wiring has been added or faulty wiring for that matter, to an existing older wiring design, it may be possible for a strange reverse current to be created. Older wiring used ground wires as returns from appliances/outlets, what have you, and the ground was the 180 out from the feed. If, by chance your microwave is drawing from an older circuit with eccess return being generated through the ground on the circuit which may be directly or indirectly connected to your furnace circuit, perhaps directly as well, not only through the thermostat, depending on your set up. This same situation may be possible directly in the electrical panel as well. I hope I've explained myself some what clearly. Best of luck to you, I would find a qualified electrician to look over the situation.

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Darwin from Sniper Developments in Acme
Date/Time2/18/2011 at 7:18:27 PM

Hi Andrew.

Your problem is far from typical. One quick and easy test is plug the microwave in to another circuit (not in the kitchen or separate circuit in the kitchen) and see if the issue is still there. Your thermostat should be 12 or 24 volt activated by a transformer near your furnace which in turn activates a circuit in your furnace control converting it to a 120v or 240v signal. If you have any type of electrical bleed there is a serious electrical problem which may be a fire hazzard. Your circuits for the kitchen and furnace should not be shared typically, therefor alleviating an issue of shorting but do not disacount this as a possibility as stranger things have been encountered by most of us who have spent signifigant time in the trades.

I am in your area and would welcome questions.

You can contact me via E-mail: Sniper and I can give you a phone number to contact me.

Dont take this lightly Andrew as electrical Shorts are a major cause for house fires. Good Luck.

Darwin Eaket

Sniper Exteriors & Renovations

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Date/Time3/2/2011 at 11:32:13 AM

To the OP. I will have to disagree with the other posters on this issue.

This is a very typical installation in "newer" homes and now many older homes that have be substantially renovated. Although not part of the electrical code, it is part of the building code. In simple language the building code requires the same amount if intake air to be supplied as there is exhausted. I would suspect that you will notice the same thing when you turn on the bathroom exhaust fan.

Likely in this case there is a low voltage wire from the hood fan to the furnace which activates the blower when it is turned on. This is done through a love voltage relay or a set of isolated contacts depending on the make of the hood fan and the furnace.

You may also find a switch located near your thermostat that has "no apparent reason" to be there. If you have one, it is likely connected to the exhaust fans in the bathroom, so when that switch is activated the fans will draw stale air out of you home and draw fresh air in.

It is very unlikely that you have a "problem" with the wiring in your house.

Best Regards

John Kuehnl-Cadwell

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