furnace fan runs when microwave/hood fan is on

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Posted by: from Calgary
2/27/2009 at 5:49:31 PM

I am the 2nd owner of a home that was built in 2005. I have a microwave/hood fan combo in my kitchen. I have a high efficiency furnace. My question is this: why does the furnace fan come on:

- when the microwave is on; or

- when the hood light is on AND the microwave door is open (the microwave is not on, just the door is opened).

I have a very basic understanding about the ventilation requirements for new homes, so I understand that when the hood fan is running, the furnace fan will run as well to replace the air that is being removed by the hood fan. Should the same thing happen when the microwave is running?

When I originally moved in, the furnace fan would start running if you open the micowave door only OR if you just turn on the light in the hood fan only - basically, the fan would come on if the unit is on (even if it is just the light that is on, or the hood fan, or the microwave). I contacted the electrical company that the builder used to install the wiring to come look at it. The electrician told me that he sees this problem all the time with new homes because of a new Code. The electrician then made an adjustment inside a square metal box that was located below the electrical panel. He inserted a small screwdriver-like-tool into a small round hole and turned it slightly. He said he lowered it as far as the the new Code would allow. I assumed he meant the electrical current or voltage. What he did made it better but I still wonder if the wiring was done correctly, or is there a setting on the furnace that isn't set properly.

Any advice or suggestions will greatly be appreciated. Thank you.

Pam Myers

Calgary AB

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Date/Time3/3/2009 at 4:58:44 PM

Hire a professional guy or company to give you an aswer,, Here you are dealing with POWER and GAS which is dangerous if you atempt to do anywork by yourself??

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Abba's Service in Huntsville
Date/Time3/16/2009 at 8:01:58 PM

Hi i do heating work and this sounds like a real problem it sounds as if they are connected together and if the are you have a problem that needs to fixed right away. I was a sub contractor in Toronto for many years with the gas company. and then a sub for sears for 12 years and this sounds weird. You need to shut the furnace down now! Sorry you should be aware that this is not a small problem it is a big problem. Please feel to email me at

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Hannah in Calgary
Date/Time11/16/2010 at 1:44:00 PM

Hi Pam - did you ever get a resolution to your problem? Only we are experiencing the exact same problem (our house is 3 yrs old).....any recommendations?

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Jeremy from Remy Mechanical in Milton
Date/Time2/3/2011 at 10:33:13 PM

No. the furnace does not make up air that is being exhasted from the house. This shouldn't be happening. You need to call an electrician

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Date/Time3/2/2011 at 11:44:38 AM

I just posted this in another your case the electrical contractor used a current sensing relay instead of a mechanical relay or set of contacts. That was the "little box" you saw him adjust. The proble with that is, like you pointed out, it may work just when the door is opened insead of when the actually mircrwave hoodfan is running.

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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Pam in Calgary
Date/Time3/3/2011 at 6:51:53 PM

To John (Welland) - thanks very much for your reply. Your are correct in your response that it is not a wiring issue, and that it was done persuant to a new building code that came into effect sometime in 2006 (I think). I believe Alberta is the only province right now that has this new code so far.

To Hannah (Calgary) - the easiest way to get around this is to install a hood fan, rather than a hood fan/microwave unit, but the furnace will still run everytime you run the hoodfan. The other option may be to get an electrician to re-wire so that the furnace blower is disconnected from the same contact or not drawing from the same electricity as the microwave/hood fan or hood fan. Hope that makes sense. I'm not an electrician (as your can tell by now) so I may have miss-used some of words in my preceeding sentence.

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