Why Does The Furnace Set Off The Carbon Monoxide Detector?

The only thing that sets off a carbon monoxide detector to my knowledge is carbon monoxide. Most people install carbon monoxide detectors that are set way too high to be of good use. By the time the carbon monoxide detector goes off the people in the home have already been subjected to at least some degree of carbon monoxide poisoning. Three different types of furnaces we are dealing with behave differently:

• Floor Furnace
  Wall Furnace
  Forced Air Furnace

Floor Furnace

If your floor furnace sets off your carbon monoxide detector you have a problem that needs to be addressed right away. Do not run the floor furnace until the problem has been looked at by someone very familiar with floor furnaces. The floor furnace probably has a rotted out exterior shell where the draft hood is. The draft hood is then funneling in the carbon monoxide from the normal exhaust path into your home. This is incredibly dangerous and must be fixed before the furnace is used again. Chances are good that the heat exchanger is okay, but the draft diverter needs to be remanufactured and reinstalled.

Wall Furnace

If your wall furnace sets off the carbon monoxide detector the first time you fire it for the season you can relax. Sure, have it checked out for proper venting, but you shouldn’t be overly worried about it. Wall furnaces will vent some exhaust gases into the home when they first start up until the flue warms up. Once the flue is warmed up (20-30 seconds) they will start venting properly. The pilot light will keep them warm enough after the initial start up. If your wall furnace continues to set off the carbon monoxide detector you will need to have a professional out to look over the system. The gas valve could be over firing, the air fuel mixture could be off, the inner flue could be obstructed or the cap on the roof could be collapsed. Improper initial installation plays a very large part in whether or not these wall furnaces will actually properly. Do not use 90’s in the vents. It is highly unlikely that this is from a crack in the wall furnaces’ firebox.

Forced Air Furnace

Your forced air furnace should never set off your carbon monoxide detector. It has so many built in safeties now that dumping carbon monoxide is rarely a problem. If you have an older furnace you could have a problem with a crack or hole in the firebox or heat exchanger. It is much more common to have a carbon monoxide alarm go off with a horizontal furnace rather than an up flow furnace. This is because of where the cracks form in those older furnaces and how big they can get. I have seen holes big enough to stick my hand through. The furnace burners fire up and fill the ducting with exhaust gases. The furnace blower motor then turns on and blows the carbon monoxide into the home and blows the flames out of the firebox. This was the problem with the Consolidated Industries furnaces (Premier) a few years back. Home could actually catch on fire.

If your forced air furnace activates your carbon monoxide detector turn it off, air out the house and call a professional HVAC company, preferably us here at 911furnace.com and we find and fix that problem for you. (The picture to the left is a Carrier heat exchanger that has suffer a catastrophic failure)