We have a 2-year old gas furnace connected to water pipes and radiators throughout the house. It just turned on for the first time this year and is blowing a large amount of very hot air directly into our basement from the front of the furnace. When and after it is lit the flue on top remains cold to the touch. It has not gotten too cold here yet and has only run a few times, but I noticed this when I was doing laundry and it was unusually hot in the room. I don't remember it ever putting out so much hot air in the past. We have a CO detector somewhat nearby and it has not been set off, but it seems like the exhaust is being released directly into the room and not exiting through the flue. We had a yearly checkup on the equipment only a few weeks ago and the tech said everything was OK, but it was still warm outside and he only ran it for a few minutes. I'm thinking this is cause for concern especially because in the next few weeks it will be getting quite cold and we'll be running it continuously. Has anyone seen a similar issue?

  • Check the chimney/flue, to make sure it's unobstructed. What is the make and model of the unit? Usually furnaces have sensors to detect if exhaust is not going out properly.
    – Tester101
    Commented Oct 26, 2017 at 12:15
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    The flue may well be a double flue - in which case the outer metal may stay quite cool even if the furnace is running. In addition, it is quite likely that your furnace is a condensing furnace (guaranteed if it is in Europe - don't know about US), in which case the flue gasses will be less than 100 C anyway. Commented Oct 26, 2017 at 13:11
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    I'm confused that you describe this as a furnace and say it's blowing hot air, but also indicate that it heats your house via pipes/radiators. Most likely it's a hot water or steam boiler, and only heats water (and should not blow air). If it's a boiler and really blowing air, then the exhaust must be compromised. If it's not actually blowing the air mechanically, just emitting a lot of heat into your basement, that's actually normal for a boiler but you can minimize it by wrapping the pipes in your basement with insulation. Commented Oct 26, 2017 at 14:49
  • Sharing a picture and make/model of the equipment would help clarify. Commented Oct 26, 2017 at 14:49

1 Answer 1


First: The situation you describe I would consider borderline hazardous if not clearly dangerous. (CO poisoning hazard) Please do not continue to run this appliance under these conditions.

Now to my practical input; I worked on boiler (we call this type of appliance a boiler) once that displayed this type of problem. The firebox in these things is lined with a fireproof insulation. One of these blocks of insulation fell from the 'ceiling' of he firebox and dropped onto the top of the heat exchanger. This blocked the normal flow path of the products of combultion up the vent and they were generously spilling into the room/house. Once I reattached that block of insualtion back in its rightful place, everything was fine. However, the boiler required a couple of hours of my time with a long handled wire brush and a shop vac to rid the exchanger of the resulting abundant soot buildup.

When natural gas does not burn properly, efficiently and completely the result can be and often is the generation of CO gas. This is a dangerous substance when inhaled especially for children and little ones; in can be fatal. Please, when making decisions regarding the operation of these appliances, always put the health of your family first.

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