What does this problem sound like?

There is a 25 year old gas furnace in the house we rent. (The guy that owns the place claims there is NO problem at all. He said the igniter is brand new and working fine.)

It blows cold air for the first 10 minutes of each heating cycle. (Sometimes this happens for 30-60 minutes.) Then heat starts to appear fine.

The "blowing cold airs" periods eventually get shorter and shorter... as the "blowing hot air" periods get longer... and eventually, after 3-4 hours, the home is at the desired temperature.

Perhaps a thermostat problem?
Does the average thermostat independently control the "on/off fan" and the "on/off flame" events?' Perhaps the "flame on" part of the thermostat is bad or incorrectly adjusted?

Perhaps a furnace problem?
Is there some timer-device that decides how long to wait between "fan on" and "flame on" events?
Perhaps this delay is set too long? Or the device is bad?

Another (perhaps totally unrelated) thing:
The problem seems worse when the furnace is trying to heat the place from 60 degrees, up to a thermostat setting of 70. (Yes, it gets THAT cold in here sometimes.)

It seems to happen less often... when trying to heat the house from 75... up to a thermostat setting of 85. (Yes, have to set things VERY high, to lessen this problem.)

3 Answers 3


Can you look at the furnace during the time it is blowing cold air and see if the flame is on?

The thermostat has the ability to control the fan, but typically the thermostat sends a "turn heat on" signal to the furnace and the furnace controls are setup to turn the flame on for a X amount of time (to warm up the air) and then turn on the fan once the timer expired (so the air coming out of the vents is not cold). When the heat is turned on is the flame and fan going on at the same time?

Not sure about your furnace, but some have a couple of LEDs that are lit up and can be used to read a status or error codes. If you have the manual you could try to decipher the code while the cold air is running and see if it gives you a clue.

I would start by trying to figure out if the problem is with the thermostat or the furnace by installing a new thermostat and seeing if the problem goes away. If the problem is still there you can just return it. I had a furnace issue where it was going off randomly and would work in heat mode but not cool mode. It turned out that the thermostat was bad and sending intermittent signals.

  • 1
    If it's blowing very cold air for 10-60 minutes... wouldn't that HAVE TO mean the flame is not lit? (We have no way of seeing if the flame is actually on/off... but wouldn't it have to be off?) Maybe we should try replacing the thermostat... but I thought you meant the thermostat isn't the thing that even controls the flame on/off status anyway. Right? So I'm not sure any part of the thermostat can be bad... or can it? (How can it be sending intermittent flame on/off signals... if it doesn't even control the flame.)
    – Lora
    Dec 3, 2010 at 21:01

Even 10 minutes is too long from time to call for heat to warm air arriving; it sounds like a two stage furnace, and the first stage is producing no heat, or there is too much heat loss from furnace to the registers; when the second stage kicks in, then you're getting heat. It could be the thermostat, or the furnace. It definitely needs to be looked at. Does more air come out when it is warm? Does it sound like the blower is running faster?

Depending on where you live, there may be regulations that require living spaces be heated to at least 65 degrees during the winter, and failure to do so is grounds for breaking a lease; I would check into it, but you need to work with the landlord if at all possible to have the furnace and/or thermostat checked.


High limit switch, (example L240-30 ) it controls fan too, also it is a bimetal switch working on heat on and off and goes erratic.

  • 4
    This is a very low quality answer. Please elaborate.
    – Doresoom
    Nov 12, 2014 at 17:23