It's still heating the house which is why we have never considered it a problem so far this season. However I recently got a smart thermostat and really started noticing how long the startup takes to get hot air moving.

Is this a serious problem? I'm thinking at minimum it's probably worth having a tech come take a look.

We have a carbon monoxide detector and it hasn't tripped or registered anything. The furnace is sealed combustion furnace so I wouldn't expect co to be an issue? I'm mostly concerned about it being efficient with gas usage or damaging the equipment.

The furnace is a Coleman DGAA070BDTB.

  • What model Tstat and what are its delay settings put at....? And what type of gas?
    – Mazura
    Commented Mar 7, 2023 at 23:48

3 Answers 3


The flame should be blue with some orange tips, if not it is more than likely a serious problem.
From your description it does not have enough oxygen - thereby depositing a lot of carbon on the inside, which is gas that did not get burned. That will have a big impact on your heat output. This will slow down the heating of the bonnet causing the fan to start later if it is thermostatic controlled as your description indicates.
I suggest you get somebody in to fix the problem. Depending on a CO detector for your life is questionable. They can and do fail.

  • 2
    Yeah, the OP should check the combustion air intake for blockages as well Commented Mar 8, 2023 at 3:21
  • In this case, the carbon monoxide is probably going up the chimney. It's the other byproducts of incomplete combustion that are the risk here.
    – Mark
    Commented Mar 8, 2023 at 4:50

I found the manual for that furnace here. It says:

Observing Burner Operation

1.Observe burner to make sure it ignites. Observe color of flame. On natural gas the flame will burn blue with appreciably yellow tips.On Propane gas a yellow flame may be expected. If flame is not the proper color call a qualified service technician for service

  • There's water in the lines...or it's running on propane, +1.
    – Mazura
    Commented Mar 7, 2023 at 23:42
  • @Mazura ... or natural gas is burning with inadequate air !!
    – user20637
    Commented Mar 8, 2023 at 10:57

It may be that this behavior completely normal. Most forced-air furnaces have a thermostat inside the furnace that, when it detects that the gas is burning, turns on the blower. And it continues to run the blower until the heat from the burner has largely dissipated. This helps blow most of the heat into the house without a chilling draft when the burner is not lit.

You should get a knowledgeable friend to look at the unit and see if its behavior seems correct.

  • 2
    that "knowledgeable friend" can also be a service tech being paid to do upkeep on the unit, Commented Mar 7, 2023 at 13:30
  • 6
    I thought we were supposed to be the "knowledgeable friend"...
    – FreeMan
    Commented Mar 7, 2023 at 19:24
  • AFAIK it's a delay timer on the board for starting and stopping, and possibly adjustable with dip switches. The high limit switch tells it not to fire anymore if it gets too hot, and that's the only 'thermostat' inside it. The new Tstat might have it's own delay settings too.
    – Mazura
    Commented Mar 7, 2023 at 23:46

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