PDA

View Full Version : Furnace fan slow to turn on, off



Bent
Sep 18, 2005, 10:13 PM
We recently (as in last week) purchased a cabin equipped with a Day and Night forced air furnace. The fan for the furnace seems to take an awful long time to come on after the burner assembly ignites, and, similarly, the fan takes an awful long time to turn off after the furnace is turned off. It's making me nervous, so I wired a jumper at the thermostat so that, for now, the fan comes on immediately when the furnace starts.

I'm having trouble finding any information about this furnace (Day and Night), so I'm trying this forum for pointers on where I should look for a problem (or is there perhaps an adjustment to be made). Limit switch or fan delay timer relay? I could use a tutorial on principle of operation; I've never worked on a furnace before. Sorry, I don't have the model number; all I can say right now (until our next trip to the cabin) is that it was made somewhere between 1969 and 1983, and it has electronic ignition.

Thanks for any help!

dherman1
Sep 19, 2005, 05:59 AM
It is not uncommon for the burners to ignite before the blower kicks in. And Vice verse. Especially on the older furnaces like yours. The following is a rundown of how furnaces work.

The thermostat signals a call for heat. (If your stat is set for 70, the temp needs to fall a few degrees below that (~3-4 degrees) in order for the stat to signal for heat)

The Furnace lights the burners.

The burners warm up the heat exchanger.

Once the heat exchanger (plenum?) reaches a pre-determined temp, the furnace then kicks in the blower.

When the 'stat reaches the correct temp (about 3-4 degrees above the set temp) it sends a signal to the furnace that no more heat is required.

The furnace shuts down the burners but the plenum still has a large amount of heat in it that needs to be dissipated. So, the burner stays on for a few minutes (5-15) to allow for more heat to be extracted. And to prevent premature failure).

So, in other words, do NOT set the blower to shut off right away, allow it to run awhile to help prevent failure of the heat exchanger.

Think of it this way, your driving in your car in the winter. Your cold so you turn up the heater fan. Do you get heat right away? Nope, you just get cold air blowing at you. You won't get heat until the thermostat senses that things have warmed up enough. After your drive, you shut down the car and walk away. In the non-winter months, and if your car has an electric radiator fan, the fan may come on. This is done to dissipate excess heat in the radiator to prevent boil over.

I hope this helps, Dan

labman
Sep 19, 2005, 07:10 AM
In the heating mode, the fan delay is usually based on a limit switch. The fan doesn't come on until the air is heated. The fan also continues to run after the gas shuts off until the switch cools. There is also a second switch that shuts the gas off if the fan fails to come on and the temperature get too high. If it takes more than a few minutes for the fan to come on, and the air is extra hot at first, you might check the limit switch. If it is delaying both start and stop, I would guess it is dirty, mispositioned, blocked, etc. It will be just downstream of the burner, sort of a box with 2 wires, and maybe a probe in the air stream.

Furnaces come with a wiring diagram on them. If you can find one on the furnace, or in an envelope at the furnace, it may help you identify the limit switches. You might also find info at the Day and Night website.

Muadi
Mar 25, 2008, 12:46 PM
Sorry, wrong post.