Gas forced air heat, instant on? Help!

[Scottpat]

I had a first today. Gas fan forced heat. When I turned the thermostat to heat, the blower came on and started blowing out cool air. It took about 2-3 minutes for it to get warm.

Shouldn't the blower be on a delay so cool air is not being blown out?

[hausdok]

Yep.

OT-OF!!!

M.

[Bill Kibbel]

Newer furnaces have an electronic control board to turn on the blower. Some have the control as part of the furnace control board. Unless the control wiring was really installed wrong, the board usually needs replacement

Most older furnaces (that also usually have a standing pilot) have the old dial type fan/limit control. Could be stuck, miswired or failed. Some fan/limits have a "heat assist" or "time delay" that turns on the blower as a function of time or temperature and this feature could have failed.

You sure it wasn't a heat pump with gas back-up? []

[paul burrell]

Originally posted by Scottpat

I had a first today. Gas fan forced heat. When I turned the thermostat to heat, the blower came on and started blowing out cool air. It took about 2-3 minutes for it to get warm.

Shouldn't the blower be on a delay so cool air is not being blown out?

Yes we call them hi lo temp control. The air blower should not turn on until the heat is at the desired temp, like wise turns off when temp lowers to pre set temp. These controls wear out and have to be repaired or replaced. Had to replace one on my furnace.

Paul B.

[Scottpat]

Thanks for the help! Out of 5,000+ homes I have never run across this condition. I can recall in the back of my mind Mike Casey telling us about this in my ITA class in 1991![:-bigeyes

[Les]

This is a perfect example of regional differences! We do quite a few inspections and have not seen anything other than a gas forced air since first of the year. All the above responses are right. I have an inspector that has worked for 9 years now and never seen a heat pump!

[kurt]

I've never seen a heat pump either, at least not while inspecting. I think I saw one on vacation in Florida.

[Les]

Got to thinking on my first inspection about Mike's response. He is not entirely right, as there are several models that will purge the entire system before any burner controls engage. In other words, the air blower comes on for few seconds, then the inducer fan comes on, then hot surface ignitor (or equiv), then ignition. Of course there are variations to this because there are 12,673+- different brands of furnaces.

[hausdok]

Hmmm,

Like you, 99% of what I see is gas, forced-air furnaces. In fact, the boiler I looked at yesterday was #10 in just under 10 years. Can't remember ever seeing one where the blower came on immediately. Exhaust inducer, ignitor, etc., but never the blower. Suppose I could be mistaken and just didn't notice that the blower came on with some. I'm wrong a lot I fear and the wife never lets me forget it.

ONE TEAM - ONE FIGHT!!! (or is it two?)

Mike

[carle3]

Just a quick note. Yesterday I had a Rheem 85% effiecient furnace that when heat was called for the cycle of operation was for the air handler to run for a minute it would then shut down and then the induction motor would turn on. After a minute the ignitor lit and then gas burners lit. After that sequence the air handler would then turn back on. As I read these post one could jump to the conclusion that the air handler should never be part of this sequence.

[Neal Lewis]

Les,

What is the purpose of purging the system. I have never witnessed that sequence of operation before.

[hausdok]

Carle3

If you took the cover off the furnace without turning the thermostat down and the heat off first, it will usually do just that as soon as you put the cover back on the air handler. Pulling the cover off the air handler with them still on and calling for heat screws up their little digital brain and then they act a little weird until they reset themselves. However, turn the stat down, let it cool itself and shut itself off and then kill power to it and take the cover off and they start up with a normal cycle of inducer, glow-wand or igniter, gas valve/ignition and finally the blower comes on.

ONE TEAM - ONE FIGHT!!!

Mike

[Les]

Carle3,

That is common and usual for Rheen, also most Amana units will do the same thing in sequence. I am under impression the purpose for this sequence is just to purge system. I have never made note of what particular brands sequence that way other than some Rheems and Amana.

I first ran across this several years ago when an inspector called me at office with the "odd" sequence and wanted to know if it was reportable. After several phone calls to hvac guys, we are of the opinion that it really does not matter. Then of course there are the 10% of gas forced air units that the air handler never stops completely and the summer/winter switch common in Mich.

[carle3]

Hi Mike,

My normal sop on furnace inspections is to flip off the furnace disconnect and remove the service panel prior to firing up the unit at the thermostat. This way I can turn the unit back on from the disconnect and be there to watch for any rollout during the firing sequence.

[Jim Katen]

Originally posted by Les

Carle3,

That is common and usual for Rheen, also most Amana units will do the same thing in sequence. I am under impression the purpose for this sequence is just to purge system. I have never made note of what particular brands sequence that way other than some Rheems and Amana.

I first ran across this several years ago when an inspector called me at office with the "odd" sequence and wanted to know if it was reportable. After several phone calls to hvac guys, we are of the opinion that it really does not matter. Then of course there are the 10% of gas forced air units that the air handler never stops completely and the summer/winter switch common in Mich.

And that doesn't include the wierdness that happens when you 'jump start' them from the circuit board. . .

- Jim Katen, Oregon

[Les]

Jim, yep that will cause permanent brain damage. 84+-yrs ago, it was not uncommon to jumper at valve to fire 'er up! When I first started inspecting, we would bring a little kindling and a lump of coal along to the inspection.

[hausdok]

ROFLMAO !

Good one Les. Is it true that you're old as dirt? You sure didn't look that old when we met at Inspection World. Was it petrification or putrification that helped you age so well? []

OT - OF!!!

M.

[hvacman]

There are a number of reasons that the blower will start at the instant you have a call for heat; t'stat wired wrong, no delay on fan board selected ( doubtful), fan relay bad ( stuck "picked" ) and if relay is on the units system board...the board is bad.......

[bodeen0]

More than likely someone installed a digital t-stat and has the switch in the electric position instead of gas.