Normal gas furnace behavior

Mike R._2

I was asked to look at a friend’s gas furnace yesterday that they think is acting erratically. There is a tag on the gas valve that says it is a delay valve. When the thermostat is turned up, there is about a 30 sec. delay before the furnace starts blowing. When you turn the thermostat down, the furnace runs for 3-5 minutes before shutting off. I am not familiar with gas furnaces – are these delays normal or should they have someone check it out?

There is terminal block with several unused points on it – the thermostat is connected to ‘W’ and ‘4’, it’s a Bryant furnace. Is there any way to know if it was connected right in the first place?

Thanks,
Mike

Mark Hunt

The delay


on the blower is normal. The heat exchanger needs to warm up before the blower comes on otherwise you would be getting cool air blowing out of the registers.

As for the fan off delay, you really need to look at the manufacturers instructions to see if it can be adjusted.

Hope this helps.

Mark H

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Jeff Lawrence_13

off delay

The off delay is a timer on the newer stuff. On the Circuit board, there are a set of pins that you move a jumper around on to get the correct off timing. You need to check several things before you adjust the timimg, including temp rise across the H/X. There will be a certain amount of residual heat in the H/X even after the 2-3 minute off delay.

Hope this helps.

J

Don_4

Normal Operation

As already stated, the delay of fan coming on after the burner starts in required to allow the heat exchanger to heat up enough to eliminate condensing problems and not blow cold air out of the registers. The delay on the shut down side is to allow the heat exchanger to cool down and remove the residual heat from the furnace. If the air blowing out of the registers is uncomfortable before the fan finally shuts off the control requires adjustment. This may be achieved by jumpers or dip switches on newer furnaces or adjusting the fan control on older furnaces.

Larry_7

Older Bryants

Older (green cabinet) Bryants have a diaphragm valve that delays on and off. The fan control also delays on and off. I think it is an 821 relay and it is adjustable between min and max. It's usually located on the right side in the cabinet. When the stat calls for heat, the valve is powered and opens on delay. 24 volts is also supplied to a heater in the fan relay and after heating a bi-metal switch, the fan starts. On off cycle, valve closes after a short delay and the fan delays off after bi-metal cools and opens the circuit to the fan. This was revolutionary technology back then.

On a no-heat call, the customer says when she turns up the stat, it just blows cold air. This simple statement speaks volumes to the service guy. 1:It's probably a Bryant or Carrier. 2:It's got 120V. 3:It's got 24V. 4:The bi-metal pilot is stopped up, out,or shot. When he gets there he finds it's none of the above and pledges never to diagnose en-route again. Things get more goofy after A/C is added with additional fan center, non-OEM valve with thermocouple and pilot wired to the burners, etc...

If your furnace is not the type I've described, please forgive the previous novel. Although they're relics, there are a bunch of them still kicking out there and are sure to deflate a few servicmen's egos (mine included) at every oportunity.

Be the Bryant...

Larry