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TheMG
Premium
join:2007-09-04
Canada
kudos:3
Reviews:
·NorthWest Tel
reply to dm19891

Re: Furnace starts, then shuts off.

Hmmm the OP is a bit unclear now that I read the post again.

He mentions that the main blower comes on, and also mentions that the gas cuts out 3 seconds after igniting.

That doesn't make sense, as normally the main blower does not come on until later in the sequence, after the burners have had a chance to warm up the heat exchanger.

Unless this particular furnace is designed a bit differently than most or something.

Anyways, one thing that should be mentioned is that the diagnostic LEDs on the controller board should indicate the fault condition. So I guess it's time for the OP to dig out the instruction manual and have a peek at the LED code.



KrK
Heavy Artillery For The Little Guy
Premium
join:2000-01-17
Tulsa, OK

2 edits

Ok you got me curious, so I went out and observed a start up. Mine is a Criterion II.

The thermostat calls for heat, then a blower starts up. But it's not the blower that blows air into the house? Or maybe it spins at a slow speed or in reverse or something... I'm not sure but it's definitely some type of blowing/venting BUT is not coming into the house yet.

Then the igniter starts firing and about 1 sec later the burners ignite. Then the burners burn for maybe 10 seconds or so and then the air starts coming out through the house vents. This seems designed so it doesn't fire a few seconds of cold air into your house first.

The only way to learn more would be to take the doors off and watch it again (with the safety interlock pressed).

--
"Fascism should more properly be called corporatism because it is the merger of state and corporate power." -- Benito Mussolini



rockotman
...Blown On The Steel Breeze
Emerging Research
join:2000-08-06
DSotM
kudos:2

1 recommendation

said by KrK:

The thermostat calls for heat, then a blower starts up. But it's not the blower that blows air into the house?
What you hear is a small draft motor towards the top of the furnace that promotes venting and fresh air in the ignition spaces during start-up.
--
Shine on you crazy diamond...


Vchat20
Landing is the REAL challenge
Premium
join:2003-09-16
Columbus, OH
reply to KrK

said by KrK:

The thermostat calls for heat, then a blower starts up. But it's not the blower that blows air into the house? Or maybe it spins at a slow speed or in reverse or something... I'm not sure but it's definitely some type of blowing/venting BUT is not coming into the house yet.
If it is anything like the furnace in our house (Most operate in fairly similar fashions as far as procedural steps to normal operation), that would be the blower that pulls in combustion air for the gas.

Essentially it should operate like this:
1) As the thermostat calls for heat, the furance kicks on the combustion air intake blower.
2) After a given period that #1 runs, the gas valve opens and the burners ignite (obviously sometime around here the igniter lights to accomplish this)
3) After the burners run for a sufficient period to heat up the heat exchanger, the main forced air blower kicks in.
-Now during this period the intake blower will continue to run as long as the burners are lit-
4) After the thermostat is satisfied and ends the call for heat, the gas valve shuts, burners go out, and the intake blower cuts out.
5) After a sufficient amount of time for the heat exchanger to cool down (some furnaces use a temp sensor, others go by a set time delay. Mine is the latter and the delay can be set via jumper on the control board), the main forced air blower shuts off and that is that until the next time a call for heat is made.

To the OP, I will concurs with the others here on the flame sensor. That seems to be the most common culprit of all especially of the furnace has not had a checkup or maintenance in a while. And it is the easiest and cheapest to check. Take it out and clean it yourself. If that does not solve it, then you can go down the next options until something sticks.
--
I swear, some people should have pace-makers installed to free up the resources. Breathing and heart beat taxes their whole system, all of their brain cells wasted on life support.-two bit brains, and the second bit is wasted on parity! ~head_spaz


KrK
Heavy Artillery For The Little Guy
Premium
join:2000-01-17
Tulsa, OK
reply to rockotman

said by rockotman:

said by KrK:

The thermostat calls for heat, then a blower starts up. But it's not the blower that blows air into the house?
What you hear is a small draft motor towards the top of the furnace that promotes venting and fresh air in the ignition spaces during start-up.
Thank you
--
"Fascism should more properly be called corporatism because it is the merger of state and corporate power." -- Benito Mussolini