Consistent failure after one cycle... troubleshooting a furnace

GopherT

Joined Nov 23, 2012
8,009
Mine had a similar problem and would restart one time as well. It turns out that most furnace companies allow one run on reset to allow you to heat your house until the furnace man arrives.

The LEDs flash an error code pattern (four short and a pause means one thing while a two short and a pause mean something else - as an example). The error code pattern should be flashing if it does not start. Look at the pcb or the circuit diagram on the inside of the front panel of the furnace. The codes should be listed in one of those places.

One diagnosed, it will likely be in order
1) vent fan not starting (high efficiency furnaces must detect a slight vacuum in the exhaust before starting). If the exhaust fan is running, the pressure sensor is faulty (this is more common than faulty fan). This will likely prevent the igniter from even trying to fire up the gas because there is a risk of carbon monoxide buildup without proper venting. The pressure sensors look like round about the size of a hockey puck. There may be two, one will check flow in the main furnace fan.

2) if flame does light on second cycle then immediately goes out, the flame sensor is not working. This is unlikely since it would be a problem on the first cycle too.

3) if the glow plug or spark igniter keeps firing (or heating) with no gas, then the valve is likely faulty. This is unlikely since it would be a problem on the first cycle too.

Again, check the codes output by the flashing LEDs - rhe exact patterns of the codes depend on the brand of furnace so I cannot advise from here.

Good luck, i know it is very frustrating sometimes.
 

Thread Starter

s_mack

Joined Dec 17, 2011
198
The pattern is 6+1 and simply indicates that it failed to start and then goes into soft lock.

The problem, if you Google as exhaustively as I have, is that this Honeywell "Smart Valve" is known to provide useless and/or false errors so its not a reliable diagnostic. That's why I didn't bother to mention the code (well, that and I wasn't asking for furnace help lol)

- Steven
 

GopherT

Joined Nov 23, 2012
8,009
Still, check the vacuum sensors. If they are essentially extensions of the circuit board. If you tap them hard, they tend to free up for a few cycles.
 

shortbus

Joined Sep 30, 2009
10,045
May not apply but what I found with the furnace in my garage/shop. It was doing the same thing, a friend said same thing that GopherT said about the vacuum sensor. Replaced it and still the same. Don't know why, but started to replace the exhaust vent but looked into it first with flashlight. There was the problem! During the summer a mouse had decided that the PVC pipe would make a great place to live. My guess was that the nest material was bunching up during the first furnace cycle and not allowing the exhaust to work and shutting down the furnace. Next time I tried it it had expanded enough to let air/exhaust through again and kept doing it over and over.

Blew out the nest from outside with air compressor and the furnace has worked since.
 

Thread Starter

s_mack

Joined Dec 17, 2011
198
Well, no mice around here... but I'll give the exhaust a good look through. You mention PVC so you must have a high-efficiency, right?
 

Thread Starter

s_mack

Joined Dec 17, 2011
198
The valve arrived early. I didn't bother waiting for the HVAC guy... I installed it, and the problem appears to be solved. Guess my "smart meter" was just a bit brain dead.

- Steven
 
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