Furnace not kicking in

[1victorianfarmhouse]

I live in a big old Victorian house. Years ago it was made into a two unit. While it is all now one house again, it still has two furnaces. The bigger one in the basement heats the basement and the main level. The smaller one in the attic heats the second level and to some extent the unfinished attic. Each has it's own thermostat.

I came home from work yesterday and thought the house felt kind of chilly for the outside temps being in the mid 40s. Upstairs was clearly warmer. I checked the basement furnace, and it was cold and trying to start. The gas flow valve on the pipe was in the correct position, and there is gas coming into the house as the stove lights up with no problem. I fooled around with the igniter and found it will heat up (I can see the igniter through the little window), but the furnace will definitely not light, kick in, whatever the correct term is. I replaced the entire motor assembly myself 9 years ago.

This whole deal worked flawlessly during the Chicago area "Polar Vortex" a few months ago.

Comments as to where the problem may be?

Thanks!

Vince

 

[bud16415]

Just a wild guess Vince. If it is a newer unit that vents out thru PVC like mine and it sounds like it is with the igniter and all. There is a pressure switch that picks up when the fan that draws the exhaust gas is running. Kind of a failsafe thing that wont allow the furnace to light unless there is a negative pressure caused by the fan.


I had mine stick I called the furnace man and he tapped on the thing with a screwdriver and it was enough to unstuck it temporarily. He said that’s it and then replaced it.


It’s worth a try.

 

[1victorianfarmhouse]

Hey Bud, thanks for the interesting reply! I did check as you suggested and there is a PVC vent from the fan that goes to the outside. The fan goes on and sounds like it is working just fine. There is an on/off switch on the back of the furnace that allows the fan to be turned on/off manually as well, and the switch works. There is a date of 12/96 written on the side of one of the inner panels in the furnace so that may give a clue of how old the unit actually is. So, still not "kicking in" and wanting to get it working correctly, but live-able.

 

[bud16415]

Then it could be that pressure switch.


The T-stat calls for heat so the furnace starts that fan. It is allowed to run for a short time and if this switch I’m talking about sees that the fan is working it allows the igniter to come on and after that is shown to be working the last thing is the gas comes on.


The switch has a couple wires and a tube running to it.


I have gotten to marking the date on things also. A furnace used to last 100 years. Now they tell me 20 years. I don’t know.

 

[1victorianfarmhouse]

Hi Bud, Thanks for the additional info. I shoud be able to find that switch pretty easily. I pasted a shot of the open furnace

 

[1victorianfarmhouse]

Thanks, Bud! attached is a picture of the furnace in question. Tomorrow or Monday I'll be able to clean it up and fool around with it a bit more. Vince

 

[bud16415]

That can in the center of the photo is what I was talking about it has a diaphragm in the can I’m guessing and the switch to the side gets made when the big fan at the bottom runs.


Again I’m not a furnace-man maybe some others will suggest best method to test it and also what else to test.

 

[1victorianfarmhouse]

Thanks, Bud! I will check that today and let you know what I find. The big fan at the bottom is not starting, that is the main problem.

 

[sadavis80]

If that 'big can' at the bottom isn't starting, then, yes .. THAT is your problem. That is the fan that forces the flame fumes out the PVC pipe and it MUST be running before the furnace will light. It's a common problem since the PVC flue 'sweats' as it cools off .. that drips WATER down into the fan and causes it to rust. If that fan has been in there (without maintenance removal and cleaning) since '96, then it's most likely rusted internally. Remove it and replace it. While you're at it, make sure the flexible pipe attached to it is clean as that is what transfers the suction to the suction switch in the center of the picture that actually activates the gas to the burners to allow the heater to HEAT
Steve

 

[Fireguy5674]

When you say the big fan at the bottom is not kicking in, do you mean the fan in the picture or the squirrel cage fan in the bottom of the furnace out of the picture? If you are referring to the fan out of the picture, it will not run until the burners fire long enough to heat up the heat exchanger. There are several things that must prove out before the burner will light besides just the pressure switch. There should be a wiring diagram on the inside of your furnace cover. It should tell you what has to happen before your furnace will light. The induced draft fan has to draw enough vacuum to close the pressure switch, then there are other safeties that must function to provide power to your gas valve so it can open. If you think your pressure switch is not working, pull the wires off the spades terminals and tie the wires together to bypass the switch. If your burner lights then you need to replace the switch. DO NOT OPERATE YOUR FURNACE THIS WAY. This is a safety switch and must not be permanently bypassed. That furnace is old enough there are not near as many things to keep it from lighting. Newer furnaces have many more safeties. Do you have a printed circuit board down on the front of your squirrel cage blower? That can cause problems as well but usually causes the blower to run continuously.

If you are talking about the fan in the picture not running, sadavis80 is correct. Your furnace will not fire if the induced draft fan is not running.

 

[1victorianfarmhouse]

Thanks for all the great info! It's taken me a while to get to playing with it and trying to figure it out. The big can in the bottom left (induced draft fan) of the picture does start. I will check the pressure switch tomorrow and see if bypassing it makes a difference. I do also have a PC board down on the big fan below and not shown in the picture. To be continued....

 

[DFBonnett]

Bad gas valve? Had that on mine. Tstat called for heat, induction fan ran, igniter clicked away, gas valve didn't open. Gas valve replacement was the fix.

 

[Fireguy5674]

A bad gas valve can certainly be the culprit but make sure you are getting 24 V to the gas valve before you decide to replace it. If you have a multi meter check to make sure you 24 volts coming to your pressure switch as well.

 

[1victorianfarmhouse]

My apologies for not replying sooner, but the weather got warm enough that the furnaces didn't need to kick in. Now, in October, the weather has started to get cold enough at night that the furnace was needed, and I could play with it some more. Today I started fooling around with it and tried to check the pressure switch by pulling off the terminal wires and connecting them. Plenty of flame, but nothing seemed to happen with holding them together. So I reconnected the terminals on the pressure switch, and .....it worked again! So I will keep an eye on it and see if it continues to operate like it should. Once again, thanks for the great info!

 

[user 4883]

might not be getting enough voltage to the gas valve to open it. take a look at the flame sensor rod, might need cleaning with steel wool or a fine grit sand paper.