Home HVAC help!

So my heater/furnace quit working. It's an older gas furnace with a pilot light. It hasn't been on for about a month(it's been pretty nice outside lately), and now that it's getting cold, it's not coming on as it should. I've checked the thermostat, it's fine. The blower works if I turn the thermostat in the "on" position. The AC works. The pilot light it lit. If I jump the two wires that the thermostat connects, it does nothing at all. I tried that at the thermostat and at the connection on the furnace. I could get the ac to kick on by jumping the wires, but the furnace didn't do anything at all. No noise, no spark, completely dead. I couldn't find a reset button. I also checked the fuse on the side of the furnace, it was fine. So what's the problem? The little control box thingy mounted on the side of the furnace? What's that thing called and where would I get one if that's the problem.

There's a thing called a 'thermocouple' on pilot light equipped gas fired appliances that will do something similar. It's sorta like a high/low temperature limit switch.

http://home.howstuffworks.com/how-to-maintain-a-furnace1.htm

They look like this:

From what I've read (on that site and others) the thermocoupler just effects the pilot light, and I'm not having any problems with that. It does sound like the limit switch they refer to in that link might cause the problems that I'm experiencing.

Interestingly enough, depending on how old the furnace is, there could be a "check engine" light as well, that will flash in a sequence, or light a combination of 2 out of 4 bulbs or some such E36 M3. I didn't know this until ours crapped out. It was the high limit switch (the limit switch craps out and therefore thinks it's on fire.) Look for lights.

The pilot light is burning, but the burners do not come on, right? That means something is telling the gas valve not to allow gas through to the burners.

The thermocouple can also cause it not to light but the pilot will still work. This happened on a gas water heater at one of my dad's properties, drove us nuts for a few hours.

I've only messed with the really old ones, like through the 1970's. Haven't seen what poopshovel describes but it would be nice if it could tell ya.

I checked for lights, no dice. This thing is friggen' ancient. So I bypassed the limit switch and controller to no avail. I couldn't find my multimeter, then had other things I had to do so I gave up for the evening. I'm starting to think it's the transformer. Does anyone have a wiring diagram of a traditional older gas furnace I could look at? I didn't have much luck finding anything online.

On a unit like yours, there is always a detector that lets the controller know the pilot light is lit, and if that signal is not received, the main burner will not be allowed to open. It's to prevent your house from getting filled with gas. The detector signaling the control is usually a thermocouple.

Your library likely has books covering gas furnaces quite well. Be they in the home improvement or HVAC sections. Online, these three are fairly decent 101 types. http://homerepair.about.com/od/heatingcoolingrepair/ss/gas_furn_trblsh.htm?once=true& http://homerepair.about.com/od/heatingcoolingrepair/ss/trblshoot_tstat.htm http://homerepair.about.com/od/heatingcoolingrepair/ss/pilot_light.htm

How have you checked the thermostat btw? Just watching the mercury roll around in the tube doesn't mean the furnace gets told the signal. Make sure the signal is getting to the controller. Lack of spark in the mercury tube is a darn good indication of no signal.

Maybe you're cold because you tore the outside walls off the house!

Sorry, I had to.

Of course SVreX had to throw in the missing walls thing.

I just thought of something else: normally the transformer for the thermostat is on the same breaker as the furnace, meaning if you throw the breaker you kill power to the furnace and its controls all at the same time (smart, if you think about it). I had a house once which was wired all funky, white wires were hot and blacks neutral, all kinds of kooky crap. That one had the furnace on one breaker and the thermostat on another, meaning you had to throw 2 breakers to disable the whole thing. Dig out your DVOM and check for, IIRC, 24 VDC across the terminals going to the mercury bulb. No voltage = no power to the T stat, start checking for breakers unrelated to the heater that are popped or ?

foxtrapper wrote: How have you checked the thermostat btw? Just watching the mercury roll around in the tube doesn't mean the furnace gets told the signal. Make sure the signal is getting to the controller. Lack of spark in the mercury tube is a darn good indication of no signal.

Well it's a digital thermostat, so I checked my old mercury thermostat to see which wires were jumped to activate the furnace, and figured it out to be the red and white. Used a jumper wire, nothing. Used a jumper wire to jump the red and yellow, and the ac fired right up. Then just to make sure I was doing it right, I disconnected my thermostat and hooked up my old mercury thermostat. On heat it did nothing, no spark or anything. On ac it sparked and the ac kicked right on. I then went down to the furnace and jumped the red and white wires on the transformer/controller thingy to make sure vermin hadn't chewed through the wires or something strange, and still nothing.

SVreX wrote: Maybe you're cold because you tore the outside walls off the house! Sorry, I had to.

Actually I was waiting for it. I know you had to.

Jensenman wrote: Dig out your DVOM and check for, IIRC, 24 VDC across the terminals going to the mercury bulb. No voltage = no power to the T stat, start checking for breakers unrelated to the heater that are popped or ?

I intended on doing that last night, but couldn't find my multimeter. My tools are still in total disarray from the challenge. I'm pretty sure the thermostat is getting power though, because it turns on the AC, and from the way it looks, the AC and heat share the same hot wire on the thermostat. There's definitely a spark when I turn the ac on, nothing when I turn the heat on.

A simplified thermostat wiring diagram that might help you:

This is for the type of 'stat which uses the mounting screws as contacts too.

OK, that sounds to me like you're not getting any power to the thermostat when it's in the heat mode, but it is getting power in the cool mode.

That would lead me to wiring diagram like Jensenman indicated. For the thermocouple has nothing to do with powering the thermostat (I think). I believe that circuit is seperately powered off a voltage reduction transformer.

But it also seems like you're saying there's only two wires to the thermostat. I'm going by memory here, but that doesn't sound right for a furnace and seperate a/c system. There should be more wires. One set for the a/c unit, and another set for the furnace. You switch between them when you throw the heat/cool switch on the thermostat. You may find the furnace wires for the thermostat still hiding back in the wall.

I'd again be going to the library. Most of the units all follow the same basic logic.

And do make sure it's not something embarassingly simple, like a tripped breaker, or the furnace power switch being off.

foxtrapper wrote: But it also seems like you're saying there's only two wires to the thermostat. I'm going by memory here, but that doesn't sound right for a furnace and seperate a/c system. There should be more wires. One set for the a/c unit, and another set for the furnace. You switch between them when you throw the heat/cool switch on the thermostat. You may find the furnace wires for the thermostat still hiding back in the wall. I'd again be going to the library. Most of the units all follow the same basic logic. And do make sure it's not something embarassingly simple, like a tripped breaker, or the furnace power switch being off.

No, there's four wires. From what I can figure, red is power, white is to the heat, yellow is to the ac, and green is to the fan. If I use a jumper wire, everything works but the heat, so the red wire is definitely getting power.

'White is to the heat'. I assume this goes to the 'box thing' you mentioned, right? IIRC there is a solenoid inside which is used to control gas to the burners. I think this solenoid runs on 110V and the thermostat uses a relay to control this solenoid, but I could be wrong; both may run on 24V. BTW, the thermocouple's job is to turn the gas off if it detects temperatures outside of a specified range, IIRC it works very similar to an air conditioner expansion valve.

In the picture below, you can see the transformer which supplies the thermostat etc on the left, the control circuit's small wires go to the top of the gas valve which is the gray box with the blue knob. Your white wire should enter the valve, or be connected to an external terminal board, at the same point the small wires from the transformer do, your red power wire for the thermostat will probably be connected there as well. Above it you see the gray wires which go to the thermostat, you can also see the orange wire nut to the right of the gas valve. The pilot and thermocouple are visible just below the metal cover, you can see the thermocouple's copper tube going below the gas manifold tube.

Is there a wiring diagram inside the heater cover? If so, probe for voltage as needed. If you decide to disassemble the gas valve, make REAL sure the gas is turned off outside and do not make any sparks!

There is some troubleshooting stuff on this page, might want to read down it. http://www.inspect-ny.com/heat/HeatFurnaceOperation.htm

At the risk of demonstrating my extreme ignorance of gas heating systems, is there a ground wire?

Not a dumb question at all. Home systems do have a ground wire, but it's not used in the same way as wiring in a car. AC electrical systems have a wire with power going out ('hot') and another so that power goes back ('return' or 'neutral'). The 'ground' is a separate wire which ordinarly carries no current. It goes from the case of a motor, transformer etc to a ground wire inside the house's electrical system which is then connected to a stake driven into the ground outside (hence the name 'ground'). It's there so that if a component shorts out, the electrical current has a safe path to the ground, as in ol' Mamma Earth. That way, if you grab the case of a component that has an internal short the electricity returns to earth through the wire rather than through you.

Jensenman wins! Thermocouple is the problem! I think the main issue I was having was not knowing what actually controlled the gas flow. howstuffworks.com didn't even mention that part. Once I actually looked at it with a fresh pair of eyes, I realized that there's two wires that go from the transformer to the gas control valve, with two switches in line, the thermocouple and the limit switch. I had already tried to bypass the limit switch with no results, so I just ran a new wire and bypassed the thermocouple and the limit switch and it fired right up. So I guess now I need a thermocouple. Are they pretty much universal? Mine looks like it was made in the 50's, so I doubt I'll find an exact replacement.

Never mind, it looks as if it is universal.

Glad you can get it going grassrooty style!

YaY! Great remote diagnosis J-man!

And I was just about to send you a beanie to keep warm!

Jensenman wrote: Glad you can get it going grassrooty style!

Heck yeah! I don't pay anyone for E36 M3 if I don't absolutely have to. I'll deal with no heat if I'm sure I'll figure it out eventually!

And David, uh, I meant, that thermocouple totally didn't work. Brrr... I'm cold.

Uh, my head and ears are getting sunburned, a bucket hat would help with that...