The Gas company came and checked the Gas pressure and all is well. You're right, they came out right away and wanted to be safe than sorry and they do this free of charge.

The gentleman that came also exclaimed that he's never seen a setup like mine. He did assure me that the control unit I described is in no way connected to the furnace but it could be that the wires go to the furnace enclosure to run the wires to another "motor" unit which controls some sort of a vent. Still strange and I don't understand it.

He did recommend that I clean the furnace (normal maintenance) and to call a specialist. That's my next step.

There's more than what you've told us.

This control unit - is a gas pressure switch for a forced air inducton blower. I don't know why you have one, maybe code requirements?

There's CO sensors under your exhaust hood on the top of the hot water tank. It is part of the control loop for the gas valves and the blower unit. Makes sure the flue is open enough and the blower is operating properly.

I'm guessing now... there's more under the hood in the furnace. My guess is they are interconnected in some fashion to use a single blower?

Second guess is that the interlock on the furnace side is improperly connected - as it won't acutuate the gas valve in the funace unless the hot water tank is sensed to be on, thus allowing the blower to fire up.

You need an expert - and insist on having him tell you exactly how it works. More importantly, why does it only work in what appears to be a fault mode? I'd ask them to re-configure it; this time failsafe!

Look here for more... http://www.fieldcontrols.com/manuals.html

I only skimmed enough to get a feel for what you've got going on in there - you can probably look around for the rest of the apparatus and fill us in with more details.

Good luck and keep us in the loop.

Our new man stew may be on to something. If the hot water heater is using the furnace's draft blower for its draft, the pressure switch could turn on the draft when hot water heater came on.

Now, if the furnace controls that turn on the blower for the furnace failed, the furnace may only run when the hot water heater already had the draft blower on. The weird connection may not be the problem. It just confused the issue of the furnace not starting the draft blower.

Might even be able to rat out the blower problem yourself. Chances are though, it is on the control board and very tough to find andfix.

Engineering degree keeps getting in the way... :)

Follow all the wires - take some more pix.

Try to draw out a schematic - might be able to figure it out if we know what all is in the circuits. This stuff is usually simple relay logic (or solid state equivalent).

There's something messed up in the control logic that interlocks out the furnace. I'd be fairly critical of the original installer - while this failure mode is relatively benign, I begin questioning what other failure modes might have been overlooked.

This one's got my attention...

Guessing now... you got one of there?

http://www.fieldcontrols.com/WD_PDFS...e30milliWD.zip

I don't know how the "post purge timer" is supposed to work, but I'd guess the blower has to be on for some time before it will allow the gas valve to open.

Depending on the internals of that little beast - I'm guessing the pressure switch on the water heater is backfeeding the furnace circuit - letting it fire the gas valve if the blower is already running and the thermostat calls for heat.

Now I'm going to wait and see what you find out.

Boy this is getting exciting :) What I have looks practically identical to this:

http://www.fieldcontrols.com/instruc...s/46285300.zip

I'm going to have to take apart the Furnace to see where the wires are going but I can't do that until Monday (military duty). I can't wait to get back. I have a hunch we're now on to the root cause of this problem.

Casey


P.S. For some strange reason the problem (after 30+ days now) has stopped without me doing anything at all. I suspect this is only temporary until the weather gets real cold again.

OK - keep us posted on your findings. We'll be waiting for your response. This one's been FUN!

By the way - on the military duty - THANKS!

Be careful!

Great, a furnace problem that gets worse when it is cold.

When things are working normal, does the small draft blower, or anything in the furnace, come on when the hot water heater is on? That would confirm my idea that the water heater shares the furnaces draft blower. Thus if the furnace controls failed to start the blower, the furnace would run once the water heater started the blower. It would be like the furnace blower controls failing and the furnace running with the fan in the on position as reported here frequently.

This is an interesting one, I thought the furnace was dependent on the water heater ,for some reason, just did not think of the power inducer. I had a setup similar to this, only in respect to the length of the flue duct work, and had a power inducer with a common flue for both units that would only come on when the water heater came on. My furnace looked very similar to the one in the picture, and if you notice you can see a built in inducer built into the furnace.

So, my guess is that is what they intended to , but for some reason either accidentally or intentionally added the furnace control circuit to the power inducer control circuit.

I wonder if they are trying to control the built in draft inducer in the furnace?

Stew great job, just hope I was able to add something relevant.

Hello Folks. I'm finally getting a chance to revisit this problem. As a refresher, the problem is still such that the Furnace only kicks in when the Hot Water heater goes on. I've even narrowed the behavior further... get this... this problem ONLY seems to happen when the temperature is very cold. If the outside temp is above freezing then I don't seem to have that problem but when the temperature is really cold then the only way to get the furnace going is by forcing the hot water heater to come on by either letting the hot water run inside the house or by going downstairs and raising the dial to a hotter setting.

So... what I've done so far was to clean the furnace. No change. When I opened the furnace to see where those wires went from the water heater it was like a rat's nest of wires in there. I wasn't like originally thought previously that the wires lead to the external blower. The wires actually connected to some sort of control unit inside the furnace and after that I was lost.

At this point the unit is beyond the warrantee period so I'm not sure I can call the builder for repairs. If anyone has any other suggestions to try just shy of calling in a specialist I'd really appreciate it. I'd hate to give up on this one.

Also I'm going to take some more pictures tonight of the wiring inside the furnace and hopefully that will provide a clue.

Casey

OK - we're here when U got more info. Let us know what we can do to help. This one is too fun... :D

I guess Stew is right, the tough ones are more fun. Glad to see him back again. Did you see the question on the sloshing Freon at the ''A'' coil?

Does the furnace have 2 firing rates? It still sounds like the furnace won't run without the inducer. The signal from the furnace isn't starting it, so it only runs when the hot water heater is on. That's my story, and I am sticking to it. It could be the furnace controls work to start the inducer at the low rate, but not the high. Great situation, the furnace won't run just when you need it the most.

I haven't forgotten about this. I've been getting home late and the lighting in the basement has been poor. So every picture I've taken so far is horrible and doesn't really show what I want it to show. So I've gotten some special lighting equipment from a friend to disperse the light (soft light) so I can take a good shot of this beast. Otherwise I really can't explain what or where all the wires go. So as soon as I get home I'm putting my photography makeshift studio in the basement :)

Casey