Day & Night furnance won't come on.

[patcmontgomery]

We have an older Day and night furnace with a HH84AA009 or P771-7007 control board. The furnance is not coming on all of a sudden. The only thing that happens is the HFR relay clicks, nothing else. The limits switches appear to test good. The fuseable links test good(the blower would just run anyway, right). The 24 volt circuit seems to be working at least 24 volts are present from the secondary's of the transformer. So could this control board be bad? Seems like the CA3098E Schmit Trigger IC may be bad? Thoughts? Won't else would cause a no start condition?

[SeattlePioneer]

When the thermostat calls for heat, gas for the pilot burner and a spark to light the pilot light should both turn on together. The pilot light should light and stay lit for a minute or two before you hear another click which should be the main burner gas supply turning on, which should light off the pilot light.


How far does that sequence get in your furnace?

[patcmontgomery]

I don't get any of that in the sequence. No inductor fan, nothing, just the one relay clicking on the control board.

[SeattlePioneer]

The circuit board you list for your furnace doesn't operate with an inducer motor.

It has a pilot light that is lit each time the thermostat calls for heat using a spark.

The one click you hear when the thermostat closes should be a relay on the circuit board turning 24 VAC on to both the spark and turning on the pilot burner gas.

Check and see if there is gas coming from the pilot burner, and check to see if there is 24 VAC going to one of the pins on the gas valve.

If the gas valve roughly a cube, or sort of a flat rectangle with a blue control knob on top? Is the ignition module a small black box roughly 2x4x2", or a larger gray box about 4x6x2"?

[patcmontgomery]

Yes, I am affraid it does have an inducer fan and motor. No, I do not get any voltages on the gas valve wires.

The gas valve is the larger one with the blue on/off knob.

[SeattlePioneer]

Ummm. As I read through you post again, I see you are specifying that it's the heating fan relay that clicks. If you can distinguish that from the other relay, you are doing well.

You should hear the HFR click on only when you turn the power on to the furnace, not when the thermostat turns on.

Is that what you are observing?

Try disconnecting a wire from the limit switch --- which should turn on the fan.


Rather than getting voltage to the point that the pilot valve and spark would turn on as I suggested in the previous post, you are losing voltage before reaching that point.

It could be a defect in the three wire pilot, too. You could try tapping that with the handle of a dinner knife or fork to see if that jogs it into operating. When the pilot burner gets old, it wont make the needed connections reliably.


Unfortunately, I can't find a circuit diagram for that furnace off hand.

[SeattlePioneer]

Originally Posted by patcmontgomery

Yes, I am affraid it does have an inducer fan and motor. No, I do not get any voltages on the gas valve wires.

The gas valve is the larger one with the blue on/off knob.

Oh well, another theory shot to heck.

You'll need to post the model and serial number of the furnace to identify it better. This can be obtained from the rating plate in the burner compartment.

Does this have the three wire pilot burner?

I'm guessing this is a BAW model --- but my guesses don't seem to be entirely reliable tonight.

Sheet metal vent pipe?

[patcmontgomery]

Yes, sheet metal vent pipe. I have tried disconnecting a wire from a limit switch nothing happens. But If I disconnet a fuseable link the fan kicks on.

I do only here the HFR relay click when power is applied. No other time.

I'll get you the model number...

[patcmontgomery]

The model is a 376BAW036075.

I'm guessing I have a bad board?

[SeattlePioneer]

Unfotunately, I can't remember the circuit diagram of this furnace well enough to identify the problem, and I can't find a copy of it through Google.

If you can take a good picture of the circuit diagram, perhaps I can help you futher....

[patcmontgomery]

How do I post a picture? I scanned the schematic.

[patcmontgomery]

This should do the trick.

[patcmontgomery]

The output of the transformer is about 27.3VAC, do think that would make the difference? It is suppose to be a 25 VAC transformer.

[SeattlePioneer]

I don't think the voltage would be a problem.


I'm afraid I need to ask you to post the list of parts that are referred to in the circuit diagram --- I can't remember them reliably.

It should be right there with the circuit diagram.

Thanks.

[patcmontgomery]

I don't mind. If it helps me in the end.

[SeattlePioneer]

Originally Posted by patcmontgomery

I don't get any of that in the sequence. No inductor fan, nothing, just the one relay clicking on the control board.

Well, by taking a good deal of time I have relearned this circuit diagram, which is more complex than most.


Based on your observation above, here's what you should test:

1. Turn on the power.

2. Turn up the thermostat and verify that 24 VAC appears at the "W" contact on the circuit board.

3. Test both sides of the main limit switch, 7h2 on the circuit diagram. If no 24VAC appears on either contact, look for the auxiliary limit switch 7h3 on the circuit diagram.

4. If voltage appears on only one side of the 7h2 main limit switch, you probably have a bad limit switch. Try putting a jumper wire across the two prongs of the limit switch and see if that turns on the furnace.

[SeattlePioneer]

Thanks for posting the parts list.

You are rewarded by the following:

The 7H2 limit switch is the draft safeguard limit, not the main limit switch as I thought in my last post.

It's in a little sheet metal box on the right side of the inducer motor housing if I'm remembering correctly.

There's a small red button to reset that switch, and if you push it that's probably going to restore service to the furnace.

But come back, if it tripped there still some more to do!

[patcmontgomery]

Bingo! It was the limit switch on the fan. I know I checked the stupid thing once though. It reads open now though. Thanks a bunch for the help!

[SeattlePioneer]

Good!
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--- but remember that it may have opened for a reason, which could amount to a hazard.

What you've got is a small hole in the side of the inducer motor housing. If the venting system works properly, the hot combustion gasses go up the vent or chimney and little comes out the hole to cook off the limit switch.

If the chimney or vent is obstructed, pressure builds up in the venting system and hot combustion gasses come out that hole and trip the limit.

So you need to take apart the vent at the furnace and inspect the vent and/or chiumney for debris, a birds nest, dead squirrel or other material plugging up the works.

It's IMPORTANT to do this! The furnace could be a hazard if the chimney or vent is obstructed.

[patcmontgomery]

So what tells the blower fan to come on and at what point?

[patcmontgomery]

By the way, it is the limit switch on top of the main blower housing that trips. 7H1 I believe. Not the limit switch down by the inducer motor. Does the solid state time delay circuit energize the HFR relay?

What is happening is the furnace lights and runs but the limit switch opens on top of the blower housing because the blower never comes on.

[SeattlePioneer]

The 7H1 limit switch is the main limit that's in the center of the burner compartment, usually a light brown plastic base with two 1/4" spade clips coming out of it and a fusible link wired to it. If you pull one of the spade clips off, the fan should start running. It's an automatic reset limit that resets itself once the furnace cools off.

The 7H2 limit is the one in the side of the inducer assembly.

The 7H3 limit is the one I presume is opening. It's located in the fan compartment itself, usually on counterflow furnaces where the fan compartment is above the burner compartment.

And it's there for exactly the purpose you have discovered --- to shut off the furnace if the blower motor fails to turn on.

I was taken somewhat aback earlier when you said the fan motor didn't start operating when you pulled the wire off the 7H1 limit. I guess now we know why.


You may find it worthwhile to pull the fan assembly out and trying oiling the blower motor to see if you can get it to work. Sometimes you can, sometimes it wont. You have to disassemble the fan completely to get to both the oil ports.

Once you have oiled it up, you need to reassemble the fan enough to run the fan for a while to see if it will run properly for a half hour or so or whether it repeatedly slows to a stop.

If it runs, you're back in business. If it slows and stops repeatedly and wont run continuously, you need a new blower motor.

If that motor is oiled every five years, it is unlikely ever to fail. But it's rare that repairmen want to spend the time to tear things apart to do that work.

Now me, I have that motor in a similar Carrier designed furnace, and I oiled that one in a rental house of mine last summer. Another Carrier type furnace is in the home of my brother and sister in law, and I oiled that one last spring.

So--- are you feeling LUCKY tonight?

[SeattlePioneer]

And yes, the Solid State Time Delay circuit operates the fan by energizing to 2E relay coil, which turns on the fan motor.

[patcmontgomery]

There is nothing wrong with the blower motor itself. It must be the circuit board.

After the furnace lights and runs for a minute or so until a limit switch opens up. Then the burner shuts off and the blower will then start to cool down the furnace. 7H2 or 7H3 must be opening due to the overheat condition.

So doesn't this sound like the solid state time delay circuit is not working?

Also if I take and push the contacts on the relay to turn on the blower when the furnace is lit, it will work fine, as long as you keep the relay contacts pushed.

[SeattlePioneer]

Yep. Bad Board.

Don't forget you still need to take out and oil the blower motor or you may well revisit this problem again.

[patcmontgomery]

Thanks for your help on this.