[spinn]

I'm new here, so please bear with me.

I have BDP 394GAW000050 furnace (no AC) that was installed when the condo was built in 1987.

Last week, we turn the thermostat on to try to get some heat. About 5 min goes by and we don't feel any heat, but we smell something like paper burning. We shut off the furnace's electrical switch and check around to make sure there's nothing burning. We don't find anything. We turn the furnace back on and the blower goes on. We check the thermostat, it's still registers a cold reading, and the furnace is blowing out air. The furnace was not lit. We checked the thermostat and forced it to turn on the furnace, but no response from the furnace. I could hear the click on the thermostat, but nothing coming from the furnace. To top it off, the blower is still blowing, even though the thermostat is set to off.

I replace the thermostat first. No change. Blower still blowing as soon as I flip on the shutoff switch. And still no ignition at the furnace. No click or nothing.

The furnace has a limit safety switch (HH12ZA251A) which I was thinking failed and caused the blower to blow continuously. Checking the line with a multimeater, it appears to be a closed circuit (0 ohms). The fusible link, located on the upside of the limit safety switch, also showed to be OK. Am I correct to think that the closed circuit should indicate that the limit safety switch is OK for now?

And if it's not a limit safety switch issue, what's causing the furnace not to respond to the thermostat signal to light up?

I've not had a chance to open up the box that houses the furnace control circuit board. Could the burning smell (which was different from the type of musty smell you get when you haven't used the furnace for months) been because of a fried circuit board? (I will not have a chance to look at this until NYE)

Should I call in and expert to look at this and face getting reamed? I know a circuit board replacement online runs for about $170. Is an expert going to actually test everything, or just tell me to replace the whole unit? Any recommendations for a reputable repair guy in the Fremont, CA area? Thanks!

[hvac1000]

If you want to TEST the limit switch try to jump it out of the circuit on a TEMPORARY basis. Usually the thermostat is the last item that fails on a system. I hope you hooked the new one up correctly.

[spinn]

I thought about jumping the limit switch, but the schematic that was on the back of the furnace door, indicates that the limit switch is normally closed. Since there was continuity, I'm guessing that the switch is OK because the circuit is still closed.

I'm pretty sure I hooked up the thermostat correctly... there was really only one setting available for the wires that I had. I will have to try the old one again to see if it was just faulty wiring on the new one.

However, that still wouldn't explain why the blower wants to run without any signal from the thermostat.

[hvac1000]

If the sensors/limit switch is OK then I would be thinking it is circuit board time. The relay on the board controls the fan from what I remember on those 20+year old units.

[spinn]

Yeah, there is a relay shown on the schematic and I will try and check that out when I have a chance.

I have photos of the unit's schematic, although not so great, here:
Carrier Furnace - a set on Flickr

[mygirlsdad77]

Did you disconnect wires from limit when checking for continuity?

[spinn]

Power was shut off at the fusebox first, before I checked for continuity. I disconnected the wires (and fusible link) and checked the link first. It was OK. I then checked the limit switch. It was OK. I connected the wires back up, and checked it. It was still OK. The way the switch works is that it opens up the circuit when there's a problem. Since there was still continuity, the circuit is still closed.

[mygirlsdad77]

Yep, but if wires are connected to limit while testing for continuity, it can show continuity through board, even if limit is open. Sounds like you have done a good job of testing limits. I agree that board or relay is the next step. I would bet board, being is the burners won't ignite, and the blower is staying on. Usually this is a sign of a bad limit or roll out switch, but you have eliminated that. So now its one more step at a time. Please let us know what you find.

[KC13]

I am very reputable. Fly me out there and I will give you a completely honest assessment of your system. I can depart from Indianapolis, Cincinnati, or Dayton, three cities with great weather this time of year. I'm cooperative like that.:D

[mygirlsdad77]

Just as well book me a ticket too. I promise I won't tell you that you need a new furnace, unless of course, you need a new furnace.

[wmproop]

Hey spinn,don`t listen to those guys,for the cost of their round trip airfare,I could bring you a new furnace and install it,, lol,:):):),
But one catch,you would have to feed me

[KC13]

Hey spinn,don`t listen to those guys,for the cost of their round trip airfare,I could bring you a new furnace and install it,,,,,lol,:):):),,
but one catch,you would have to feed me

Who said anything about round-trip? It's California! Swimmin' pools, movie stars...

[KC13]

You should check the transformer to assure it has not failed, as continuous blower operation will result if it has.

[mygirlsdad77]

Id even be willing to carpool. Don't mind us, we are just having some fun with each other, Just waiting to hear back from you.

[letmetellu]

Does the blower ever turn off? What about if the thermostat is off?

This heater has a time controlled fan switch, so when the thermostat calls for heat the fan starts timing, after a set time (30 - 45) seconds and will run until the furnace goes through a heat cycle, so I guess if the fan does not stop it is a warning that you have a problem with the heating cycle. I don't remember reading where you checked the voltage at the gas valve, did you?

Every thing you have said about your furnace sounds like a bad limit fuse link, but you sound rather sure that you checked that all right, I would check it at least one more time.

[spinn]

Turns out it was a bad transformer. $32 later, it's fixed.

[hvac1000]

Bad transformer? Interesting and I thought all this time you had power.

[spinn]

So once again, I have the same problem. Fired up the heater after a long haitus, it worked for a while and then no heat, only blower. Played with the thermostat and I don't hear the furnace ignite any more.

Last time the transformer was bad; the limit switch was fine.

I have yet to re-open everything up again.

Can anyone explain why a bad transformer might cause the blower to be on continuously, based on the schematic?

My guess is that the transformer was shorted and voltage was going directly to the blower and bypassing the limit switch. Can transformers go bad that quickly?

Anyone have a clue?

[KC13]

The heating-speed relay on the circuit board utilizes normally-closed contacts to control the blower, which are held open when control voltage is present. In the event the board loses control voltage, the contacts close and the blower runs. If the transformer has failed again, something is overloading the control system. As you do not have a/c, the problem lies somewhere in the furnace itself. Start small by thoroughly inspecting the wiring within the unit.

[spinn]

KC13, thanks for the info.

Based on the earlier schematic, what would you suggest be my next steps in determining where the overload is coming from? All I see is the wiring to the furnace door switch which is routed to the transformer.

I'm not quite sure what I would be inspecting with the wiring. Do you mean to look for any exposed wire contacting the board? Inspecting the wiring inside the switch shows nothing unusual. The transformer does not looked burned out, but testing it shows that there is no step-down voltage, thus, another bad transformer.

Your help is appreciated.