# furnace randomly shuts down



## sspeed (Dec 17, 2008)

The furnace says it is a:
Consolidated Industries Model MBA 060 NH2R
Smart Valve is HW SV9500

I think the Electronic fan timer, which pretty much runs everything, is going out. The fan will come on, the pilot will light, and then sometimes the burners come on. Sometimes it runs perfect, other times it runs for 2 seconds, sometimes it doesn't come on at all.

I had a furnace guy out and it performed beautifully the whole 2.5 hours while he was here so he could not diagnose the problem. He said if he had to guess, he would replace the smart valve and accompanying burner for $485.

Tonight when the furnace shut off prematurely I heard a faint electrical buzzing. I pulled the cover off and it was the electronic fan timer, I pushed on the outside of the plastic of the electronic fan timer and the furnace came to life.

The name is deceiving, it seems to be the main control unit for the whole furnace, and also has diagnostic led's that are reporting normally of course.

The part number of that control unit (electronic fan timer) is Model ST9141A1002, part # 406650. I've duplicated getting the furnace to turn on after it shut off prematurely at least a half dozen times now simply by applying pressure to the outside of the plastic case. How often do these go bad? There is not a pressure switch inside of that unit?

It looks like to replace it I simply shut off the power, draw a diagram of the wires, take out 4 bolts, and reverse the order to install a new one. I see the ST9141A1002 406650 online for $85 to $189.


----------



## hvaclover (Oct 2, 2008)

i installed over three hundred of the MBA series furnace. I think you might be in for a shock if you only change the board. The Smart Valve on this type and model furnace has been a royal pain in the ass for the HVAC industry.So much so it's earned the nick name Dumb Valve.

You would do well to change both valve and board together as when one goes the other goes in a short time after.
The suspicion is that what ever causes one to go it damages and weakens the other.

I worked on the model you have this very work day. At my age now I'll never know why I sold so many 31" tall furnaces. I have to lay on my back to replace an igniterLOL.

But that's what you need to do.

Just curious, I was putting MBA furnaces since 1993. Where are you located, just wondering if it's one of my Installs. Probably not but I thought I'd ask.


----------



## sspeed (Dec 17, 2008)

I'm in Denver... That's a bummer to hear, but I definitely appreciate the input. It was basically the same thing the HVAC guy that came out mentioned... I'd be comfortable doing the controller on my own, I wouldn't say the same for the smart valve... I have to narrow it down more before I have him out here again, last time he looked at it for 2.5 hours while it ran perfect.

I downloaded the controller information tonight http://customer.honeywell.com/techlit/pdf/PackedLit/69-0780.pdf and have had the multimeter out to check everything is alright. So far it is, when it shut off prematurely I was going to see if W stayed energized or if it was de-energized. That way I could at least rule out the limit switches since the thermostat was replaced, but tonight the dang thing is working fine. The controller never gives LED codes after stopping, so that either indicates the switches are fine or the controller is slacking. According to the operating sequence it looks like the LEDs only flash while the switch is open, once it returns to normal operation the LEDs resume their bright-dim sequence?

I have absolutely no doubt you are right, the unfortunate part is the controller doesn't have an Operating sequence for smart valve failure, nor does it have an LED for it. When the furnace shuts off prematurely, or fails to start, I'll simply have to listen for the sequence and check some voltages.

The controller and the smart valve are an expense I wasn't hoping for...


----------



## SKIP4661 (Dec 3, 2008)

Wiggle the wires on the valve when it fails again. If it fires its the valve.


----------



## hvaclover (Oct 2, 2008)

When a limit opens you will get a flash on the board. All the burner limits are manual reset. If it goes out on main limit three times in five miutes it will lock out.


Limits aren't the problem.


----------



## sspeed (Dec 17, 2008)

So here's what I've got:

- Furnace set to 71 as a test
- Furnace comes on and runs, shuts off at 69, fan is still running (didn't confirm if it was the circulating fan or blower).
- Test "W", it's still energized at 30V, this tells me based on the operating sequences, that the thermostat hasn't ended the call for heat, nor have the limit switches opened as any of those situations would have de-energized "W", also no LEDs
- Wiggle the smart valve wires, nothing
- Apply presure to the LED side of the control unit, control unit clicks a few times and furnace fires up

Is my control unit bad, or has it lost good ground? I guess I could run a ground wire from one of it's mounting screws? It looks like it gets the ground from the mounting screws?


----------



## hvaclover (Oct 2, 2008)

Same answer as before: Control board and Valve.


----------



## beenthere (Oct 11, 2008)

The thermostat energizes W.
So weather a limit is open or not, you would still have voltage there.


----------



## sspeed (Dec 17, 2008)

Thanks hvaclover, I agree, I just want to have all the ducks in the row when the hvac comes out again... I don't want to spend another 3 hours looking at it while it performs beautifully, or have him tell me it's only the valve if the controller is acting up.

A question on the limit switches... From the schematics and operating sequence, it says if a switch is open, the thermostat de-energizes. From my limited understanding of schematics, it looks like the "R" path to the thermostat becomes an open circuit when a limit is hit and the switch opens. I assumed possibly incorrectly, that when the thermostat is no longer is getting power via "R", the thermostat would not be able to provide power to the board via "W"? Am I misunderstanding "R"?


----------



## hvaclover (Oct 2, 2008)

Limits break the heating circuit electrically after the W.

Stat could be closed and limit open and you will still have 24v at W.


----------



## sspeed (Dec 17, 2008)

Awesome, thanks for the clarification! I think with what everybody said I have enough info to call the hvac guy again and get the right diagnosis this time, thank you!


----------



## hvaclover (Oct 2, 2008)

Didn't give you that info to second guess another tech. Was assuming you wanted to replace it your self.


----------



## sspeed (Dec 17, 2008)

Oh, I definitely will replace the control unit myself, I'll need him to do the smart valve. The tech was at my house the other day for 2.5 hours and left telling me just to watch the furnace for failures and try to collect as much information as I could. Because everything worked great while he was there, he was unwilling to throw parts at it.


----------

