HVAC residential furnace help - any experts here?

[El Chupacabra]

Need some help diagnosing what's going on with one of our furnaces -- any HVAC mags here? A search of old posts shows there are (were) some, but dunno who's still around.

Here's the (tl;dr) way-too-long description of what's going on:

Furnace
1994 Day & Night Plus 90, model 350MAV048100, run on propane. Installed early 1995, but (long story) house wasn't finished and occupied full-time until 2008, so this furnace doesn't have 20 years of run time on it -- maybe more like 10 years total. OTOH, it is old.

This furnace is installed in a utility closet in the basement (i.e., dry, warm place, not dusty). It is a condensing unit, so the intake and exhaust flue are a concentric PVC pipe setup.

Symptoms
When furnace gets a call for heat from the thermostat, the furnace cycles in its normal startup routine:
- Blower runs ~90 seconds, without flame.
- Small fan runs ~30 seconds, thermocouple ignites (visible through window into combustion chamber)
- Gas flow starts, all burner tubes ignite, blue flames look normal

Then, this is where the problem is:
- Once the furnace switches to "on" and the main blower starts, then the flames flicker/sputter and go out.
- Furnace shuts itself down.
- Furnace then restarts (standard start-up cycle).
- Restart cycle runs 3 times, then furnace shuts itself down (lock out) for 3 hours. Per instructions, this is normal.

Troubleshooting done already
- Last winter, HVAC repair guys replaced the main circuit board. Furnace ran normally afterwards.
- There is sufficient propane in the tank; the other furnace runs fine, as do other gas appliances in the house.
- Thermostat functions; batteries are good.
- Filter is good.
- Opened combustion chamber, vacuumed out some dust, bugs, small debris.
- Removed condensate drain box, emptied water, cleaned out box and hoses.
- Removed flame sensor, no soot; wiped it off and reinstalled.

Service call yesterday
HVAC service guy came out to check out furnace. He said the heat exchanger looked a little rusty, and suspected it could be the problem, since he and I had ruled out most of the other usual suspects.

Service guy also said the flame sensor was giving out low mA readings -- lower than expected. Before replacing any other parts, I think I'll try swapping the flame sensor from our other furnace (same model), and see what happens.

Furnace functions right now
Somehow, the furnace fired up and ran normally, after service guy fiddled with it awhile. His guess is that after running it through the start-up cycle a few times, it warmed up the heat exchanger enough such that any small leaks somewhat closed themselves off (metal expansion) -- at least enough to get it to run.

Our thermostat is programmed to kick down to 61* overnight, which usually means that it does not run at all overnight. This morning, the problem furnace fired up normally, and heated to the programmed temp.



So -- any HVAC mags have any ideas? Any cheap parts I should replace (or easy parts to poach temporarily from the other furnace)?

One final item -- if it's the heat exchanger, then this furnace is part of a class of furnaces built by Carrier/ Bryant/ Day & Night that had problems with the heat exchanger, leading to a class action settlement that covers replacement of the heat exchanger. Any way to definitively diagnose the problem as being the heat exchanger?

[fatnslow]

As a rule of thumb the heat exchanger won't shut off the flame unless you have a significant leak due to a crack/leak. I would swap out the control box/main board. I had a similar situation on a 80% efficiency and it was the controller which you stated was previously replaced BUT take a look anyway. Do you have flashing LED that gives a code on the controller ?When you have the main board out and take a look on the back side for any arced/burned solder joints , if you see damaged solder joints just resolder them. If you can do basic p-texing you can resolder . The following link suggests that your board is known to arc some joints. Most HVAC guys won't do this type of repair or trouble shooting to avoid call backs.




http://www.diychatroom.com/f17/bryan...-blower-70375/

[Rideski]

Sounds like you could be short cycling due to bad pressure switch.
This is wht mine looks like your is probably different.
http://www.hvacpartsoutlet.com/Press...-B1370142.aspx

If you are in an urgent jam for the night you can take the hose off the nipple and stick a needle into the nipple to see if there is gunk buildup, might buy you another day or month.

[iamchappy]

Thermo couple, could even be a limit switch stuck open

[El Chupacabra]

As a rule of thumb the heat exchanger won't shut off the flame unless you have a significant leak due to a crack/leak. I would swap out the control box/main board. I had a similar situation on a 80% efficiency and it was the controller which you stated was previously replaced BUT take a look anyway. Do you have flashing LED that gives a code on the controller ?When you have the main board out and take a look on the back side for any arced/burned solder joints , if you see damaged solder joints just resolder them.

Thanks - good tip. I'll take a look at the backside of the board. There is a LED that flashes and gives 2-digit codes to self-diagnose. With the problem I'm having with the furnace, it's a code that indicates ignition lockout due to failed startup -- but no other code giving details as to what's causing the failed startup.

Sounds like you could be short cycling due to bad pressure switch.

If you are in an urgent jam for the night you can take the hose off the nipple and stick a needle into the nipple to see if there is gunk buildup, might buy you another day or month.

I looked at the pressure switch on my furnace, but mostly was checking for any water in the hoses, or any tears in the hoses (none). Is there any way to test a pressure switch?

Thermo couple, could even be a limit switch stuck open

I'll try swapping the thermocouple (same thing as a flame sensor, right?) with the one on the identical other furnace, and see if it makes a difference.

[fatnslow]

You are pretty lucky to have a second furnace to pull parts to trouble shoot, intermittent issues can be pretty expensive if you pay tech to stand there and watch after swapping parts. I would try thermocouple first since the tech noted a low output with flame then swap out the board.

[El Chupacabra]

You are pretty lucky to have a second furnace to pull parts to trouble shoot, intermittent issues can be pretty expensive if you pay tech to stand there and watch after swapping parts. I would try thermocouple first since the tech noted a low output with flame then swap out the board.

Thanks - will try that first. As of right now, it's working normally. Tomorrow may be different...

[Rideski]

I looked at the pressure switch on my furnace, but mostly was checking for any water in the hoses, or any tears in the hoses (none). Is there any way to test a pressure switch?



.

There is. I'm not an hvac guy but if I remember right, you take the wire connecters off the back and short them, and then kick the furnace on, something indicative happens at that point. The youtubes might know, I can't remember at the moment.

[RShea]

With 2 of the same furnaces as mentioned, swapping parts to see if it follows should be the way to go. I'm not an HVAC expert at all, but the circuit board could be intermittent. Fatnslow sounds like he thinks try the thermocoupler first, so again do only one of the 2 if the problem comes back and see if the problem follows to the other unit and the part swapped resolves the intermittent failing unit. (Of course intermittent failing is always tricky as you do not know for sure the mystery just goes away without doing anything and are the worst- I say to my customers just the threat of calling the repair guy maybe enough to fix an intermittent problem - for awhile).

Also if there is a factory known issue, maybe a call to the manufacturer tech support to discuss could be worth a phone call to discuss the mentioned symptoms and if the exchanger could be any part of the issue. Most they can tell you is they do not know or only discuss the issues with a HVAC trained tech, but could be helpful and offer first and second rounds of parts recommended based on your symptoms.

[stevesmith7]

Our furnace did this a few years ago. It was an exhaust obstruction. Check the vents.

[Rideski]

^ That's a good call, intake filter being clogged same thing.
With all the work already done can't imagine that's what it is, but it would cause that symptom.

[El Chupacabra]

The malfunctioning furnace did it again -- ignition lockout. So, I swapped over the flame sensor from the other (working) furnace, and fired it up. It did the startup sequence, and fired normally.

Keeping fingers crossed that this is all it is -- if so, that's an easy & cheap fix.

stevesmith - thanks - I was thinking about trying to disassemble the exhaust and intake pipes to blow them out with a leaf blower. We do have birds and mice that like to nest wherever they can, and it's totally possible that something crawled down inside the vent. I previously tried cleaning out the exhaust by stuffing my shop vac hose down it as far as I could -- didn't sound like anything came out, but figured it couldn't hurt. I should put screens over the vents during summer to keep critters out.

[stevesmith7]

On a super cold morning I had our exhaust ice over and plug up. Same symptom.

[El Chupacabra]

It's been in the 50s each day for high temp here, for the past week.

[fatnslow]

When my son was 2 he and a another kid stuffed rocks into my friend's furnace exhaust outside his house costing him a few hundred bucks for a service call. Luckily for me the other kid was his daughter so I didn't feel compelled to pay for the call.

[El Chupacabra]

The malfunctioning furnace did it again -- ignition lockout. So, I swapped over the flame sensor from the other (working) furnace, and fired it up. It did the startup sequence, and fired normally.

Keeping fingers crossed that this is all it is -- if so, that's an easy & cheap fix.

OK, that wasn't it. Ignition failure again.

I then tried swapping the pressure switch. Again, fired up normally last night. But this morning, ignition failure again.

I'll try cleaning the intake and exhaust later, but any other ideas of sensors or switches that might be the cause?

[SB]

Probably not in your system but there is a thing called a sail switch whitch senses the air pressure of the fan or blower. Usually its a micro switch with a plate on it in the duct work, it will make with the fan on. If fan fails for whatever reason the sail switch will open and shut down the burners so it does not over heat the heat exchanger.

If the sail or plate is dislodged it would affect the operation of the switch.

[fatnslow]

I would try to swap out the main board. You don't know if your HVAC guy sourced a reman board or new.

[markcjr]

I'll see if my bro will come down with me from tahoe when I come grab those bindings. He has a hvac business. buy him some beer and I'm sure he'll give you a look see.
Mark

[El Chupacabra]

Mark - awesome!

Fatnslow - the replacement board has a 1 year warranty, and I'm within it. I still need to examine it closely for any burned out looking solder joints, as suggested above.

Scotty - I'll search for that. I found the original manual and circuit diagram for the furnace online, and printed a copy.

[Snow Dog]

From the original description the controller is dong what it should -- the furnace fails to starts so it shuts off the gas. That points to one of the sensors (flame or air flow) or the connections to the sensors at either end. There could be:
- exhaust air flow (vane or sail or pressure switch)
- duct air flow
- flame detector (optical or temperature)
- air exchanger high temp limit (the last resort)

If you're feeling daring you can bypass the various switches with jumpers. Or you can put a volt meter across the switch and measure the voltage. It's probably 5 or 12 VDC and falls to zero when the switch closes.

It's always possible that the switch detection circuitry on the controller is broken which would mean a new controller.

[davidchin]

My furnace was doing likewise a couple of weeks prior. The HVAC service tech guys replaced the control board and all is well at this point. They told me that it had probably gotten singed in a power surge.