HVAC went weird on us this AM. Any thoughts on why?

LVSteve

LVSteve

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Lying in bed half asleep the morning when I heard the HVAC go into its heat sequence. Here is what should happen. Flue fan comes on, igniter heats up, gas valve opens and the gas ignites, after a delay the circulation fan for the house comes on.

Today, I heard the gas valve click, followed by the quick whoosh of the gas igniting, and then after ten seconds or so it all shut down. Huh? Rinse and repeat a couple of time and the "Oh dear" moment is upon us.:eek:

Last time this happened, a cover on the side of the unit wasn't put back quite right after chasing a vibration in cooling mode. This created an airflow issue for one of the sensors. Could it be the panel has vibrated loose? Have I lost the thermocouple that detects that the gas has lit? Got up, put on some scruffy clothes, collected the ladder and tools to open the attic hatch and up I went. While getting these bits I also set the thermostat to OFF.

Once I got the hatch open it was evident that the covers were all snug. The "oh dear" sentiment is fast turning into "oh something we can't say on the forum". Sent my wife downstairs to turn on the system again at the 'stat in case I can see anything obvious. Flue fan comes on, the igniter glowed fiercely, the gas valve opened and the gas ignited, and...everything worked perfectly. :confused: Color me baffled.

I wonder if the system does not like this between weather that tends to make the heat run in short bursts about every 35-40 minutes in the morning. This is with the "swing" set to 1°F. Thinking this might be an issue, I changed the swing to 2°F and let the system run. It hasn't run since because the sun is warming the house.

So, does setting the 'stat to "OFF" reset the control board in some way. Had some sensor hung up and the system just needed a timeout? Have to say I'm relieved not to have to call "The Man", but I do wonder what was going on with the equipment.

Expert comment is invited!:D
 
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This is what I do for a living.
If the main burner ignites for 5-10 seconds then goes out...clean the flame sensor with a damp paper towel. Careful, don't break.crack the ceramic isolator.


Obviously you'll want to turn off power to the furnace first.


Some folks will recommend a scotch-brite pad, sand paper or steel wool. Trust me on this one, a wet paper towel is all you need.
 
I agree with 9mmPatriot. I remember you had a new installation not that long ago, maybe a warrantee issue?

My MO is clean the original to get back up and running then install a brand new one and save the used one for problems in the future.

Ivan

Being in a condo complex when the neighbors replace a furnace or A/C, I strip the parts I can use on mine. I help out the few neighbors that will talk to a grey haired old man, what could I possibly know that would interest them?
 
All of the modern HVAC units rely upon computerized circuitry, and every sensor includes a sense-of-humor chip that understands exactly when the unit's warranty expires, as well as monitoring conditions to determine the best possible time to fail in order to cause you maximum inconvenience and expense.

That is the best explanation I can provide.
 
Sounds like the flame sensor needs cleaned. Taking the bottom cover off will kill the power and reset the furnace, not setting the t stat off and on. It may work after that but you should clean the flame sensor.
 
Ivan the Butcher said:
I agree with 9mmPatriot. I remember you had a new installation not that long ago, maybe a warrantee issue?
Click to expand...

The new unit was for the upstairs. If I ever have to use it on heat the glaciers will have reached Reno.:D The downstairs system is enough for our needs in all but the most extreme weather here, especially since I had the attic insulation updated.
 
The safeties on the access door will shut certain parts of the furnace off but you can still set a fatal shock. Shut the electric off at the service switch or breaker panel to be safe. I would set the thermostat "Fan" switch to "On", so you can verify the power is off. (I assume that you don't have test meters, but always verify that the power is off before getting in the wiring, sometimes breakers are mislabeled, or servicing required they be moved!)

Ivan
 
Ivan the Butcher said:
The safeties on the access door will shut certain parts of the furnace off but you can still set a fatal shock....
Click to expand...
Definitely OT but this reminded me of an apocryphal story about Alan McFee, one of our iconic and beloved radio broadcasters on the CBC up here.

Back in the 40's when he was just starting out, he couldn't afford to eat in the staff canteen, so he would bring his own hot lunch. But where to put it until lunch-time?

Well, his route from the staff entrance to the studio went through the transmitter room- which apparently was unmanned - and he discovered that the cabinets that held the enormous transmitter tubes, which gave off a lot of heat, weren't locked, and there was a convenient shelf above them. So he would briefly open the cabinet, put his lunch on the shelf, close the cabinet and head down to the studio. At lunch time he would go and retrieve his lunch.

Apparently they never found out why the transmitter shut down for a few seconds twice a day. The cabinets were indeed unlocked, but as there was very high voltage inside, the doors were fitted with a safety interlock!
 
An electric meter is always a handy tool for any home owner, and learn how to use it. At minimum one of those tappers, as we used to call them, is handy also. You just touch a wire and it will glow and usually beep when there is power present. Safety first! I've seen rare cases where the switch by the furnace is wired to break the neutral (usually white) wire instead of the power wire. It will disable the furnace but there is power available to shock your world. This is the same with light switches, too. Yeah, clean the flame sensor as 9mmPatriot recommended; it doesn't need to be polished. Usually a longer-handled 1/4" magnetic nut driver comes in handy for removal & replacement of the sensor. Many furnaces have a little about 1" diameter window you can look through and see a little blinking light on the circuit board that gives you a code while it's locked out, but you need to look at it before you turn anything off. Count the blinks then look at the little chart on the cover to give you an idea what's going on. In your case its probably the sensor. On very rare occasions I've seen all but the last burner where the sensor is located light then it lights the next time, but it's usually when the furnace is located near or in the laundry room or some other dusty area.
 
Well, the wacky thing is the system appears to be working (touches wood, crosses fingers) with the increased swing, but I will certainly look at pulling the flame sensor. My biggest issue is access, as the thing is up in the attic. It is right next to the hatch, but the crazy roof layout puts space at a premium.

One thing I noticed doing more Internet research was that the pressure sensors can get cranky under certain conditions. The weather took an odd turn here this week, with very dry air coupled with high pressure. Jeff's comment about dust made me wonder about that as the system is in the attic.
 
If it happens again, don't shut off the power or open the panel. If there is a fault, that will reset the fault and you have accomplished nothing. You will see a light blinking on the board. Count the number of blinks and reference the manual (or internet) to determine the cause of the issue. This will eliminate the guesswork and dread of the unknown.My tagline is Recycled Bricklayer, I have been a commercial HVAC service tech for a some years now.
 
Last edited:
My furnace used to give up on about the coldest night in the Winter months.
I don't really know what I'm talking about, but there is a little
hose that gets clogged up and that's what causes it to stop. I just have
my furnace repair guy come by before cold weather and blow it out, and
then it runs all winter. My fingers are crossed.
 
bricker said:
If it happens again, don't shut off the power or open the panel. If there is a fault, that will reset the fault and you have accomplished nothing. You will see a light blinking on the board. Count the number of blinks and reference the manual (or internet) to determine the cause of the issue. This will eliminate the guesswork and dread of the unknown.My tagline is Recycled Bricklayer, I have been a commercial HVAC service tech for a some years now.
Click to expand...

Getting where I can see through the little window is a bit of a challenge, but should the need arise I will bear your advice in mind.
 
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