Furnace not responding to thermostat

[Mitsu3000gt]

I am having an issue with our thermostat, not sure if anyone here has any suggestions.

Basically, the furnace responds to the thermostat only when it has dropped like 8C. We will set it at 22C on "permanent hold" (holds the temp 24 hours a day until changed), and the furnace won't kick in until hours later when the house drops to 15-16C and we're freezing. Then, it will stay on for a long time warming the (older) home up to 22C. It is supposed to be able to maintain a temperature within 1F so something is definitely not right.

I have been though the whole manual multiple times, I've tried setting schedules (which it doesn't obey, or starts to obey completely at random, or hours after the scheduled start/stop times), and I have tried the simplest "permanent hold" setting which it also doesn't seem to care about. The thermostat will display "heat on" but the furnace won't be running and indoor temperatures continue to fall.

The system is set to "heat" all of that is OK. When it finally signals the furnace to turn on, it works great and as expected. The furnace fires up, and a minute or so later the fans kick in and we get lots of heat from the registers.

Any idea why the furnace doesn't seem to want to talk to the thermostat? It's a fairly new Honeywell touchscreen programmable thermostat.

[JordanLotoski]

Just had that issue...Had a higher end filter in swapped it to a cheapo one and seems to be working fine now.

Also check your exterior venting, maybe its iced up

Let me know if that works

[carson blocks]

Also check to see if it's firing then turning off when you call for heat. I had a similar problem and just needed to clean the flame sensor.

[Mitsu3000gt]

Thanks guys, I will have a look and see what I can find both inside and outside of the house.

There is one other strange issue, maybe you have some ideas about it as well - when the heat is on, all registers blow hot air except one (upstairs bathroom). It blows air just fine, it's just cold. What possible reasons would there be good airflow, but not heat, to only one register when all other registers blow hot during that time? Only thing I could think of is maybe the pipe leading to that register is too close to an exterior wall or something and it's cooling off before it gets all the way upstairs but I really have no idea.

I am starting to get a new appreciation for my old condo's baseboard heating

[Mitsu3000gt]

Everything looks good on the outside of the house as best I can tell.

I don't know how to check or clean the flame sensor, I've never tinkered with a furnace in my life and I don't want to mess with anything haha.

The thermostat makes a "click" noise when it turns on the heat, presumably a solenoid or whatever completing a circuit. You can turn it up as high as you want, it won't send that signal to the furnace until it randomly feels like it, or the temperature drops like 6-8C, at least as far as I can tell. It knows when it *should* be heating though, because the display reads "heat on" but the furnace won't be running.

There is also a normal light switch on the side of the furnace labelled "FAN" that is in the off position, I have no idea what that is, because the furnace fan seem to work just fine when it decides to kick in.

[asp integra]

Hmm, I would suggest throwing some new batteries in the thermostat. I had a similar problem and then it got worse as the furnace would blow air, but it would not be warm and the heat would only come on at certain times. I just had the flame sensor replaced and now everything's working like a dream.

[quick_scar]

Originally posted by asp integra
Hmm, I would suggest throwing some new batteries in the thermostat.

This. When I first got me place, batteries had enough power to display the screen and temp, but not enough power to send the start signal. That was an expensive lesson to learn.

[Mitsu3000gt]

Hmmm sounds like it could be the flame sensor. I have heard the fan kick in before with no heat, but it always shuts off like 1 minute later. Are those expensive to replace?

Thermostat has new batteries and is this one, except it strangely doesn't have the "Fan" section on the left. I couldn't figure out how but maybe a hard reset of the thermostat could help.

It has been better the last couple days, but it might just be random.


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[Vdubbin]

Found this in how to test if it's your thermostat (I assume no warranty if you mess this up and fry something.)

The thermostat control is low-voltage and poses little risk, but best practice is to turn off the power to avoid injury or damage to equipment. The sub-base is screwed to the wall. You will probably see two or four wires coming from a hole in the wall and connected to screw terminals on the sub-base. If you have four wires, this indicates wiring for both a furnace and central air conditioning. If you see just two wires, those are for the furnace.


Before you disconnect anything write down which wire connects where or take a photo. Sure, you think you will remember, but 15 minutes from now you will be thinking "is it the red wire or the green wire." Even more important, if you disconnect any wires, secure them so them don't fall back through the hole in the wall. Use a small spring clamp or wind the wires a few times around a pencil to prevent the wires from dropping through the hole and into the wall.

To test the heater you will need to connect the "R" (or "Rh") wire and the "W" wire. This bridges the two wires and has the same effect as turning on a switch. Turn the power back on and listen again to the furnace for any sounds. If the furnace starts up, then the problem is almost certainly the thermostat and you should replace it. Do not leave the furnace running like this for an extended period of time, it will not shut off and could overheat or cause a fire. If the furnace is completely silent, make sure the furnace has power. Check the circuit breaker and use a tester to make certain there is power at the furnace. If it has power but does not start, then the problem could be a break in the wire running from the thermostat to the furnace or with the furnace itself.

http://www.acmehowto.com/hvac/test-thermostat.php

[carson blocks]

Originally posted by Mitsu3000gt
Hmmm sounds like it could be the flame sensor... Are those expensive to replace?

To test the flame sensor, go look in to your furnace with the cover off if it will let you, and have someone crank the thermostat up. If it's the flame sensor, you'll see the flame come on for a few seconds, then the gas will cut off as the flame sensor isn't registering the flame.

The flame sensor will likely be a metal rod extending in to one of your burner tubes. It looks kind of like a meat thermometer probe. Mine has one 1/4" bolt and one wire. To remove it, turn the power off at the furnace with the light switch thing near the furnace, remove the wire and bolt, and the sensor will lift out.

Over time it gets carboned up and you can get it working again simply by cleaning it with a scotchbrite pad or similar. My furnace guy says you're supposed to clean it a couple times a year as maintenance but really who bothers until it stops working. Get it reasonably clean and shiny and reinstall it, remembering to hook up the wire and turn your furnace back on. Don't overtorque the bolt, it's cheap shit metal and the part isn't going anywhere.

I don't like working on furnaces at all and it's an easy 5 minute job anyone who can work a ratchet will have no problem with. Come to think of it, I'll do mine as it's been 2 years since I cleaned it. If you're stuck I can take pictures of the process.

Edit: Fixed bolt size

[carson blocks]

Ok, got thinking there is no time like the present and cleaned my sensor, took some pics to guide you through it. I hate working on stuff like furnaces too but this is dead easy.

My furnace is a Lennox Elite, I was told it was mid-efficiency, but I don't know a damned thing about furnaces. Yours might be a little different.


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1. Tools needed
- 1 Beer (Loncium Sweet Krampus 7% from my Beer Advent Calendar)
- Appropriate socket+ratchet (mine is 1/4" bolt, and I used 1/4" drive ratchet and deep socket)
- Scratchy things (I found a scotchbrite and a dollar store brass brush lying around)


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2. Turn it off. There will be a power switch by your furnace. I also flip the breaker off because I'm a pussy and electricity is magic.


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3. Open it up. The flame sensor is a little metal rod poking down in to the burner. Also my furnace apparently needs cleaning, but that's 'Future Carson's' problem.


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4. Remove sensor. Take the wire off, it's just a spade connector on mine. Undo the 1/4" bolt and the sensor will lift right out. Probably friggin' dirty, eh? Mine isn't terrible but has some carbon and scorched spots and whatnot. It didn't look much worse than this when the sonofabitch stopped working 2 years ago though.



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5. Clean it up. I spent 2-3 minutes total with the wire brush followed by the scotchbrite. If you use something less than ideal like a SOS pad just make sure there's no steel wool bits hanging on or no residue or anything. Have some beer, you earned it.


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6. Put it back together. Don't torque the shit out of the bolt. Remember to reconnect the wire. Remember to turn the switch/breaker back on. Fire up the furnace and see if it behaves a little better. If not, at least you maintained something and earned a beer.

[bjstare]

Great DIY

Also +1 for replace filter, if you haven't done that yet. Cheap, and probably needs to be done anyways.

Last week, we woke up to a cold house. Furnace would turn on when the thermostat requested, but would turn off very shortly after. Turns out the filter was just dirty, once it was replaced everything was good to go. To be honest, I never expected a dirty filter would cause problems like that.

[mix123]

I think new high/mid efficiency furnaces are very particular about pressures. So if anything seems wrong they shut down.

[suntan]

Originally posted by cjblair
Great DIY

Also +1 for replace filter, if you haven't done that yet. Cheap, and probably needs to be done anyways.

Last week, we woke up to a cold house. Furnace would turn on when the thermostat requested, but would turn off very shortly after. Turns out the filter was just dirty, once it was replaced everything was good to go. To be honest, I never expected a dirty filter would cause problems like that.

I was told by the Arpi's repair guy that came to fix the furnace blower years ago that by far the #1 reason furnaces don't work is a clogged filter. Sometimes they've never been replaced, so he had to tear them apart to take them out. Also he said stop using fancy filters.

[killramos]

Originally posted by cjblair
Last week, we woke up to a cold house. Furnace would turn on when the thermostat requested, but would turn off very shortly after. Turns out the filter was just dirty, once it was replaced everything was good to go. To be honest, I never expected a dirty filter would cause problems like that.

"Your filter is dirty" is the favorite tool of an HVAC repair guy to bill you for an hour and get to come back next week to bill you again for the real problem.

Source - Friend who works with HVAC guys ( He does ducting himself )

Solution is any time you are having issues with furnace change the filter and see if it fixes the problem before calling the repair guy, that way you can tell him right off the bat the filter is brand new and clean and he can glare at you for 5 seconds and start actually fixing it. Cleaning the flame sensor is also a good one to give a shot before bringing someone in.

3 years ago when I was starting to have furnace problems I had a repair guy pull that on me. Furnace shit the bed again that night. Fortunately the bill went to my builder and not me. Guy was just waiting for my call... He never actually fixed anything in the end, jsut a bandage solution, and I had to bring someone else in the next year, this time on my bill.

/rant

[Mitsu3000gt]

Thanks guys, great DIY Carson.

By the sounds of it, most likely a filter change or flame sensor clean should do the trick assuming my thermostat isn't faulty.

[Mitsu3000gt]

I haven't had a chance to clean the sensor yet, but I believe I have confirmed what's happening.

The furnace starts, lights completely, and looks like it's good to go (intense blue flames, fan on, etc.). After about 30 seconds it turns itself off, and the Thermostat will still register "heat on" until the next time it checks or resets itself or whatever.

Sometimes it works fine and stays on until the desired temperature is reached, but several times in between it will start up normally, fully lit, for about 30 seconds then turn off. What this does is makes it such that the house drops 6-8C before the furnace decides to stay on long enough. It's a 2006 Lennox "Elite" furnace very similar to Carsons.

There are also un-labeled switches on the furnace, so I don't know which turns it off.

[BerserkerCatSplat]

That does sound like a dirty flame sensor. I had similar symptoms when mine needed a clean, although it cut out faster than 30 seconds. 0000 steel wool or a Scotch-Brite pad and you should be back in business.

[carson blocks]

Originally posted by Mitsu3000gt
There are also un-labeled switches on the furnace, so I don't know which turns it off.

Just hit the breaker, that's what I do just to be sure.

[Mitsu3000gt]

I actually found the diagnostic LED's and I am 99% sure it's the flame sensor as well based on the code it is displaying.

EDIT: Cleaned the flame sensor, hasn't failed to stay on since