HVAC help - furnace limit switch?

RatLabGuy

RatLabGuy

You look like a monkey and smell like one too
Joined
May 18, 2005
Location
Churchville, MD
Noticed last night our furnace (typical Bryant Plus 80 gas) seems to be running constantly, but the house isn't getting warm. Like it runs, fan runs for a while, shuts off... 3 mins later, run again.
Its cold but not that cold (around freezing).

I recently changed the filter (it was nasty... oops), and turned on the whole-house humidifier for the first time 'cause the dry skin was getting too bad.

It feels to me like the air coming our of the vents isn't very warm. Normally its quite hot.
I'm suspicious the high limit switch has gone bad, and is cutting off way too soon, so there isn't much warmth added to the air.

Is there any good way to test this, aside from just the obvious test of continuity or not?

And now for the better question. A switch is only $20 but its Sat afternoon and all the local hvac places are closed.
Is there any way to jerry-rig this thing so it will fire longer? Just need to make it to Mon morning.
I was thinking of maybe wrapping a little tape around the sensor... but I feel like this is pushing the boundaries of safety...
 
Could be the sensor that detects the flans and shuts off the gas if it detects no flame and just blows cold air instead.
 
No, it's firing and burning like normal, all 5 chutes have a nice blue flame.
It just burns for a little while, the fan kicks on, the burners kick off, then it blows luke-warm air for about 3 mins. and stops... repeats again 5 mins later.
 
Is the thermostat calling for a run, then turning off too quickly? The thermostat should be turning the gas on and off, if you hit the high limit it should be shutting the furnace down (that is a fail safe, on mine it kills the pilot and you have to reset the furnace). I am only really familiar with standing pilot systems and I do not know what you have exactly. The fan limit switch senses the increase in temperature of the furnace and switches the fans on when it reaches a predetermined temperature, cuts them off when the air box drops below another temperature (after the gas turns off). The ducts are probably not getting very warm so the little bit of air you are getting has to heat up the ducts but it never runs enough to get really warm air back into the house.

If you have a digital thermostat, you can probably listen for the relay clicking to call for gas or not and see what it is doing. If you have an old school analog thermostat, check that the heat anticipator is adjusted correctly (do a google search for your thermostat model). Could be that the thermostat has crapped out.
 
Well, I'll be damned. What we have is a crappy old school mercury t-stat.
As it happens, just the other night (thurs?) I was messing w/ it trying to figure out how its mounted, I want to replace it w/ a "smarter" one. The little mercury capsule got pushed aroudn a little....

so I just went to the hallway and cranked it all teh way up. Also just fiddled w/ the mercury ball and the parts inside there... Low and behold, it kicked the furnace on and let it run awhile, hit 7-0 degs inside in no time (so I pulled it back down).

Guess I've been barking up the wrong tree.
So, it's teh t-stat that tells it when to turn off the burner? I thought thats what the upper limit switch was for...
Mine is labeled as a 190-40 degree drop
 
You're like me, you need to learn to keep your damn hands off things. :D glad you got heat!
 
Well sort of.
It still isn't acting right, still cycling over and over. Its just that if I set the t-stat waaaay ahead (like up to 80) it'll run longer and get hot. But then it runs again and again once it gets close.
Its like its short-cycling to try and raise it just a little bit, instead of allowing a couple of degrees swing like it should.

So for the time being I can manually crank it up, let it get warm then turn it down so it will stop.... and then re-crank again when needed...

Ugh there's no model name/# on the stat (its an old-school round one, but with GIANT numbers - PO was an old lady) and can't find any pics like it online... first world problems I guess o_O
 
I believe you have a Honeywell thermostat, or similar. Did a quick e-bay check, nothing right off, but was directed to sponsor site Old honeywell thermostat. Several sites, & they have them. But I would be looking for a Modern update. LOL!
 
Just get a digital programmable. Ours is programmable, but here lately, we've just been leaving it at 70f, with a bump to 72f first thing in the morning.

Don't waste any money on a nest, unless you absolutely need to check the thermostat from your phone.
 
shawn said:
Just get a digital programmable. Ours is programmable, but here lately, we've just been leaving it at 70f, with a bump to 72f first thing in the morning.

Don't waste any money on a nest, unless you absolutely need to check the thermostat from your phone.
Click to expand...
My fossil burner of 1982, doesn't have the wiring for a programmable. I could do digital, but wouldn't save me anything.
 
Your limit is just that. Has nothing to do with cycle time of furnace. It only comes into play if air stream gets to hot via dirty filter/blower bad or slow running/coil is impacted/ crushed ductwork. A programmable t-stat does not require any more wires that you have now. red/yellow/green/white. Maybe a blue as a common. remove cover from t stat and put a jumper between red and white. If problems returns its not your t-stat.
 
rodney eppes said:
My fossil burner of 1982, doesn't have the wiring for a programmable. I could do digital, but wouldn't save me anything.
Click to expand...

most of the "wiring" is inside the programmable T-stats...as long as your tstat doesnt have relays inside...the tstat would just adjust its set point based on yoru programming... and use a micro processor to trip the switch instead of a mercury switch...

all that said..Im no HVAC guy and I know at our old house we had some specific Trane unit that used 7 wires that none of the programmable ones could control.
 
I've seriously considered getting a Nest, almost pulled the trigger on Thursday, but didn't (see below)
I really have no need from cell-phone control - aside maybe for that moment when we're halfway into a multi-day trip when I remember "oh, should have turned down the stat.".
However I do really like the idea of it either "learning" or being able to be programmed to adjust to very specific schedules, self-adjust the tuning of fan to burner based on actual response rate, tying to other devices to figure out if you're actually there and adjusting accordingly etc. We do a lot of algorithm/modeling stuff and work and I can really see where there's room to improve efficiency.

All that being said, what I've gleamed from careful reading regarding Nest is that its a great idea but the execution form the company leaves some serious concerns. Especially from some notes by actualy HVAC guys that have ben responding to service calls in teh last 4 months involving them.
Like, its cool that they are always adding features and updating the firmware... but it just gets auto-pushed to teh stat w/o any warning or control by the user... so when that update doesn't work quite right for your system, or causes odd behavior... you are immediately and unexpectedly stuck w/ HVAC problems. When a cell phone gets a patch that changes features and/or breaks something, it's irritating but life goes on. But when you change my stat and the heat dosn't work in 20 deg weather... that is a serious problem that dramatically upsets my family's life. I've read about people spending hundreds of $$ trying to diagnose HVAC problems only to discover it was Nest's fault. Even some trying to recover expenses incurred (good luck w/ that...)
So until it has manual "on demand only" update feature... no, thank you.
Plus now being owned by Google? I'm no paranoid alarmist... but there is something just potentially very scary about that... Google (along w/ all the other things they track) knowing exactly what your in-home behaviors are... when you're on vacation...all of that is already routing through Nest's server for the phone app...
But they did recently open the firmware SDK to other developers so thats going to be interesting.
So my thinking on Nest is to give it a year or so before jumping on that ship. Potentially very powerful... but not ready for my home yet.
 
RatLabGuy said:
Guess I've been barking up the wrong tree.
So, it's teh t-stat that tells it when to turn off the burner? I thought thats what the upper limit switch was for...
Mine is labeled as a 190-40 degree drop
Click to expand...

Yes sir. The T-stat controls the gas valve in the furnace to cycle it on and off. For 99% of the systems out there you can switch out to a cheapo $30 programmable from any store you find one at. They have batteries in the housing that power the display and backlight that need to be replaced once every couple of years on average.

So it sounds like you tweaked the mounting for the bi-metallic spiral that suspends the mercury switch. That sets the temperature, then the heat anticipator regulates how much longer the unit runs after hitting the set temperature. It is designed to prevent just such "short cycling" that you are experiencing, the idea being that it runs the air temperature up above the setpoint and creates a window of time in which the house air is at or above the t-stat setpoint for a little while (as things gradually absorb the heat from the air and drop the ambient temperature of the air again) so the cycle can start over.

If you have jacked it up you can fiddle with it and eventually get it working right again (helps to have an IR temp gun) or just get a digital unit and call it done.

I am not HVAC trained either but have been dealing with these systems in my homes for quite a long time. My current gas-fired furnace has had a few components quit and I have done extensive troubleshooting and with my electrical background understand how the basic circuits are laid out and what the components do within the system.
 
A lot of good info from all of you! Mine is a Carrier heat pump, & to be So old, & using the mercury switch, it Does have seven wires. I'll be getting a New unit this Spring, unless it dies before then.

Backing up what RatLabGuy said, I had an icing up prob, for over 6 months. Eventually found it was a fan failure, but still couldn't find the reason. Maybe 4-5 techs, from different companies looked at it.
I know 1/2 didn't know what they were doing. A friend of mine brought his independent HVAC man over, & since "I" was jumping the switch on the out door unit, to make it run, He decided it was a bad circuit board.
They went & got a new one[$100], & while trying to switch them, he broke the old one. I was at work, so my friend told me what was done. Guy said, well it was bad, so what? he put the new one in, & fan did not come on. During all this my friend was looking over the inside unit, as the cover was off. He was wiggling all the wires, when all a sudden, "YEP" the **** fan came on. 1 LOOSE wire, that Nobody, had checked for! + I was stuck with the new $100 circuit board, because he ripped the old one out. Seems nobody checks the Simple stuff, First.
 
I have a nest sitting in the box. I bought the first gen Nest and when it went from summer to winter it killed itself. Nest blamed it on my brand new unit but the new unit worked fine with a cheap lowes tstat. Nest replaced the tstat twice for the same issue the second replacement sits in the box and I learned a $200 dollar lesson. From what I gathered the nest is brittle compared to a regular tstat that can handle voltage swings. The nest could not handle this an would pop an internal resistor. Nest the company would then put your HVAC brand on the "will not work" list.

Dave
 
there is also a vacuum actuated switch on the exhaust fan. if there is a leak in the rubber hose or the switch has some dirt floating around in it you can get some funky symptoms.

I got a crash course in all things furnace related last year. I started replacing components one at a time on mine. My background is in office equipment repair so I figured I could handle this no problem. After following what I thought was a pretty well thought out diagnostics and flow of checks I still hadn't repaired my furnance. after spending waaaay to much money on new parts and no results I called in the pros.

A friend of mine owns a well respected HVAC company in town. He and I sat down and went back over the diagnosing steps. The whole time I fully expected him to find something stupid or obvious that I missed. Nada. to my relief I didn't miss anything. To my horror, after 6 hours with me and a pro I still had no heat, and no components left to replace. That meant it was time for a new unit $$$$$. We still don't know what the problem was. but it was 15 years old. after 6 hours it just seemed like the best thing to do.


I hope you have better luck than I did!
 
I played with the heat anticipator a bit, turning it up did help but still wasn't the same... it also warmed up outside so who knows.

Went ahead and went to HD, bought a $35 digital programmable 5-1-1 model as a temp fix until I know for sure what I want in something fancier and get that later. Actually I'm pretty sure I bought something identical to this for like $150 about 8 years ago in our old house, lol.

Here's what I don't get. Your basic analog old-school stat is between 20 and 30 bucks.
you can buy a basic programmable digital one for $25.
Why would anybody bother with those old ones?

Its also frustrating knowing damn well that the only difference in the features between the $25 and $80 (non wifi, touch screen etc) units is about $5 in parts and a change in firmware. Yeah, I know thats how the electronics business works...

Anyway thus far it seems to work. Not real happy with how much of a temp swing this one seems to have but I'll give it some time.
 
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