So I had the professional come in. He discovered that the filters were dirty and that the limit switch was off. He cleaned the filters and reset the limit switch, and it worked for a few hours or maybe even days (It got warm so it was hard to tell}. Now it is cold and now it is broken in the same way. I pulled out the filters reset the limit switch and it worked again for maybe half an hour. Given what I have told you so far, what is/are the likeliest cause of this problem.
Put a thermometer next to the limit switch and see if it’s tripping at too low a temperature.
What are the symptoms that are causing you to ‘reset the limit switch’?
The limit switch turns the main blower on and off as well as shutting the entire unit down if it overheats. Depending on the style of limit switch that you have, you might even be able to watch the dial spin as it heats and cools. Also, on some, if it’s malfunctioning a light smack with a blunt object will get it working again for a bit.
But, if you’ve reset it twice, either it needs to be replaced or there’s a bigger problem and you need to find out why it’s tripping. So, again, what’s the symptom?
Here’s the sequence.
TStat calls for Heat
Inducer comes on
Gas/Fire come on
Limit switch trips low limit
Main blower comes on
TStat reaches temp
Gas/Fire turns off
Inducer turns off (after some set amount of time)
Limit switch falls below low limit
Main Blower turns off
To make any guesses, we’d need to know where this is sequence is breaking down. You could have a bad limit switch, you could have bad airflow, you could need a new flame sensor, but it’s hard to say based on what you gave us.
Is the light on the control board flashing?
To add to Joey P, either the limit switch is working or not. Another way of looking at that is, either the switch is the problem, or the switch is doing it’s job (and keeping you safe and alive) and the problem is somewhere else.
IME, it’s usually not the switch. If that’s true, it’s air flow related. Anything that would inhibit airflow could cause it to run hot. (you should never, ever defeat, jumper or remove a limit switch.) In my anecdotal experience it is usually caused by one (or more) of these things, ranked by likelihood:
- Dirty filter
- Dirty evaporator coil
- Too many registers closed, or rugs over the return air grate
- A dirty blower wheel. The ‘vanes’ on a blower wheel are shaped like this -----> ). The vane ‘cups’ the air and ‘throws’ it. When that ) gets filled with dirt it doesn’t cup the air; instead, air ‘slides’ right over the vane.
- The furnace was changed to a bigger furnace and the unit work can’t deliver the air through the now too small ductwork
- Remodeling over the years has resulted in registers being removed etc and there being fewer registers etc.
One thing to check…
Most furnaces design around a 35° to 70° ‘temperature rise.’
What that means is that the furnace will heat the air 35°to 70°.
So…if the return air is 65°, the temperature should be 100-135 at the registers; thats between a 35 and 70 rise.
Most times it will be closer to 35. When its closer to a 70 rise, or higher, that usually indicates reduced airflow.
ETA
There are other things that could cause this…common enough but less likely…
- A cracked heat exchanger
- A sooted up heat exchanger causing “flame roll out”
- A clogged flue or chimney (if its an older furnace that doesn’t use a draft inducer or pressure switch)
- Elevated gas pressure
And there are others. This isn’t a DIY fix I’m afraid.
A couple more things and I’ll shut up, ok?
Your blower is probably a 3 speed or 4 speed blower. The highest speed (High) is usually reserved for cooling. Usually Low is set for heat, and the other 1 or 2 speeds are “parked” and not used.
Have the tech set your motor so High is for cooling and the next highest speed is set for heat.
Also…If you have one of those dense washable polyester filters throw it away. They do a good job of filtering but they’re hell on furnaces and A/C systems. The blower can’t overcome the high resistance and they tear up furnaces and A/C systems.
Get a good pleated filter and change it as needed.
The LED in the board is signaling faulty limit switch. I know it doesn’t necessarily end there , but it seems a good place to start.
The board doesn’t know if it’s faulty. It just knows that the fault is the limit switch circuit.
It could be the limit is faulty, or it could be that the limit is ok and doing it’s job.
There is no furnace I know of that can determine whether a limit is bad or not.
Today a salesman from the HVAC came out and attempted to confince me that the best thing to do was to have everything replaces in all three systems. Not such a hot idea as far I as I am concernet.d
I’m not sure what you mean by replace everything in all three systems, but if you want to look at the sequence I mentioned up thread and tell us where it’s stopping, it’ll give us a really good jumping off point.
Here it is again:
TStat calls for Heat
Inducer comes on
Gas/Fire come on
Limit switch trips
Main blower comes on
TStat reaches temp
Gas/Fire turns off
Inducer turns off (after some set amount of time)
Limit switch falls below low limit
Main Blower turns off
So you understand, when your furnace turns on, you’ll first hear a small motor kick on. That’s the inducer. After about 30 seconds or a minute, you can hear the flame, after about 30 more seconds (give or take) you’ll hear the big motor, the one that blows into the house, that’s the main blower.
What we need to know, is where exactly this sequence stops and how it’s stopping.
Also, you can, depending on the model, pull the cover off your limit switch and see it turn. If it’s going all the way up to the third little switch, you may have a problem.
There’s a big difference between “The the flame comes on for 5 minutes but the blower never kicks on and then the whole thing shuts down” and “The main blower runs for 30 seconds and then the whole thing shuts down” or even “the flame comes on for 3 seconds, goes out and the whole thing shuts down”.
Ok, I will try.
The thermostat calls for heat.
Then the next three are skipped.
The the blower motor comes on and stays on.
It seems to not be on all them time, simply part of the time.
A couple of times, while fiddling with the reset on the limit swich I could hear the inducer motor come on, and a couple of times, the fire lit, but only for a very short period of time. Mostly it blows room temperature air.
That is odd. If it’s calling for heat A)the main blower shouldn’t turn on until the plenum is hot and B)the inducer is the first thing that should kick on. The inducer also has nothing to do with the limit switch. I suppose your control board could be on it’s way out, but I’ll let Raindog weigh in on that one. I only say that because you mention the inducer isn’t coming on. As soon as your TStat calls for heat, that’s typically the first noise you hear. I almost wonder if the control board is allowing the main blower to come on early by having some issues with AC or FAN signals getting confused. That brings up something else. There’s a very slim possibility that your actual Tstat bit the dust and when you call for heat nothing is happening or it sends the wrong signal (FAN). BTW, have you tried changing the batteries in your TStat if it’s electronic?
Based on nothing more then what’s you’re saying, I’d probably start by swapping out the limit switch, but it’s not a cheap fix. As I recall, they run about $100 or so. At least it’s easy to do. A few screws, a few wires and you’re done in 5 minutes. Take a picture first and before you unhook the old one make sure the new one has the same amount of terminals. Some have two, some have three, I think some have 4 or 5.
While you’re at it, and you have everything opened up, make sure the drain tube is clear. If that’s backing up it’ll cause everything to shut down as well and if you don’t expect it or don’t know it’s a possibility you might not think of it. Though I’d expect the HVAC guy to have pulled the hose off.
I have always felt that HVAC was way to complicated for me, what with safety features and feedback loops, etc. One day, in the middle of a 90 degree july evening when hot air started to blow out the registers despite the fact that the AC was on. That was a circuit board.
So after it was fixed and broke again, the guy is coming back tomorrow. They seem to think it is the air exchanger clogged up, and that that would make replacing the whole unit worthwhile. I think I will just get it repaired, if that is indeed the problem.
So, I had an hvac guy over again today, who looked into things a little more deeply than he had a few days ago. Now he says that the heat ex changer is all watered up due to the fact that propane has lots of water in it. He suggested replacing the whole thing, heat pump, gas furnace, the works. I am sure I would save on energy but do not want to replace something that has given me little recent trouble. He says if I just replace the exchanger I am asking for trouble. I said I will take that under advisement, and if he is correct I will hire him to replace the whole thing. Sounds fair to me. And replacing the whole unit seemed to be jumping the gun, so to speak.
So in my carrier heater the heat exchanger has a permanent warrenty. But just installing it seems very straightforward, and in my opionion should not add up to the 1200-1500 he says it will cost. I mean, he showed it to me and it was not at all hard to get to. If his time is worth 100/hour it does still not seem like a 14 hour task. Am I being ripped off or totally unreasonable.
How old is the unit? I’ve seen some cases where a cracked heat exchange, under warranty, will net you a new furnace because they aren’t making that part anymore. If that is the case, you could still end up paying that much after labor, some sheet metal, a new air cleaner etc…
I doubt that’s the case but I just wanted to put it out there.
I’ll let raindog comment on the costs associated with having a pro replace a heat exchange under warranty. That’s beyond the scope of my knowledge.
For a major job you should always get more than one bid.