Is it Normal for furnace to reach high-limit switch often?

[cn]

I will ask my HVAC tech to do routine checkup soon.

But from owning different houses and different furnaces and watching them go through a few complete cycles, I have made the following observation:

1. Thermostat calls for heat
2. Electronic pilot ignites gas, temp rises until it reaches "Low-Limit" (around 120F)
3. Fan kicks in
4. Burners continue to work, temp continues to rise until it reaches "High-Limit" (around 180F), then the gas is cut off, fan continues to run until it cools down to minimum (around 120F) then furnace shuts off.

My questions are:

a. Is it normal for furnace to reach High-Limit on every cycle?
b. Is it a good idea if the HVAC tech adjusts the GAS VALVE down a bit so when burners run, they do not heat too much causing the HIGH-LIMIT switch to trip?
Potentially saving gas and money?

Thanks.

cn
1991 Tempstar 80% Furnace

[simpleman]

If your furnace is going out on limit then you have airflow problem.

Or you are keeping the stat setting way above 80 degrees.

Its could be overfired but, I doubt it.

[cn]

-Air Filter changed often (every 3 months)
-Thermostat set at 68-69F.

[simpleman]

Originally posted by cn


1. Thermostat calls for heat
2. Electronic pilot ignites gas, temp rises until it reaches "Low-Limit" (around 120F)
3. Fan kicks in
4. Burners continue to work, temp continues to rise until it reaches "High-Limit" (around 180F), then the gas is cut off, fan continues to run until it cools down to minimum (around 120F) then furnace shuts off.

[/B]

What you have posted does not indicate that the furnace is goingout on limit.But actually it sounds like that stat is taking it out.

[cn]

Originally posted by simpleman

Originally posted by cn


1. Thermostat calls for heat
2. Electronic pilot ignites gas, temp rises until it reaches "Low-Limit" (around 120F)
3. Fan kicks in
4. Burners continue to work, temp continues to rise until it reaches "High-Limit" (around 180F), then the gas is cut off, fan continues to run until it cools down to minimum (around 120F) then furnace shuts off.

What you have posted does not indicate that the furnace is goingout on limit.But actually it sounds like that stat is taking it out. [/B]

and see the dial in the Limit Switch keeps rising until it hits the HIGH-LIMIT, then the flame goes out but the blower still runs.

[dash]

If you see it going out on high limit,then I don't doubt it.

Going out on high limt is likely due to lack of air flow,as stated above.

Furnace has X btus of heat,if air flow is not sufficient ,the furnace gets too hot,and goes out on the limit.Not what you want to have happening.


Might want to try it for aa few minutes without the filter,this will aloow a little more air flow,so it should run longer,or not trip the limit.

What type filter do you use,if it's a 3M ,that could easily be the problem.

[cn]

Originally posted by dash
If you see it going out on high limit,then I don't doubt it.

Going out on high limt is likely due to lack of air flow,as stated above.

Furnace has X btus of heat,if air flow is not sufficient ,the furnace gets too hot,and goes out on the limit.Not what you want to have happening.


Might want to try it for aa few minutes without the filter,this will aloow a little more air flow,so it should run longer,or not trip the limit.

What type filter do you use,if it's a 3M ,that could easily be the problem.

I will try to run without the air filter and see what happens.

On the other hand, I'd like to see furnace tuned so at peak operating temp, it should be around 160-170 F or so, but not 180F that trips the high-limit.

Thanks

[deejoe]

Cn; If your furnace is reaching it's high limit setting (1800 then it is due to one or all of the following;
obstructed/dirty filter.
restricted return air
fan speed set too low.
Several supply air dampers /registers, or runs are not fully open and are causing too high a temperature rise at the plenum or near the heat exchanger, thus tripping high limit.
Normally on a T stat call for heat the burner will fire up,then a few minutes or so later at approx 120-130 degrees in the plenum the blower will kick in. The burner will continue to run until the T stat is satisfied, then shut off while the blower continues to run until the supply plenum temp gets down to approx 90-95.

Some questions for you;
When did this "high limiting" start occuring?
Did it just start acting up like this overnite.Have any adjustments been made to the system which would cause this problem?
What size is this unit (btu output) and what size of home.?

[simpleman]

Originally posted by deejoe
What size is this unit (btu output) and what size of home.?
[/B]

None of the above will cause a high limit problem.

[cn]

cycle during the last year that I own it.
Serviced by HVAC (not sure they watch a complete cycle).

I will ask if they can adjust the gas valve down a bit.

All registers are open.
Blower healthy blowing lots of air
I will try w/o the air filter and see what happens.

Maybe it was over-adjusted from day one (1991)!
Probably no harm as the high-limit shuts the burners instead of the thermostat.

[tatted1979]

i got a call about this happenin also, went to the house she had rugs covering the return air grills, plus all the registers were closed. took the rugs off and opened the registers problem fixed.

[beenthere]

Doubtful adjusting the gas valve is the real solution.

You more then likely have an air flow problem as others have said.

Your dust work can still be undersized and the regs blow out alot of air, because there aren't enough of them for the furnace size.

Your furnace is probaly over sized for your house and duct system.

[smokin68]

Tripping out on high limit,wanting tech to adjust gas valve, you need a service call, where airflow/safeties are checked and valve/combustion can be dialed in.

[seaeagle2]

Course on a 15 yr old tempstar, it could be the combo fan/limit switch has a weak spring, or the heat exchanger has a partial plug....

[tatted1979]

ive seen where the homeowner has tons of cats and the A coil is plugged with tons of cat hair, blockin the air flow.

[MikeJ]

Originally posted by cn
cycle during the last year that I own it.
Serviced by HVAC (not sure they watch a complete cycle).

I will ask if they can adjust the gas valve down a bit.

All registers are open.
Blower healthy blowing lots of air
I will try w/o the air filter and see what happens.

Maybe it was over-adjusted from day one (1991)!
Probably no harm as the high-limit shuts the burners instead of the thermostat.

I see a lot wrong with your thinking. Some has been picked up on.

160-170 is the temp you shoot for? Who told you that?

Unit has been service by a company? Not sure if they ran a complete cycle? I would hope they checked a few things:

Gas intake and manifold pressure: They are rated by manufacturer and posted on name plate.

Temp rise: Also posted on manuf. label. Best to follow their advice.

Clean blower wheel: A dirty one just spins in the air, it can't grap and throw air.

Not sure if constant overheating is harmful?
What planet are you from? Go out and start your cold car in the morning and once it starts just floor the acelerator for about 10 seconds. Won't hurt the car a bit, right? After all you floor it on the freeway all the time, right?

I'm thinking you better rethink your thinking.

/Anyone thinking of a defective heat exchanger/seal allowing hot combustion air to influence the limit?

[Edited by MikeJ on 10-17-2006 at 12:07 AM]

[deejoe]

[QUOTE]Originally posted by simpleman

None of the above will cause a high limit problem.
************************************************** **
oh yes,lol, the size of the unit and the size of the home has a lot to do with it.
but thanks anyways for that JOKE statement.(lol)

[cn]

Originally posted by MikeJ

Originally posted by cn
cycle during the last year that I own it.
Serviced by HVAC (not sure they watch a complete cycle).

I will ask if they can adjust the gas valve down a bit.

All registers are open.
Blower healthy blowing lots of air
I will try w/o the air filter and see what happens.

Maybe it was over-adjusted from day one (1991)!
Probably no harm as the high-limit shuts the burners instead of the thermostat.

I see a lot wrong with your thinking. Some has been picked up on.

160-170 is the temp you shoot for? Who told you that?

Unit has been service by a company? Not sure if they ran a complete cycle? I would hope they checked a few things:

Gas intake and manifold pressure: They are rated by manufacturer and posted on name plate.

Temp rise: Also posted on manuf. label. Best to follow their advice.

Clean blower wheel: A dirty one just spins in the air, it can't grap and throw air.

Not sure if constant overheating is harmful?
What planet are you from? Go out and start your cold car in the morning and once it starts just floor the acelerator for about 10 seconds. Won't hurt the car a bit, right? After all you floor it on the freeway all the time, right?

I'm thinking you better rethink your thinking.

/Anyone thinking of a defective heat exchanger/seal allowing hot combustion air to influence the limit?

[Edited by MikeJ on 10-17-2006 at 12:07 AM]

Now I am rethinking my thinking....

My point is that: if the "high-limit" is set at 180F (is this industry standard), then if the furnace is adjusted in such a way the temp hovers around 160-170F, a bit below the High-Limit then it seems common sense to me.
BTW: Is 180F industry standard?

(Like a car with temp gauge below the HOT zone, rather than running into the HOT zone).

The rest of the heating system is checked:
- all air ducts (return and supply unobstructed)
- A-coil cleaned last year. House has no pets, no carpet ( a few rugs and that is it)
- No smokers
- Air filter brand new but it is 3M type
- blower cleaned with vacuum machine. No dusts.

- Blue flame in the burners about 2 inches high or so (per tech).

[dash]

It is almost certain that you have an air flow problem.

Ducts could have been undersized since day one,or the coil was only surfaced cleaned and is impacted(has dirt and stuff in the middle),or a duct could have been crushed by the cable guy,or etc.,etc..

Furnaces are not designed to cycle on the limit,call a Pro ,to get the problem solved.

[cn]

1. How to clean A-coil properly? I hear some people use a spray???

2. If ductwork undersized then I doubt there is anything I can do about it.
But back to my original question: what is wrong with asking the tech to adjust the gas valve a tad so it does not trip the high-limit?
House is nice and warm and air flow seems fine.

The other way to phrase this question is: is there any harm to adjust the gas valve down a tad?
Isn't this what HVAC techs do when installing the brand-new furnaces anyway?
I suspect the furnace was not adjusted properly even from day one.

The reason I say this is because from seeing the HVAC setup and ductwork in this house, I know it was done by inexperienced people (I am the second owner). The register layout is another story!!!!