High Limit Opening

charliechicago

Hello To All

I'm working on a condensing furnace and the high limit was opening. Took it out and stuck two different thermometers into the air flow at the same depth as the limit and was reading 160deg, limit is set for 210deg. Replaced limit and same thing happens. Replaced it again to be sure first one was not defective and still happening. Checked gas pressure, secondary heat exchanger cleanliness, blower speed, blower cleanliness, combustion is fine, ductwork is all good. Its been working for 7 years without a problem. I feel like the heat from the face of the furnace where the limit screws in is radiating heat to the limit.

Any suggestions?

LRCCBJ

I feel like the heat from the face of the furnace where the limit screws in is radiating heat to the limit.

Highly unlikely. This would have occurred 7 years ago when it was installed.

I presume the limit is not a mechanical switch?

If so, the furnace is reading some type of resistance from the switch and, if the main board has wandered out of calibration for unknown reasons………………..

charliechicago

Hi thanks for responding. I agree its very very unlikely. Its a standard mechanical high limit.

pecmsg

https://forum.heatinghelp.com/discussion/199464/high-limit-opening

combustion is fine, How was that determined?

A thermometer in that 1” opening will read low.

EdTheHeaterMan

You will need to block off the opening in the same manner as the switch blocks off the hole in order to get an accurate temperature reading there. Are your air filters clean? Are any of the supply registers closed for balancing? Are there balancing dampers in the ductwork between the trunk and the registers that may be closed? Does the secondary heat exchanger have any dust or lint buildup on it? This sounds like an air flow problem.

The air moving over the heat exchanger is slower than it was before you started experiencing the high limit cycling. (Unless you have always had this problem and are just noticing it now). That would mean the duct system does not hafe enough capacity for the size of the furnace.

JUGHNE

There are probably several high limits in the furnace. Are you sure this is the limit that opens? Would show 24 volts across the 2 terminals when opening while running.

Many might require manual reset. But look for another auto reset limit.

charliechicago

Thanks for responses. Firstly, why would not blocking off give me a low temp reading?

Filter is clean. No dampers. Return is not blocked. All registers are open. It’s for sure the main high limit that’s opening, not an auxiliary. I did combustion analysis and 2ppm co, flue gas temp is on target etc. I pulled out the blower and inspected secondary heat exchanger. All is good bit limit is still opening.

pecmsg

if that limit is opening then there’s probably lack of air flow.

JUGHNE

Not blocking off the air flow may let that hole act as a venturi opening….sucking in air as the supply air moves upward at a high velocity.

IIWM, I would install the limit and wire connections. Connect a voltmeter across the 2 wire terminals……..then drill a small hole in the plenum near the limit….just make the hole large enough for your thermometer.

Fire the furnace and note the temp when and if the limit opens (showing 24 volts on your meter).

I have seen this problem when the filter was too restrictive and/or a humidifier by-pass was added.

charliechicago

Thanks Jughne for the explanation!
It ended up being ONE vent that was not open on the basement. Once I opened it the furnace ran fine. But I was still getting way below the 210deg cut out even with the hole blocked.

pecmsg

A little extra air flow makes a big difference.

What are the temperature readings in and out of the unit?

charliechicago

In is 70 out 120

pecmsg

and the max listed on the ID Tag?

charliechicago

Temp rise between 40-60. So its in range.

JUGHNE

IMO, if one vent makes the difference then the ductwork must be marginal in size and design.

SuperTech

How about static pressure measurements? Is there an evaporator coil after the furnace? This sounds like an oversized furnace or undersized ductwork. Are you sure it hasn't been cycling on the limit since it was installed?