hi temp cutout

zeke

Have a carrier upflow forced air furnace, vintage 1990.
Burner going off on hi temp- set for 190. I measured 125 in duct about 2 feet downstream from limit switch. Unit was changed for same problem. Why the difference in the two temps.
Thank you

Constantin

Insufficient Air Flow?

Are the registers and ductwork

• unobstructed
  
• adequately sized
  
• feature the right damper setting (if any)

for the amount of air you're trying to push through? How many CFM should the unit be moving vs. what it is moving? Does the furnace have a speed setting for the fans depening on heating vs. cooling?

zeke

airflow

Constantan,
Thanks for the reply.
Registers unobstructed
Ductwork mostly flex @7" for straight runs less than 40'
Single zone,1 damper set OK
Have no way of measuring CFM. Book says 1670 @ .5" esp.
If it is a flow problem, why the difference in cutout temp(190) and measured temp(125)?

zeke

why temp diff

Quote myself:
"If it is a flow problem, why the difference in cutout temp(190) and measured temp(125)?"
--------------------------------------
Again, why do I have that much temp difference. Would very much appreciate reply.
Thank you.

Charles G.

Air temp

Do you have central a.c. @ furnace? Limit switch is upstream (beneath) of evap coil. Take the temp downstream (above) the coil and you get a BIG difference if coil is dirty. Pull out the blower assy, get on your back and look up past the heat ex at the coil. You may not like what you see...

Also, what "unit" was changed? Another possibility would be if the heat exchanger has failed. Serpentine-style H.E. are famous for ballooning so flue gases short circuit from turn to turn. Too much heat vs too little surface area = high flue temps.

zeke

Charles,
Thanks so much for your reply. What was confusing me was that I assumed that the 190 degree air was the "average" temp past the heat exc and so I felt that the temp downstream should be about the same. From your remarks it appears that the temp at the limit switch is localized and does not represent the "average" air past the heat exc. Please correct me if this is wrong.
Answering your query.
Yes I have an A coil above heat exc and the locations of limit switch and recorded temp downstream are as you suggested.
And the "unit" mentioned was a limit switch that the tech
replaced, since he could not find the problem.
My final question is, what should the normal temp difference be for a clean coil, i.e. the two temperatures in question or is this a function of the configuration of the furnace?

Empire_2

Zeke

If you look on the name plate, you will find the accepted temp rise, usually 40 to 70 aprox. Since the limit is tripping, you MUST take a temp reading at the limit control usually just above or below. The switch if fact may be faulty, but that is the only way to tell. You also might want to contact Carrier or Bryant corp, because I believe at one time there was a deflector for that limit.

I don't understand why someone replaces a furnace because it was tripping on hi limit.... Is this a hi EFF> model or 80%'er?

Mike T.

zeke

Mike,
Thanks for the reply. The limit switch was changed out by the tech and is doing the same thing as the old one, so it may be a low airflow problem due to a plugged A coil;, but in spite of this, I am puzzled as to why I get such a large temp difference between temp at limit switch and a point 2 feet downstream of it.

Dale

limit trip

If your temp rise is in range, I would suspect a hole in the heat exchanger that's affecting the high limit. Your furnace is old enough for this to be an issue. You could raise the fan speed to see if the unit still trips. Someone with an oxygen analyzer could see the change when the blower starts. A co reading on the cells on either side of the limit should change if there is a hole.

Empire_2

Air acts in funny way's

If ya think about it, air will take the path of least resistance. Say the "A" coil is more plugged on the side over the limit control. That side will get hot....too hot to quickly. Downstream you are seeing the accumulation of a # of things, but we still have to find out why the limit (if ok) is reaching 190*. Secondary HX partially plugged?, A coil same? Combo of both? What about the take offs on the ducting.....Is there any crazy immediate bends or "U" turns? Funky designs and air flow do not get along. Why do you think Air veins are used. They keep the air flow as "Less turbulent as possible after the bend, thus Les restriction.

Dale

temp difference

Another thought on the temp, the limit is close between sections of the heat xchanger. There is allot of radiant heat there. The usual way to check air temp is out of direct line with the heat exchanger to avoid the radiant gain. If you have an A coil above that is ok. Also many limits have a piece of sheet metal under them to move the air to where the manufacturer decided was a normal sensing spot.

Charles G.

Airflow

Well, I guess it wasn't a bad limit switch after all. A clogged A coil will not permit all the air to flow through per design. If the blower is moving 1200 cfm, but the coil is only flowing @ 800 cfm, you have all that HOT air just sitting there and it's getting hotter and hotter. And then the limit trips. Temp drop through the A coil should be minimal. Sure, the coil will absorb some heat @ start up, but again, average temp drop should be minimal. A quick check would be to remove limit switch from mounting point. A clogged coil will produce a HUGE amount of airflow through the opening. Another way to guesstimate how dirty the coil is is to look at the blower motor and wheel. All the dirt that missed the filter will visit the motor and wheel on its way to the coil. And if you think inspecting the coil is a pain, wait til you try to clean it.