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

[dash]

Duct systems can often be improved to and acceptable static,by adding turning vanes in accessable 90° elbows ,often those near the furnace.

Often the return drop9main return duct along side the furnace),is to small or doesn't allow turning vanes to be added,but it can be replaced,with a wider one that allows vanes.

Post pictures of the furnace and duct,sizes and model number to get suggestions.

[simpleman]

[QUOTE]Originally posted by deejoe

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)

Why you're laughin? If you assume that a oversize furnace
and a small home would have the furnace going out on limit then I should be the one that laughin.

Oversize furnace and a small home will cause short cycling.
LOL!

[MikeJ]

It is what each manufacturer deems appropriate for that model of furnace. Temp rise can vary but should fall within manufacturers specs.

Let's say the label inside the furnace says that a temp rise of 35 - 70 degrees is on the label inside the unit.
\
I have 68 degrees coming into the furnace, usually taken at the filter and a temp reading a few feet down the trunk line away from turbulance caused by the a/c coil or plenum of 103 degrees. That is at the very bottom of the specs:35 degree difference.

Now, let's say the temp down the trunk line was 138 degrees. That would be a 70 degree difference, that is, at the top end of the specs. If I let the unit hover around 165 degrees down the trunk line, I would have almost a hundred degree temp rise, far over specs. My warranty on the heat exchanger would be invalid if the company knew this, or, my heat exchanger would be stessed out and probably fail prematurely. Constant overheating kills athletes as well as furnace heat exchangers.

Too low of a temp rise causes condensation to buildup in the heat exch., venting, inducer housings, and chimneys. Not good either.

Hope this helps.

BTW, changing the manifold pressure rather than finding the real cause is also ill-advised. Tweaking down to 3.25 probably won't make a diffence anyway.

If your return temp is 78 degrees and you had 148 in the trunk line you still would have a 70 degree temp rise.

3M makes several filters. Which one are you using? What is the temp without a filter?

[beenthere]

If your using the 3m filtrete, its very restrictive.

Adjusting down the gas valve could cause condensation in the flue or chimney. And cost you more to repair then fixing the real problem.

[amickracing]

[QUOTE]Originally posted by simpleman

Originally posted by deejoe

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)

Why you're laughin? If you assume that a oversize furnace
and a small home would have the furnace going out on limit then I should be the one that laughin.

Oversize furnace and a small home will cause short cycling.
LOL!

Well... both of you are correct. "normally" an oversized furnace would just short cycle. But lets say the 1st system was correctly sized, then Bubba's heating came in, they always oversize a unit by one size... now the duct is too small, and this problem comes up.

The beauty of this trade is all of the variables that pro's consider when they do their job properly.

[Black Adder]

For starters, fan switch should be coming on before the unit reaches anywhere near 165-170 degrees unless you have the house temp at 100 degrees. That is way too high a temp rise on any furnace on the market. I can see if the fan is coming on at 170 degrees it could easily hit 180 before it has a chance to get rid of all that built up heat. Bottom line, as you have been told quit dicking around with this thing and get someone in there who has a clue...to fix this unit. You talk about the system being H.O. installed and here we now another H.O. diagnosing and repairing it. Two wrongs don't make a right.

[simpleman]

[QUOTE]Originally posted by amickracing

Originally posted by simpleman

Originally posted by deejoe

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)

Why you're laughin? If you assume that a oversize furnace
and a small home would have the furnace going out on limit then I should be the one that laughin.

Oversize furnace and a small home will cause short cycling.
LOL!

But lets say the 1st system was correctly sized, then Bubba's heating came in, they always oversize a unit by one size... now the duct is too small, and this problem comes up.

Then it goes back to being a airflow problem.I've never came across a furnace be it three time to big for home that had proper airflow going across it that would trip the limit.

[MikeJ]

Originally posted by Black Adder
For starters, fan switch should be coming on before the unit reaches anywhere near 165-170 degrees unless you have the house temp at 100 degrees. That is way too high a temp rise on any furnace on the market. I can see if the fan is coming on at 170 degrees it could easily hit 180 before it has a chance to get rid of all that built up heat. Bottom line, as you have been told quit dicking around with this thing and get someone in there who has a clue...to fix this unit. You talk about the system being H.O. installed and here we now another H.O. diagnosing and repairing it. Two wrongs don't make a right.

From his very first post:
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.

This is probably a joke thread, but whatever:

T'stat should still be calling for heat as unit shut down during it. The high limit did it's job. Then when the heat ex cools down to 120, the burners should relite and his sequence should begin again till stat is satisfied.


Those dials can be misleading tho yours may cut off at 180. Most are about 200 for a high limit setting and are factory preset.

[deejoe]

Originally posted by amickracing
.


Well... both of you are correct. "normally" an oversized furnace would just short cycle. But lets say the 1st system was correctly sized, then Bubba's heating came in, they always oversize a unit by one size... now the duct is too small, and this problem comes up.
************************************************** ********
Well said, and I especially like your bottom line ;....

"The beauty of this trade is all of the variables that pro's consider when they do their job properly." [/B]

[beckservmngr]

I only read in this post a few times about ductwork. Everytime a furnace is installed, the ductwork must be checked to see if it can deliver the apropriate CFM. In my area, "pro's" have constantly dissapointed me in that they will put a furnace in and tie on 14" Flex for a return. they don't seem to grasp the fact that you have to maintain the temperature rise designated on the furnace rating plate. generally speaking, you never want a temp. rise over 60*. but every unit has there own design temp. I guarantee your problem is in the ductwork, not saying you might have a few other problems, but the ductwork is KEY. I take it that this furnace has been overheating from day one, make sure the pro you call is in fact a pro, and have them check the eyelets on your heat exchanger, because I will gurantee they have busted.

[cn]

From his very first post:
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.

This is probably a joke thread, but whatever:

Those dials can be misleading tho yours may cut off at 180. Most are about 200 for a high limit setting and are factory preset.

Actually not a joke thread at all. To respond to other recent posts.

HVAC installed by an HVAC company (in Omaha Yellow Page since the 1980's; company good or not I don't know, I am the second H.O.).

I am not a HVAC tech or try to be, but I read quite abit to understand the furnace system to see how it works. So not "2 wrong's do not make it right". It is the same idea as trying to understand how the car works before talking to your auto mechanics so both sides understand the issues and fix the problems well.

On a different topic, I just had HVAC tech in to install B.I. gas line (kitchen range), and even they admit some of their techs are "rookies" and make mistakes and they send out the "senior techs" to fix the problems caused by the "rookies".

I trust my HVAC company but I'd like to learn as well.

The above sequence of events 1-4 was what I observed. Right or wrong, I don't know.

Temp rise is easy to understand but temp gradient along the path is also "steep", well the furnace is like a stove in a sense. QUESTIONS:

a. At metal surface of HX, probably 300F+; right or wrong?

b. At the location for High-Limit Switch; during operation Temp is probably between 120-200F.
- The High-Limit Switch is located inside the HX, is this correct?
- Factory setting, what is considered "standard", 170, 180, 190, 200F???

c. The switch does its job by shutting down the burners at 180F, but the real question is:

"given a fixed condition (a typical return ductwork, supply registers, a specific furnace brand, a specific home, duct lay out etc. and etc.), what is considered PROPER TEMP measured at the High-Limit Switch location?
My guess is something like 160F???

d. Temp at supply register, what is the "standard"? I can take some measurements there.

Thank you all for your advices, I simply need to learn more before talking the my HVAC techs. There are simply too many opinions out there.

[B] ADDENDUM: Previous post by beckservmngr re temp rise of 60F is valid.
According to this document by York (Coleman), page 2 specifically mentions "Air Temp Rise Range of 45-75F" and "Max Outlet Temp of 165F".

http://www.dgatprogram.com/doc/035-20527-001-A-0804.pdf

I will ask my HVAC tech to adjust so under normal operating condition (unobstructed flow, yes I will make sure return and supply registers are clear! AF is new 3M but I will experiment without it; then if needed go back to the "old-fashioned fiberglass" filter!!!).

The temp at the High-Limit Switch area should hover around 135-145F or so (with 165F as max).

[Edited by cn on 10-18-2006 at 03:40 PM]

[deejoe]

[QUOTE]Originally posted by cn

IThe above sequence of events 1-4 was what I observed. Right or wrong, I don't know.

Temp rise is easy to understand but temp gradient along the path is also "steep", well the furnace is like a stove in a sense. QUESTIONS:

a. At metal surface of HX, probably 300F+; right or wrong?
IT MAY BE,DEPENDS ON THE DESIGN OF THE UNIT

b. At the location for High-Limit Switch; during operation Temp is probably between 120-200F.
WHEN PROPERLY OPERATING THE TEMP SHOULD ONLY BE AROUND 120 TO 140

- The High-Limit Switch is located inside the HX, is this correct?
NORMALLY IT IS LOCATED JUST ABOVE THE HEAT EX. OR JUST BELOW OR WITHIN THE SUPPLY PLENUM

- Factory setting, what is considered "standard", 170, 180, 190, 200F???
DEPENDS ON THE UNIT DESIGN, BUT APPROX IN THE 180-200 RANGE


c. The switch does its job by shutting down the burners at 180F, but the real question is:
THAT'S WHAT IT IS MEANT TO DO, IN CASE OF AIR (BLOCKAGE)RESTRICTION,BROKEN FAN BELT,ETC (IF EQUIPT)FAULTY BLOWER MOTOR, ETC. IT STOPS THE BURNERS OTHERWISE THE HOUSE MAY BURN DOWN.ITS MAIN FUNCTION IS *SAFETY.*


"given a fixed condition (a typical return ductwork, supply registers, a specific furnace brand, a specific home, duct lay out etc. and etc.), what is considered PROPER TEMP measured at the High-Limit Switch location?
My guess is something like 160F???
IF EVERTHING IS WORKING PROPER, THE TEMP SHOULD IN NO WAY BE EVER THAT HIGH AT OR NEAR THE SUPPLY PLENUM. THE FAN ON/OFF SWITCH CAN SOMETIMES BE INCORPORATED WITHIN THE HIGH LIMIT., AND IT USUALLY WILL KICK THE FAN ON AT APPROX 120-130.EVEN IF IT IS A SEPARATE HI-LIMIT SWITCH IT WILL NOT BE THAT FAR AWAY FROM THE FANS ON/OFF CONTROL SO WHY WOULD THE TEMP BE THAT EXTREME IN DIFFERENCE?

d. Temp at supply register, what is the "standard"? I can take some measurements there.
TAKE A TEMP READING JUST "OUT OF SIGHT' OF THE HEAT EXCHANGER,(AT THE MAIN HOT PLENUM TAKEOFF.READING SHOULD BE APPROX 120-140 (MAKE SURE UNIT BURNER HAS BEEN OPERATING FOR 5 MINS OR MORE)
NOW TAKE A TEMP READING AT THE RETURN INLET JUST BEFORE THE FAN COMPARTMENT. IT SHOULD BE PERHAPS IN THE ROOM TEMPERATURE RANGE OF 68-72 DEPENDING ON YOUR T STAT SETTING.
EXAMPLE OF TEMP RISE; SUPPLY 130.
RETURN 68.
TEMP RISE=62

THE ABOVE TEMPS ARE "IN GENERAL" ONLY AND ARE AVERAGED GUIDELINES TO GIVE YOU A GENERAL IDEA.

I HOPE THIS HELPS.

[smokin68]

Originally posted by Black Adder
as you have been told quit dicking around with this thing and get someone in there who has a clue...to fix this unit.

Here's the only advise you need. NOW LISTEN!!!

[MikeJ]

You keep asking what is standard. What is your exact model and serial number of the unit in question.

You said it was installed in about 1991 and have inferred it probably has been cycling on high limit since day one.

(If so, I would seriously have the heat exchanger examined very carefully as was told to you.)

I asked what kind of 3M filter. Not all 3M filters are overly restrictive. I don't like the purple ones because they are. In your case, I would go with the $1 fiberglass type which is just like using no filter at all. Which you should be doing if you are testing air flow restriction (causing overheating) caused by a filter.

Then there is this:

----b. At the location for High-Limit Switch; during operation Temp is probably between 120-200F.
- The High-Limit Switch is located inside the HX, is this correct?----


Not exactly. What is inside of the heat exchanger is the fire from the burners, gases of combustion from same, and very hot air that is drawn into the burners and some of it is burnt in combustion. All this stays withing the inside of the heat exchanger cells. One cell for each burner. All these combustion products leave the heat exchanger cells and go up the flue piping to the chimney. Unless there is a crack in the heat exchanger or a split in a welded seam. This can cause hot gases to exit the heat exchanger and heat up a limit switch causing premature shut down of the unit.

No, the blower sucks air from the return registers and draws it thru the filter and into the furnace where it goes around each cell of the heat exchanger. It is here that this air is heated to a temperature that is comfortable for the unit being heated. Once out of the furnace and plenum it should be at a temperature that falls withing the temp rise specs of the manufacturer.


---At the location for High-Limit Switch; during operation Temp is probably between 120-200F.----

If you understand the above and all else that has been told to you, you would understand that the temperature near the high limit switch WOULD NOT be near 200 degrees.
Unless something was defective as stated above.



--- Factory setting, what is considered "standard", 170, 180, 190, 200F???-----


What is your model of furnace? That model has a standard. This may vary by manufacturers and even with different models by same manufacturer. But most Honeywell dial type limits are preset for 190-200 if that helps.

BTW, do you have a honeywell dial type limit?

Ha, smokin posted whilst I was typing. Best advice given yet.

[cn]

Update, HVAC tech came, great guy, knowledgeable.
We did a few tests and they are all OK.
(Tempstar furnace 80% from 1991, exact model ???)

- HX inspected...OK

- Then we ran a few COMPLETE cycles (took almost an hour to do it because the Honeywell thermostat is programmed to run only 6x/hour)

- Remove 3M Filtrete filter and re-inserted back in: no difference in OUTput temp (measured at 3 feet above HX in the plenum; above A-coil)...so ruled out 3M as a cause.

- In fact OUTput temp was at 145F or so, a bit high and no big deal, but tech says 125-130F is good enough so he adjusted the gas valve a bit, and temp settles down to 125F (outdoor temp now 28F in Nebraska).

- High-Limit still set at 175F (Yes I have a dial Honey Limit Switch). This is OK. Set for

- Furnace runs fine

-As others have said, the tech made sure temp rise is between 45-70F. We measured around and it is within spec.

So all is good. Thanks all! You guys are great!

(An informed customer is a happy customer!)

[simpleman]

[i]Originally posted by cn

- In fact OUTput temp was at 145F or so, a bit high and no big deal, but tech says 125-130F is good enough so he adjusted the gas valve a bit, and temp settles down to 125F (outdoor temp now 28F in Nebraska
(An informed customer is a happy customer!) [/B]

Sorry to bring this up to the top again.If I may ask, what type of readings on the combustion side did you all end up with after the adjustment was made to the gas valve?

[cn]

The HVAC tech thought you guys are correct.

We looked at page 2 of this chart together:
http://www.dgatprogram.com/doc/035-20527-001-A-0804.pdf

And as the above link (Coleman furnace manual) suggested:

- MAX for furnace is 165F
- Temp rise between 45-75F. So for typical house temp of 68F, temp on combustion side (measured about 2 feet above A-coil) should be between 113-150F.

- BEFORE adjustment: 145F
- AFTER adjustment: 125F (which is kind of the average
of (113+150)/2 = 131F)

I agree with him, when in doubt go for the average.
He kind of laughed and mentioned that furnace is like a gas stove. If all you need is to boil the pot of water then turn your gas burner just to get enough gas output to boil the pot, any excessive gas creates extra flames that "lick" the outside of the pot and lost to the air.

Adjusting OUTPUT temp down to 125F (from 145F) should save some gas (I will give a follow-up post later in the winter).

The bottom line is at 145F, alot of heat is lost to the flue because the HX can only extract that much heat to heat the house (at the fixed blower speed and with the given ductwork, filter, return and supply registers layout). At 125F, there is less heat loss through the flue (I can attest to it with my hand feeling the flue pipe BEFORE and AFTER adjustment).

The problem with 80% furnace is when there is called for heat "this Honeywell smart gas valve" only has one setting that puts out a fixed BTU. So whether it is 20 or 40F outside, it still puts out the same BTU.

In contrast, modern 2-stage furnace has a different gas valve that "titrates" the BTU according to demand.

House is still nice and warm, no change noted. I will keep everyone posted later. It is nice to have a furnace operating below the High-Limit.

Thanks all for your help. You guys are great!

[Edited by cn on 10-20-2006 at 03:13 PM]

[simpleman]

Originally posted by cn


Adjusting OUTPUT temp down to 125F (from 145F) should save some gas (I will give a follow-up post later in the winter).
[Edited by cn on 10-20-2006 at 03:13 PM]

I doubt it.By reducing the radiant heat and the convective
transfer to the hx to me would increase the gas bill.

This why I ask what were the readings on the flame side.

No worry..If you're happy then so be it.

[cn]

[QUOTE]Originally posted by simpleman
[B][QUOTE]Originally posted by cn
[B]

Adjusting OUTPUT temp down to 125F (from 145F) should save some gas (I will give a follow-up post later in the winter).

[Edited by cn on 10-20-2006 at 05:05 PM]

[cn]

Originally posted by simpleman

Originally posted by cn


Adjusting OUTPUT temp down to 125F (from 145F) should save some gas (I will give a follow-up post later in the winter).
[Edited by cn on 10-20-2006 at 03:13 PM]

I doubt it.By reducing the radiant heat and the convective
transfer to the hx to me would increase the gas bill.

This why I ask what were the readings on the flame side.

No worry..If you're happy then so be it.

When I cook spaghetti, when water is boiling, I keep the gas burner low, any more heat will create flame that "licks" outside of the pot and is lost as heat.

Of course there is the "recommended" temp rise of 45-75F in the mfg manual, and my HVAC tech adjusted to 60F temp rise (65 Home temp + 60 rise= OUTPUT temp of 125F).

What do the pro's in this forum adjust their customer's gas furnace OUTPUT temp to? Just curious!