Lennox G24M...Need help
I am having a huge issue with my furnace that heats my garage. As stated it's a Lennox G24M. The issue we are having is that is flipping it's primary limit switch after a short while. We have measured the heat at the heat exchanger (170 degrees) and in the plenum(92 degrees). There seems to be too much heat being held at the heat exchanger and not being pushed up into the plenum and along the duct work. We have checked the heat exchanger for damage and found none(looks like new), the furnace is approx. 5 years old. The chimney is clear and to spec., the vent motor is working correctly, the rollouts are working too,we have also checked the air flow capabilities in and out for correct capacities(all good), we have correct gas pressure as asked for in manual, primary cut off switch works correctly too cutting out at 170 degrees. The flame looks perfect too, nice and blue non flickering. We have vacuumed out all tubes in the heat exchanger for obstructions and found none. Physically checked heat exchanger and found nothing out of the ordinary. The blower motor working fine and on high at the moment, we have tried all settings there too.
So....why aren't we getting enough heat into the plenum? We have checked for obstructions everywhere and found none. The issue is that we have plenty of heat around the heat exchanger but not enough above it and can't figure out why.
Anyone have any ideas or suggestions???
The cold weather is coming and I need heat soon.
Thanking you in advance
Lack of airflow, find out why.
Karst means cave. So, I search for caves.
The duct trunk is 8X14 with four 6" and two 5" down spouts coming off it. The main trunk is approx 12' in length with the down spouts being approx 8'. All 6" downs are run off the main trunk but the two 4" are taken off the plenum as an after attempt to get better flow.
My next plan is to temporarily remove the joiner to the main trunk and see if that will still restrict the flow. We have also enlarged the intake to way more than it possibly could need. We have 1000cmf passing through the furnace from our calculations. We have also tried plugging and capping off one burner tube and readjusting pressures for that with hopes of running the heat at 3/4s capacity to see if restricting the heat would help. It allows it to run longer before tripping. We have also upped the blower speed to high and also tried all speeds without success. We have also lengthened the chimney to try creating better draft, same result. Checked exhaust blower for debri found it clean and like new. Chimney clear also. This furnace has been in my garage for almost 5 years and was near new when it was installed then. So it hasn't seen much use at all. I now want it working as I have more projects in mind to work on. The furnace doesn't have any aircon or anything other than heat attached to it to restrict flow either.
Any help is greatly appreciated...thanks
call a local contractor to correct the airflow problem and have it serviced
please read site rules
I dont warranty Tinkeritus
Sounds like low air flow. Could be that part of the heat exchanger is blocked due to improper fittings. Pictures would help.
Could be improperly sized (over sized) orifices.
Get a pro to check it out.
Remember, Air Conditioning begins with AIR.
I agree with the DIYers....the other half of my team is a licienced man and we are stumped that is why the postings....just trying to help his mental quest and maybe help solve the issue. Many brains working together is always better than one...isn't it??
This is why I can offer the stats he is finding to you...I do understand enough to be damgerous and that is why he is the one doing all the work. I hate gas and propane just for the obvious reasons....very dangerous to my health.
I know enough to make furnace work incorrectly...that is why I have asked for liciensed help...my family is worth much more than heat in my garage.
Like I have said before any help is greatly appreciated. maybe someone's suggestion will help him to see whatever it is he is missing.
Orfices are of correect size also. he has had his hands all over the heat exchanger and found it to be in excellent condition. No obstructions there.
Pictures could help.
If the heat exchanger is too hot, then either there is too much heat going in or not enough heat coming out. Air flow is what takes the heat out. Gas flow with a flame puts it in. Did you clock your meter?
Uneven air flow across the heat exchanger can also cause the problem, I would need to see it.
What kind of air filter are you using and what size and when did you change it last?
?Is there a cooling coil installed? Is the coil dirty?
Last edited by Kevin O'Neill; 11-11-2007 at 09:34 AM.
Reason: additional questions
Remember, Air Conditioning begins with AIR.
No cooling coil or filter, but the area being used to pull air it greater than the filter size through a 90 bend to pull from the ceiling of the garage.
Yes he clocked the meter(sorry I need the type of measurement to jog my memory on his results to give to you...units?)
He plugged off one burner and capped off the hole the burner was using to not allow any unburnt air to affect things that way. From what I remember to not effect exaust flow or draft, it did make sence when he explained it to me.
Yes i agree with the idea that flame makes heat air cools it and air speed over exchanger determines rate of heat flow. We have a smooth tranition from plenum to duct work. He redused meter reading from 3.5 to 3.1 to help reduce heat also for the plugging off of one burner also. The plugging off of one burner helped solve problem for a while but still trips switch just takes a bit longer. Furnace is standing upright also.
Pictures aren't my strong piont with the posting aspect of them.
1000cfm through 14x8 duct and you arent listing to anyone here
stop messing with the furnace and put it back the way it is supose to be
altering the furnace isnt going to go anything but hurt you or someone else
if this guy is a pro then he should know how to do static pressure and size duct
airflo is your problem
the supply is to small and i will also bet you return is even more restricted
stop messing arround and get someone to do it right before someone gets hurt
Last edited by tinknocker service tech; 11-11-2007 at 10:04 AM.
I dont warranty Tinkeritus
If you using the G24M4-75, according to the manual, the furnace on high speed, .50"w.c. delivers 1600 cfm. 1000cfm is too low. to get that through a 8x14 duct, the air would need to move about 27mph!
Not trying to upset you here but....YES he is a pro and has sized the ducting and intake, says it is adequate(spelling?) I just opened up the main ducting and it still trips with the same temp results. The intake is larger than the filter size opening which is making us wonder why it isn't getting enough air through the furnace.
Believe me I am not looking to do it incorrectly in any way...I am quite capable of bypassing all limit switches and running it that way. I want it done right...hense the questions and request for help. So please if you don't want to contribute in a construtive manner....Please don't ...Thanks to all who are willing to help us out.
As for CFM through 8X14 duct...isn't that possible? Is he missing something?? I don't know so please explain what it is that he is missing.
Model is....Lennox Elite Series G24M2-60A-10
I don't know who and how it was used before I got it other than it was used for 6 months in a shop and was too small to heat that area and he upped the BTU size in his shop. It was a brand new furnace when he bought it. He too was a pro as he was the guy who installed our heating system in our farm buildings(3 houses and 2 work shops). I never asked if he ran it with propane or gas. I expect the orfice sizes are different from one to the other. So if that is the issue here, not that my guy say it seems it is. How do I find the correct orfice sizing??? As you may have guessed I am on my own as I type. I am willing to ask anything if I can offer him something to help him get through this problem information wise that is.
OK...I went ahead and removed the top off the plenum and ran it. Plenty of air flow I'm sure...?
It still tripped the switch but only got up to 140 before it did....why??? Switch tested good when tested and even got the same results when we switched out the old switch for a new one.
Gas is off again and impatiently waiting for my buddy to get a few minutes to fry his brain again...lol
I have a few pics now of the set-up and will try and post them soon....may be harder than getting furnace to run ..lol I also hate computors!!! Or is it they hate me???