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A combustion analyzer is a great tool but will not always tell you that there is a cracked hx. If the crack is large enough you definetly will have readings out of range but if it's just starting to crack, around an eyelet, out of your air stream, etc. You might not see it with a combustion analyzer. Learning how to do a proper visual inspection for different hx's is invaluable. All hx's have certain places they will fail first and is good to know.
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I look at the duct first, if the returns are piss poor and its an old Payne (the most common 80's furnace around here) I'm usually pretty certain there is a crack, or cracks. I keep nice looking 14X14 patches on the van so I can cut a hole in the plenum to visually inspect the HX if its an upflow, if its a downflow I will pull the blower and climb into that somb!tch. There are many times you wont see a crack with an inspection camera that you would see with your head directly above the HX
America; first we fight for our freedom,
then we make laws to take it away.
-Alfred E Newman
New Guest
what is the max recommended for co ppm?
Professional Member*
0.0Originally Posted by crazyjohn
America; first we fight for our freedom,
then we make laws to take it away.
-Alfred E Newman
Professional Member*
99 and stable.
Regular Guest
Just had one today. A Duomatic Olsen condensating nat gas furnace...Looked like a tech before me had cut the sheet tin on the furnace itself up near the plenum, looking for a bad hx, then plated it. the condensate pump was laying on the floor, with the top half seperated from the bottom half and the drain line off the furnace was in a bucket. The furnace had a spark-ignited pilot module and was tripping out and it looked like the A/c was added as an after thought by the wiring of a relay and other componenets inside the furnace...Hx had rust spots in it so I failed it on the basis that the H.O. would have been throwing good money to bad with me trying to get it going again then find out it had a bad hx anyways with flame roll out. Basically The time has come to start over with the heating unit. It wasn't designed to do what someone was making it do in the first place and now it needs a possible couple hundred dollar control module...Time to do things right.
Professional Member*
Combustion analyzer, works sometimes.
Visual inspection, works better.
I have noticed that 18+ year old furnaces were built much better. I have come across 20+ year old Yorks that I tried to find a crack and couldn't. The heat exchangers were built better "back in the day".
I always find my cracked heat exchangers on the appliances that are 10-15 years old. Sometimes just due to age and sometimes due to an oversized furnace or incorrect A-Coil/plenum on top restricting air on the outer most cells of the exchanger.
Another trick that was told to me by a gentleman that has been in the business for quite some time is,
while the furnace is running and heating, use a spray bottle and mist a brine (salt water) solution into the intake of the fan. As you do this watch the flame structure. If there is a crack you will see the flames change.
The biggest cracked heat exchanger I've ever seen on a residential was a 15 year old natural gas. I pulled the blower assembly, laid on my back, wiggled inside and when I turned on my flashlight I didn't even need my mirror. It was 10 inches long, in plain sight and in the center it was about 1/4 inch of separation.
You can usually take the back panel off of oil furnaces to inspect. It's only like 20 screws...
~smoke~
"That motor's done, he let the factory smoke charge out!"
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Not true.... there are some furnaces that are positive pressure.
Warning: Just because I am over the head injury doesn't mean I'm normal!
The day I stop learning.... I'm dead!
Professional Member
get the book from heat exchanger experts. better yet get your company to bring him in for training
Professional Member
If I got CO reading 10 ppm at supply air,I would red tap it. If the reading smaller than that, I would checked the condition of the furnace. I might use smoke candle to double check.
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which ones?
Professional Member
only lennox pulse that I can think of
Professional Member*
Trane Voyager positive pressure forced draft. But it depends on where the crack is at that can determine if the air from the blower motor is entering the heat exchanger or pulling flue gases from heat exchanger and putting them in the airstream.
It's just rocket science. It's not like it's heat and air work or something.
Professional Member*
If you see 10 ppm or any other number in the supply airstream, that tells you absolutely nothing about the source of the CO. It just tells you the furnace is blowing CO around. If a furnace was producing 400 ppm of CO in the flue and was somehow dumping all that into the airstream, you would see less than 9 ppm in the supply air due to dilution. Any time you have a vented appliance that has CO of higher than 400 ppm air free in the flue, you have to red tag it per ANSI Z21 until repaired.
The correct way to find the source of CO inside a building is to test CO levels in the flue of all fossil fuel appliances in the building.
Professional Member*
Plus the Rheem drum style heat exchanger also has positive pressure. They probably never have higher pressure than the supply static pressure from the blower so it is still unlikely combustion products would ever make it into the airstream. Except possibly the Pulse as the high pressure pulses would probably puff into the airstream. Always do a pressure test when working on a Pulse furnace!!!!
Professional Member*
Yep yep..... the pulse, the voyager, the drum, oil burners, some trailer furnaces and a whole bunch of old converts I used to work on from the early last century.
Warning: Just because I am over the head injury doesn't mean I'm normal!
The day I stop learning.... I'm dead!
Professional Member
Spraying a brine (table salt and water) into a furnace burners while it is operating was done because most of us techs have a refrigerant leak detector, since table salt is sodium chloride, the leak detector picks it up. Just put your leak detector at a supply register and if there was a heat exchanger leak, the detector would go off. I was told to spray it directly into the flame not the blower.
Regular Guest
Didnt understand what you meant by if you think that? What I meant was that I will go as far as I have to in pulling a furnace apart. I also use the poke through method although I use my fingers usually. I agree with the gut feeling in so much as if it looks very rusty or otherwise unsafe I will completly dissasemble and remove the HE completly to cover my rear.I never leave anything thats suspect.So what I was saying is Ill dissasemble as far as I have to, unless the customer just says replace it. Learned long ago that they will fail in the most inacessable places sometimes.I own an inspection camera also but there are still cases I find myself removing the HE completly and find rusted through holes or cracks.Also in my opinion seperated seams are worse the small cracks.They should be red tagged also.
Professional Member*
just an observation, the hx's often crack from operating at above their rated heat rise and rust from operating below their rated heat rise. cleaning up a rusty furnace and doing normal (correcting the heat rise) service to a furnace with no crack or rusted open spot may leave you as "the tech of record" on a furnace with eggshell thin metal where no one will call for service for a very long time. don't feel too guilty about poking the hx to be sure this isn't the case. we make judgement calls with our careers and other peoples lives at stake. sleep well.
i was born under a wandrin star.
Professional Member*
they rust because they have a venting problem, not because the temp rise isnt high enough
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