Cracked heat X ????

[joey83]

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.

[chuckcrj]

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.

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.

[chuckcrj]

only lennox pulse that I can think of

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.

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!!!!

[grasshopper]

which ones?

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.

[Williamsma]

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~

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.

[stickinit2thman]

Lol... You may think that, but there are several companies in major class action law suites because of that mindset.

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.

[stonefly]

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.

[Gross]

and rust from operating below their rated heat rise.

they rust because they have a venting problem, not because the temp rise isnt high enough

[Gross]

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.

add a couple ice cubes and spray directly onto a hx while the furnace is running...you'll be amazed at how many cracks you find...

[stonefly]

they rust because they have a venting problem, not because the temp rise isnt high enough

so...... do you have some way to support that a low heat rise does'nt reduce the furnaces ability to reject the moisture in the products of combustion?

"venting problem" is kinda vague. since you've corrected me why don't you expand on that a little so i know you aint just woofin.

[Gross]

so...... do you have some way to support that a low heat rise does'nt reduce the furnaces ability to reject the moisture in the products of combustion?

"venting problem" is kinda vague. since you've corrected me why don't you expand on that a little so i know you aint just woofin.

sure, you either need to figure out why you have insufficient draft on a ND or oil furnace OR install a post purge kit on an iduced draft furnace

[Gross]

in addition...rusting of a HX is caused by moisture and flue gases being trapped in the HX...not because it didnt get hot enough. Removing the the flue gases (and moisture with it) solves this problem. 4-5 minute post purge is sufficient

[stonefly]

ok here's my analogy. iv'e just completed a clean & service to an old natural draft furnace with a lot of rust in the firebox. i run the furnace 3 different times for 10 minutes allowing it to cool to ambient temp. before restarting. one cycle each at low. med. & high speeds. would you hold that the same amount of moisture passed through the flue cap to atmosphere on each of those cycles?