heat exchanger rusting possible dangers?

[bgrich2003]

I went to a customers house whos heat exchanger was rusting on a 20 year old furnace. I told her to replace due to possible carbon monoxide poisoning. She said she had detectors and asked weather there could be possible danger of a fire breaking out. I wasnt sure if it was so just wanted some info of the possible dangers if the heat exchanger cracked on a furnace

[davidr]

Anytime you see rust in a heat exchanger that furnace has had something impede it's ability to vent properly.

A properly venting furnace should have no rust in it at anytime.

[bgrich2003]

thanks for the info but that doesnt really answer the question of what the possible dangers are once it is rusting

[amickracing]

Once it's rusting it has weak spots. Those weak spots can be a perfect place for cracks and holes to occur. Once you have a HX that has cracks or holes (no matter how small) there's a chance for CO to leak out and into the house. Obvioulsy once you have a small crack it will only get bigger as it heats up and cools down.

I doubt I'd have to give too much info on the down sides of CO getting into the house... nothing good can come from it.

[mark beiser]

Originally posted by davidr
Anytime you see rust in a heat exchanger that furnace has had something impede it's ability to vent properly.

A properly venting furnace should have no rust in it at anytime.

I see tons of heat exchangers with standing pilots that haveh quite a bit of rust that has nothing to do the venting in the heat mode. ;)

In my area furnaces are installed in the attic, or in a closet that draws its combustion air from the attic.

Burn a pilot flame on one side a piece of steel that has 120º+ air on the side with the pilot flame and 74º-80º blowing across the other side of the piece of steel.
The results are inevitable!

The dedicated horizontal furnaces that were made by Ducane and Colsolodated, but sold by just about every name brand, are really bad about rusting around here if the pilot is left on.
The ones that Lennox sold that had spark ignition, or furnaces that had the pilot turned off every year during the cooling season, rarely have any rust.

The Ducane made ones finally rust a hole in the top of the HX chamber the pilot is in.
The Consolodated made ones will get holes all over the place in the chamber over the pilot, assuming they didn't split open at a weld or crack at the expansion joint over the burners before they got a chance to rust through, lol.

[contactor]

I agree with Mark on the rust issue. A 20+ yr old furnace will invariably have rust. To what degree is another matter. As to your questions bgrich, rust alone doesnt cause CO. nor a fire danger.Unless it's so excessive the heat exchanger is blocked by it,or it has caused a breach,I would say light rusting is normal,depending on the exchanger material,humidity, over or under firing,and equipment age,and other factors.
Is your question about rust,or cracked exchangers? Sureley a person qualified to condemn a furnace would already know these answers, I would assume,If I were the customer, and you were the tech I hired.
The effects of rust are obvious. Severity is the issue. Is it somewhat rusted with no breaches or blockages and everything else checks OK. Or severely rusted?

[davidr]

Originally posted by contactor
I agree with Mark on the rust issue. A 20+ yr old furnace will invariably have rust. To what degree is another matter.

I would have to disagree on this, I have seen 30+ year old belt drive furnaces in crawl spaces with no rust in them whatsoever.

If the rust is on the inside of the HX there has been or is a venting issue.
The flue gases are condensing on the inner walls of that heat exchanger at some point.

[davidr]

Originally posted by mark beiser

Originally posted by davidr
Anytime you see rust in a heat exchanger that furnace has had something impede it's ability to vent properly.

A properly venting furnace should have no rust in it at anytime.

I see tons of heat exchangers with standing pilots that haveh quite a bit of rust that has nothing to do the venting in the heat mode. ;)

In my area furnaces are installed in the attic, or in a closet that draws its combustion air from the attic.

Burn a pilot flame on one side a piece of steel that has 120º+ air on the side with the pilot flame and 74º-80º blowing across the other side of the piece of steel.
The results are inevitable!


So you are seeing rust on the outside of the heat exchanger then?

[mark beiser]

Originally posted by davidr

Originally posted by mark beiser

Originally posted by davidr
Anytime you see rust in a heat exchanger that furnace has had something impede it's ability to vent properly.

A properly venting furnace should have no rust in it at anytime.

I see tons of heat exchangers with standing pilots that haveh quite a bit of rust that has nothing to do the venting in the heat mode. ;)

In my area furnaces are installed in the attic, or in a closet that draws its combustion air from the attic.

Burn a pilot flame on one side a piece of steel that has 120º+ air on the side with the pilot flame and 74º-80º blowing across the other side of the piece of steel.
The results are inevitable!


So you are seeing rust on the outside of the heat exchanger then?

No, rust on the inside of the HX due to condensation of the byproducts from the pilot flame usually, sometimes just from the attic air if the people keep it particularly cool in the house.

Imagin the conditions inside a horizontal furnace in a Texas attic on a sunny, hot and humid day.
Lets say it is 95º and 65% RH outside and 130ºF and 30% RH in the attic.
The homeowner runs the indoor temp down to 72º for whatever reason(really common), so the return air temperature entering the furnace is around 75º.

Now you have 75º air on the outside of the HX and 130º 30% RH air on the inside of the HX.
Even without a pilot flame adding water vapor to the equation, you will get some condensation because the air outside the HX is below the dew point of the air inside the HX.

Toss in a pilot flame adding water vapor and it doesn't even take much in the way of ambient humidity to cause a problem.

There is only ever a little bit of condensation, rarely enough to form water droplets, and it drys out between cycles, but it is enough to cause some serious corrosion over time.

Some evidence of this is that the rust usually only forms on the walls of the HX, and is fairly uniform on the whole surface of it. Any baffles at the top of the chambers, and parts of the burner and collector boxes that inside air doesn't make contact with, rarely have any rust on them.

[Edited by mark beiser on 11-04-2006 at 04:38 PM]

[mark beiser]

Originally posted by davidr

Originally posted by contactor
I agree with Mark on the rust issue. A 20+ yr old furnace will invariably have rust. To what degree is another matter.

I would have to disagree on this, I have seen 30+ year old belt drive furnaces in crawl spaces with no rust in them whatsoever.

How often is the temperature of the return air in the cooling season lower than the dew point of the air in those crawl spaces?

It is relativly common during parts of the year around here that the return air temperature with the AC on is lower than the dew point of the attic air that the furnace is located in.

[Edited by mark beiser on 11-04-2006 at 04:33 PM]

[contactor]

[i]posted by davidr[i]
[b]If the rust is on the inside of the HX there has been or is a venting issue.
The flue gases are condensing on the inner walls of that heat exchanger at some point.[b]

Flue gas condenses on the cool walls of the heat exchanger everytime the unit initially fires up. A venting problem would no doubt cause more rusting,but furnaces with no vent problem still condense slightly upon firing up untill exchanger warms up.
I still disagree with no rust on 20+ units. Cant say Iv'e ever seen a rust free steel exchanger ever over that age. I have seen minimal rust on these units. Stainless and aluminized exchangers of course dont rust.

[mark beiser]

BTW, I'm not arguing that venting issues don't cause heat exchangers to rust, I'm just saying that venting issues are not the only thing that causes them to rust, and the rust can be totally independant of any venting issue.

[davidr]

I just read the last part of one of your post Mark & it appears our definitions of what degree of rusting we were talking about varied.


What is odd is that on any furnaces that we had previous rusting issues with once fitted with a barometric damper pretty much disappeared.

[markj]

Originally posted by davidr
Anytime you see rust in a heat exchanger that furnace has had something impede it's ability to vent properly.

A properly venting furnace should have no rust in it at anytime.

That is not the cause of rusting heat exchangers around here on most natural drafting furnaces. Many are located in damp basements. In the summer the heatexchangers often condense internally. Not much you can do other that get the customer to condition the basement and or run a dehumidifier.

[Carnak]

Originally posted by mark beiser

Originally posted by davidr

Originally posted by contactor
I agree with Mark on the rust issue. A 20+ yr old furnace will invariably have rust. To what degree is another matter.

I would have to disagree on this, I have seen 30+ year old belt drive furnaces in crawl spaces with no rust in them whatsoever.

How often is the temperature of the return air in the cooling season lower than the dew point of the air in those crawl spaces?

It is relativly common during parts of the year around here that the return air temperature with the AC on is lower than the dew point of the attic air that the furnace is located in.

[Edited by mark beiser on 11-04-2006 at 04:33 PM]

Yes you could have the high 70s dewpoint air mark.

imagine if you had the coil of the return,

[mark beiser]

Originally posted by davidr
I just read the last part of one of your post Mark & it appears our definitions of what degree of rusting we were talking about varied.


What is odd is that on any furnaces that we had previous rusting issues with once fitted with a barometric damper pretty much disappeared.


I doubt a barometric damper would help out the average furnace with a mild steel HX in a north Texas attic, if I could even find a supply house that knows how to order a barometric damper for me, cuz it definatly won't be in stock, lol.
They get a nice one two punch of short cycling in the winter due to being oversized, and condensation inside the HX in the summer.

About the only thing we can do about the oversizing here is sell 2 stage furnaces, wich we do. Usually the minimum capacity furnace that has the size blower we need for AC is grossly oversized for our heating needs.
People are becoming more aware of the benefits of radient barriers and proper attic ventalation, wich helps knock the dew point of the air in the attic to more reasonable levels.

I have never actually seen a barometric damper in a residential system, just on a couple of gas fired commercial boilers.

[Carnak]

Radiant barrier has nothing to do with the attic dewpoint. Attic vents have a lot to do with it

[davidr]

Originally posted by mark beiser


Usually the minimum capacity furnace that has the size blower we need for AC is grossly oversized for our heating needs.




We have the same issues here, thankfully we have been able to obtain furnaces with better blowers & less heating capacity.

[jrbenny]

Originally posted by davidr

Originally posted by mark beiser


Usually the minimum capacity furnace that has the size blower we need for AC is grossly oversized for our heating needs.




We have the same issues here, thankfully we have been able to obtain furnaces with better blowers & less heating capacity. Both of you should become Carrier dealers. ;)

[davidr]

Originally posted by jrbenny

Originally posted by davidr

Originally posted by mark beiser


Usually the minimum capacity furnace that has the size blower we need for AC is grossly oversized for our heating needs.




We have the same issues here, thankfully we have been able to obtain furnaces with better blowers & less heating capacity. Both of you should become Carrier dealers. ;)


:D

I am working on it, can't seem to quit selling those Water Furnaces. :eek: ;)

BTW your fine company was one of the ones that came to mind when I posted this. :)

[Carnak]

Originally posted by markj

Originally posted by davidr
Anytime you see rust in a heat exchanger that furnace has had something impede it's ability to vent properly.

A properly venting furnace should have no rust in it at anytime.

That is not the cause of rusting heat exchangers around here on most natural drafting furnaces. Many are located in damp basements. In the summer the heatexchangers often condense internally. Not much you can do other that get the customer to condition the basement and or run a dehumidifier.

I have never thought much about that angle, but too low of an entering air temperature can also cause condensation. Back in 1990, in my hometown, new construction you were forced to put in an HRV if you had a furnace requiring a B-vent.

Part of the reason was to prevent a fresh air intake on the return air side of furnaces not designed to condense. Get people setting back the thermostats overnight plus drawing in some sub-zero F fresh air and could also turn that 80% furnace into a condensing one.

Commercially and industrially could also be a problem in morning start up, or if there was a lot of fresh air. See stainless steel HXs with condensate drains as well as higher excess air like 10% O2 on low fire as well to reduce and allow for inevitable condensation.

On the 'economical' packaged roof top stuff, say under 25 tons and 400,000 Btu gas, a lot of times they limit fresh air to 25%. Even with a stainless HX, those are doomed to failure in the cold climates.



[Edited by Carnak on 11-05-2006 at 09:15 AM]

[Carnak]

Some one like skuttle had a kit where hot supply air would get blown back and used to temper the fresh air to try and get around this problem as well

[mark beiser]

Originally posted by Carnak
Radiant barrier has nothing to do with the attic dewpoint. Attic vents have a lot to do with it

I realize that, but the companies in my area that are doing attic ventalation are also doing radient barriers, so people are getting both done at the same time.

[Carnak]

Not venting the attic will lower the summer dewpoint is what I meant to say.

[davidr]

Originally posted by Carnak
Some one like skuttle had a kit where hot supply air would get blown back and used to temper the fresh air to try and get around this problem as well

Field Controls also makes one of these kits that works very well.