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nunya
LXI 483
Premium,MVM
join:2000-12-23
O Fallon, MO
kudos:13
Reviews:
·Charter
·voip.ms

Heat exchanger question

Looking for some feedback from HVAC guys.

The story is, last year my 34 yo NG furnace started to stink. When it came on, the whole house would stink. It was an odd smell. It happened every time the furnace ran (it wasn't the "first of the season stink" either). It got worse through the heating season. Almost like the smell of welding on galvanized metal - if you've ever smelled that, you know what I mean.

At the same time, one of the burners started to flame roll back.

I thought cracked / damaged heat exchanger, right?

I had the gas company check it. They said the burner was dirty from "debris", but the HE was just fine. He cleaned the burners and told me it's probably time for a new furnace.

I got a new furnace this year. I basically had to beat the old furnace apart to get it out. Lo and behold, the 34 yo heat exchangers were in remarkably good condition. No holes, cracks, etc... TBQFH, they almost looked brand new.

Then... I looked *inside* the heat exchangers. They were practically clogged with rust / scale / crap. It looked like the inside of the Titanic, but dry. The "roll out" burner was the worst.

What causes this? I thought NG was clean burning. It's a moot point now, but I'm just curious. Was it the burner / exchangers fault? Could it have been the gas itself?

I've heard of plenty of cracked HE, but never a "clogged" HE.
--
If someone refers to herself / himself as a "guru", they probably aren't.

TheMG
Premium
join:2007-09-04
Canada
kudos:3
Reviews:
·NorthWest Tel

3 edits
said by nunya:

What causes this? I thought NG was clean burning. It's a moot point now, but I'm just curious. Was it the burner / exchangers fault? Could it have been the gas itself?

The main combustion products of natural gas are CO2 and H2O (water).

This water vapor will corrode the heat exchanger eventually. The corrosion is accelerated if the water vapor is allowed to condense and remain on the inside of the heat exchanger which can occur if the furnace short-cycles or intake air is very cold.

Also, natural gas contains a small amount of sulfur, which definitely doesn't help.

Beezel

join:2008-12-15
Las Vegas, NV
reply to nunya
Also non-proper flame adjustment can add to it. "Yellow flames" instead of the nice pretty blue ones.

Bob4
Account deleted

join:2012-07-22
New Jersey
reply to TheMG
said by TheMG:

The corrosion is accelerated if the water vapor is allowed to condense and remain on the inside of the heat exchanger which can occur if the furnace short-cycles or intake air is very cold.

I replaced my thermostat several years ago with a Honeywell electronic model. In its normal settings, it would run cycles as short as 15 or 30 seconds. Sometimes the gas would shut off before the blower even came on! I changed the thermostat to the 'hot water' setting to increase the cycle times. Now it takes longer to call for heat (so the inside temperature swings a little bit more), but the cycle times are good, and I'm sure will increase the life of my heat exchanger and flue.


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
reply to nunya
said by nunya:

What causes this? I thought NG was clean burning. It's a moot point now, but I'm just curious. Was it the burner / exchangers fault? Could it have been the gas itself?

I've heard of plenty of cracked HE, but never a "clogged" HE.

I googled it and it seems all NG is not the same composition. It varies depending on the source and other factors.

Chemical Composition of Natural Gas


jrs8084
Premium
join:2002-03-02
Statesville, NC
kudos:1
Reviews:
·AT&T U-Verse
reply to nunya
I gather a good cleaning of the HEs hasn't been part of your annual maintenance.

My furnace is going 40 years old this season and the HEs are clean as a whistle. Well-some surface rust by the pilot, but that is it.

But yes, they slowly do corrode. How fast depends on various circumstances.


fifty nine

join:2002-09-25
Sussex, NJ
kudos:2
reply to Jack_in_VA
said by Jack_in_VA:

said by nunya:

What causes this? I thought NG was clean burning. It's a moot point now, but I'm just curious. Was it the burner / exchangers fault? Could it have been the gas itself?

I've heard of plenty of cracked HE, but never a "clogged" HE.

I googled it and it seems all NG is not the same composition. It varies depending on the source and other factors.

Chemical Composition of Natural Gas

It definitely is not. Neither is crude oil, coal or any other fossil fuel for that matter. There are "sweet" and "sour" varieties of oil and natural gas.

MaynardKrebs
Heave Steve, for the good of the country
Premium
join:2009-06-17
kudos:4
reply to nunya

A byproduct of natural gas combustion is sulphuric acid - the more 'sour' the gas, the more H2SO4 you get produced.

Under a number of atmospheric states, the sulphuric acid gas will condense on the inside of the clay chimney liner (or bare brick if you house is of a certain age) and begin to eat away at the liner/brick/mortar - small particles of which will spall-off and fall down the flue to the heat exchanger. There they will get 'cooked' into something like 'clinker' and clog the heat exchanger.

Did your old furnace have a stainless steel liner up to the roof? The SS liner is to prevent what I described above, and to prevent CO gas from seeping into the house via damaged brickwork and mortar.

Masonry chimneys located on exterior walls cool down more than chimneys wholly within the building envelope, as such the combustion gas cools more quickly on the way up the chimney - and is more prone to condensing. On the other hand, chimneys on the inside of homes have 4 surfaces exposed to the inside of the home - these are usually found in older homes.


nunya
LXI 483
Premium,MVM
join:2000-12-23
O Fallon, MO
kudos:13
reply to nunya
Laclede has low H2S gas (sweet).

The flue pipe is galvanized, and clean all the way up. I can see straight up it.
--
If someone refers to herself / himself as a "guru", they probably aren't.


jack b
Gone Fishing
Premium,MVM
join:2000-09-08
Cape Cod
kudos:1
reply to nunya
The "sweet" odor you detected was likely caused from aldehydes, and other volatile organic compounds associated with incomplete combustion of a gaseous fuel. A good amount of CO is also almost always present in this particular scenario.

The corrosion of a heat exchanger can be accelerated from condensation of the gas products of combustion, which is usually slightly acidic. Among other acids that forms from this condensation is carbonic acid, that can attack the steel heat exchanger and which dries leaving a white powdery dust.
--
~Help Find a Cure for Cancer~
~Proud Member of Team Discovery ~


jrs8084
Premium
join:2002-03-02
Statesville, NC
kudos:1
Reviews:
·AT&T U-Verse
reply to nunya
Considering our race to the bottom in terms of product longevity, rest assured that your new furnace will provide a time lapse glimpse of 34 years of corrosion in a much shorter time span.

Since you are familiar with working with a parts house, it won't be as bad getting the draft inducers, control boards, etc.


cdru
Go Colts
Premium,MVM
join:2003-05-14
Fort Wayne, IN
kudos:7
said by jrs8084:

Considering our race to the bottom in terms of product longevity, rest assured that your new furnace will provide a time lapse glimpse of 34 years of corrosion in a much shorter time span.

To some degree it's the price we pay for higher efficiency appliances. That 34-year old furnace might have been 60 or 70% efficient new. 30-40% of the heat when up the chimney resulting in considerably hotter drafts, more draft flow, as well as longer burn times with equally sized furnaces. With your ultra-high efficiency condensing furnaces, more heat makes it into the home resulting in shorter burn time possibilities, cooler drafts, and less draft flow.

It's not just a matter of cheaper equipment. Even if manufacturing processes from 34 years ago were used now, I bet you'd still have the same problem, if not more. Plus, I know with my last Carrier furnace the primary heat exchanger has a limited lifetime warranty and the secondary has I think a 20 year warranty now.

ke4pym
Premium
join:2004-07-24
Charlotte, NC
Speaking of ultra-high efficiency condensing furnaces - why do they condense? My water heater is a condensing unit. What is the purpose of it condensing?


jack b
Gone Fishing
Premium,MVM
join:2000-09-08
Cape Cod
kudos:1

1 recommendation

said by ke4pym:

Speaking of ultra-high efficiency condensing furnaces - why do they condense? My water heater is a condensing unit. What is the purpose of it condensing?

To extract the latent heat of vaporization.

In other words, more heat energy is extracted from the combustion products when they are further cooled to below the dew point.
--
~Help Find a Cure for Cancer~
~Proud Member of Team Discovery ~


mityfowl
Premium
join:2000-11-06
Dallas, TX
reply to nunya
I would be worried about a 30-40 year old furnace.

No matter what anybody says.


mityfowl
Premium
join:2000-11-06
Dallas, TX
reply to nunya
I've got a 30 year old lawn mower but peoples lives don't depend on it.


nunya
LXI 483
Premium,MVM
join:2000-12-23
O Fallon, MO
kudos:13
Reviews:
·Charter
·voip.ms
reply to nunya
I bought a cheapie replacement furnace. I'm hoping this will be my last heating season in this house. The only thing I did spring for was 2 stage.

The city hasn't picked up the old furnace yet, I'll see if I can get a picture.
--
If someone refers to herself / himself as a "guru", they probably aren't.


mityfowl
Premium
join:2000-11-06
Dallas, TX
said by nunya:

I bought a cheapie replacement furnace. I'm hoping this will be my last heating season in this house. The only thing I did spring for was 2 stage.

The city hasn't picked up the old furnace yet, I'll see if I can get a picture.

Good job.

I don't know if you have a family but I sure don't want them dead. I don't want you dead either.

At worst a wash.


jack b
Gone Fishing
Premium,MVM
join:2000-09-08
Cape Cod
kudos:1
reply to nunya
Forgot to ask: did/do you have (working) CO detectors?


nunya
LXI 483
Premium,MVM
join:2000-12-23
O Fallon, MO
kudos:13
I have interconnected hardwired CO detectors on each floor. They never went off.
--
If someone refers to herself / himself as a "guru", they probably aren't.


tp0d
yabbazooie
Premium
join:2001-02-13
Carnegie, PA
kudos:6
Heat exchangers get nasty like that not due to natural gas, but contaminated air. A lot of basements have laundries in them, and lint/fabric softener is quite corrosive. Not to mention high humidity, construction dust, and pet hair. Which is (one reason) why newer high efficiency furnaces bring their combustion air from outside. Some 80+ units allow for this too, but its rare.

-j
--
if it aint broke, tweak it!!
currently on FiOS (kick aZZ!)


SwedishRider
Rider on the Storm
Premium
join:2006-01-11
not Sweden
kudos:2
reply to nunya
nunya, did you need to pull permits to replace your furnace? Or since it was a "repair" was it exempt?

I'm just curious... my furnace is almost new, so I'm hoping not to have to replace it for many, many years (knocking on wood as I say that )