I put in a high efficiency furnace two years ago. That left my gas water
heater alone on the chimney.(Upstate NY)
The HVAC company and the gas utility assured me that it was safe to do
because the chimney was small enough, tiled, and ran up through the center
of the house; but I never felt good about it. I was just too cheap to
actually install a metal liner as some people recommended.
Anyhow, it seems to me that if I insulated the exhaust on the water heater
(it is about 5' from the heater to the chimney) it would stay hotter and get
out of the chimney without condensing better.
Does this make any sense? If so, is there any draw back to insulating the
exhaust? It wastes a little energy in the winter, but saves a little in the
summer, so it is probably a plus.
You ask why? Because the exhaust is hot. When things get hot they can
catch on fire. In this case the proposed insulation is at risk.
Fire is hazardous and is a drawback regardless of how small the
Of course it can be done, so what? This guy is a homeowner. This is
a water heater not an industrial boiler. He won't save any energy by
doing this. He can wrap his entire heater with glass if he wants to
save energy but wrapping the exhaust is something I have never seen
done and don't recommend.
Didn't seem to me he cared about saving energy. Sounded like he
wanted to increase his stack temperature a little bit by not losing
the heat on the way to the stack. Sounded like a reasonable question
As Edwin indicated, there's lots of insulation that won't burn, for
The quotation is about the company, not the product, necessarily. My company
serves the industrial and commercial markets, yet 60% of our product ends up
in homes. We just don't sell to homeowners. Fibrex, evidently, has chosen
to sell their products through certain distribution channels to those
markets. Yes, the residential market for their product would be miniscule,
but that does not forbid its use.
If you read the question, the reason for doing it had nothing to do
with saving energy. How could it? The stated purpopse was to keep
the flue gases as hot as possible before entering the chimney, because
he's concerned about the size of the chimney allowing the gases to
condense. And of course it can be done, withhout any fire hazard,
provided he uses the right insulation. I doubt it will make enough
difference to matter though.
I've seen Toller make enough posts here to indicate he could do the
job properly, without creating any fire hazhard.
I've seen Toller make enough posts that I'd worry about him setting
himself on fire! =:O
T, I really don't know how much insulating the first 5' would affect
the flue gas temperature. The gas is at its hottest right there so I
don't think that you have a condensation problem at that point. What
is prompting the question?
Now that's not very nice; expecially since you don't even know that I routed
some frame rails upside down this morning. (fortunately they are small and
Exactly, the gas is hottest there, and the conductivity is highest. Since
heat loss is proportional to the temperature difference and the
conductivity, the gas will lose a lot of heat in those 5'. Which means it
will be cooler when it gets to the chimney, and more likely to condense. I
am trying to prevent that.
No need to replace them. Finish the upside down piece and list it on
You are worrying about the wrong thing. Initially, with the exposed
metal flue at room temperature, the heat transfer would be the most
rapid when the water heater kicks on. Once the metal flue gets to a
certain temperature, the heat transfer is _much_ slower. That certain
temperature will be arrived at very quickly. The flue will then only
pick up enough heat to offset the heat loss to the room air. Since
the heat transfer to the air outside of the flue is predominantly
radiation and convection, it is far less of an efficient heat transfer
mechanism than the heat loss through the metal flue where it enters
the masonry. That will not change whether you insulate the ducting or
You already know that removing the lion share of the chimney's work -
the burner output - has serious implications on the size of the
chimney. Your chimney is too large and all of the insulation in the
world won't change that. Condensation in the metal flue? Won't
happen to any appreciable degree, and even if it did, the metal
ducting is trivial and inexpensive to replace. Your real problem is
the too-large, unlined masonry chimney. Condensation has far more
damaging and expensive effects in a masonry chimney.
You are asking if insulating the metal ducting will help your
chimney. The answer is no. Lining the chimney to the correct size
based solely on the water heater output will.
The risk is ZERO. No risk at all. The insulation will not burn if you hold
a torch to it as it is designed to take the high temperatures of an exhaust
stack. Rather than be negative, find out what is available and how to
properly use it.
The man asked a question so he could find if there is a material or method
that is safe. All he has to do is go to a plumbing supply house and ask
about it. Comes in sizes from 4" to 36". No drawbacks as there is NO
BTW, he was not trying to save energy, he was trying to get more h eat to
the chimney to prevent condensation.
This is not about being negative. It is about being helpful. Many
things are available and possible. That doesn't make it a good
A bit of condensation on the inside of an exhaust flue is nothing to
worry about and certainly no reason to change established safe
practice. While I have already admitted it can be done, it never
is. It is good and fine to learn and experiment with new practices.
This does not fall into that category.
To say there is zero risk is irresponsible. I challenge you to post a
link to a material specifically designed for such a residential
purpose. Residential heater exhausts are never wrapped for good
reasons. I have never seen it and nobody here has claimed that they
To call it unusual would be an understatement regardless of any
theoretical scenario which may exist. The best advice in a public
newsgroup is standard, safe practice not experimental scenarios which
may or may not be good advice.
Toller specifically asked if there were any drawbacks. That was his
question. I have stayed on topic and answered his question. Who else
has done so??
Your challange is an impossible one for good reason. There is insulation
made for exhaust stacks and a temperature range is given. It does not state
residential, or commercial, or industrial as it works on all applications.
There is no restriction for residential use.
You may be interested in this though http://www.protechinfo.com/pdf/2049.pdf
Unusual, yes, possible, very. In most cases you want the excess heat to
heat the basement so for that reason it is not usually done. Personally,
I'd not do it for t hat reason. There are other situtations in the world and
I don't see anything wrong with addressing them in a proper manner with the
Your yearly chimney inspection will tell you whether the
water is causing any problems in the chimney long before
those problems become serious. If that occurs, then
the appropriate solution would be to put in the metal
flue-liner. Insulating the exhaust pipe between the
heater and the chimney stack doesn't really solve the
problem of condensation (assuming there is one).
I have never heard of anyone getting a chimney inspected, outside of buying
a house or having it cleaned.
My understanding is that chimneys need to be cleaned once every cord or two
of use. I burn about a quarter face cord a year, so I don't have it
inspected very often. Are you serious?
Inspected, not necesarilly cleaned. It just means someone should
climb up there at the beginning of every burning season
and look for damage, blockages, dirt, wild animals, etc.
If you're having to clean a wood-burner's chimney every cord,
there's something wrong, you're burning crappy wood, or you're
running short, smoldery fires.
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