Eric in North TX wrote:
Does it really need stainless to the weather cap? I've had the same
flue on my tank type for 10 years since I moved in, and I'm pretty sure
it has been there much much longer than that. I could see a couple of
sections near the heater needing to be SS, & maybe I'm fussing about
nothing, the whole thing can't be much more than 10'. If it is so hot
on the stack that stainless is required, how close can it come to wood?
Will it be necessary to sawzall the ceiling and roof and put in metal?

Most gas/propane fired tankless unit produce a HOT exhaust, and the WH
manufactures generally spec a UL listed cat III system - in the US
the choices I'm aware of are FasNSeal, Z-VentIII and Saf-T Vent - and
than it be installed "per manufacturer's instructions".

So you need to read *both* manufacturer's instructions, and then follow
them.

For example the SS pipe specified for most tankless heaters is 2x wall
material, but still requires clearance to combustibles - FasNSeal for
instance requires 12" at the top, 8" to the side and 4" below a
horizontal run, and even then if it's other than a one or two unit
residence they specified a non-combustible enclosure.

http://www.protechinfo.com/pdf/1150.pdf

In addition, these are engineered systems, so to maintain their
ratings each manufacturer is going to require that in so fas as
possilbe the entire system be composed of exclusively of their
components, and installed per their instructions, and ther only
exceptons they allow will be BFPs, wall caps, ect. specified by the
appliance manufactuer for use with cat III systems.

Other frequent problems I encounter in tankless retrofits:

- Insufficient combustion air: many tankless WHs are rated at 180,000
input BTU or above, which is more that most GFAFs, often no provision
is made to insure increased combustion airflow, resulting in a
negatively pressurized utility area (sometimes, an entire house) which
can result in back drafting of exhaust vents, etc.

- Insufficient gas supply: most take type units are fed by a .50"
line for at least part of the run, most tankless units REQUIRE a .75"
line all the way to the unit or they may not operate at full output. In
addition, depending on the run and other appliances attached, it may be
necessary to replumb the entire supply system, upgrade the gas meter,
etc.

This is not a just a "theoretical" problem, either. The *majority*
of service calls by Takagi for "defective" newly installed units
are the result of an undersized gas supplies, and in the last year I've
had to upgrade the newly installed gas meter on a building feeding a
Takagi tankless WH and a GFAF because the gas company disregarded our
specifications and installed a "standard" sized meter undersized for
the actual requirement, and on another project the plumbers had to tear
out and ungraded the entire length of newly installed gas supply line
because they had failed to size it based on the fact that it would be
feeding multiple tankless heaters.

- No backflow preventer in freezing climates - many tankless units
require a backflow preventer to reduce the risks of freezing the coils.
Sone units (for example some Takagis) have a built-in in coil heater,
but this is NOT designed or intened to take the place of a BFP.

So you really have to plan such installations correctly, and then check
cafrefully to make sure they are in fact installed as planned.

Michael Thomas
Paragon Hone Inspection, LLC
Chicago, IL

847-475-5668