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But, for residential guys getting the
draft over to the positive side seems strange
and to some even wrong. Keep in mind
that to run at zero to slightly positive you
must have a burner that will run at those
pressures, and when it's time to leave, you
are always better off with a chimney that is
producing some negative draft.
With pressurized flues, like those connected to powerventers or direct vent
systems, the biggest mistake I see are due
to air leaks. You really need to follow the
rules. Everything should be sealed from
end to end, and I don't believe in vacuum
breakers, sorry. I like my airflow pressure
switches instead, Figure 1.
Since so many are using oil boilers
for gas conversions and even new installs
(as they do in Europe as blown gas), it's
important to note that when considering
a fuel conversion if you have a draft (airflow) problem on oil, it might just be an
even bigger nightmare on gas.
Correct sizing and positioning of the
draft regulator, if needed, is not only a
must for proper operation, Figure 2, since
they are listed devices and if installed
wrong create a code violation. I've already
beaten that to death, so we move on. By
the way, Figure 2 is from Field Control's
directions on proper installation of one of
their RC Model regulators.
Another problem area is the grossly
oversized chimneys that heating appliances are installed into. If you have a
typical modern boiler, it probably comes
with a 6" fluepipe that provides 28 square
inches of surface area. The OEMs make
one-size-fits all smokehoods to keep
costs down and use that one six inch
smokehood to fit most, if not all, of their
residential line.
However, an 86 percent efficient boiler
with a firing rate of 0.50 GPH to 0.65 GPH
connected to a 35 foot high chimney with
no more than 10 feet of fluepipe needs a
four inch chimney liner. That liner will
provide 12 square inches of surface area
and if you are over 12 square inches you're
starting to get too big. In addition, the
math never lies; watch where this goes. If
the chimney is too big, it may never pro-
duce an adequate supply of thermal draft
except on the coldest day of the year. You
may be starting to see the benefits of a high
static pressure burner?
Let's look at Figure 3. If you start off
with a six inch flue, with 28 square inches
of surface area, and go into a four inch
liner, with 12 square inches of surface area,
your burner fan will actually be forced
to create more velocity (speed) to push
those flue gasses out. In addition, since the
temperature is dropping as it moves along
headed to the chimney (or vent), there is
less fluegas to deal with. You want to keep
www.fueloilnews.com | FUEL OIL NEWS | JUNE 2013
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