The Glitch
Overview: A heating contractor,
who’s tired of coaxing 500-pound cast-iron boilers into homes, decides to try
out a wall-mounted mod/con boiler because of its light weight and side wall
venting.
He pipes it up as shown below. All flow to the
distribution system supplying several radiant panel manifolds passes through
the three-way thermostatic mixing valve. When purchasing the boiler, the
installer was told to use a pair of closely spaced tees to separate the boiler
and distribution system, so he installed some closely spaced tees as
shown.
Exercise:
Can you spot at least four things that should be changed in this layout?
The Fix
Well, those are closely spaced
tees under the boiler, but they don’t serve to hydraulically separate the
boiler form the distribution system.
Another poor choice is to install a 3-way
thermostatic mixing valve (or any mixing assembly for that matter) between a
mod/con boiler and the distribution system. It makes no sense to operate the
boiler at a temperature higher than needed by the distribution system, and then
mix down the water temperature. This reduces the ability of the mod/con boiler
to operate with flue gas condensation and thus reduces efficiency. Furthermore, if that 3-way thermostatic
valve has a low Cv ― which is typical of valves of 1-inch pipe size or less ―
it’s going to act as a significant flow restrictor to the distribution system.
Low flow leads to wide delta T on the radiant panel circuits and very likely to
insufficient heat delivery.
It’s also likely that reduced flow through
the mod/con boiler will lead to operating problems, especially in boilers with
low mass heat exchangers.
A good microbubble air separator also needs
to be added to the system. For the best performance install it where the fluid
is hottest, near the boiler outlet.
Finally, take a look at how the
circuit purging valves are installed on the zone returns. In this orientation the only thing they will
be purging is the return manifold.
The corrections are shown at right.
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