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 "
