The Glitch
The schematic
shown here is for a system that needs to supply higher temperature water to
three zones of baseboard, and low- to medium-temperature water to three
different types of radiant panels.
These reduced temperatures are created by
motorized mixing valves that sense both mixed supply temperature and boiler
return temperature. The system also supplies domestic hot water via an indirect
heater. A conventional cast-iron boiler supplies all loads.
Exercise:
There are at least four design errors in this schematic. Can you spot them?
The Fix
The motorized mixing valves cannot protect the boiler from sustained
flue gas condensation without a mixing point to boost boiler inlet temperature
when necessary. This requires the additional circulator and hydraulic separation
via the closely spaced tees. A flow restrictor valve is included to prevent a
high flow through the bypass — which could reduce flow through the other zone circuits.
The
indirect tank in the original schematic was not piped for counterflow heat exchanger.
Natural convection causes a rising current of water in the tank, so flow in the
coil should be from top to bottom for the highest heat exchanger
effectiveness.
The short inlet piping leading to the higher temperature
zone circulators is not a good detail. Allow at least 12 diameters of straight
pipe upstream of all circulators. This is indicated by the “12D” notations on
the drawing.
Finally, the mix supply sensors for the motorized valves
should be located downstream of the associated circulators to ensure complete
mixing before flow passes by the sensors.
