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.

Links

• April 2007 The Glitch & The Fix