A.John Siegenthaler responds: There are
several ways of combining low-temperature radiant floor heating
with higher-temperature distribution systems, such as fin-tube
baseboard convectors. Above all, the system must be designed 1)
to prevent condensation within the boiler, and 2) with a
specific type of control system.
Conventional boilers must have return water temperatures
high enough to prevent sustained condensation on the fire-side
of the boiler, or within the flue pipe. Water vapor is a
byproduct of combustion, and if allowed to condense, it can
cause severe corrosion. Flue pipes are especially vulnerable,
and can fail in a matter of weeks when condensation is present.
This could allow toxic gases to be released into the
building.
Typically, the return water temperature for a gas- or
oil-fired boiler should be 140°F or higher to prevent
condensation. Since radiant systems operate with a return water
temperature in the range of 80° to 100°F, their return
water must be mixed with hotter water before it is sent back to
the boiler. There are two simple ways to do this — with a
four-way valve or with injection mixing.
The four-way mixing valve lowers the temperature of the
water supplied to the radiant floor system by mixing return
water into the radiant loop, as shown in Illustration A. To
avoid condensation in the boiler, the four-way mixing valve
maintains a relatively high return water temperature by mixing
some hot supply water into its return flow.
To control the four-way valve, you’d ideally have a
motor-operator regulated by an outdoor reset control. This
measures outdoor temperatures and automatically adjusts the
valve to maintain a suitable water temperature in the radiant
floor to match the required heating load. A less expensive (and
less exact) control system for the four-way valve is to control
the circulator in the floor heating loop with a room
thermostat. This way, the four-way valve is set at the
design-load temperature of the floor heating system and left
there. When heat is needed, the circulator comes on to deliver
hot water to the area. The thermostat should have a low
differential (one or two degrees), to minimize swings in room
temperature.
Another way to lower the water temperature of the radiant
floor is through injection mixing (Illustration B). In this
system, the high-temperature zones have individual circulators
(C1) that are controlled by room thermostats, just as in a
standard multi-zone system. Water circulates continuously
(using circulator C2) through the radiant floor loop during the
heating season, and a zone valve opens to allow hot water to
flow into the loop when heat is needed. This zone valve can be
controlled by a thermostat or for more precise control, a reset
control.
The hot water from the zone valve is mixed with cool return
water at the tee downstream from the valve. A balancing valve
determines how much hot water flows into the radiant loop when
the zone valve is open. To prevent condensation, the circulator
in the main system loop (C3) must operate when the zone valve
is open to shunt a significant portion of hot water back
towards the boiler.
The injection mixing hardware (zone valve plus reset
control) is significantly less expensive than the four-way
valve system with a reset control (about $300 vs. $800).
Detailed information about both approaches can be obtained from
Tekmar Control, 4611 23rd St., Vernon, BC V1T 4K7, Canada;
604/545-7749.
John Siegenthaler, P.E., owns
Appropriate Designs, a building systems engineering firm in
Holland Patent, N.Y.