Mixing Methods with Radiant Heating Systems
4. The last method is with an injection pump. This
method has been used since the early 1960s. Back
then, a controller would cycle the pump on and off
to inject bursts of hot water into the radiant zone.
Today there are control companies that will control
the speed of a water-lubricated, impedance-protected,
wet rotor pump. Instead of turning the pump on
and off, the control increases or decreases the speed
of the pump.
How to choose
Here are some general concerns for mixing:
Is one mixing method preferred over
the others?
Not really, all of these methods work, but each method
does come with its own benefits as well as its own limitations.
1. For example, two-way valves should only be used
for small loads where the amount of the injected
flow rate is a small percentage of the radiant zone’s
total flow rate, typically less than 25%.
2. Three-way, self-contained, thermostatic valves are
relatively inexpensive but can only provide one
fixed temperature. This causes the zone’s thermostat
to cycle the zone pump on and off. This type of
operation is fine for a small radiant zone but not
recommended when the zones become larger.
3. Injection pumping with a variable speed controller
has become popular over the past few years. This
method of mixing, which uses common wet-rotor
circulators, provides many benefits to a radiant system
such as full temperature modulation and boiler
return protection from cold water. It is limited only
by the pumping capacity of these wet-rotor circulators,
which are typically around 35–40 gpm. In a
typical radiant system, that flow rate equates to
approximately 1,000,000 BTU/H.
4. Three-way and four-way valves, when used with
actuating motors, have been installed in many
radiant systems very successfully for years. The
actuator adjusts the valve’s position to supply the
appropriate mixed water temperature based on the
heating load of the zone. The only real limitation
to this method—compared to the cost of a wet rotor
circulator—is that the valve and actuator are more
expensive than an injection pump.
What happens if I use only one pump with the
mixing device?
There will only be one mixing point, which will control
the supply water temperature to the radiant zone, but not
the temperature of the water returning to the boiler. Besides,
the flow rate through the boiler will vary, decreasing
the efficiency of the boiler.
Why should I use two pumps?
With two pumps and a mixing device, you create two
mixing points. This allows you to control the temperature
of the water going back to the boiler as well as to the radiant
zone. In addition, the second pump provides constant
flow through the boiler, increasing the boiler’s efficiency.
Why should I be concerned with the temperature
of the water going back to the boiler?
Most oil-fired boilers are of the non-condensing type.
This means it is important that the flue gases, released
from the combustion process, are vented out of the boiler.
When the water in the boiler is at a temperature below
the dewpoint of these flue gases, the gases will condense
back into water inside the boiler. The results can be very
damaging. In commercial applications, boiler thermal
shock is another important reason to control the return
water temperature.
Is there a preferred way to pipe the mixing
devices and the two pumps?
The preferred method is to use primary-secondary pumping.
This method, which has been around since the 1950s,
14 ICM/September/October 2019