George R. Carey, Jr.
Vice President,
Fluid Industrial Associates
gcarey@fiainc.com
twitter: @Ask_GCarey
Boiler Facts
Mixing Methods with Radiant
Heating Systems
When designing a radiant heating system, it
becomes obvious that this system has characteristics
different from the conventional
baseboard-type of heating system. One quick difference
is the temperature of the water circulated through the
tubing. Most radiant systems can be classified into two
types.
The first is a “wet system” in which the tubing is
installed into concrete. The second type is a “dry system”
where the tubing is either stapled-up from underneath
the floor or laid down on a sub-floor and the final flooring
placed on top of it.
The average water temperatures are 110°–120°F for
the concrete type and 130°–140°F for the staple-up application;
of course, there are exceptions where hotter
or cooler water may be required. Unfortunately, most
oil-fired boilers can’t operate at such low temperatures
without experiencing flue gas problems. The best way of
overcoming this problem is by using some type of mixing
device, which lowers the supply temperature to the
radiant zone(s) while allowing the boiler loop to maintain
a temperature high enough to satisfy its requirements.
There are numerous mixing methods available.
Mixing concerns
Here are some common concerns regarding the subject
of mixing:
What is mixing?
Mixing is when you take some cooler return water and
“mix” it with some hot boiler water to supply a temperature
of water that is lower than the boiler temperature
but warmer than the return water.
Are there different methods available
for mixing?
You can use a two-way valve, a three-way valve, a fourway
valve or a circulator. All four devices can be used to
supply a mixed water temperature.
How does each of these methods work?
1. A two-way valve works on the injection principal.
There is a boiler loop with a circulator and the radiant
loop with its own circulator. These two circuits
are inter-connected through a supply pipe and return
pipe that are spaced close together. A two-way
valve is located on the supply pipe and has a controller
that measures the radiant loop’s supply water
temperature. The controller will cycle the valve open
and closed based on the water temperature in the
radiant zone. When the valve is opened, it injects
bursts of hot water into the radiant loop. There, it
mixes with some cool return water from the radiant
zone.
2. A three-way valve mixes cool return water with hot
boiler water to supply a “mixed” temperature. It has
three ports, one for the return water from the radiant
zone, one for hot water from the boiler loop and a
mixed port to supply the radiant zone. These valves
can be manually set to maintain a fixed temperature
or they can have an actuator that repositions the
valve according to the load.
3. A four-way valve is very similar to a three-way valve
except it has four ports instead of three. Two ports
go to the boiler and two ports go to the radiant zone.
This valve can be set manually or used with an
actuator to modulate the water temperature based
on the zone load.
12 ICM/September/October 2019