Reprinted with permission for Bell & Gossett Counter Point
Volume 6 Issue 1 May 1999
What is Mixing? Mixing is when a portion of return water
from the system is "mixed" with a portion of hot water
from the boiler to supply a specific water temperature that is
lower than the boiler temperature but warmer than the return temperature.
What is the difference between each method? A three-way
mixing valve has three ports and a four-way valve has four ports.
Mixing blends cooler return water into one of the valve's ports
with hot water that is entering another port. The two temperatures
blend and exit the supply port. With a four-way valve, any of
the return water that isn't used to mix with the hot water is
returned back into the boiler. Injection mixing injects bursts
of hot water into a constantly circulating loop. A two-way valve
opens and closes, or a pump's speed changes, to introduce the
right amount of heat.
Why would someone use mixing in a hydronic system? There
are three major uses:
· Radiant heating that requires lower temperatures than
most boilers can produce without experiencing flue gas condensation.
· Outdoor reset. By matching the supply water temperature
to the load for the building, the heating system will operate
more efficiently. Unfortunately, the required water temperatures
are lower than most boilers are designed to handle.
· Hydronic systems that incorporate different types of
heat emitters such as in-floor heating, panel radiation, cast-iron
radiation and hydro-air coils. Each type requires a different
supply temperature but all receive their water from the same boiler.
What happens if I use only one pump with my mixing device?
There will be only one mixing point. This will control the supply
water temperature for that particular zone, but not the temperature
of the water returning to the boiler. Also, the flow rate through
the boiler will vary, decreasing the boiler's efficiency.
Why should I use two pumps? Using two pumps, with a mixing
device, establishes two mixing points. This protects the boiler
by controlling the temperature of the returning water. The second
pump also provides constant flow through the boiler, improving
the boiler's efficiency.
How should I pipe the mixing device and the two pumps? Use
primary/secondary pumping so the two pumps will not operate in
series with each other. Another benefit of primary/secondary pumping:
you can efficiently size the mixing device.
Why should I be concerned with the temperature of the water
returning to the boiler? If you are using a non-condensing
style boiler, it is important that the flue gases released from
the combustion process be vented out of the boiler. When the water
in the boiler is at a temperature below the dewpoint of the flue
gases, these gases will condense back to water inside the boiler.
The results can be very damaging. Boiler thermal shock is another
reason for controlling the return temperature.
What is flue gas condensation? During combustion of the
fuel, many by-products are formed including carbon dioxide, sulfur
compounds and water vapor. Low return water temperatures will
cause the compounds to condense, forming corrosive liquids in
the boiler stack and heat exchangers. The amount of damage that
will occur depends on the design and materials of construction
used in the boiler, as well as the specific compounds in the flue
gas. Always check with the boiler manufacturer to find the recommended
return water temperature.
When using a mixing device, how do I calculate the flow rates
to achieve the desired mixed temperature? The answer can be
found in the example:
Radiant zone load = 500 Btu/h designed at 20° temperature
drop.
Design radiant zone flow rate = 5gpm
Radiant design supply temperature = 120°F (based upon 20°F
temp. drop, return temp. of 100°F)
Boiler loop supply temperature = 180°F
The design temperature difference between the two loops is 80°F
so…
5gpm x 10,000 BTU per gallon
80°Fx500 = 1.25 gpm
This is the amount of 180°F boiler water needed to "mix"
with 3.75 gpm of 100°F return water from the radiant zone
to supply 5 gpm of 120°F water.