wirsbo DuoMix 201, Installation Instructions Manual

Page: 26 / 32

DuoMix Installation Handbook 26
Variable Speed Injection Mixing For
Hydronic Heating Systems*
The purpose of this essay is to explore the
practical application of Variable Speed Injection
Mumping to effect precise heat transfer from high
temperature to low temperature hydronic heating
system loops. Often the available heat source
produces higher temperature water than is required
by the heat delivery system. To effectively achieve
this lower water temperature, the high temperature
water must be injected into the system at a con-
trolled rate to mix with the system return water,
raising the temperature just enough to meet the
heating load. Various devices and plumbing ar-
rangements can be used to accomplish this.
In some instances, a heat source - condensing
or electric boiler, geo-thermal heat pump, etc. - can
be operated at low temperatures and dedicated
solely to operating a low temperature radiant
heating system. In the vast majority of systems,
however, mixing is required because:
• A boiler minimum operating temperature is
required
• High temperature water is required for other
system needs
• Water temperatures vary over a wide range,
i.e. solar heat sources, waste heat utilization,
wood fired boilers, etc.
When the water temperatures of both the high
and low temperature loops are constant, with a
constant heating load, a simple manual bypass
balancing arrangement (Figure 1) can be used to
maintain a steady water temperature since it con-
tinually injects high temperature water at a fixed
rate.
*Text and diagrams used with permission from tekmar
When the high temperature supply fluctuates,
and/or varying water temperatures are required in
the heating system at a constant load, an on/off
valve or pump (Figure 2) can be used to intermit-
tently inject hot water at a fixed rate whenever heat
is required.
In most heating systems, the high temperature
supply fluctuates, the system loop temperature
requirements constantly change and the heating
load constantly changes. These factors, combined
with the common need to maintain a safe minimum
boiler return temperature, require that the rate of
heat injection be precisely controlled and con-
stantly readjusted.
The easiest, most reliable and most cost effective
way to accomplish this is to use Variable Speed
Injection Mixing with the Wirsbo DuoMix 201 Sys-
tem Controller.
Variable Speed Injection Mixing (Figure 3) can
be used to transfer heat from a Primary, high
temperature boiler loop to a Secondary, low tem-
perature heating loop. The speed of the Injection
Circulator is automatically adjusted to deliver the
desired volume of hot water to the low temperature
loop. Constant flow can be maintained in both
loops regardless of the circulator speed, and more
or less heat is injected as the circulator speed is
increased or decreased. The circulator speed is
constantly adjusted as the heating system load and
the supply water temperature change. If the boiler
return temperature becomes too cold, the circulator
can be slowed down to reduce the heat injection
rate, resulting in an increased boiler return tem-
perature.