User Guide

PA03 – 2
Reset Ratio
The Reset Ratio sets the relationship between outdoor temperature and supply water temperature. It determines the
amount the supply water temperature is raised for every 1 degree outdoor temperature drop. For example, if a Reset Ratio
of 1.2 is selected, the supply water temperature is increased by 1.2 degrees of every 1 degree of outdoor temperature drop.
DESIGN
In order to properly accomplish this mixing method, the
following piping details should be considered.
When the injection pump is turned off, there must be no
heat transfer from the boiler loop to the system loop. In
order to avoid this unwanted heat transfer, primary/sec-
ondary piping techniques are used as shown in Figure 1.
This piping arrangement requires that the injection pip-
ing be at least one pipe diameter smaller than the piping
of the boiler and system loops.There must be no more
than 4 pipe diameters between the tees in the boiler and
system loops (Note 1), in order to prevent ghost flow
when the injection pump is off and the system or boiler
pump is on.Also, there must be at least 6 pipe diameters
of straight pipe on either side of the tees (Note 2), in
order to prevent the momentum of water from the boiler
and system loops from pushing flow through the injec-
tion loop. Finally, there should be a minimum of 1 foot drop in the injection loop in order to create a thermal trap (Note 3)
in order to prevent convective heat transfer through the injection loop.
PUMP SIZING AND SELECTION
In order to properly size the pump, follow the design procedure below:
1. Determine the design operating temperatures of the system loop and boiler. (Ts and Tb from Figure 1)
2. Determine the flow rate and design temperature drop (∆T - Delta T) in the system loop. If one of these variables is
unknown use Equation 1 or 2 to calculate the other variable.
3. Compute Tb - Ts. Look up the ratios in Figure 2.
4. The design injection flow rate for direct injection is calculated from Equation 3. If the injection flow rate is greater than
40 US GPM, a 3-way or 4-way valve may be required.
5. Decide whether or not to include a balancing valve in the injection piping. A balancing valve allows adjustment when the
injection pump is larger than needed.A balancing valve also provides the possibility of manual operation of the system by
turning the injection pump fully on and adjusting the balancing valve to obtain the desired supply water temperature.
6. The injection piping size and model of Taco 00-VR pump to install can now be looked up in Figure 3. Do not oversize
the injection system. If the injection system is not able to provide enough heat, the boiler’s aquastat may be increased.
Eq. 1: System Flow Rate (US GPM) =
Design Heating Load (BTU/hr)
500 x ∆Ts ( °F)
Eq. 2: ∆Ts ( °F) =
Design Heating Load (BTU/hr)
500 x System Flow Rate (US GPM)
Eq. 3: Design Injection Flow Rate (US GPM) = System Flow Rate (US GPM) x Flow Ratio
Fi
g
ure 1
Balancing
Valve
Boiler
Loop
System
Loop
(Ts)
(Tb)
00-VR
Injection Pump
∆Ts
= Boiler supply temperature
Ts
= System supply temperature
∆Ts
= System temperature drop (Typically 20°F for
convectors and 10°F for radiant floor heating)
Note 3
Note 1
Note 2
Note 3
Note 1
Note 2
Tb