TW2000 WOOD–FIRED BOILER – INSTRUCTION MANUAL
TW2000 13 Jan-08
2.6.2 PARALLEL PIPING SYSTEM
In the PARALLEL SYSTEM the water is circulating between the two boilers by means of an
auxiliary circulator. In this system the flow of water from the existing boiler (supply) is teed and
piped to the return of the Kerr TW2000 boiler. The return pipe to the existing boiler is teed, and
piped to the outlet nipple of the Kerr TW2000 boiler. Therefore the TW2000 is maintaining heat
in both boilers. If the TW2000 is not being utilized the system warm return water is not keeping
the wood boiler warm.
In the parallel system an auxiliary circulating pump keeps the temperature even between the
two boilers. It is controlled by a separate aquastat on the Kerr TW2000 boiler. When the water
in the Kerr TW2000 boiler is hotter than the set point of the single aquastat, the circulator
comes on and circulates the hot water between the two units.
This system allows the domestic hot water coil in the existing boiler to supply the household
with hot water without an additional coil in the Kerr TW2000 boiler.
2.6.3 BOILER RETURN WATER TEMPERATURE
Steel boilers are very susceptible to corrosion caused by condensation of flue gases. In order
to prevent this the return water temperature must not fall below 140°F. Most hydronic systems
are designed for a delta T of 20°F. Therefore, the low limit should be set at no lower than
170°F, which accounts for the -10°F differential of the triple aquastat. Heating systems that
require lower temperatures such as cast iron radiators and radiant systems require some form
of mixing device to ensure the return water is at or above 140°F. The warranty will not be
valid for steel boilers with return temperatures below 140°F. For more information refer to
B214-01 Installation Code for Hydronic Heating Systems (or equivalent).
2.7 CONTROL SYSTEM
The Kerr TW2000 boiler control system serves the following functions:
1) Maintaining the boiler water temperature by controlling the combustion air entering the
boiler. A double pole, double throw aquastat and the air damper control this function.
2) Protects the Kerr TW2000 boiler from an overfire by overriding the home’s thermostat and
by putting any excess heat into the living area. Opening the “normally open by-pass zone
valve” and turning on the circulator accomplish this.
3) To allow the Kerr TW2000 boiler to operate as a gravity heating system in the event of a
power failure, the “normally open by-pass zone valve” will automatically spring open in a
power off situation.
4) Kerr TW2000 boiler “BLOW-DOWN” fire extinguishing system. In the case of extreme
over firing the pressure in the boiler will exceed 30 PSI. When this happens, a relief valve
will open and allow steam to escape into the firebox area, damping the fire. See Fig. TW-
9 showing the fitting for the installation of the fire extinguishing system.
IMPORTANT: Upon completion of the installation, a change over from one fuel to the other
shall not require manual adjustment of any controls other than the thermostat.
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