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AOS WPC - Tech Training 67 of 72 Ashland City, TN © 2007
Servicing should only be performed by a Qualified Service Agent
VF BOILER SERVICE MANUAL
OPERATING TEMPERATURES
Temperature rise through the boiler: 20 - 40°F.
The temperature rise (ΔT = Delta T) through the boiler should be maintained between 20 -
40°F while firing at 100%. This flow rate maintains a balance between the harmful effects of
calcium/lime accumulation (low flow rate) and velocity erosion (excessive flow rate). The ΔT
will be lower when the firing rate is less than 100%. Install and throttle a flow control valve in
the outlet line to achieve 20 - 40°F ΔT. DO NOT throttle/adjust the outlet valve unless the
boiler is firing at 100%. Never throttle the inlet valve to the boiler it must remain completely
open at all times.
Minimum Inlet Water Temperature
The inlet water temperature should be a minimum of 120°F during normal operation to
prevent excessive condensate from forming on the heat exchanger. If this minimum inlet
water temperature cannot be maintained the boiler may be undersized. On domestic water
systems DO NOT connect the cold water make-up line to the boiler return (inlet) piping as
this will make maintaining 120°F minimum inlet water temperature more difficult.
Bypass Lines
Bypass lines should be installed on all VF boilers
to maintain a minimum inlet water
temperature of 120°F. Bypass lines feed a portion of the outlet water back to the inlet of the
boiler which raises the inlet water temperature and helps prevent condensation from forming
on the heat exchanger. A bypass line with a flow control valve should be installed between
the inlet and outlet lines of the boiler as shown in the illustration below. Bypass lines must be
installed on the “system side” of the boiler circulation pump. The bypass line should be the
same size as the inlet and outlet piping on the boiler. As the boiler reaches operating
temperature throttle the bypass valve to achieve minimum inlet water temperature of 120°F.
VW models are shipped with a factory installed pump. VB models are not. VB models will
require a properly sized field installed pump. Pump must be installed on the inlet side of the
boiler as shown in the illustration below. See footnotes for field installed pumps on page 37.
Domestic Hot Water Piping
Single Temperature
With building recirculation