Honeywell DHC, Service Manual

Page: 159 / 262

SDC / DHC Hydraulic Components
EN2H-0221GE51 R0812 159
The pre-running time determines the switch-on delay of the
burner and thus the pre-running time of the respective shut-off
device used (motor valve, motor throttle) to ensure trouble-free
circulation within the heat generator when switching on the
burner.
The setting of a pre-running time is only relevant if a shut-off
device (e.g. motor throttle) is used instead of a boiler circuit pump
at a variable output. Actuators with reversible motors must be
operated via an auxiliary relay with a switch-over contact
(separate control phases L
open
/L
closed
).
When the burner switches off, a boiler circuit pump is switched off
after a delay based on the set time to prevent a safety switch-off
of the heat generator at high temperatures.
The extended running time depends on the type of heat
exchanger used and is to be adapted accordingly.
An external heat generator disable affects the output of the boiler
circuit pump.
7.2.17 Return increase
To prevent the return flow temperature from dropping below the
minimum return temperature required by some heat generators,
the control system features various options for raising the return
temperature. Once one of these return control options is active, a
menu is activated where the appropriate settings can be entered.
The parameter minimum return limit determines the lowest
allowable return flow temperature for systems with direct or
indirect return control. When the heat generator return
temperature drops below the set limit, the respective return
control device is activated and raises the return temperature until
the set temperature is reached or exceeded.
In Software Version 3.1 the return flow control is linked to the
controller. This means that there is a fixed assignment between
the heat generator and the return temperature mixing circuit. The
output of the mixing valve pump is working according to the same
function as the boiler charge pump; the same follow-up time is
used. If the heat generator is switched off (no heat demand) the
valve is controlled to the setpoint as long as the follow-up time
has not expired – after that time the heat generator is
disconnected from the waterflow and the mixing valve is
completely opened.
Boiler circuit pump pre-
running time
Boiler circuit pump
extended running time
NOTE
Function