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UNIT HEATER
It is recommended that flexible duct connectors are used to reduce duct born noises.
The dimensions of the connecting ducts must be maintained to those of the heater spigot for at least
twice the major duct dimensions. This should ensure that adequate air flow will pass over the whole of
the heat exchanger cross sectional area. Also there must be a duct length of at least 1.2m in front of the
heater before any downward vertical turn in the ducting to ensure that the burner may be withdrawn
from the front of the heater.
All joints between the heater and the ductwork should be made as air tight as possible.
When installing a CUHB or CUHC heaters onto ducting it is possible for the duct installation to
change the natural convected air flow through the heater before the fan is turned on. Under these
condition it is sound practice to use an electrical design (such as a 60 second time delay relay) to force
the fan to run and not rely on the fan thermostat to turn it on (see suggested scheme in Section 5).
Any such device must be in parallel with the fan thermostat so that the fan run-on operation will still
take place. CUHD heaters are fitted with two combination thermostats, at the front and in the rear inlet
spigot to cater for this effect.
Contact Combat Engineering Limited Design Department for advice regarding duct resistance
and design.
3.7.1 CUHD Heaters
For CUHD heaters it is essential that the airflow in the duct system is at least that specified in the Data
Sheets in Section 2 and in the correct direction across the heat exchanger as indicated by the arrow on
the heater. Higher air flows are acceptable but it should be remembered that as the air flow rises the
temperature rise across the heat exchanger will fall.
It is recommended that the fan is positioned to blow the air through the heat exchanger.
It is important that the duct is designed as described in 3.7 above to ensure that there is a homogenous
air flow across the whole of the heat exchanger. Failure to provide a suitable air flow properly distributed
across the heat exchanger will cause heat exchanger failures.
The fan motor or its control should contain a method of overload protection and be supplied via a local
electrical isolator if it is installed remote from the heater. Where such a local isolator is installed it
should be positioned and labelled to prevent inadvertent operation
3.8 Electrical Supply
A constant 230 volt 50 Hz single phase supply is required for all heater models connected to terminals
L, N & Earth.
It is important that the "L & N " polarity is correct, particularly for automatic ignition heaters as
incorrect polarity will prevent the burner control box from operating correctly. It is also important on
these heaters that the voltage between N and Earth does not exceed 15v.
All heaters and controls must be correctly earthed.
All external wiring must comply with the relevant IEE regulations and be carried out by a qualified
electrician.
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