HAMWORTHY HEATING LTD
WESSEX-HE 200 BOILER
500001008/K
8
boiler at all times when firing. If the water flow rate
falls below the minimum, the boiler waterways might
be subject to premature failure due to scale formation.
Particular attention should be paid to the restriction of
external flow circuits during periods of low heat
demand.
8.9 Waterside Pressure Drop
The module waterside pressure drop is shown in
Figure No. 3.
NOTE:
If boilers are controlled via a time clock, a
pump overrun (not HHL supply) should be fitted which
must run for minimum of 3 minutes on shut-down of
the boiler.
8.10 Control Schemes
8.10.1. Temperature Controls.
An adjustable control thermostat is fitted to each
module and is set to operate within the range 65-
90°C for standard applications.
If a higher water temperature is required (and
providing sufficient head on the water system is
available) the thermostat may be adjusted to operate
in excess of 90°C.
For high temperature applications, a "special to
contract" controls thermostat with a higher range may
be required and it should be noted that on boilers
applied to systems having flow temperatures in
excess of 100°C, additional insulation should be
applied to the front heat exchanger tube plates
(Contact Hamworthy Heating for this special
insulation kit).
A temperature limiter (hand reset limit thermostat) is
also fitted to the module and is normally set at 100°C.
A higher range limiter may be required for high
temperature applications.
NOTE!
The minimum difference between control
thermostat and temperature limiter must not be less
than 10°C. For adjustment see section 14.7.
8.10.2 Water Flow Controls
Any external mixing valves or similar controls should
always ensure that the minimum water flow rate
shown in Figure No. 3 is maintained. It is
recommended that a water flow switch is fitted to the
system. The switch should be connected such that
the boiler cannot fire unless the water flow has been
proved.
When multiple boilers are installed it is recommended
that the water pipework is connected using the
reverse flow principle to ensure equal flow through all
boilers
8.10.3 Frost Protection.
Consideration should be given to fitting a frost
thermostat set at approximately 4°C.
8.11 Minimum Water System Pressure
To comply with Guidance Note PM5 from the Health
and Safety Executive the minimum static water
pressure at the highest point in the circulating system
must be calculated as follows: -
If the boilers are to be installed as single units the
minimum pressure must be equal to the gauge
pressure equivalent to the saturated steam
temperature obtained by adding 17°C to the required
boiler flow temperature but never less than 2 m.
e.g. 1 Required Flow Temp.
= 90°C
Safety Margin
= 17°C
_____
Equivalent Saturated Steam Temp = 107°C
From Steam Tables -
corresponding Gauge Pressure
= 0.3 bar
= 3.06 m head of water
If the boilers are to be installed in a modular formation
the minimum pressure must be equal to the gauge
pressure equivalent to the saturated steam
temperature obtained by adding 17°C to the sum of
the required mixed flow temperature and the
temperature rise across the boilers.
e.g.2 Required mixed flow Temp = 82°C
Temp. Rise across boilers = 11°C
Safety Margin
= 17°C
____
Equivalent Saturated Steam Temp = 110°C
From Steam Tables -
corresponding Gauge Pressure
= 0.42 bar
= 4.3 m head of water
e.g.3 Required mixed flow Temp. = 82°C
Temp. Rise across boilers
at minimum flow
= 13°C
Safety Margin
= 17°C
____
Equivalent Saturated Steam Temp = 112°C
From Steam Tables -
corresponding Gauge Pressure
= 0.57 bar
= 5.8 m head of
water
8.12 Unvented Pressurised Systems
See Figure No. 2 for typical layout of a pressurised
