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GENERAL

1.1

Description

Construction of the Dorchester DR-SA heaters is in

accordance with the European standards for gas fired

water heaters for sanitary use (EN 89). The heaters
comply with the European Directive for Gas Appliances
and are permitted to bear the CE mark.

It is an open-flue appliance without a fan and with

combustion products discharge safety device (category
B

11BS

). The heaters are suitable for use with a working

pressure of up to 8 bar but on un-vented systems this is
limited to 5.5 bar due to the pre-set of the un-vented kit.
The cylindrical tank is made from sheet steel containing

one vertically placed flue tube.

For protection against corrosion, the inside of the tank is

glass lined. The tank is also fitted with one magnesium

anode for extra protection against corrosion. A thick
insulating layer between the tank and the steel jacket
helps to reduce heat loss.

When the heater is completely filled with water, the

system is under continuous water pressure. When hot
water is drained from the heater, cold water immediately

enters the heater. For effective heat transfer, four flue

baffles are included in the flue tube. Heat from the flue

gases is transferred to the water by means of radiation

and convection. The flue gases are discharged by natural

thermal draught (see figure 1).

Dead legs on a hot water installation are undesirable.
Where possible they must be avoided. Where the
inclusion on the system of a dead leg is unavoidable the
following restrictions must be applied:
-

For pipes not exceeding 19 mm inside diameter;
maximum length of dead leg permitted 12.0 metres;

For pipes exceeding 19 mm but not exceeding
25 mm inside diameter; maximum length of dead leg
7.5 metres;

For pipes with an inside diameter exceeding 25 mm;
maximum dead leg 3.0 metres.

1.1.1 Delivery

All Dorchester heaters are factory assembled and
delivered individually packed and shrouded in polythene
on a wooden pallet. Within the packaging, wrapped
separately is the draught diverter complete with the
combustion products discharge safety device attached by
its capillary sensor. A separate package is included that
contains the installers kit, a ½” manual gas shut off valve,
a ¾” drain valve with Tee and plug, a special ¾” NPT to
BSP threaded nipple and a plastic cover for the drain cut-
out in the casing.

1.1.2 Options

The Dorchester DR-SA heaters can be supplied with

additional variations, either ready fitted or for on-site

assembly.

a) LPG – The heaters can be supplied adjusted for use

on propane or butane – see section 2.7. It is important
that the heater is correctly adjusted for the gas to be

fired.

b) Un-vented supply – The heaters can be supplied

with a purpose designed and sized un-vented kit
comprising pressure reducing valve, non-return
valve, expansion vessel, expansion relief valve and
temperature/pressure relief valve, to enable the heater
to be coupled directly to the mains water.

c) Top to Bottom Recirculation – In order to give

enhanced temperature control to aid compliance with
HSE guidance and Health Technical Memoranda,
a pump circulation kit can be supplied comprising

230 volt single phase pump, pipe work and sufficient

fittings to connect between the hot water flow and the

bottom drain connection.

d) Electrical Anode Protection - The electrical

conductivity of water, measured in micro siemens per

centimetre, is an important parameter in sacrificial

anode protection. The lower the value, the less
effective the magnesium anode becomes.

In areas where the conductivity is likely to be less than
200 ¼

/cm, it is recommended that the magnesium

anode be replaced with an electrically powered inert
anode that requires an uninterrupted electrical supply