Technical data

System Head
Guidance Note PM5 Health and
Safety Executive
This note states that "hot water
boilers should have an automatic
control apparatus to cut off fuel to the
burners of gas fired plant when the
water at or near the boiler flow outlet
rises to a pre-determined temperature.
This should provide a margin of at
least 17°C below the temperature of
saturated steam corresponding to the
pressure at the highest point of the
circulation system above the boiler."
To comply with this recommendation,
the minimum system pressure is
dependant on system design flow
temperatures and in the case of
modular installations, the temperature
rise across each module.
Single Installations
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. The
highest point of the circulation system
above the boiler should never be less
than 2m (6.5ft).
n Required flow
temperature 90°C
n Safety margin 17°C
n Equivalent saturated
steam temperature 107°C
From steam tables
corresponding gauge pressure
0.3 bar – 3.0m head of water.
Modular Installations
The minimum pressure should be
equal to the gauge pressure equivalent
to the saturated steam temperature.
This is obtained by adding 17°C to
the sum of the required mixed flow
temperature plus the temperature rise
across the modules.
n System ∆t 11°C 20°C
n Required mixed flow
Temperature 82°C 80°C
n Temperature rise
across modules at
minimum flow rate 11°C 20°C
n Safety margin 17°C 17°C
n Equivalent saturated
steam temperature 110°C 117°C
n From steam tables
corresponding gauge
pressure 0.43 bar 0.80 bar
(4.40m) 8.20m)
From steam tables
corresponding gauge pressure
at 11°C∆t 0.43 bar, 4.40 m
From steam tables
corresponding gauge pressure
at 20°C∆t 0.80 bar, 8.2 m
Air Supply and Ventilation
An adequate supply of fresh air for
combustion and ventilation must be
provided in accordance with BS 6644.
The air supply should be free
from contamination such as
building dust and insulation
fibres from lagging. To avoid
unnecessary cleaning and
servicing of the burner, we
recommend that the boilers are
not fired whilst building work
is being undertaken.
The air supply should be achieved
using:
n Natural ventilation supplying air
with a low level opening and
discharge through a smaller
sized high level opening.
n A fan to supply air to low level
with natural discharge through
a high level opening.
n A fan to supply air to low level
and discharged by means
of a fan at a high level.
Note: Fans must be selected
such that a negative pressure is
not created in the boiler house
relative to outside air pressure.
The air supplied for boiler house
ventilation should be such that the
maximum temperatures within the
boiler house are as follows:
n At floor level 25°C
(or 100mm above the floor level)
n At mid level 32°C
(1.5m above floor level)
n At ceiling level 40°C
(or 100mm below ceiling level)
Where natural ventilation is used
suitable permanent openings at low
level and high level connected directly
to the outside air should be provided.
These openings must be fitted with
grilles that cannot be blocked or
flooded.
The free area of the grilles should be
as follows:
Low Level (Inlet)
4cm
2
per kW of net heat input.
High Level (Outlet)
2cm
2
per kW of net heat input.
Ventilation
Where a boiler installation is to
operate throughout the summer
months, e.g. for domestic hot water
production for more than 50% of
the time, then additional ventilation
allowances are required. Refer to
BS6644 for more detailed information.
22
The Building Regulations 2000
Conservation of fuel and power 2006 edition
Approved Document AD L2A New Buildings, other than dwellings
Approved Document AD L2B Existing Buildings, other than dwellings
These new regulations came into force 6 April 2006. Compliance with the latest regulations now requires a whole building
approach to reduction in carbon emissions. The 2006 edition requires the use of heat generating plant as detailed in the
supporting 2nd tier guide - Non Domestic Heating, Cooling and Ventilation Compliance Guide.
Seasonal Efficiency
The efficiency data used for evaluating commercial boilers is known as the heat generator seasonal efficiency and this guide
states that for new buildings and existing buildings, the minimum heat generating system seasonal efficiency is 84% gross for
natural gas.
The heat generator seasonal efficiency for Purewell VariHeat boilers exceeds the minimum requirement, Individual boiler figures
can be found on pages 6 & 7.