Domestic Hot Water Mode (Condensing Combi) -
When
there is a demand for hot water (from a tap, shower, etc.) the
water flow operates the flow switch. The Hot Water pump starts
and circulates water from the primary store through the
domestic hot water plate heat exchanger, heating the incoming
mains cold water.
The hot water produced is mixed with incoming mains cold
water in the thermostatic blending valve to automatically ensure
that the leaving hot water temperature does not exceed 65˚C.
When the hot water demand ceases (the hot tap or shower valve
is turned off), the flow switch is no longer operated. The Hot
Water pump will continue to run and the burner continues to
fire until the primary store reaches the required temperature and
the store temperature control is satisfied.
The boiler will return to the central heating mode, if there is a
heating demand from the timer/programmer and room
thermostat, with the Heating pump and burner running.
Note:
As the temperature of the boiler will now be higher than
the boiler thermostat setting, the burner may not fire
immediately, but only after the primary flow temperature has
fallen to below the thermostat setting.
If there is no heating demand from the timer/programmer and
room thermostat, the Heating pump will not operate.
For optimum performance the thermostatic blending valve has
been factory set to provide a hot water temperature of
approximately 50˚C.
If the water in the boiler is already up to temperature when a hot
tap is opened, there will be a delay before the burner fires to
maintain the hot water temperature in the boiler (to heat the hot
water).
During the combustion process, hydrogen and oxygen combine
to produce heat and water vapour. The water vapour produced
is in the form of superheated steam in the heat exchanger. This
superheated steam contains sensible heat (available heat) and
latent heat (heat locked up in the flue gas). A conventional boiler
cannot recover any of the latent heat and this energy is lost to
the atmosphere through the flue.
The Condensing Combi condensing boilers contain an extra
heat exchanger which is designed to recover the latent heat
normally lost by conventional boilers. This is done by cooling
the flue gases to below 55˚C, thus extracting more sensible heat
and some of the latent heat.
To ensure maximum efficiency, the boiler return temperature
should be 55˚C or less, this will enable the latent heat to be
condensed out of the flue gases. The boiler will achieve nett
thermal efficiencies of 100%.
To achieve maximum performance from a Condensing Combi
boiler, it is recommended that the heating system is designed
so that a temperature differential of 20˚C between the flow and
return is maintained.
The boiler will however still operate at extremely high
efficiencies even when it is not in condensing mode and
therefore is suitable for fitting to an existing heating system
without alteration to the radiator sizes. The boiler is capable of
a max flow temperature of 80˚C.
To achieve the maximum efficiencies possible from a
Condensing boiler, the heating system should be designed to
the following parameters:
Radiators:-
Flow temperature
70˚C
Return temperature
50˚C
Differential
20˚C
Underfloor:-
Flow temperature
50˚C
Return temperature
40˚C
Differential
1O˚C
1 Size radiators with a mean water temperature of 60˚C.
2 Design system controls with programmable room
thermostats or use weather compensating controls to
maintain return temperatures below 55˚C.
The boiler should not be allowed to operate with return
temperatures of less than 40˚C when the system is up to
operating temperature.
5.14
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How Grant condensing boilers work
5.15
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Condensing heating system design considerations
Outdoor Combi 90 V3, Combi Max and Condensing Combi
25
5 - GENERAL BOILER INFORMATION
