DHW Modes of Control

Combination Central Heating Boiler

The combination central heating boiler is basically a central heating boiler which becomes an instantaneous multipoint water heater when there is a demand for domestic hot water.

Heat exchanger

Most combination boilers use a gas to water heat exchanger where the heat from the main gas burner is used to heat the water contained within the water ways of the heat exchanger.

Calorifier

A calorifier is a water to water heat exchanger which consists of a tightly wound copper coil fitted within a cylindrical casing. The tightly wound coil typically consists of 20 feet of 12mm copper tubing and as the domestic-hot water passes through it picks up heat from the surrounding hot water on the outside of the coil and within the cylindrical reservoir.
On the more recent combination boilers the design of calorifier has been updated and changed. The earlier design of a closely coiled tube has been replaced by a plated heat exchanger design. In this type of calorifier the heating water and domestic hot water runs across alternative plates within the heat exchanger and there by transfers heat from the boiler into the domestic hot water. These type of heat exchangers are more efficient and reliable than earlier ones.

Double pass heat exchanger

This type of heat exchanger consists of two tubes one inside the other. When the boiler heats up, the heat from the outer tube is transferred to the inner tube containing the domestic hot water and so no separate calorifier is required for this type of system as the boiler water and the domestic hot water are kept separate at all times.

The Combination Central Heating Boiler

The combination central heating boiler is basically a central heating boiler which becomes an instantaneous multipoint water heater when there is a demand for domestic hot water. Although the basic principle is the same for all combination boilers there are various different methods used to obtain the production of hot water, dependent on the manufacturer.

The four basic designs are:

1. Domestic hot heater heated in a calorifier with the use of a diverter valve.
2. Domestic hot water heated through the main heat exchanger with a diverter valve.
3. Domestic hot water heated through the main heat exchanger without a diverter valve. (pump used only for central heating mode)
4. Domestic hot water heated through the main heat exchanger using non- return valves and a separate pump for domestic hot water to that used to circulate the central heating water.

Type 1

Utilising heat exchanger, diverter valve and calorifier.

Hot Water Mode
The operation of the domestic hot water draw off tap leads to the mechanical movement of the diverter valve and the change of operation of the boiler to that of an instantaneous water heater.

• When a hot tap is turned on the mains cold water flows through the water section of the diverter valve and causes it to lift.
• The diverter valve now does 2 jobs, it diverts the water from the central heating into the calorifier and the centre spindle which acts as a pushrod makes contact with a double microswitch which switches the boiler to hot water mode.
• The water is now diverted into the calorifier. The calorifier must keep the boiler water separate from the potable water, and be capable of transferring the heat as quickly as possible.
• The water is then transferred into the main heat exchanger to be reheated. When the hot tap is turned off the diverter valve returns to central heating mode, and the pushrod returns to its original position.

Celtic Chaffoteaux

Type 2

Utilising single heat exchanger and diverter valve.

Hot water mode
The operation of the domestic hot water draw off tap again leads to the mechanical movement of the diverter valve but due to the design of the main heat exchanger does not require a second heat exchanger (Calorifier)

• When the hot water tap is operated, the diverter valve stops the water from circulating around the central heating circuit. It diverts it, to circulate within the boiler. It also operates a double microswitch, to change mode.
• The heat exchanger now heats tip the separate incomming flow of cold water as it passes inside the pipes that form the 2 pass heat exchanger.

Potterton Lynx

Type 3

The operation of the domestic hot water tap causes the main burner to light and heat the water in the main heat exchanger and transfer its heat to the domestic hot water without the need for the use of either a pump on a second heat exchanger (Calorifier).

Hot water mode

• The incoming cold water is heated in the heat exchanger (outer spiral).
• Hot water mode is activated by a flow switch (the pump does not run).
• After running hot water the pump is activated to remove residual heat in the heat exchanger.
• If the summer/winter knob is in the winter position the heat will be quickly dissipated into the central heating circuit.
• If the summer/winter knob is in the summer position the pump will keep circulating the water around the boiler until the heat is dissipated (this could take some time).

Glow-Worm Express 80

Type 4

This type utilises 2 pumps, one runs on operation of domestic hot water tap the other on demand for central heating. Each pump contains a nonreturn valve to prevent reversed circulation within the boiler when the opposite pump is running.

Hot water mode

• When a hot water tap is turned on, the flow switch (or sensor) is activated.
• The domestic hot water pump now circulates the water from the heat exchanger into the manifold and out into the calorifier, transferring the heat into the domestic hot water coil.
• It then leaves the calorifer and enters the air separator.
• Because of the non-return valve fitted in the central heating outlet. The water does not circulate into the central heating but returns to the heat exchanger.

Myson Midas B