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Priority Domestic Hot Water (PDHW)

Central heating systems comprising of a gas boiler and hot water cylinder using a standard S or Y plan configuration are set to run at one flow temperature which will be the temperature for the cylinder at around 65°C - 75°C operating with a combined heat output for the hot water and heating demand. Running at these high flow temperatures limits the potential for the boiler to condense effectively, if at all, and that's probably for most of the year. 

Priority domestic hot water installations however control the hot water cylinder and central heating system at different flow temperatures maximizing its efficiency and creating a higher potential for the boiler to sit in condensing mode for longer periods of time. It also reduces the heat output for the whole system saving energy consumption.

How does it work?

This method of controlling the hot water and heating allows the hot water cylinder to take priority over the heating circuit when there is a demand from the cylinder sensor, the hot water is heated to a higher flow temperature in relation to the central heating flow temperature.

Combination boilers also use this method, when the hot tap is opened, the flow switch senses movement and diverts a valve over allowing the boiler to heat the mains water instantaneously producing hot water out of your taps taking priority over the heating circuit, when the tap is closed the boilers diverter valve reverts back to heating mode and reduces the boiler water temperature to the set level on the control panel of the boiler.

The Advantage of Priority Hot Water

PDHW technology allows the system to be run a two different flow temperatures. The hot water cylinder can be reheated at a considerably quicker time frame using a high flow temperature and operates at the boilers set maximum kW output with all the energy being used to heat the coil in the cylinder. Once the hot water is satisfied the boiler then reduces its flow temperature and kW output permitting the heating circuit to now operate at its designed heat output, the boiler can then start to condense.

Controlling and diverting the water would incorporate the use of either 2 port zone valves, a 3 port valve or an integral diverter valve depending on the boiler, set up and control options used for the specific system.

Example of a recent project:

  • Worcester CDI classic 35kW system boiler; left at factory settings.
  • S Plan heating system using 2 port zone valves.
  • 250L Unvented Indirect Cylinder, coil rated @ 19kW.
  • Building heat loss of 12kW.
  • Hive 2 channel controller.

Adjustments made to this system for PDHW:

  • Worcester CDI classic 35kW system boiler; range rated to 19kW, integral diverter valve installed.
  • 2 port zone valves removed and pipework adjusted.
  • 250L Unvented Indirect Cylinder, coil rated @ 19kW.
  • Building heat loss of 12kW.
  • Worcester 2 channel 'weather compensation' controller installed.

This system is now running at a 19kW maximum load, the heating is controlled using 'weather compensation', adjusting flow temperatures, reducing the kW output further and creating a higher energy efficient system than previously installed.

Converting your existing system to PDHW

Most S and Y plan heating systems can be converted and upgraded to incorporate PDHW as show above. Firstly you will need a boiler that is capable of offering this technology! 

If your existing boiler is over 10 years old, does not incorporate duel flow temperature capability and have range ratable functionality then our advice would be to upgrade the boiler to a model that can be set up for this control method.

Not all manufactures offer priority domestic hot water technology and the appropriate advice would be needed from your heating engineer or manufacture of choice when having a new boiler installed.


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