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What is Priority Hot Water (PDHW)

Updated: Mar 8

Traditional central heating systems, made up of a gas boiler and hot water cylinder connected using an S or Y plan configuration are set to run at one flow temperature. This usually is somewhere between 65°C - 75°C, meaning that the heat output supplied by this system is designed for both hot water and heating demands. However, running at such high temperatures can limit the potential efficiency of the boiler.

A priority domestic hot water installation however separates the heat settings for the hot water cylinder and central heating system, allowing higher efficiency thanks to increased potential of the boiler to stay in condensing mode. This also reduces energy consumption as it reduces the overall heat output of the system.

What is the process?

The system of managing hot water and heating allows the hot water tank to be prioritized over the heating circuit when a signal is sent from the cylinder sensor. Hot water then reaches a warmer flow temperature than the central heating’s flow temperature.

Combination boilers, as well, make use of this method. When someone turns on the tap, the flow switch detects movement and shifts a valve over that permits the boiler to instantly heat up mains water, giving it precedence over the heating circuit. Once the tap is turned off, the boiler's diverter valve switches back to heating

mode and lowers its water temperature to a level specified on its control panel.

The Benefit of Priority Hot Water

Using PDHW, it is possible to run the system with two different rates of flow temperature. The water in the cylinder can be reheated much faster using a higher rate and receive the maximum kW output from the boiler directly for heating the coil inside the cylinder. When it has been filled with hot water, the boiler reduces its flow temperature and wattage, allowing the heating circuit to work at its anticipated output so that the boiler can then begin to condense.

To control and move the water around, you must use either 2 port zone valves, a 3 port valve or an integral diverter valve according to the boiler set-up and control options used for this unique system. Smaller gas boilers can be installed if only one process - either heating or hot water - will be operating at any given time.

If you have an S-plan or Y-plan heating system, you can convert it to incorporate PDHW.

If your existing boiler is over 10 years old and does not have the ability to provide priority domestic hot water, you should look into upgrading the boiler. Not every manufacturer provides PDHW technology, so be sure to get advice from a heating engineer or manufacturer before making a decision. When installing a new gas condensing boiler, some of the recommended manufacturers include ones that will allow you to retain your existing S-Plan or Y-Plan configuration and simply make adjustments to the piping and control systems.

For example, a Viessmann 100-W Compact (WB1B) heat-only boiler can be used for priority domestic hot water and wired to an outdoor weather compensation kit, so that it's DHW Cylinder Demand Terminal Box Z007933. Changing all Viessmann systems to PDHW is possible with addition components.

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