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 (DHW) 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. Smaller gas boilers can be installed as only heating or hot water will be in operation at any one time.
Example of a recent project:
This Worcester CDI Classic 35kW system boiler 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.
See other Priority Domestic Hot Water installations here.
Above: Viessmann 100-W Compact (WB1B) heat only boiler piped as priority domestic hot water and wired using outdoor weather compensation kit - DHW Cylinder Demand Terminal Box Z007933.
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!
Boiler Manufactures offering PDHW
Below is a list of boilers that offer a Priority Domestic Hot Water feature, some manufactures offer the capability out the box, whilst others require extra controls to adapt the boilers hidden features. We will continue to update this chart when other boiler manufactures include this feature within their products.
| Manufacture | Model |
| Viessmann | Vitodens 200, 100, 050 |
| Worcester Bosch | Greenstar CDI, i, 8000 Range |
| ATAG | i Range |
| Vaillant | Green iQ, 600 Range |
| Alpha | E tec 15, 20 |
| Vokera | Evolve S, Mynute |
| Ideal | Vogue |
| Keston | System 2 S30 |
If your current boiler is more than 10 years old, lacks dual flow temperature capability, or doesn't offer range rateable functionality, we recommend considering an upgrade to one of the boiler models listed above that supports this control method for your next installation.
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 upgrading any gas condensing boiler the below manufactures should be taken into consideration, as these will offer the option of converting an existing S-plan or Y-plan configuration to PDHW. If you are lucky enough to have one of these already in place then adjustment's to the pipework and control system would suffice.
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