what is priority hot water (pdhw) ?

In simple terms, it means the hot water demand takes priority over everything else. If you were to do this on most of the boilers on the market, it would cause problems. The majority of boilers generate an amount of heat and leave it to the system to determine where to distribute that heat but, if you install a boiler capable of giving different flow temperatures on different circuits, then you can “tell” the boiler exactly what you want it to do. By telling the boiler that you need hot water means it will run at a higher heat output to warm the hot water cylinder up quicker. Once the hot water cylinder has reached temperature the boiler will lower its output to keep the central heating running at its required design Kilowattage. 

To take it a step further, installing a high recovery hot water cylinder along with a boiler capable of PDHW increases reheat time massively, and also allows you to install a smaller hot water tank that will still keep up with demand. One of the most frustrating things I see when looking at existing systems in houses is seeing a huge hot water tank because the customer “doesn’t want to run out of hot water.'' With PDHW you do not need to heat up 300L of water, you can have a system that can store 117L, which will reheat in less time than it takes to run a bath.

To do all this costs no more than installing a “standard” system