What is Priority Domestic Hot Water (PDHW)
Priority Domestic Hot Water (PDHW) refers to a hot water and central heating system designed to prioritise the rapid delivery of hot water.
Its key advantage over traditional systems is its rapid hot water regeneration time, combined with a huge increase in efficiency.
Old fashioned heating and hot water systems (often called Y-plans, or S-plan system) were designed back in the middle of the last century. Yet, remarkably your average plumbers still fit these out of date systems. In fact, they are common and are in almost every home that has a hot water cylinder.
The basis of the old fashioned system is that the boiler is so powerful it can run the central heating system, as well as slowly heating up the hot water.
Intuitively this sounds great. But it requires the boiler to be vastly overpowered in order to run both heating and hot water simultaneously.
The average house requires around 6-8KW of power to heat during the depths of winter. Yet most houses have vastly oversized boiler…anything up to 40KW in some circumstances.
The best analogy I have found for trying to describe this is this: Imagine you have a normal family car, and its used for normal commuting and visiting friends etc. Now imagine it has a formula 1 engine. Every time you drive the car it goes for 0-100 in 4 seconds. Great! But as soon as you put your foot down it’s already time to slow down. Now imagine driving from London to Edinburgh, with the car accelerating to 150 MPH, then down to 20 MPH, then back up to 150 MPH. Sure you will certainly get to Edinburgh, but what state will your car be in…and how much fuel would you have used?
It is much more fuel efficient to drive at a steady 60MPH, arrive at the same time, and save a huge amount of wear and tear and wasted fuel.
That’s Priority Domestic Hot Water.
Combi Boilers Already are PDHW
Combi boilers already use the same basic principle of PDHW. Simply put, the boiler prioritises hot water over central heating. For those who have a combi boiler, they may (or my not) know this already…and this is key. When the central heating is on, and you run a hot tap the heating is switched off and the boiler sends all power to creating hot water. The key is, although the central heating is off for 5, 10, 15 minutes; it’s not noticeable. As soon as the hot water demand is switched off the boiler reverts to heating. The short period the heating was off hasn’t affected the temperature in the house.
PDHW with a Cylinder
In a modern heating and hot water installation PDHW operates in a similar way as the combination boiler, but with some key benefits. A well planned installation separates the flow temperatures required for central heating and hot water. A good installation will have the homes internal temperature, as well as the outside temperature fed back to the boiler. Using this information the boiler will modulate its output to use the correct amount off heat required. Rather than simply overheating, and shutting off…just like the formula 1 engine. The exact amount of fuel and power are used to deliver exactly the correct amount of heat to where it needs to be, when it needs to be there. Conversely the hot water cylinder sends live data back to the boiler with temperature information. When the boiler detects a drop in temperature it will send the correct power and heat to the cylinder in order for it to regenerate itself within minutes, rather than hours. PDHW cylinders typically are smaller, but have larger coils in order to regenerate quicker. Typically they can fully reheat an entire cylinder in 15-20 minutes. Once the hot water is fully heated the boiler will revert back to central heating. This can help maximise condensing and the fuel efficiency of the boiler.
Advantages of PDHW over S-plan and Y-plan
Old fashioned boilers and systems cannot differentiate between hot water and heating flow. The boiler is ‘unaware’ of where the heat is going when it is on. A hot water cylinder needs to maintain a relatively high water temperature to combat bacterial growth. Because of this the heating will always be relatively hot. Most likely too hot. This will mean the boiler burns too much fuel, overheat and switch off. Just like the oversized car engine.
Hot radiators and hot water intuitively sound like a good thing. But what we need is the correct temperature, not excessive heat.
Modern heating systems are designed to run at much cooler flow temperatures. The days of 70-80 degree radiators are coming to an end. Hot flow temperatures put unnecessary pressure on the pipework and radiators, speed up chemical decay of system and affect air quality inside the property due to excessively hot air currents.
PDHW systems help deliver a modern solution that’s kinder to the fabric of the building, the people inside in, all while maximising fuel efficiency. On top of that your house is still warm, your water is still hot and can fully regenerate from cold in 15-20 minutes…and you use less energy. It is a no brainer.
How can you get PDHW
PDHW systems are often cheaper to install than the old S and Y plans. However, it’s a bit harder to find am installer capable of fitting one.
Not many boiler manufacturers are up to speed with PDHW. Most installers aren’t either. Our preferred manufacturers offering PDHW are ATAG, ALPHA, IDEAL and VAILLANT.
Call us to discuss how PDHW can help you.
#PDHW #PHW #ATAG #Vaillant #ALPHA #unvented #hydronics #energysaving #homeheating
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