November 5, 2020 - 6 minute read

Why Hot Water Priority is the reason S and Y Plan should be banned

Hot Water Priority (or PDHW as its widely now known) is certainly not a new concept. Invented in the late 20th Century around the same time as S & Y Plan systems, W Plan (as it was historically referred to) allowed the hot water cylinder to be reheated without the space heating circuit being heated (should they both have demand at the same time).

In the past when we fitted high temperature non-condensing boilers, hot water priority was pretty pointless. The only time it would have been considered, was when a very small boiler was fitted in a property and it wasn't sufficiently sized to cope with both space heating and hot water demand at the same time. So in the days before condensing boilers, S & Y Plan systems were prevalent and king.

So the point of this blog, and the question I'm asking is... Why in 2005 when A-rated condensing gas boilers were mandated by law (as the only boilers installers could fit), were S & Y Plan systems not outlawed on energy efficiency grounds?

There is a simple known fact amongst low temperature heating system installers; Traditional S & Y Plan systems do not allow condensing boilers to 'condense' when the hot water cylinder is being reheated! Allowing this mistake has been responsible for millions of kWh's of needless gas wastage over the past 15 years.

It's a good job we don't recommend or fit high temperature S & Y Plan systems in 2020 right?!

Lets have a look at what I'm going on about and why this matters.

I have previously written an article on why your new gas boiler isn't efficient, so I won't cover too much of this topic again. The most important fact to remember is condensing boilers don't condense when return temperatures back to the boiler are over 54°C. Fundamentally we should all know this, but what the majority of installers and homeowners don't always know is that condensing gas boilers are only 89% efficient (not the 93% ErP rating shown on an A-rated badge) when the return temperature back to the boiler is 50°C. So, if we design an S Plan system with radiators correctly sized for 70°C flow and 50°C return, in theory the system will run at 89% efficiency at the external design temperature when the heating is on. But what happens, if we now demand hot water?

Lets say we have correctly sized the boiler and the cylinder as per the Domestic Heating Design Guide and in this example we assume this house has a heating load requirement of 8kW and a hot water coil rating of 3kW. Therefore, we size the boiler for 11kW. So, at -3°C outside our heating is working as per our system design and if we demand hot water that works perfectly too? Well, not quite.

What actually happens is this...

The 70/50 designed (89% efficient) heating system works ok until the point hot water has demand. Most cylinders are not fitted with a flow rate gauge (ok lets face it none are), so even if the installer has fitted a gate valve or flow restrictor to the return pipe to slow the flow of water through the coil to 3kW heat input (which on a DT20 boiler is approximately 129 litres of heating water through the coil each hour), they wouldn't know where they are setting it to. Most installers don't fit a flow restrictor to the return of the cylinder, which then allows the system pump (usually set for a much higher system flow rate) to push the system water through the cylinder coil way too fast, which means return temperatures come back to the boiler higher than 54°C and nearly as hot as they are sent from the boiler. This is goes on for hours and really not good for that 89% condensing boiler efficiency!

You don't need to be trained in heating system design to understand this system is inefficient? Lets just remember one key fact... Condensing boilers actually 'condensing' was the reason we ditched standard efficiency boilers in 2005.

So again, we ask the question... Who has allowed this to happen and why are some heating manufacturers still pushing S & Y Plan systems in 2020?

"To improve condensing boiler efficiency, we bring modulation controls into the equation."

In my last article, I mentioned Load and Weather Compensation Controls are the absolute best way to get the boiler operating more efficiently, but there is a key issue here when we now want hot water recovery. Modulation controls like Honeywell Home evohome, produce low flow temperatures for space heating and high temperature for hot water recovery. However, the nicely controlled heating system with stable air temperatures in your home (thanks to lower and consistent system flow temperatures) now needs 75°C flow temps to recover the hot water cylinder.  If we use S or Y Plan, we are now going to throw 75°C into a space heating circuit that was just operating at maybe 45°C. If we do this, all our nice stable room temperatures are going to over shoot, and yes, the controls might jump in to turn off space heating when it detects a sharp temperature rise, but this will be after a serious quantity of energy is needlessly wasted into the space heating circuit.

This is where we enter, the concept of two flow temperatures with hot water priority. One flow temperature directed towards space heating and another flow temperature directed towards the hot water cylinder. We need to make sure that at the same time we demand high temperature recovery of the hot water cylinder, we prioritise the flow of system water to the cylinder coil. However, in the same breath we don't want hot water recovery taking 2-3 hours like this and we want to keep the boiler in condensing mode for as long as we possibly can. Therefore, we introduce cylinders with bigger surface area coils (we sell cylinders with coil ratings in excess of 36kW) and have boilers that can be range rated for both space heating and the cylinder coil output.

With PDHW (Priority Domestic Hot Water) we have the right heating output, at much lower flow temperatures. This ensures efficient heating operation. We also have superfast recovery of the hot water cylinder with the boiler remaining in condensing mode for much longer. Also because of the enlarged cylinder coil size, recovery is actually so fast, we can install significantly smaller cylinders (which also reduces cylinder standing losses, material waste and embodied carbon of its manufacture).

My favourite way of installing hot water priority is using 'X Plan'. This is very similar to S Plan, but the heating valve is always open (normally open zone valve) and only closes when the hot water zone valve on the cylinder has opened.

Intergas boilers have had PDHW out of the box with just the addition of a cylinder sensor for many years. There are other boiler manufacturers that also do priority hot water and dual flow temperatures too, but not all boiler manufacturers understand the concept or why we need to do this.

Joined up thinking really costs nothing. Its just a shame the right intentions are not thought through to the end. In a so called 'climate emergency', the concepts that have not been correctly adopted in the UK, need very quick implementation into our every day working practices. Its easy for us to carry on what we've always done, but now is the time for UK manufacturers and installers to lead from the front and innovate once again - future generations will not thank us for complacency.


Author - Richard Burrows, Director, The INTERGAS Shop

If you are interested in efficient heating system design, system specification or controls and PDHW training, please send us an email at and we can assist you.