Priority domestic hot water or S plan?

[Notch7]

my prehistoric boiler has just died ( the cast iron hex started piddling out water).

A heating engineer / plumber very kindly came around the same day + quoted which I’m going ahead with (system boiler using existing vented hot water cylinder)

He has suggested a system boiler with an S plan, but a bit of googling seems to indicate an S plan will mean return will always be too high a temperature for condensing and weather compensation won’t work….so boiler won’t be operating at its most efficient.


My feeling is I should take the advice of the plumber (he is the expert) and keep things simple and do the S plan rather than complicate the install.


I would be grateful for any advice

 

[MNW67]

I will likely be making a similar choice in the near future. I'm pretty sure I will go with priority hot water. Mainly because I think a house feels nicer with cooler radiators giving out a gentler heat. Also, weather compensation appeals to me. Any extra efficiency savings would be a bonus. Which boiler have you been quoted for? I don't think it's a massive job with some boilers to change from S-Plan to priority hot water at a later stage, but don't hold me to that!

Was the heating engineer familiar with priority hot water and weather compensation?

 

[Notch7]

Which boiler have you been quoted for?

It’s a Main eco compact system boiler (identical to the Baxi 600 series)

I know the boiler can do weather compensation and is open therm compatible (although threads on here suggest not fully)

Was the heating engineer familiar with priority hot water and weather compensation?

To be fair when he came around to look our boiler had broken down on one of the coldest days of the year so he was quoting for the easiest cheapest system to fit.

At the time he suggested an S plan and basic thermostat and 7 controller, to keep price and speed of replacement down.

I ask him afterwards about it and he said he would ring Baxi and ask what options were available


My understanding is that S plan is a 2 pipe system so the cheapest to install - my old boiler was an old fashioned gravity hot water and Un pressurised hot water and boiler is going in a new location.

 

[Mister Banks]

my prehistoric boiler has just died ( the cast iron hex started piddling out water).

A heating engineer / plumber very kindly came around the same day + quoted which I’m going ahead with (system boiler using existing vented hot water cylinder)

He has suggested a system boiler with an S plan, but a bit of googling seems to indicate an S plan will mean return will always be too high a temperature for condensing and weather compensation won’t work….so boiler won’t be operating at its most efficient.


My feeling is I should take the advice of the plumber (he is the expert) and keep things simple and do the S plan rather than complicate the install.


I would be grateful for any advice

This is a good read.......

PDHW Pt 1 Why new S&Y plan systems should be BANNED - Heating Academy

This is Part 1 of a series of PDHW articles I am writing. This is a general background. Putting a brand new, high efficiency boiler onto an S or Y plans system is like putting triple glazed windows into a rotten wooden frame. All of the performance benefits of the new window are lost as cold air...

heatingacademynorthampton.co.uk

 

[Madrab]

He has suggested a system boiler with an S plan, but a bit of googling seems to indicate an S plan will mean return will always be too high a temperature for condensing and weather compensation won’t work….so boiler won’t be operating at its most efficient.

The return temperature has nothing to do with what 'plan' the system is setup as. A system properly setup and system temps managed properly, rads correctly sized and setup will dictate efficient running and correct temps to maximise condensing.

Weather comp and separate system temps can now be managed by Open Therm or equivalent, so the system doesn't need to be hard set @ 70-75 deg for HW. Doesn't mean that PDHW cant be setup of course rather than S or Y plans but that would need a change to a new high efficiency cylinder to take advantage of that high HW system temps so it heats up as fast as it can and that high system temp runs for as little time as possible.

 

[MNW67]

Weather comp and separate system temps can now be managed by Open Therm or equivalent, so the system doesn't need to be hard set @ 70-75 deg for HW.

Thanks. That is interesting. More like a software fix rather than a hardware fix.

So, as long as your boiler uses OpenTherm or any of the proprietary bus systems, are you saying you can set a higher flow temperature when the HW is on than when the CH is on by itself?

 

[Madrab]

Yep .... That's one of the major advantages of OT.

System temps can be set specifically to requirements, that's how PDHW works. It has to set the HW call to the higher system temps to minimise HW heat up time and then drop the target temps back down when in CH mode, also incorporating weather comp. It differs slightly from an S plan whereby, in simple terms, when the cylinder stat calls for HW then the cylinder automatically gets priority flow so shuts down the CH flow and directs all the system water to the cylinder @ an increased temp. Either that or it uses a controlled diverter valve.

 

[Mister Banks]

Yep .... That's one of the major advantages of OT.

System temps can be set specifically to requirements, that's how PDHW works. It has to set the HW call to the higher system temps to minimise HW heat up time and then drop the target temps back down when in CH mode, also incorporating weather comp. It differs slightly from an S plan whereby, in simple terms, when the cylinder stat calls for HW then the cylinder automatically gets priority flow so shuts down the CH flow and directs all the system water to the cylinder @ an increased temp. Either that or it uses a controlled diverter valve.

That’s PDHW just done a different way.

 

[MNW67]

I'm still not 100% getting it. Apols.

I currently have a standard S-Plan. If I wanted my new system to work so that, when the CH is on by itself, the flow temperature is managed by the bus system and is relatively cool most of the time. Too cool to ever make my HW hot enough. But when the HW is on, either by itself or at the same time as CH, the flow changes to a higher flow temperature. Can I do that with my current standard S-Plan design, as long as my new boiler is using a bus system?

 

[Notch7]

This is a good read.......

PDHW Pt 1 Why new S&Y plan systems should be BANNED - Heating Academy

This is Part 1 of a series of PDHW articles I am writing. This is a general background. Putting a brand new, high efficiency boiler onto an S or Y plans system is like putting triple glazed windows into a rotten wooden frame. All of the performance benefits of the new window are lost as cold air...

heatingacademynorthampton.co.uk

thank you very much, Ive read it through and am trying to understand it at the moment

 

[Mister Banks]

I'm still not 100% getting it. Apols.

I currently have a standard S-Plan. If I wanted my new system to work so that, when the CH is on by itself, the flow temperature is managed by the bus system and is relatively cool most of the time. Too cool to ever make my HW hot enough. But when the HW is on, either by itself or at the same time as CH, the flow changes to a higher flow temperature. Can I do that with my current standard S-Plan design, as long as my new boiler is using a bus system?

Your boiler has to support PDHW and you will need to have a normally open 2 port valve on the heating circuit. There will then need to be some wiring changes in the wiring centre.

 

[Notch7]

The return temperature has nothing to do with what 'plan' the system is setup as. A system properly setup and system temps managed properly, rads correctly sized and setup will dictate efficient running and correct temps to maximise condensing.

Weather comp and separate system temps can now be managed by Open Therm or equivalent, so the system doesn't need to be hard set @ 70-75 deg for HW. Doesn't mean that PDHW cant be setup of course rather than S or Y plans but that would need a change to a new high efficiency cylinder to take advantage of that high HW system temps so it heats up as fast as it can and that high system temp runs for as little time as possible.

thank you very much for this.

would a high efficiency cylinder be something like one of these:

I am wondering if fast recovery would mean a smaller tank would be suitable?

Ultra High Recovery 117 Litre Vented Hot Water Cylinder (900 x 450)

Ultra High Recovery 117 Litre Vented Hot Water Cylinder (900 x 450) | Order Online Today @ The INTERGAS Shop | Next Working Day Delivery Available

theintergasshop.co.uk

Super fast recovery Indirect Copper hot water cylinders | Re-heat in as little as 5 minutes

Super fast recovery copper hot water cylinders. Reheat time as little as 5 minutes allowing a smaller cylinder to be used.

www.gasapplianceguide.co.uk

 

[MNW67]

Your boiler has to support PDHW and you will need to have a normally open 2 port valve on the heating circuit. There will then need to be some wiring changes in the wiring centre.

I know the traditional way to do PDHW. I had thought @Madrab was saying there was a way to do achieve similar using a standard S-Plan as long as you were using OpenTherm controllers. Negating the need for a high recovery cylinder.

 

[MNW67]

It’s a Main eco compact system boiler (identical to the Baxi 600 series)

You might want to get it confirmed that boiler can actually do PDHW, and how it is implemented. If you read Parts 2 and 3 of that PDHW article, I can't see any mention of Baxi/Main. This is all quite new stuff, I believe.

 

[Madrab]

That’s PDHW just done a different way.

That's exactly what I said.
=========

I know the traditional way to do PDHW. I had thought @Madrab was saying there was a way to do achieve similar using a standard S-Plan as long as you were using OpenTherm controllers. Negating the need for a high recovery cylinder.

An S plan can easily be converted to a PDHW by some easy wiring as long as the boiler supports OT. That is rather than pure PDHW whereby one of the NC 2 port valves is replaced by a NO.

Would still need a new cylinder though TBH with U/high recovery, to take advantage of the increased flow temps on HW and ensure the cylinder is satisfied as fast as possible, minimising energy costs.

Main eco compact system

I believe that boiler is OT and weather comp compatible