W-Plan Wiring

[Set Square]

As many of you will know, I have posted many times about Y-Plan systems and have (hopefully) helped a few people to diagnose problems with them.

Until recently, I hadn't really studied W-Plan systems (which have a diverter valve rather than a mid-position valve). However, the subject has recently arisen in another thread.

Looking at the W-Plan wiring diagram in

it looks to me as if you can't have CH until HW is satisfied - even if HW is turned off at the programmer - because the valve is driven to the CH position by a feed from the cyl stat's satisfied contact.

Is that the way it's meant to operate? Seems a bid odd to me!

Wouldn't it make sense to have a HW-OFF wire from the programmer (like Y-Plan does) connected to 5 in the junction box? You could then have CH only if you specifically didn't want HW.

[ARWadsworth]

that that programmer should be of the type where heating cannot be selected without hot water.

If I am following you correctly then I presume you are suggesting a HW off connection from the programmer to terminal 5 to give seperate controls.

I have only ever seen one W plan and I cannot see any good sides to it

Adam

[Set Square]

Yes. There may be occasions - maybe when returning home from holiday to a cold house, for instance - when you might want the heating on without waiting for the hot water to heat up first.

Unfortunately, I can't get the pdf file, which you cite above, to load - so I can't see what the rationale is for not having a CH-only mode.

[Set Square]

I've now managed to get the pdf file to load.

It comes pretty close to contradicting itself - referring to independent control of HW and CH in some places and then saying that the programmer must not allow heating to be selected without hot water.

I can't see that it matters *what* the programmer does in this respect - the effect will be the same. Even with no HW feed from the programmer, the HW will still be heated (and CH not heated) until the cyl stat is satisfied. So if you want CH only, you have to turn the cyl stat down below the temperature of the water in the cylinder.

My suggestion of a HW-off feed from the programmer would enable the same thing to be achieved without needing to frig about with the cyl stat.

[Sadly]

Is it usual for programmers on a Y-plan system (or even S-plan) to allow for HW priority? There seems very little point in buying a quick-recovery cylinder and finding that it still takes about an hour to heat the tank because all the radiators are on!

[Set Square]

No - neither Y-Plan nor S-Plan provides for HW priority. They both allow for HW and CH to be on concurrently with *equal* priority.

W-Plan does, of course, give HW priority - but can keep the CH off for a very long if you *haven't* got a quick recovery cylinder.

[ARWadsworth]

Pretty close to contradicting itself is generous, I thought it was a direct hit on the Heating Only Requirement section.

I agree. So the only reason would be so the user can see a neon/led in the programmer to say the water is on. There is no other reason

Your suggestion would work fine IMHO. I might try it out as I have to fit an electrical supply for an immersion on a W plan over Xmas (they need a back up in the event of a boiler failure).

Adam

[Sadly]

Maybe I will come up with a cunning way to add one of these "1-hour boost" type switches to my Y-plan system to allow the HW to take priority for a while - this means that when taking a bath I can hit the boost so that the hot water takes priority for the next hour...

Maybe I will first get the system working as standard and see how long it does take to recover!

[Set Square]

I have a Y-Plan system with a Danfoss programmer which has a 1-hour boost facility for the HW. But that just turns the HW on for an hour at a time when it otherwise wouldn't be on. It doesn't turn CH off for that period.

It seems to me that you either need a very fast recovery cylinder and a HW priority facility OR a method of allowing the HW to get hot while the CH is on without interfering unduly with the CH. I have a balancing gate valve on my HW circuit which can restrict the flow a bit so as not to interfere with the CH. Obviously the water would take a bit longer to heat when this was partially closed. This is not a problem unless you want a lot of baths in quick succession.

My current modus operandi doesn't use the gate valve, but runs the HW and CH asynchronously. I use the main programmer to time the HW, and a programmable room stat to time the CH - so that they can be on at non-overlapping times. This means that the gate valve can be fully open - allowing the water to heat up as fast as its (non fast-recovery) heat exchanger allows. Admittedly it doesn't cater explicitly for unscheduled hot water demands - but has never caused any problems that I am aware of.

[Sadly]

We have a huge bath, probably 225ltrs (ISTR that this is the max without needing to inform the water board) and using just an immersion heater (set very hot with a blending valve on the output) there isnt enough hot water to fill the bath. This should be better with the boiler running as I expect that with the immersion heater it only really heats the to 2/3rds or so.

My concern is that when running a bath I want to have the boiler pumping loads of heat into the cylinder to keep it warm enough that I can get a decent bath-full.