Pros and cons of a W-plan system?

[Mathew Newton]

I have just been reading the heating controls Wiki page

and it seems quite dismissive of the W-plan system. However, if my understanding of weather compensation in many modern boilers is correct is the W-plan not perfect given that it prevents simultaneous CH and HW operation thus allowing completely independent control of the CH and HW temperatures? The Y-plan, which the Wiki seems to imply is preferential, would surely negate weather compensated benefits for those times when both CH and HW are being demanded.

The reason I am interested is that I am considering purchasing a new boiler, with weather compensation, and will be moving away from my S- plan system given the potential requirement for pump overrun and desire for optimum efficiencies. I am thus looking at Y-plan and W- plan and am viewing the benefits of the W-plan as being somewhat simpler (less complicated valve, no microswitches/electronics to fail) and facilitating independent temperature control for CH and HW operation. The general feeling of the Wiki is putting me off though!

Any comments?

Mathew

[Roger Mills]

I think all the plans - S, Y, W, (even C!) - provide independent control of HW and CH temperatures, with a separate stat for each. *However*, in all cases, the DHW cannot get hotter than the boiler flow temperature so, if you have weather compensation which reduces the flow temperature on a warm day, the DHW ain't going to get very hot *unless* you can disable weather compensation while it is being heated.

This may well be easier to achieve with W-Plan because, as you say, it only does one thing at a time. If you go with W-Plan, it's important to have a fast recovery cylinder so that you're not without the CH for very long when re-heating the tank after someone has had a bath.

[Mathew Newton]

Certainly the boiler I'm look at, a Viessmann Vitodens 100-W Compact, has seperate CH and HW demand inputs so one can only assume this is to allow the weather compensation to be disabled during HW demand.

I think I do - it's a Range Tribune 150L UV indirect cylinder which seems to take around 30 minutes to heat a full tank from 'cold' to 60c on the stat.

Mathew

[Andrew Gabriel]

When I was choosing a condensing boiler 10+ years, there was one which had two outputs; one for heating (with optional weather compensation), and one for the hot water cylinder. They had separate temperature controls. It might have been an Ecomax, but I don't remember for sure now.

[ARWadsworth]

The reason most people overlook the W plan is that you have always got to heat the water.

Drivel will be along soon to tell you to use 2 boilers, one for the HW and one for the CH.

[John Williamson]

Only because he'd get *two* kickbacks... ;-)

[Heliotrope Smith]

"ARWadsworth" wrote in news:jjau0h$ph3$ [e-mail]:

Don't forget he will also want you to make up a heat bank out of an old tank and a plate heat exchanger or two nailed on!

--- Posted via news://freenews.netfront.net/ - Complaints to [e-mail]

[John Rumm]

To be fair the FAQ is showing its age a fit there (the later articles in the wiki might be worth reading)

W used to be less desirable because it could deprive the house of heating for a significant time while the cylinder reheated, since it had priority. With slow recovery cylinders that could only absorb heat at around 5kW that meant quite a delay and lots of boiler cycling (also inefficient with older boilers).

With modern fast recovery cylinder, however it can work well.

If you already have S plan, then stick with it and add a bypass valve if you need to. That way you can have any control mechanism you want with suitable wiring.

[John Rumm]

Indeed - you need a boiler capabile of dual temperature operation (or one that can be told when its re-heating water and not running the rads).

Indeed.

[Roger Mills]

Well, not quite. It's true that if you turn the HW off at the programmer with the cylinder full of cold water and the cylstat set normally, you'll not get HW *or* CH - because you only get CH when the cylstat is satisfied, and that ain't going to happen.

However, if you turn the cylstat to minimum, its demand will be deemed to be satisfied - even though the tank is still cold - and you can have CH only.

[ARWadsworth]

But would you really want to have to do that every time you only wanted CH?

[Mathew Newton]

Can you elaborate on this?

If, say, my HW it timed to only come on once a day 5am-6am if I draw off a tankful during the day will the CH not be able to come on in the evening?

That can't be right, can it?

Mathew

[hugh]

In message , Heliotrope Smith writes

Don't forget to find a use for a supercap.

[John Rumm]

Are there going to be many times where you don't want HW?

[Mathew Newton]

My main concern, and correct me if I'm wrong here, is that without an explicit HW demand a weather compensated boiler may only be providing output at, say, 50C during periods of CH demand. If the cylinder stat is set to 60C then it is never going to be satisfied hence you won't get CH... (until after the HW timer has kicked in later).

Mathew

[ARWadsworth]

Yes. The most common one being when you have just had a bath.

[John Rumm]

Indeed, my comment was really addressing a particular point about W plan systems that is often raised as an objection.

With a weather compensation system and lower flow temperatures, then W plan is not really a suitable system. S Plan or S Plan+ would make more sense.

[John Rumm]

Someone else might ;-)

(unless it was a very quick bath, the water will probably be reheated by the time you are out anyway)

[Roger Mills]

'Fraid so. W-Plan relies on the cylstat being in the satisfied state to drive the diverter valve to the CH position. If the tank temperature is below the stat setting, but there is no HW demand to remedy that, you ain't going to get any CH. You could, of course, turn the cylstat down to a point where it becomes 'satisfied' but, as Adam has said, you don't really want to have to keep doing that.

W-Plan works best when the programmer is set to Constant for HW. That way, you'll always have a tank of hot water (which may waste heat, of course, if you don't need it) and the CH will work more or less[1] whenever it's programmed to do so.

I have to admit that by re-visiting the W-Plan wiring diagram in order to contribute to this thread, I've learned a few things of which I wasn't previously aware.

[1] By which I mean subject to HW priority if the tank isn't up to temperature

[Roger Mills]

No, I wouldn't! I was simply qualifying your assertion that you *must* always heat the water. You don't *have* to - although the alternative may not be that appealing.