Diverter valve mystery

[newshound]

I'm nervous about posting this, because it sounds so much like a user error somewhere.

I've just replaced a combi with a system boiler and indirect cylinder, and for various reasons decided to go for a W type arrangement with a diverter rather than mid position valve.

So I have a new Honeywell V4044 in position, not yet wired in electrically to the control box. No cylinder thermostat control yet, just running both circuits at whatever the boiler supplies.

Boiler flow goes to AB, A goes to radiators, B goes to cylinder.

On the manual setting (for filling, flushing, etc) I get heat to both circuits. With it set to auto and *no* electrical power to the valve, I get flow to the cylinder but not to the radiators. With the valve *energised*, I get flow to the radiators. (I'm not sure if there is flow to the cylinder in this condition because it's a well insulated vertical run, so stays hot for a while).

It's as if the ports are incorrectly labelled, which just seems incredibly unlikely. But before breaking into the wet side to look, I just thought I would say:

HELP!

[Andy Wade]

Then it's doing what it should, isn't it? The V4044 data sheet says

"Valve opens to Port A [...] under power[...] "valve opens to port B [...] under spring return"

If you want flow to the cylinder when powered you need A to the cyl and B to the rads.

[newshound]

Oh. The data sheet that came with it doesn't say anything about which way round it goes under power. But you are absolutely right, the V4044 sheet on the web does say "opens to A under power".

I *thought* I had read that you want "power on" to be the less frequent condition with "power off" as the default. This would be consistent with the rads as the normal condition, with the cylinder heated only when the thermostat says it needs to be. My controller gives me power when the cylinder thermostat says the cylinder is under-temperature.

The data sheet that came with my V4044C definitely says B to the cylinder, A to the rads. So I fitted it that way without thinking any more about it.

Problem solved! At least now I don't have to take it off to experiment, I can just swivel it round, hopefully without losing too much water. Thanks for the quick response.

Steve

[Toby]

Surely you want to leave it like it is, otherwise your cylinder takes preference over your heating doesn't it?

So with a cold system, and a cold house, you won't get any heat to the rads until the cylinder is satisfied, or the thermostat on the cylinder is turned off/down!?

[ARWadsworth]

Do not swap it round. Port B is for the HW.

[John Rumm]

It sounds like its doing exactly what you would expect...

If you look at the wiring diagram:

on from the programmer, should feed power to the cylinder stat call for heat connection. This is fed via the common to the boiler/pump to fire them. That leaves the valve unpowered, and hence in its spring return position sending the flow to the cylinder.

Once the cylinder it hot, the stat will switch to its "satisfied" position, which in the absence of a demand from the room stat/programmer will cut the power to the boiler and pump.

A call for heat from the programmer/room stat will now re-energise boiler/pump and feed the common terminal of the cylinder stat. This in turn should power the valve via its satisfied connection to drive it to the CH position.

Any time the cylinder stat decides to call for heat it will switch position and release the valve. This gives the HW priority over the heating.

Note that the programmer needs to be told its a W plan system so that it always calls for HW when its calling for CH - otherwise you can get a deadlock situation where the cylinder stat calls for heat and hence disables the CH, but there is no power from the programmer to fire the boiler etc and heat the cylinder. Hence it just sits there waiting for the cylinder to heat, and you won't get any CH until its done!

[John Rumm]

That is the purpose of W plan... HW priority. Ideally suited to fast recovery cylinders than can absorb the full power output of the boiler.

Yup. That's how its supposed to work.

[John Rumm]

Yup, default position is HW port on spring return.

That would be desirable, but its not the way W plan normally works.

You have it fitted the correct way now, if you want it to work in the classic W plan scheme.

If you try to wire it the other way around, (i.e. with CH priority) you may well find you create a situation where you can't heat the cylinder without also have the CH on. Probably not what you want in the summer.

[ARWadsworth]

You could turn down the room stat :-)

[Ronald Raygun]

It may be doing what *you* expect, but evidently not what *he* expects.

Are you therefore saying he should not have gone for W plan at all?

That's not true. The wiring diagram as shown ensures that the cyclinder will get heat when its stat calls for it and when there is (programmer and room stat) demand for CH even when there is no programmer demand for HW.

[John Rumm]

No not really. Especially since I don't know what the OPs selection criteria were in the first place. It may just be a case of a slight misunderstanding of how this style of system usually works. W plan works well with fast recovery cylinders, or with directly heated heat banks - since it minimises recovery time. Its not so good with conventional indirect cylinders since they can't usually take the boilers full output (or for that matter its minimum modulation output if its a modern boiler) - and the mid position of a T plan system would actually be handy to prevent boiler cycling.

>>> (snip)

Yup, sorry I was subtly mis-remembering the limitation... you don't have to have the HW demand present at the programmer for the CH to run, since the room stat / programmer can indeed call for heat by itself and fire the boiler.

The limitation is that it will still be the cylinder that gets heated even if the programmer is set for CH only, since the only way the valve can be energised to move to the CH position is via the cylinder stat moving to its "satisfied" position. (you could of course turn the cylinder stat right down)

[newshound]

Thanks to all for the helpful comments, I knew the collective brainpower would sort me out (and I was right to be worried about posting my incompetence).

To explain how I got into this position, I decided to go for W plan as I have a rapid recovery cylinder and an old house with high thermal inertia: so losing radiator heating while recovering the cylinder on occasions of heavy hot water demand didn't seem like a problem. Also the diverter valve seemed a little simpler and perhaps more reliable than a mid-position.

I went for a Vaillant boiler with their fancy electronic 140 timeswitch which needs the VR 65 control unit. The timeswitch tells this what to do via a low voltage link (described as a bus, so presumably some sort of digital link). I hadn't been able to find the datasheet for that on the web at the planning stage, so I didn't realise until the hardware arrived that it notionally only supports Y-plan (mid position) or S-plan (zone valve) systems, and not W plan. But by then I had a diverter rather than mid-position valve plumbed in as described with B (closed on power) to the cylinder and A (open on power) to the radiators.

But I thought never mind, I can ignore the timer-controlled supply which opens the CH side and use the S plan timer controlled DHW supply to actuate the diverter valve. That was when I thought the diverter valve OPENED to B on power.

In the summer, I'll have a timer program to provide DHW only so that on a cylinder thermostat demand, the boiler will power up and the diverter valve will operate so I'm only heating the cylinder. In the winter, I'll have a different program calling for CH and DHW so timed CH demand will heat the rads except when there is a DHW demand as well, at which time diverter valve will operate until the demand is satisfied, in other words W plan logic.

I have TRVs throughout except for a towel rail, but the new boiler has an automatic bypass and fancy protection so I guess I could put that on TRV as well.

Or am I still really missing something? For the case where I have a cold house and a cold cylinder I suppose I could have another program for CH only although, to be honest, that's probably where I would fire up the woodburner and be wanting hot water ASAP.

[<me9]

Surely (it does on my system) the HW sat comes from the NC cntact of the thermostat /and/ that of the programmer.

It may be harder to obtain programmers with c/o contacts. Tank thermostats usually have them.

[ARWadsworth]

So if you turn the divertor valve around and power it from the DHW ON connection in the V65 it would do what you want.

If you then call for HW then the valve will open and you will heat your HW. If you call for CH and HW then the valve will open heat up the HW until the cylinder stat reaches temperature at which point your CH will be on.

And if you just call for CH you will get just CH.

Adam

[Dave Starling]

Yes and that approach will save a few watts for the motor and also increase the lifespan of the valve since the alternative is to power up the valve when the radiators are calling which will be many times more than the cylinder is calling if its a high recovery variety.

Y plan is the same in that the default open port is B to cylinder. This means the valve is energised/stalled when the radiators are calling. Reversing the logic would work but cause the next person to fix it to complain it was wired incorrectly in the first place. Of course the standard way means =A3=A3 to repairers for replacing and more money to the manufacturers and so keeps the wheels of industry turning.

Dave.

[newshound]

Thank you both for the confirmation; that was just what I figured, I implemented it yesterday and it seems just fine. And being a nice sort of person, I'll write up a little note and leave it inside the VR65 box for any future investigator!

[John Rumm]

Indeed, in the classic W plan wiring scheme the source of power to the cylinder stat can be either the programmers HW output or the room stats call for heat output. However the only connection to the the valve itself is from the cylinder stats satisfied output. Hence until the cylinder stat is satisfied, the valve stays in the HW position.

[Ronald Raygun]

I agree, it's a much better option for the valve to be actuated for HW and unpowered for CH. One particular reason for this is that in summer, when the CH is likely to be off for months on end, with the boiler firing up only when HW is demanded (which typically it is likely to do at least once a day), this will keep diverter valve exercised, and less likely to stick through non-use.

Whether it's wired "correctly" is partly a question of whether the plan to which it's wired is appropriate. A repairers mistake would be to assume that the plan in question is "standard" W-plan. Clearly if a more appropriate but less conventional plan is being used, it's be a good idea to place a diagram of the system (as it is) into the wiring box, together with a notice drawing attention to the fact that it may be "nonstandard".

You're an old cynic, but absolutely right!