Constant flow automatic bypass valves

[Chalked]

Fit a vaillant vrc doesn’t need a bypass, as it opens the motorised valves.

 

[decomplexity]

Re Constant flow automatic bypass valves reply: interesting that you have W/C installed on a Viessmann 100-W S-Plan system. I also ordered W/C. I got a far as having the outside sensor fitted and cabled, then we (me, electrician and GS engineer) realised that the cylinder demand box's role was in effect to override W/C 's recommended flow temperature when giving DHW priority - which becomes a little more difficult when the system has more than one DHW cylinder as I have. Viessmann were blunter: you cannot use W/C with S-Plan.
Back on thread: I currently use a towel-rail (no TRV) as bypass as it is fitted on the boiler side of the MVs and thus get some benefit from the few minutes of overrun (it is also the usual fail-safe against all the TRVs shutting). But intend to try out a C&P 4l/m low-pressure constant flow isolator valve since I cannot (yet...) see any reason why it shouldn't work - and it is half the price of an adjustable pressure-driven ABV!

 

[Riley]

I would say your choice of faux abv will void any boiler warranty

 

[chris watkins]

Viessmann were blunter: you cannot use W/C with S-Plan.

Oh yes you can & I sent them the wiring diagram to show them how it should be done. Would you like a copy?
The call signal to the cylinder demand box is from the HWS two port end switch so adding a second cylinder should not be a problem but you may need a relay to separate the cylinder satisfied's to power the heating, looks do-able & would save all this with the by-pass.

 

[decomplexity]

Wiring diagram ? Yes please - via Contact.
I also had an 'interesting discussion' with Viessmann about this.
Good boilers though!

 

[chris watkins]

As requested

 

[decomplexity]

Cunning! So as soon as DHW cylinder demand is satisfied – but not before, CH can heat if (and only if) the room stat demands, and this is independent of W/C. This leaves W/C to ramp the flow temperature up or down in line with the output from the external sensor.

In effect, this creates DHW priority without e.g. needing a diverter valve – just the additional wiring link back to the programmer from the cylinder stat’s “satisfied” terminal and a link in the programmer itself from DHW 'off' to CH selector. It also allows W/C and room stat to coexist rather than the W/C superseding the room stat which seems to be Viessmann’s intention.

Have I understood correctly?

 

[chris watkins]

Yes very good, in basic terms if there is a demand for hot water the boiler flow temp is ramped up to 80 degC & the heating is disconnected, as soon as the HW is satisfied & there is a demand for heating the flow temp is controlled via the W/C & the particular slope selected on the boiler sat. I use 4 as I have an old 1930's house, although I have done my best to insulate the dumb W/C would not on it's own give tight enough control (unless heating was left on) so I kept the digital stat & it all works well.
I have the HW set for 4omins before the heating is required (AM only) & timed heating morning & night. The only time there is a problem is on the odd PM heating when it is a cold winters early evening as the internal temp has dropped but the outside is still relatively warm but dropping fast. It just takes too long for the house to get to temp but it isn't a problem we just move the boiler stat up to increase the water temp, as the boiler is in the kitchen this isn't a problem.
For what the W/C kit cost it was a no brainer to go with it.

If you have 2No. cylinders it could still be made to work by using a small 240V relay so if there is a demand from either the satisfied from the stats is only allow on to 5 when both cylinders are up to temp.
If you need any help with that let me know.

 

[decomplexity]

Perhaps CW could kindly clarify one point re “wired so heating valve only closes when there is a demand for HW in that way there is always an open pathway for pump overrun even in the summer”.

If in the circuit diagram DHW demand is satisfied and then either the programmer or room stat turns heating demand off, both MVs would appear to be shut. The boiler will be notified of this no-demand via the link from terminal 12 of the wiring centre but overrun would appear to start with closed MVs.

 

[chris watkins]

You are quite right re the heating valve but you have to remember what the over run circuit is for, in this case it is only to dissipate excess heat from the heat exchanger (not for the pump), I believe it only comes into operation if the water temp rises in the HX & the high limit stat is in danger of going to lockout.
If the programmer or stat turn off the heating it is likely that the system is already up to temperature, with the boiler running on its lowest modulating setting. As it is under W/C control the water temp is 50 - 60deg C max.
The pump is under pressure control so likely to be running at reduced speed as TRV's are closing a rooms get up to temp.

I have been thinking back to when I first put it in, the system had an ABV installed for the old boiler but I wanted to use the speed control of the new pump so I closed it to see how it all would work.
All I can tell you is it has been working fine for >7 years now.