Central Heating Controller CH OFF terminal

[casparcat]

I am currently specifying a system that will provide back-up CH/DHW to a proposed solar thermal installation. The current boiler setup is W-Plan (CH-ON powers the valve giving CH only, 3-wire Cylinder Stat "call" cuts CH-ON power to diverter and diverter goes to DHW only).

The Solar DHW will be provided by a Plate Heat Exchanger from the Thermal Store, but with an S-Plan DHW zone valve to call the DHW circuit from the boiler as back-up, activated by a two-wire flow switch. While a 3-wire Cylinder Stat could cut the live feed to the CH Zone valve while simultaneously powering the DHW valve (so diverting boiler all heat to DHW) it would seem that I will need to specify a relay to achieve the same with the two-wire flow switch in an S-Plan circuit, (or can it be achieved by using the CH OFF terminal of the controller)?

 

[matt1e]

I am currently specifying a system that will provide back-up CH/DHW to a proposed solar thermal installation. The current boiler setup is W-Plan (CH-ON powers the valve giving CH only, 3-wire Cylinder Stat "call" cuts CH-ON power to diverter and diverter goes to DHW only).

ok understood so far, as for the next bit, confusing! (flow switches ??) so I have tidied it up and altered it to what I think you mean

The Solar DHW will be provided by a Plate Heat Exchanger from the Thermal Store, but with a zone valve to call the DHW circuit from the boiler as back-up,
boiler activated by the valves end switch
While a 3-wire Cylinder Stat could cut the live feed to the CH Zone valve while simultaneously powering the DHW valve (so diverting boiler to DHW only)
it would seem that I will need to specify a relay to achieve the same as the valves end switch in an S-Plan circuit
(or can it be achieved by using the CH OFF terminal of the controller)?

Maybe, but a schematic and discription of your proposed layout would help

Matt

 

[casparcat]

Thanks for the encouragement.

The zone valves are on a conventional S-Plan layout on a 10-way board with a controller on the other side. The power feed is taken off the leads to a (removed) analogue timer. The old Flowsure timer has a 6-way terminal block which is actually [1/6:HW-ON][2/6:L][3/6:S/L PumpL1][4/6:X][5/6:N][6/6:CH-ON].

The 1/6:HW-ON is the live feed to the Flow Switch, which somewhere in the relay must switch the live feed to the diverter valve off, to give DHW priority - if I could understand where that is, I could use it to close the feed to the CH in the S-Plan.

I have tried to show the idea by blending a Flowsure diagram with an S-Plan one, with a box for the new relay [Relay HW-On = CH-Off and HW-Off = CH-On]. I have also attached the Flowsure process diagram, so you may be able to work out if the required relay switching is already there.

Hope that makes more sense now?
View media item 42324View media item 42325

 

[matt1e]

The zone valves are on a conventional S-Plan layout on a 10-way board with a controller on the other side.

The 1/6:HW-ON is the live feed to the Flow Switch, which somewhere in the relay must switch the live feed to the diverter valve off, to give DHW priority - if I could understand where that is, I could use it to close the feed to the CH in the S-Plan.

I have tried to show the idea by blending a Flowsure diagram with an S-Plan one, with a box for the new relay [Relay HW-On = CH-Off and HW-Off = CH-On]. I have also attached the Flowsure process diagram, so you may be able to work out if the required relay switching is already there.

Hope that makes more sense now?

No not really mate, a planned system layout is what I meant ie boiler, thermal store solar pipework etc
the two drawings make sense though
one shows a typical S-Plan system (you said it was w-Plan earlier BTW) and the other is the boilers internal schematic, which is an oil fired combi

Is this boiler also heating a cylinder/thermal store at the moment or are you planning it to?
the flowswitch, relay and divertor valve are only necessary for the instant HW operation of the boiler, the "HW on" connection on the block just isolates the HW function of the boiler

I am currently specifying a system that will provide back-up CH/DHW to a proposed solar thermal installation

Is this for a customer ? No offence but I think you may be out of your depth

Matt

 

[casparcat]

Is this boiler also heating a cylinder/thermal store at the moment or are you planning it to?

It is not (yet) heating the thermal store - that's why I am trying to sort out some diagrams to get quotes for solar thermal. However, it's in anticipation of the solar thermal that when the Flowsure diverter valve died (and I had a heart attack at the price of a "Baxi Special" replacement, that I put in the S-Plan zone valves. At the time, I had failed to notice that the Baxi/|Myson "3-way" was in fact a two wire diverter W-Plan (ie no DHW-ON call wire). In-for-a-penny-in-or-a-pound sourced an S-Plan kit with a modern digital programmer, so the primitive Flowsure electro-mech clock has also been replaced.

After a lot of prodding, Baxi finally responded with advice that the Flowsure will work in this configuration, as long as a relay is inserted so that the Flow Sensor "DHW" call can open the DHW zone valve and close the CH zone valve at the same time, to send all the boiler heat to the plate heat exchanger.

[No offence but I think you may be out of your depth?

No offence taken - as you are probably correct. I am still trying to figure out the Function Flow diagram and the MT326230 Relay logic, newver mind trying to see if the sort of relay I think I want even exists ... !!

 

[matt1e]

Is this boiler also heating a cylinder/thermal store at the moment or are you planning it to?

It is not (yet) heating the thermal store - that's why I am trying to sort out some diagrams to get quotes for solar thermal. However, it's in anticipation of the solar thermal that when the Flowsure diverter valve died (and I had a heart attack at the price of a "Baxi Special" replacement, that I put in the S-Plan zone valves.

After a lot of prodding, Baxi finally responded with advice that the Flowsure will work in this configuration, as long as a relay is inserted so that the Flow Sensor "DHW" call can open the DHW zone valve and close the CH zone valve at the same time, to send all the boiler heat to the plate heat exchanger.

I am still trying to figure out the Function Flow diagram and the MT326230 Relay logic, newver mind trying to see if the sort of relay I think I want even exists ... !!

right I think I know what you are getting at but I dont think baxi do
I take it the diverter in the flowsure has failed and because of the price of replacement and the fact that you want some sort of solar preheating a termal store , you also want the boiler to heat the store,
I think baxi think that you want the boiler to heat the store and fit a plate heat exchanger in place of the internal flowsure one but with the same configuration ie you open the tap ,the flowswitch operates, powers the relay which in turn opens a valve on the flow to the now external heat exchanger while olso closing off the flow to the store
it will be a lot easier if you tell us what thermal store you are planning on getting then better advice can be given on wiring etc
I will explain the relay logic if you like

Matt

 

[casparcat]

if you tell us what thermal store you are planning on getting then better advice can be given on wiring etc
I will explain the relay logic if you like

The intention was to get round to that a bit later - but with the Flowsure having blown a fuse and out of action, life was a bit chilly - and an S-Plan "kit" is easy to source as (theoretically) a quick way to get warm again, rather than shelling out (and waiting) for a Baxi/Myson special - which in any case looked like a swine to replace with it's non-standard pipe fittings.

Two flaws in my original logic became apparent after the "dead" diverter was out - one that the Baxi valve was two wired only (I had just assumed that all three way valve had four wires that could be re-routed to give S-Plan separate CH-On and HW-ON) and secondly that the Baxi electro-mech controller would at least have standard HW-ON & CH-ON terminals that could simply be re-wired to an external controller. Wrong on both counts.

Re-wiring in the new external controller has not been a problem as the old controller L&N have been fed out to S#1 & S#2 and the S/L from the old 5 wired out to S#4 to give CH-ON, so I do at least have a working boiler with timed heating (and DHW with manual valve changes).

From what I have (had to learn) about W-Plan diverter two wire valves, I think they are simple beasts, with "power on" routing priority to CH and with "power off" closing the spring and routing to priority DHW. With the Flowsure the default is where the clock#5/constant switch powers the diverter for CH priority. To turn this off, the DHW "call" from the flow switch must presumably interrupt the CH-ON line - possibly through the built-in relay?

That's the wiring I would like to understand and/or use before looking to Baxi's quoted solution of sourcing an external relay and a CH savy sparks to enable emulation of the W-Plan switching by bridging on the CH-ON and HW-ON on the standard S-Plan 10-way block.

If you can explain the relay logic that might help, and reduce the time needed with an avo to work it out.

 

[matt1e]

ok mate will do but have got a rehearsal tonight so will do it later on

Matt

 

[casparcat]

... so will do it later on

Matt

Thanks but no rush - the heating's back on now which was the original pressing need

 

[casparcat]

....Thanks but no rush - the heating's back on now which was the original pressing need

Had some time to look at the wiring and "guess" at what the relay is doing to the CH "call" and HW "call" wires.

Looks like I was hunting in the wrong place for how the Flow Switch closed the Diverter S/L - I think that it achieve the same effect by actually cutting the Diverter N between 10>2 on the relay.

While not really understanding the relay switching, I have tried a bit of lateral thinking on how to power the S-Plan zone valves to replicate the old W-Plan feeds to the relay.

By taking the existing live feed ex-diverter to the CH Zone-L (br) then returning the CH Zone-N (bl) via the ex-diverter common, the diverter connections to the relay should still work as originally planned.

In the same way taking the S/L feed for the Flow Switch to S-Plan#6 where it gets power from the Programmer's HW-ON, this can feed the HW Zone-L (br) with the HW Zone-N (bl), which in turn can be fed back to the Flow Switch N, which in turn feeds Relay 2.

I shall try that scheme tomorrow with an avo to see if it cliks the right switches in the correct order.