Adding multiway switching to a boiler ...

5 May 2019
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United Kingdom
Just moved into and old house and trying to get the CH and HW bill under control.

The CH and HW run on two separately pumped zones but re-combine into one at the boiler for heating.

Current programmer has HW on as a constant default, with the option to have CH on in addition.

So the HW needs to be on to also then call for CH.

I am installing a Wiser smart therm with 2 zones.

The HW zone works fine as 'boiler live' and 'pump live' are connected (fires boiler and pumps HW to cylinder)

The CH zone doesn't (it cant fire boiler as not connected to 'boiler live' but does pump the CH circuit)

How can I insert some logic (relay?) to allow the smart thermostat to pass switched live to the boiler via EITHER the CH or HW live output from the smart control so I can run CH and HW separately.

Pic attached to help explain!


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Yeah I fitted a 5 pin 240v relay to sort this issue.

And a hw stat for economy.

Normally water valves with end switches would do the job , but you may not want to fit them
You had better concentrate on the replies in the Plumbing Forum as they are questioning the set-up.
The way I connected relay was.

Contact common out to boiler.
NO to live feed
NC to HW stat (then hw timer)

Coil CH stat
Coil neutral

Very similar to your diagram!
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It is unusual to have two pumps, and I suspect that the HW was originally gravity fed.

You need to investigate the piping to ensure their is adequate expansion/pressure relief, before altering the control system.
A smaller relay is the Finder Part Number which plugs into socket Part Number 95.83..3 which has screw terminals.

This combination fits nicely into a two module enclosure such as


NOTE when selecting relay and socket be aware that some relays have pins on a 5mm pitch (40.51) and some have pins on 3.5mm pitch (40.31)
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... Normally water valves with end switches would do the job , but you may not want to fit them
Indeed. Whilst, if a 'quick/easy fix' is what is required, then what is being proposed/discussed would work, it would seem far nicer to get rid of one of the pumps and install two motorised valves (or even just one 3-port valve). As I see it, that would offer several potential benefits:

1...Only one pump to go wrong (and, indeed, one would gain a spare).

2...Probably a little cheaper to run, since only one pump would be operating when both HW and CH were required

3...Maybe not an issue, but would eliminate any possibility of some hot water 'leaking' into the radiators (by convection), through the (non-operating) CH pump, when only HW heating was required.

Kind Regards, John
Mid position valves are kind of out ruled these days.

While motorised valves would offer economy on HW system. I wouldn’t bother removing a pump.
running the system less (adding controls), is only likely to make it safer.

How do you know it's safe at the moment? For example, I wonder what would happen if the how water pump valves were closed.
How do you know it's safe at the moment?
As I think Andy was probably implying, no matter what the answer to that question, it is (at least for me) hard to see how doing what the OP proposes could make it 'worse'/'less safe'. After all, he will not be changing anything about when the pumps are operating, but merely changing when the boiler is fired up.

Kind Regards, John
I found to my cost, turning pumps off/on does not work, thermo syphon still takes place on upper floors even when pump is off, most important is for a by-pass valve on central heating, as if pump is running water must flow, on the domestic hot water not such a problem any thermo syphon will just keep water warm.

But motorised valves have micro switches built in, the pump does not, so you have no way to tell boiler if required or not, so you will need some relay. But next before working out how, is look at the boiler, with an oil boiler may as well fit two motorised valves and use the micro switches to turn boiler on/off, but with gas, often the boiler is designed to modulate (turn up and down) oil may modulate but 18 kW to 24 kW is not worth worrying about, 6 kW to 28 kW from gas boiler is however some thing needing consideration.

So with a gas boiler with no thermostat on the wall the central heating water is controlled by TRV's (Thermostat radiator valves) on each radiator and as they close the return water raises in temperature which tells the boiler to reduce output, the by-pass valve will open if all radiators as switched off, and once the boiler has modulated fully, then it starts to cycle, anti cycle software reduces the cycling but will not stop it. So a wall thermostat is often placed in a room which is kept cool, is down stairs, has not alternative heating, and no outside door, if it exists, if not the hall is often used.

However with some gas boilers you can gain access to the bus, this means you can electrical turn down the boiler, not off/on but down/up, as to how this is used depends on boiler, some like Bosch you have to use their thermostat, Wave, but others allow third party called OpenTherm.

So you can have with some boilers where each radiator has an electronic head on the TRV which tells a central hub what heat is required as well as controlling flow through radiator and in turn the central hub tells boiler what output is required, so when boiler turns off it is already cool so less energy lost through the flue.

However depends on boiler, some times best option is some thing like the Terrier i30 heads where you can set temperature and time you want that temperature for each room, and boiler is controlled by return water.

There are many other methods, Myson for example make fan assisted radiators where instead of using TRV to control output it controls fan speed, ideal in kitchen as will fit in kick space, bit expensive to use else where.

I used MiHome Energenie TRV heads, and yes they worked, better on supply side of radiator, on return very careful control of lock shield valve is required, I had intended to fit a Nest thermostat with will work with Mihome, then found my Bosch boiler is not OpenTherm enabled, so Nest was never fitted.

So design your final system, work out what will work with your boiler, then decide what order to fit the bits, with a by-pass valve there is no need for a motorised valve to turn off radiators, the TRV's will do that, if you can use a modulating thermostat no point in using old fashion digital (off/on) and if the boiler is modulating without assess to bus, then you don't want a thermostat with anti hysteresis software, since all good wireless have anti hysteresis software, you need a hard wired thermostat. Cheap wireless thermostats don't fail safe, batteries got flat and heating may not turn off, so don't use cheap wireless, but cheap wired are OK.

I jumped into mothers central heating thinking I knew what to do, as mine worked fine, but my house was open plan, one thermostat down stairs and TRV's upstairs it worked great, however mothers house had doors and bay windows, the morning sun could over heat living room, and evening sun the sitting room, it required a different approach.

Tado and Evohome do get close to one system fits all, but expensive, and for people who go to work in the day, speed is important, as the geofencing detects your phones it wants to heat up main room in half an hour, so needs a fan assisted radiator to get that speed, but a retired couple may prefer under floor heating where it takes hours to heat up and cool down. So it has to fit what you want, in my new house using oil, so modulating thermostats really pointless, 18 kW is far too much most of the year, so has to be off/on system.
I found to my cost, turning pumps off/on does not work, thermo syphon still takes place on upper floors even when pump is off...
That was, of course, my point (3) above - but, as I implied, I was not sure to what extent it was an issue in practice.

I would assume that whether or not a significant amount of heat can 'flow' through a pump which is 'switched off' (other than that due to conduction through the casing of the pump) will be crucially dependent upon the design of the pump and, with some designs, the actual ('mechanical') position in which the pump has stopped?

Kind Regards, John
As a PS I did look at the idea of fitting a MiHome relay to turn heating off, theory one can use IFTTT (If this then that) to tell the relay when all TRV heads are satisfied then turn off boiler, however in practice could not find apps to do it, maybe now apps are written? but not sure I want to rely on the internet for my central heating?

I have not been impressed with MiHome as until you buy it, you don't know what it can do, I bought a socket to turn an extruder alarm off/on wanted 4 times per day, but it only had 3, had to use IFTTT to get extra time. Other makes are just as bad, it seems to be pot luck. It seems even the pros get it wrong, my central heating in this house has a by-pass valve next to boiler, however the boiler has a by-pass valve built in, and TRV's fitted to return on radiators rather than feed, so radiator gets hot then the TRV gets hot so temperature over shoots, the valve is bi-directional however if radiator gets hot before valve senses it does not matter how good the valve is, it will over shoot.

Hind sight, and hind sight is easy, swapping TRV to supply side and using Terrier i30 valve heads would likely have worked better than the expensive Mihome heads, in this house, however had the boiler been OpenTherm then the MiHome would have worked better.

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