2 x central heating boilers wiring ??

Thanks for the update. If you don't want to use the electric boiler to heat the hot water, are not going to use it as part of the S-Plan and have just piped it straight to the radiators. From a controls aspect, I can see that would work.

From a plumbing point of view, when you use the oil boiler for the heating, and the motorised valve is open, what prevents the water taking a short cut back through the electric boiler to the return, rather than flow through the radiator circuit?

By second thermostat, I assume that you mean a second room thermostat that is only controlling the electric boiler. What make / model of thermostat is it? If it was all 230V it would be fairly straight forward, but the voltage free and 230V difference in boiler control voltage makes it more tricky, unless the second thermostat has voltage free contacts.

 

Plumbing. I fitted 5 ball valves that you can shut boilers inlet and outlet. I piped oil boiler inlet and outlet together and fitted 5th ball valve on the linked pipe which in normal operation is closed and open only in the winter when electric boiler is in use so when temperature lowers below 5C oil boiler can work on very short loop preventing freezing.

Electric. 230V to 0V. Switch boiler on wire (no 12 in my junction box) is connected to 2 way switch instead of the junction box. Goes back to disconnected wire to switch oil boiler and send 230v. For electric boiler. That is where second thermostat comes in placed. 230v is sent to receiver. Neutral and earth are connected back to junction box. Then 0V signal from receiver is connected to the electric boiler as it would be connected in any modern gas boiler to turn it on. Room stat is set for max temp so it is always on.

Both boilers are literally controlled by hive and on/off light switch between two. So simple but took some time to figure it out.

 

So you are just using the new thermostat's receiver as a relay to provide a voltage free contact. It will work OK, a relay would have been simpler and cheaper though.

Personally, I would be a bit worried about the manual valves because it would be easy for someone to have the manual valves set to a different configuration to the controls. It wouldn't have been that difficult for electrical engineer to have automated it so that you could have a fully working S-Plan that could have been interlocked and give full auto control. But if you're happy with it that's what matters.

Once it's been up an running for a while, and you have time to compare 'like for like' data, I would be interested to know what the difference in running costs before and after are.

 

I have been trying to configure my boiler wiring, the only way is to draw it out, then consider the logic will it do what you want. My problem was the original system used the switching of a pump to select domestic hot water only, domestic hot water and main house heating, and domestic hot water and flat heating, and all running.

It has been found the water flows in reverse direction in other central heating system once a pump starts, and once heating pump turned on it will continue with thermo syphon when turned off again. So plumber is due to fit motorised valves, but even then we can't turn off domestic hot water if central heating running, and when only domestic hot water is required the only control is the time the boiler runs for, found one hour a day is ample.

However I have one big advantage over you, I am using Nest which has the com, N/O and N/C for central heating and domestic hot water not connected to mains, so I have DHW common feeding the boiler and N/O has line feed, and N/C connects to line through the micro switch in the motorised valve.

But without drawing it all out I would have never followed it.

So I started with a photoshop image of pumps, motorised valves, heat link, boiler and thermostat and wired it as I thought it would work, then constructed a truth table and coloured wires red for each option to see what would happen.

So first some safety considerations to ensure nothing burns out.
1) If micro switch in motorised valve fails will a by-pass valve open?
2) If you turn off boiler, is there some where for the heat already in the boiler to go?
My system fails on first count, if micro switch sticks the pump will likely cavitate, I am taking a chance as simply no room for by-pass valve, the second is OK as domestic hot water is thermo syphon so any heat in water on switch off will heat domestic hot water.

So truth table.
C/H main house off, C/H flat off, DHW off = boiler off.
C/H main house on, C/H flat off, DHW off = boiler on, C/H main house on, and DHW heated anyway.
C/H main house off, C/H flat on, DHW off = boiler off, flat will not be heated.
C/H main house off, C/H flat off, DHW on = boiler on and DHW on.
C/H main house off, C/H flat on, DHW on = boiler on, C/H flat on, DHW on.
C/H main house on, C/H flat on, DHW off = boiler on C/H main house on, C/H flat on, and DHW heated anyway.
C/H main house on, C/H flat on, DHW on = All on.

So you need the same truth table for your system, and if there are manual valves then consider what happens if a valve is not turned off or on when you change over, as the system gets more complex then there comes a point where a PLC may be the best option, but in essence the Hive or Nest is a PLC or building management system.

One option I have missed, is if the immersion heater is switched on what happens? Specially if boiler turned off for example because of low fuel levels. In my case the FCU switches off all pumps and motorised valves, so there will be no flow through central heating, however the original set up had three FCU's so the pump could be run without the boiler, as to if the pump could suck hot water from the hot coil and heat the radiators with it I never tried.

I think if I was designing your system I would be using motorised valves to switch between the two boilers and would use the built in micro switches to ensure all swapped over.


So this is my system you need to draw out some thing similar for yours. As to if my system works I don't know, as yet still waiting for plumber, but it works good enough with a bridge wire, until plumber has done his stuff.

 

You will have probably taken this into account when installing the manual valves, but I thought I would mention it just in case. Nothing should be fitted into the boilers direct route to the open vent that is there to allow for the expansion of water. If a boiler is switched on without an open vent to allow for expansion, it would create a very dangerous situation. The vent pipe should always be completely open. Even the pump & motorised valves, will be teed off the side of it, so that the vent is completely unobstructed.

 

Yes. I did that for the electric boiler which has separate outlet with a 3 bar safety valve that is piped unobstructed back to the top of gravity tank.

Haven't done it yet for a oil boiler which at the moment won't be shut off for the safety and electric boiler will be heating it unnecessarily but I can live with that. It is only few meter of pipe and boiler itself. Boiler has a way if expansion to top of gravity tank unless I close those valves.

I can always add very small circulation pump and expansion tank for the oil boiler loop to work in a close system.

 

Well thought system, My system works, might not be the best and I might update it over the time but so far so go. To answer your questions.

1) What ever happens to my motorisdes valves all hot water will be pumped to the top of expansion tank which is in the loft. If by any chance to much water accumulate there it will be drained through the pipe to outside of the house so I'm safe here.
2) Electric boiler has a heat switch that will kill the boiler above 90C, no idea about the oil boiler. and again if pump works, all water will go back to the tank in the loft.

 

It should not go to the expansion tank, that tank is often made from thermal plastic and if it gets hot it can fail, see this report you can get thermal setting and metal tanks, for when solid fuel is used.

It is not the personal safety but boiler safety I was referring to, boilers need to cool, there are two basic methods, one is a run on timer, the other is a thermo syphon with no valve, I have latter, not good as it means domestic hot water in winter is same temperature as central heating water which is really too hot.

There were some boilers where it was simply turned off, however I am not a heating engineer, I'm an electrical engineer so not really my field. I note with my boiler the domestic hot water can be controlled by a motorised valve as long as it does not also remove the connection to top-up/expansion tank, in my case the point of separation is in the airing cupboard so the motorised valve would also need to be there.

I am sure one could set up an over run system to allow boiler to cool. But one can't simply ignore the need to cool down.

I will admit had I not got a plumber to look at my system I may have fitted a motorised valve to the thermo syphon return in same way as fitted to central heating return pipes, however once he pointed out the need for boiler to cool I realised this was not an option.

As to electric heating of my home, should the oil boiler lock out and the immersion heat is switch on, then it may suck water through the cistern and use the immersion heater to heat the house, but electric wise easy enough to bell out wires and work out how it's wired up, but without removing floor boards etc, not so easy to work out how the system is piped up.

So I have two oil filled radiators for emergency, however in real terms more likely to lose electric power to oil power, so the real fall back heating is an open fire.

 

Never heard about the tanks failing so thanks for the info. Will get them check, although in my case it would not happen with electric boiler as I mentioned before it has a safety switch that will turn it off and boiler just cool itself down as kettle would do. If any excessive pressure will build up over 3bar due to heat a safety valve will open and send water at the top of expansion tank. We are talking here about maybe 2 litres of very hot water so it will not melt the tank after mixing with existing cold water.

Not sure about an oil boiler yet. Have to go through manual and see what the safety futures are implemented there.

Luckily our tanks are above bathroom and bedroom cupboards so not so dangerous for us humans. I can always replace them with something better. Im thinking of converting my open vented system with close system which would solve the issue. I can always run return vent pipe outside the house instead back to the tank. There is plenty options.

 

A bit confised by your last post, you say you are considering converting to a sealed system, but you also say that you have a 3 Bar PRV ? which is for a sealed system, where is the 3 Bar valve fitted ? is it part of the Electric boiler ?

 

Yep. Its a part if electric boiler. Outlet has 2 pipes. Second is controlled by 3 bar valve that goes unobstructed by any valves to top of the loft water tank.