Anybody have any links or pointers to controls/controllers for setups using 2 (or more) boilers to supply larger-than-usual loads? I recall someone on this list did such a thing for a church he's involved with. I'm looking at an installation which has a couple of Baxi Solo 2s but no integration (just a user-unfriendly array of switches and valves to operate to bring one or both into service as required).
I'm inclined to just have one boiler on load all the time and an outside thermostat to bring the second in during cold weather (sort of weather compensation scheme) but it would be interesting to know what's available (and how much it would cost) to do the job 'properly'.
Yes, that was me. We did it the cheap and cheerful way as a proper sequencer would have cost four figures, not justified, and the ones I looked at were more suited for a Mon-Fri 8-6 heating regime rather than the varied times and temps you have with church heating (we had 18C for services, 19C for longer sitting still meetings, and 15C for flower arrangers and cleaning).
I've just dug out an old pipework schematic and uploaded it as
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haven't got a circuit diagram for the controls unfortunately, but basically a CM67 master controller goes went a two-way switch to the boiler A and boiler B call for heat terminals. This switch set the lead boiler. Between the switch L1 and L2 terminals were two thermostats in series - if both are closed the second boiler runs too. The first was a second CM67 set around 0.5C lower than the main one, also (for our usage) time/temp set to kill the second boiler around 20-30 minutes before the end of a heating session, the second a pipe stat at the bottom of the boiler shunt pipe which opened if the return temp rises above (depending on setting) 55-60C - which it would if the fan convectors were running at low speed or not all on.
For someone with the electronic knowledge to make a controller, an alternative control algorithm might be to pull in the second boiler if the first has been running for 10-15 minutes and to hold it off for a similar period when the call for heat stops. From cold this would give a slower initial warm up but once the building was at a temp where you needed one boiler (or less) the second would never get called. Might run into problems though if the first boiler modulates down to 80% (being all that is needed on a mild day) and the second boiler is then needlessly pulled in.
If both boilers are fan flues there isn't going to be a significant heat loss via the idle unit. I'd be incliined to simply parallel the two (might need a relay in the call for heat circuitry depending on the boiler innards) and set one boiler thermostat higher than the other so it does the majority of the work. Not perfect but simple.
In theory you need them to stop reverse circulation, but the resistance of the 1.5m 42mm shunt pipe (aka low loss header) is next to nothing compared with the 22mm pipes back to the other boiler, its internal pump and its heat exchanger.
Point. Might just try that first. Though with standard-efficiency boilers (which is what we have here) where the boiler's thermostatic control operates on the flow temperature I suspect the one set to the lower temperature would just cycle on and off, which is not ideal.
Yes, simple is best (less to go wrong) but also, on reflection, flow in the off boiler would behave just like the flow in the shunt pipe so no degradation in performance.
I'd like to think that the second boiler would only be required for fast warm up and during the coldest period of the year so you could extend the life of the installation by only running 1 except when absolutely necessary and periodically swapping which one is the primary if you want to balance the wear.
Say only have boiler 2 on when the return temp is below a certain level or in combination with an outdoor temp stat? Simple relay logic control could manage that.
Watch out for over complexity and maintainability, I am reminded occasionally of this when kind people ask who would maintain x, y & z if I got run over by a bus tomorrow. The answer is universally, "I wouldn't give a f'ck" but you may be more concerned :-).
Think you would be better off monitoring the flow and return temps and fireing both boilers until the difference got less than say 10C then drop one of the boilers off. The logic can probably be done with simple series connection of flow/return pipe stats, they might need a change over switch but fairly sure they all have that.
Back feeding one boiler to the other can be controlled with a 2 port valve wired in the normal way, call > valve > valve switch > boiler.
I like the system Mr Bryer posted, particulary the switch to set which boiler is "lead". When one goes phut it's simple to use the other albeit with reduced capacity on the system.
Finally it might be worth looking at a PLC (Process Logic Controller) to do the logic and switching rather than hard wired stats and relays. The heating/HW system here is effecively four stat controlled zones, but with two pumps (2 zones on each pump) from a single boiler with pump overrun. It works but the logica isn't perfect, if the pump overun is active for one zone and another calls for heat the previously active pump overun doesn't drop out. I could probably add more relays, it already uses six, but if I'd gone for a PLC it would have just been a tweak of the porogramming.
I think we're going round in circles here - I originally muttered that I was thinking of using an outside stat to determine when to bring the 2nd boiler in with the 1st!
Indeed. That's rather the situation I'm in now: seems whoever designed (or perhaps "designed") the original system didn't leave (or have :-)) much clue as to how it was supposed to work so I'm trying to work that out now from what I can see.
You could use an "intelligent relay" here. You can get them with 4 n.o. mains-rated contacts and 6 or 8 digital inputs. They include built-in time switching, timers, logic etc. unfortunately the programming software and lead is usually charged for, but not as much as for a proper PLC! Most manufacturers do their own flavour of these. Have a look at the Crouzet Millenium 3 range. This is a complete kit:
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course, a real tech-head would build a board with a PIC and relays on it and program it using LDmicro ladder logic for free!
Interesting. Probably overkill for this application but not enough for something like the wood-burner + boiler controller I built recently which more than half a dozen outputs to pumps & motorised valves.
Presumably the programming s/w is proprietary, closed-source and windoze- only?
I thought Arduinos were current flavour of the month? :-)
But even if I were au fait with stuff like that it wouldn't be doing any favours to my clients, who would have to find someone else to maintain the system if I got run over by a bus.
I forgot to add that I have an aquaintance who lives in an ancient pile and has a two boiler system. I've never asked him how it works, but he is a microwave engineer and his solution to one piping problem was to run the return concentric with the feed. I perhaps would really want to know how the system as a whole works - I saw the boiler room once and it was like something out of Para Handy's puffer.
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