Just thinking about our holiday let, want to prevent loss of pressure from use on our side, so can I pop another cylinder on the boiler (Worcester greenstar oil boiler)
Heating will be zoned and fettled via interweb thermostat) I guess cold water pressure is a further conundrum….

Gravity fed systems with a top up tank – I guess you could but with some rather complex plumbing and you would need some sort of 3 way valve on the circuit to heat the tanks. pressure restictor in the cold side to each part would help as well with stopping one shower effecting the other but two complete systems would be better

I am not a plumber

Purely on the basis of the commercial systems I’ve seen where #boilers doesn’t = #cylinders then I’d have thought so

Can’t see why not. TJs solution probably wouldn’t work as unlike balancing radiators the load wouldn’t always be equal. E.g. the holiday let is empty you don’t need to be pumping hot water through it and heating it above the set point just to top up the heat in your own cylinder.

You just need a 2nd mid position valve and a 2nd controller. Someone probably makes a dedicated one but the generic Honeywell one would do it, just call one cylinder “heating” and attach the cylinder thermostat to it.

IANAPlumber

Hang on. I don’t think it’s that simple, unless there are already dedicated setups for it.

You would need separate control valves for both of them, much like the heating and hot water in our system, but you’d have to bodge the heating side, and you’d have to clearly label everything in such a way that it never falls off. All the controls on the controller would be all about heating, the little icons on the screens etc.

You’d have to check the regs as that has ‘hack’ written all over it.

As far as boiler is concerned all you’re doing is adding another zone, just a hot water zone rather than a central heating zone. There’ll be the associated plumbing involved with having an unvented cylinder* but no reason it can’t be done.
*or vented, either would work.

It’s also part of the building regs to notify the council of the installation of a hot water cylinder, so don’t forget that if you do go ahead.

Kewl, would like to not have to buy a second boiler!

Depending on other factors, might it be easier just to run the showers off an immersion heater? Might cost a few quid a week to run, but offset against the holiday let income and it not sitting there wasting heat on the empty weeks (although I suppose it’s just flicking a switch either way).

+1 for immersion heater in the cylinder.

Surely a lot simpler and isn’t electric cheaper than gas these days?

We’re on oil. Would like to weft solar thermal in there at some point so will spec the cylinder accordingly but cost would prevent that right now.

You just need to suss out the logic of all the different configurations and how to control them. Should be too hard in theory but the control system might be tricky to pull together assuming there is nothing already commercially available. You just have 4 states: no heating to either cylinder, heating to both cylinders, heating to cyl 1 only, heating to cyl 2 only. Individual cylinder stats can trigger motorised valves to divert heating water to the cylinder coils and trigger demand for boiler to fire up. Don’t see why it would or should be massively complicated.

Need to establish if your boiler is big enough though for times when both cylinders are demanding heat and the central heating simultainiously.

Could be done relatively easily, I think. Wiring (and plumbing!) the system in parallel would allow either tank to run the boiler until they’re both satisfied. Never tried it myself so don’t know if there would be balancing issues, but this could be remedied with automatic valves in line to each cylinder.

You won’t be able to control them independently with an off-the-shelf wifi thermostat without creating two separate “houses” in the control app, so make sure it has that capability before installing.

Probably easier and cheaper to use an electric shower though. When you say “our side”, is it like an annexe or something?

I have done this, to be more accurate had somebody competent put my idea into operation, about 8 years ago when running a small hotel. The context was to switch from gravity-fed to mains pressure and to renew some rather knackered infrastructure. The problem, to some extent, was high variations in occupancy, from a low of just my wife and I during much of the winter, to a maximum of 15 guests all wanting roasting hot showers at 7 litres/minute for parts of the summer.

The solution we implemented was to have two large (300 litre) unvented cylinders heated indirectly by a coil from a oil boiler. There were isolation valves for the coils and for the stored water. The manufacturer of the cylinders was involved in the planning; short answer was that they were not aware of anybody having tried this approach, could see why it should work for us, and did not anticipate any problems… no problems encountered.

Happy to share more details if they would be useful.

@julesf7 Sounds pretty spot on, might not need the full 300 but yeah the principles should be the same. (You’re going to tell me this was expensive aren’t you 😬😬😬😬😬😬😬)

It’s also part of the building regs to notify the council of the installation of a hot water cylinder, so don’t forget that if you do go ahead.

Is this right? We’ve just had a new bathroom put in and the installer changed our system from a vented tanked one with an unused immersion heater in the airing cupboard, to an unvented system with a whopping great tank in the loft. Nothing was mentioned to us about needing building regs approval?

Yes it is just another zone, so you end up with S plan +, which can have as many zones as you want in theory.

Maybe if unvented you should consider an accumulator to help flow rates unless it is suitable.

All very easy to do and any competent plumber should be able to advise.

Heating two boilers independently is not that difficult it should only require an additional timer, thermostat and motorised valve.

The challenge might be isolating the tank outputs so they only allow flow when hot. If you don’t have this in place then you could be combining the output from a hot tank with the unheated one.

Another issue might be if one tank is only in intermittent use is the growth of legionella. Also another good reason to prevent the flow from a unheated tank.

It might be worth simply looking at a larger tank and having a longer heating window.

It is simple to heat multiple cylinders, too much overthinking.

It is simple to heat multiple cylinders, too much overthinking.

+1

Boiler doesn’t know nor care how many HW cylinders it’s heating, just wire them in parallel with their own solenoids and thermostats.

Nothing was mentioned to us about needing building regs approval?

plumbers can self certify an unvented installation, DIYers cannot do them

If you’re DIYing, a vented gravity fed setup with shower pump would work. controls wise, it’s just an extra zone, not a difficult addition to an existing heat + tank setup.

It’s dead easy, just another zone.

You might need a new controller with an extra zone (if they are all used on the current one), another valve feeding the new tank and wiring up.