Pipework for adding unvented cylinder to combi

Hello

I’m working on adding a couple of en suite showers but currently have a combi boiler which I fear won’t be adequate for the increased demand.

From what I’ve read it seems I might need an indirect hot water cylinder. Am I right in thinking a cold mains feed into it is heated by a coil that’s connected the central heating flow and return pipes? Then a separate electric immersion coil acts as a top up. Presumably this is the only way to get hot water in the summer when the CH is off. Can it run on a low rate overnight tariff or would it have to be Economy 7?

I’ve got an old airing cupboard where the cylinder could go but there’s no head room so it would be unvented. I know I’ll need a G3 plumber, and that the pressure and flow needs testing first but in anticipation I could run pipework to the cupboard while the floorboards are up.

I’ve got 22mm hot and cold mains water pipes and 15mm central heating pipes in the vicinity. It’s on an outside wall so the discharge pipe could go out easily enough. There’s already a dedicated electric circuit from an old immersion.

Can I just tee the CH and the hot and cold pipes into the cupboard and let the installer do the rest or will the pipes have to be split so the boiler still feeds some outlets directly? If so, how? Will there be anything installed at the boiler or will it all be in the airing cupboard?

Lots of questions I know, I just want to understand how it all works so I can be prepared. Thanks in advance for your help.

SJRSJR said:

Presumably this is the only way to get hot water in the summer when the CH is off. Can it run on a low rate overnight tariff or would it have to be Economy 7?

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You could still run your combi, in the summer, to provide hot water, but having an immersion heater available, could be useful in providing hot water when the combi is out of action. E7, is only usually worthwhile, when you have E7 for storage heating too.

15mm CH pipes wouldn't normally have the capacity to cater for a modern high recovery HW cylinder/coil, 22mm would be the norm, run from the main system 22mm backbone, controlled by a 2 port valve and interlock.

HW in the summer would probably still be cheaper to create using the boiler rather than electric, especially if it is used daily.

Thanks for the replies.

Madrab said:

15mm CH pipes wouldn't normally have the capacity to cater for a modern high recovery HW cylinder/coil

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So does it have to be 15mm all the way from the boiler? I could maybe run 22mm part way (first floor section) but it would mean too much tearing apart downstairs to get all the way.

Alternatively, would a direct cylinder be feasible? Could that feed just the showers, with the bath and kitchen running off the combi? I guess it would be expensive to have the immersion on all the time.

Madrab said:

controlled by a 2 port valve and interlock.

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Is that at the boiler or the cylinder end, or doesn’t it matter?

15mm pipework @ a standard heating circulation velocity (1.5ms) will typically carry around 5Kw odd of heat - a typical modern unvented cylinders high recovery coil capacity can be anywhere from 15Kw to over 30Kw.

So running the primaries to the cylinder coil in 15mm will restrict the cylinder to that amount of heat input, significantly increasing the recovery time of the cylinder.

Wouldn't want anything less than 22mm TBH

Madrab said:

Wouldn't want anything less than 22mm TBH

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Rob and others helped me install a HW cylinder and I ran 22mm pipework as primary circuit from boiler to cylinder coil. The boiler and DHW cylinder should be as close to each other as possible to minimize losses which improves recovery time. My 160 litre cylinder connected to a 19kw boiler takes about 20 minutes to heat up water to >40degC from mid 20degC. By the way, the boiler is not running at its full load during these 20 minutes.

Thanks. I had earmarked the upstairs airing cupboard for the cylinder especially as it was close to the en suites but perhaps I should see if it’s feasible to put one next to the boiler downstairs where it should be easier to pipe 22mm between them.

However, the room is a wall of 60cm wide floor to ceiling kitchen cupboards and it’d have to go in the second one from the exterior wall (the first one has the boiler).

It’ll no doubt have to stand on the floor so it’ll need quite a bit of butchering, assuming it’d fit in a 60cm wide cupboard in the first place. Plus there’s no dedicated electric cable.

Time to call a G3 plumber…

SJRSJR said:

I’ve got 22mm hot and cold mains water pipes and 15mm central heating pipes in the vicinity. It’s on an outside wall so the discharge pipe could go out easily enough. There’s already a dedicated electric circuit from an old immersion.

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You would normally have 22mm pipes, entering, leaving the boiler, plus in the ceiling/floor, as the main run, and so only a matter of getting to those, from your cylinder.

Unfortunately the ceiling run is 15mm copper in that woolly blanket type insulation.

I think it might be easier to change the pipe run than butcher the cupboards downstairs. It means it would go in a larger cupboard upstairs which is further from the boiler but closer to the en suites.

How important is it that it be close to the boiler.? I suppose it’s recommended so as not to lose heat as hot water travels from the boiler. But then it will lose heat as it travels from the cylinder to the outlets so is it swings and roundabouts?

SJRSJR said:

How important is it that it be close to the boiler.? I suppose it’s recommended so as not to lose heat as hot water travels from the boiler. But then it will lose heat as it travels from the cylinder to the outlets so is it swings and roundabouts?

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Not an heating engineer, but....

I would suggest short runs to outlets, for rapid hot at the taps, and less heat wasted from pipe loss. Your boiler will be reheating the cylinder much less often, than you will be drawing hot water, and so less opportunity for loss from the 22mm pipes.

I buy into that train of thought too. Don’t wanna be standing outside the shower too long waiting for the hot water to arrive