Using combi in stored hot water system

[Martin Pentreath]

Now I know I'm on dangerous ground here, and I really don't want to unleash combi war, round 481, but I would be interested in answers to this.

I have a combi which works very happily - an Ariston Microgenus 27kW. It's a perfectly good combi, but I'm toying with the idea of having a pressurised cylinder installed to provide for stored hot water at mains pressure. Before I get Mr Plumber in to suck his teeth and declare the whole system to be in need of ripping out, is there any reason why the combi can't be used to provide stored hot water in this way? My primitive understanding of combis is that they are conventional boilers, with an additional heat exchanger bolted on which provides the hot-water-on-demand function. Presumably my scheme would render the secondary heat exchanger redundant and I would just be using the primary heat exchanger to provide central heating and hot water as a conventional boiler would. Is this about right?

Martin

[Set Square]

Yes, it is perfectly feasible to use the CH side of a combi to power both the CH *and* stored hot water. You will need some sort of zoning control (either a 3-port mid-position valve, or two 2-port zone valves) in order to have independent control over the CH and stored HW.

In order to provide stored hot water, you can use a conventional indirect cylinder with a header tank, or a mains-pressure unvented cylinder (which must be professionally installed) or a thermal store/heat bank system - you pays your money . . .

Many people who do this sort of thing continue to use the original HW side of the combi for providing instant hot water to the kitchen sink - and use the stored hot water for baths and (maybe) showers.

[Andy Hall]

Yes you can.

You could use the instant part of the combi as now to deliver DHW for (e.g.) the downstairs taps.

Then have the cylinder installed with the coil fed via a diverter valve or zone valves from the CH circuit. The valve(s) would need to be fitted before the first radiator and a thermostat for the cylinder added, but that is broadly it.,

.andy

To email, substitute .nospam with .gl

[IMM]

Firstly, yes, a combi can be used as you want. Secondly, why do you want to add a cylinder? What type of cylinder? Cold tank and cylinder? Unvented cylinder? Heat bank cylinder? How many baths, showers, etc?

[rob w]

This is exactly what i have done, in my new house. the plumber was a bit perplexed at first but a quick sketch and he was off, the elecy had a bit of bother with the controls but i was using a fancy controller so it was not standard anyway. carefull not to get the combi hot water and the stored hot water mixed up as it seem our plumber has well it looks like he has plumbed both to one shower it has no cold!!and we are still waiting for him to come back!

[Aidan]

Yes, I did this a few weeks ago. The combi instantaneous hot water supply was used only for a washing machine & a 300 litre unvented HWS cylinder was installed. If a pressurized unvented DHW storage system is to be fitted, then you need to control it's temperature with a

2-port spring-return valve. A 3-port mid position valve on it's own is not suitable. See the Honeywell S-Plan (two 2-port valves) wiring diagrams at;

will get a suitable 2-port valve with the unvented cylinder package. The installer should have passed a suitable training course (CITB, IoP, etc) traing course on unvented HWS systems.