Using combi in stored hot water system
 

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

In an earlier contribution to this discussion,
Martin Pentreath wrote:

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

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.
--
Cheers,
Set Square
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On 3 Oct 2004 07:29:05 -0700, (Martin
Pentreath) wrote:

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


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

"Martin Pentreath" wrote in message
om...
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

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?

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;

http://content.honeywell.com/uk/home...20S%20Plan.pdf

You 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.