combi vs conventional

I know - done to death.. A customer has asked me this question. I have come up with the following list. Any other thoughts - sensible ones.. no need to start a debate about the merits - just need a list!
Conventional Pros Stored hot water - so water (hot and cold) available if mains supply fails Immersion heater backup - hot water if boiler or gas fails Warm airing cupboard Cons Cylinder needs replacement (in this case to comply with building regs) Storing hot water not as energy efficient More complex controls More costly than combi to install
Combi Pros High pressure showers without pumps Only heats hot water you use No stored water (flooding issue) Frees cupboard space Cons slower bath filling No warm airing cupboard
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On Thu, 17 Jun 2004 21:20:48 +0100, "Colin eclipse 2"

Conventional wins hands down for me,especially in higher demand situations.. It seems to me that storing hot water isnt really a big issue becuase of the turnover rate, i.e not stored for a great length of time. Well insulated storage vessels and lagging plus possible use of plastic piping would lower heat losses even more. Compare this to lower flow rates and wastage of water run off and for a family home,its conventional all the time,preferably pressurised stored system.
Just my views for what they are worth.
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On Thu, 17 Jun 2004 21:40:20 +0100, tarquinlinbin wrote:

I add my support for this view. The issue of HW backup during a boiler failure is a valid issue too. Also building the HW storage system from multiply sourced generic components rather than relying on the boiler manufacturer is bound to have a some value.
Also the foundations for extending the controls should the house become extended or a user wish to add zones.
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wrote:

come
to
supply
regs)
An in-line electric instant heater can be installed.

I fail to see where. A combi is designed by exerts to get the best efficiency.

That can easily be done with a combi. If a house is extended the same issues apply to any system conventional or combi. If an extra bathroom is added you can always add another combi.
You say you are a pro. Are you sure?
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Do you understand what most mean by hot water?
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On Sat, 19 Jun 2004 01:58:10 +0100, Dave Plowman

I don't think he does, other than getting into it.......
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wrote:

The in-line heater is backup until the combi is up again. Small, simple and gives hot water one tap at a time. Some do two taps.
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Have you now gone off the two 'combi's' now and transferred your allegiance to electricity?
If you have, try looking up immersion heater. It's a new idea that can provide all the hot water a family would normally need - if used with some thought. Cheap, to, as a back up.
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Please can you suggest a make and model for such a backup electrical heater. Also where would I plumb it, in series with the output from the combi, or in parallel?
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wrote:

too.
and
There are a few of these heaters around. Reding make a few, I thing even Screwfix a few too. Install it in the draw-off. make sure it has an adjustable throttle, so it can be opened up.
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wrote:

OK, I've had a look at the data sheets for a number of these. They seem to have a few common factors:
* They require a minimum of a 40A circuit. * They have small inlet and outlet size (3/8" seems typical) * They all recommend "spray taps" and most say they are not suitable for baths, however it seems that they can supply a shower.
When you say to install in the draw-off, I assume you are meanng that I should install it so that all of the hot water that is normally heated by the combi flows through the electric heater (turned off normally!). I can see that this would produce the desired result, but am concerned that the small bore of the inlet and outlet pipes of the electric heater would restrict the flow. Is this the best way to go, or should I fit it in some other way.
I assume that normal taps will work with the electric heater, but maybe I won't get the hottest water possible in this case (or should I fit spray taps and use them all the time?).
A 40A circuit seems a lot, is there a lower power option?
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wrote:

issue
simple
There are models aroud with 15mm connections.

Spray, or aerated, tap are advisable for all installations as they use less water. They are the normal in most of Europe, except in backward Britain.

Yes.
You may only want to fix it on a section of the draw-off that does not require a large flow: kitchen and basin for e,g. Larger bores are available.

You could fit it in parallel and have a manual 3-way valve that diverts the flow to the taps or through the install heater.
Depends on what you require as backup. You could have a higher flow storage heater on the inlet to the combi. Then only cold water is in and out when off. When combi is down and on, it heats the water and it runs through the plate heat exchanger of the combi doing no harm. It will not have a great amount of water in it. An electric storage heater requires a smaller supply too.

That is so. The temperature will be fine, but flow not as great. It is for backup only.

Electric storage heater as above.
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<snip>
less
How do you work that out, unless you only read the marketing blurb, a bowl full of water etc. is still a bowl full of water - the only difference is that it's taken longer to fill from either a spray or aerated tap.

Most of Europe is backwards were plumbing (and electrical) installations are concerned from what I've seen....
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says...

Is it OK to restrict the output of the combi to a 15mm pipe then, I would have thought that this would limit the flow rate.

Sounds interesting... I guess they don't use them on the bath though.

I'm still concerned about restricting the flow though.

Hmm, This would need some careful planning. Perhaps split the output of the combi into 1 x 22mm and 1 x 15mm. Take the 15mm through the electric backup heater and on to the kitchen and all sink taps. The 22mm can co to the bath taps (giving good flow, but not working in backup mode - which is OK because the electric heater will not do a bath anyway). What about the shower, should this be in the high-flow-no-backup or the low-flow-with-backup? I would prefer a high flow shower, but I don't like the idea of being without either bath or shower if anything were to go wrong with the boiler.

Sounds maybe even better, but wouldn't this be a complicated little set of pipework? I tried to sketch it out, but it starts looking like an oil refinery!!

Also sounds like a possible option, but I don't understand the benefit of fitting instant on the output, but stored on the input.

Is this like a small water cylinder with an immersion heater, or something more like a heatbank?
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Most combi's have a 15mm connection.

They do.

15mm on both legs.

Have the shower and sink go through the electric heater, then hot water at kitchen and shower.

No. The combi draw-off to the 3 way valve . One outlet of the valve to the electric heater, the other to the taps. The outlet of the electric heater, tees into the draw-off.

Stored has a limited capacity, so fine for sinks, no good for a shower.

Like a small water cylinder with an immersion heater.
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<snip>
Unless you do have your shower in the kitchen are you not replacing the cost of a hot water cylinder and immersion heater with the cost of a more complex system of pipe work (especially the property has more than one floor) ?!...
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the
heater,
You could have one cheap 1/4 turn full bores valve. From combi's DHW draw-off insert a full bore valve. Fit a tee before and after the valve and have the electric heater between the tees. When the valve is open, most flow runs around the electric heater. When close all runs though the heater. Simple and effective.
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On Fri, 18 Jun 2004 23:01:29 +0100, IMM wrote:

Of course, a stock-in item at all merchants. Approx rating required? 3 phase supply to hand?

There are,of course,some losses associated with the primary circuit pipe work and storage losses from the storage cylinder. The former can be reduced by lagging pipework the latter is also true for a thermal storage combi, but having being designed by 'exerts' the laws of physics will doubtless have been designed away.
On a direct race I'd guess that there would be little to choose between the _delivered_ HW efficiency from a well installed storage system powered by a state of the art condensing boiler against a thermal storage combi.

Yeah right. By foundations I confess I probably didn't much more than : a time switch and zone valves having being already installed there is less work to add another zone.

Thanks for the laugh.
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In an earlier contribution to this discussion,

I'm not at all sure that a system based on a conventional boiler *does* have more complex controls. More complex external controls, perhaps - but the diverter valve in a combi is at least as complex as (say) a mid-position 3-port valve - and seems to be at least as unreliable based on posts in this NG.
Also on the Cons side for combis, you only get a decent flow of hot water if you've got sufficient heating power in the boiler *and* a mains cold water system with sufficient flow capacity. Many houses don't have sufficient cold flow capacity, and *need* to have stored water systems.
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wrote:

have
this
Not so. A few combi;s don't have 3-ways valves in them.

if
cold
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