*Small* gas combination boiler?

I am thinking of replacing my conventional central heating and hot water system with a gas combination boiler, for my four bedroom extended semi, with one bathroom. My present system is 25 years old, when I did the heating calculations and ended up with a 10kw boiler, which works perfectly well with ample heat output, even in the coldest weather. When I did the heating calcs, the house was *very* well insulated, and I have continued adding insulation/double glazing over the years, so it is now *extremely* well insulated. I am therefore looking for a combi with less than 10kw output to central heating, plus enough say 10l/m to the DHW.
Objective is to save space.
I have looked in Google Groups for old threads, and tried to answer the questions which may be asked.
Kitchen tap gives 16l/m Bath tap hot water gives 9l/min Soft water.
Googleing for combination boilers only gives huge 20+kw models does anyone here know of anything smaller?
As I will now be pressurizing the central heating pipes (copper with Yorkshire fittings) does anyone expect problems with this?
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Dave Fawthrop <dave hyphenologist co uk>
"Intelligent Design?" my knees say *not*.
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Dave,
AFAIK, You have to look at Heating and DHW separately. Whilst 10Kw might be enough for Heating, you need much more to get a decent temperature rise and flow for DHW. It looks like 24Kw gives a 9l/m flow with a 35C rise in temp, and you wouldnt want any less than this. In fact, having had 24Kw boilers, I would definitely buy a more powerful one the next time - for DHW, not heating.
Hope this helps - Others who are experts may give a better answer.
--
Richard Faulkner

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Dave Fawthrop wrote:

> central heating, plus enough say 10l/m to the DHW.
You want a boiler with as high a kW rating as possible, to get the best hot water flow (you said one bathroom - bath or shower?), but which will modulate down to 10kW or lower for the heating.
As IMM isn't here right now, I'll make the obligatory suggestion of getting two small boilers, use each on alternate days for the heating and run both together for the hot water, replacing all your Yorkshire fittings with plastic pipes cut with a hacksaw. A win, win, drip drip gurgle situation^Wdisaster.
Owain
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firstly ignore the heating requirement and concentrate on the hot water side - 40kW gives 16 litres per min approx - look for one that will modulate down to your 10kW for the heating FWIW - I have just had an Alpha CD50 fitted and am well impressed - this modulates from 6.3 to 29.7 kW on heating and upto 32kW for water, it has a small store for instant hot water and is wall mounted ( in loft on gable end ) LMK if you want more info
Regards Jeff
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With a modulating condenser, the maximum heat output isn't so much of an issue, since unlike older boilers it will actually increase efficiency when not running at full output.

Then it's a good choice.

This is the existing storage system? Normally, the bath is fed from larger bore pipe and should give much better flow - even although if a floor higher and with less 'head'.

With a combi, you need to size it for decent hot water flow - especially if you like big baths. This is the real problem with them - they generally can't fill a bath anywhere as fast as a *decent* storage system.

No - they'll be fine.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Sun, 28 Aug 2005 18:12:22 +0100, "Dave Plowman (News)"
| This is the existing storage system? Normally, the bath is fed from larger | bore pipe and should give much better flow - even although if a floor | higher and with less 'head'.
Completely conventional two header tanks, and indirect (coil) HW cylinder. Two circuits, downstairs and upstairs which only gets used in very cold weather
The only oddity is that it will work by gravity in power cuts, but as we have had no long power cuts in 25 years, that is going.
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Dave Fawthrop <dave hyphenologist co uk>
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Does this refer to the heating? It must be pretty old if there's no electrics in the boiler. And if it works without the pump running this will need sorting, as it will waste energy with the new boiler.
If you mean the hot water, no electricity is needed to access stored hot water.
--
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Mon, 29 Aug 2005 09:02:43 +0100, "Dave Plowman (News)"
| > Completely conventional two header tanks, and indirect (coil) HW | > cylinder. Two circuits, downstairs and upstairs which only gets used in | > very cold weather | | > The only oddity is that it will work by gravity in power cuts, but as we | > have had no long power cuts in 25 years, that is going. | | Does this refer to the heating? It must be pretty old if there's no | electrics in the boiler. And if it works without the pump running this | will need sorting, as it will waste energy with the new boiler.
It uses a conventional controller with pump but could be switched to manual in the case of another "Winter of Discontent" All that will be ditched when a new boiler is installed.
--
Dave Fawthrop <dave hyphenologist co uk>
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Please explain?
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Mon, 29 Aug 2005 18:18:21 +0100, "Dave Plowman (News)"
| > | Does this refer to the heating? It must be pretty old if there's no | > | electrics in the boiler. And if it works without the pump running this | > | will need sorting, as it will waste energy with the new boiler. | | > It uses a conventional controller with pump but could be switched to | > manual in the case of another "Winter of Discontent" | | Please explain?
Maybe you are too young to remember the "winter of discontent" when we had long power cuts c1976. Too much effort to explain as the whole lot is for the bin.
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Dave Fawthrop <dave hyphenologist co uk>
"Intelligent Design?" my knees say *not*.
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Dave Fawthrop wrote:

I think he ment please explain how it worked in "manual mode" rather than the winter fo discontent.... then again maybe he is younger than we thought ;-)
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Cheers,

John.

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Heh heh - I can well remember the 'Winter of Discontent' being one of the very discontented workers - fed up to the back teeth with government trying to control prices and incomes - but only partially succeeding in controlling incomes for some - including me. While prices rocketed.
I was interested in how a pumped system could be switched to manual. A hamster in a wheel to turn the pump?
--
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Wed, 31 Aug 2005 18:35:23 +0100, "Dave Plowman (News)"
| I was interested in how a pumped system could be switched to manual. A | hamster in a wheel to turn the pump?
OK I give in. I was an Engineer designing little power stations for aircraft, so thinking outside the box was normal for me.
I put two loops of one inch pipe, one on each side of the house, which worked by gravity a treat, when tested. I shorted out the pump with a gate valve, and shorted out the motorized valve which cut off the upstairs with another stop valve. I also put in an extra pipe to feed the downstairs radiators when the upstairs motorized valve was shut. In the case of a power cut, open the two gate valves. The only problem left was to feed the gas valve from an invertor from a battery. Unfortunately, in the next 25 years, we never had long power cuts, so it was a complete waste of time and effort :-(
--
Dave Fawthrop <dave hyphenologist co uk>
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Dave Fawthrop wrote:

> power cuts, so it was a complete waste of time and effort :-(
Not at all. Had you not gone to all that trouble you would have had power cuts several hours a week, every week, for years.
Owain
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Sounds like flawed thinking given I've got a plumbed in invertor that will run the heating all day off a standard car battery.
But like you it's never been used in anger. ;-)
FWIW, I had a rented flat with an ancient completely non electric central heating system. All the pipework in iron barrel - and the timeswitch on the boiler clockwork. It worked very well. Think it dated from the early '50s.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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The way gas prices are going, and the over dependence on gas for power generation makes me wish I'd simply plugged the boiler in rather than use an FCU, the drawbacks of not bodging! I've an inverter as a last resort but I'd probably fire up the generator instead and keep the lights and TV on too.
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Dave Fawthrop wrote:

Save space as it "loose the tanks etc" or really save sapace as in the boiler needs to be tiny?

I would start by downloading the free boiler database program:
http://www.sda.co.uk/qsedbuk.htm
That will give you the stats for most boilers.
As the others said you can pretty much ignore the heating side, however it is worth looking for a model that will modulate as low as possible. In a well insulated house however even a small boiler will cycle a bit since it is not often going to be cold enough to keep above the minimum modulation point.
Based on my experiance with my HE35, you may find something like the Ideal Isar HE30 about right for your needs. It is physically very compact, and will modulate down to 8kW. There is also the ECO Hometec EC23s which will modulate right down to 4kW and still gives 28kW to DHW.
(personally I would not install anything smaller than about 35kW for water heating though).

With the pipework no. With radiators, only if they are close to knackered anyway. With very old lockshield valves - they may weep and need replacing - but most won't.
--
Cheers,

John.

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On Sun, 28 Aug 2005 12:26:22 +0100, Dave Fawthrop wrote:

The lowest gas rate I've seen is an 18kW Vaillant unit made around 15 years ago. This will provide an even more limited flow of HW than the 'standard' 24kW units.
It might work for a small dwelling with no bath.
--
Ed Sirett - Property maintainer and registered gas fitter.
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