Vented/Unvented Boiler? Also cheap/good suppliers?

Hello all wise oracles of DIY (and those of a less than divine nature :o)
Some help required, but then again, when is it anything else... I'm looking to replace our current boilers, the last owners of this money pit had one water on demand boiler and one potterton work horse for the central heating. All would be well except the water on demand boiler has sprung a small leak in a very inconvenient location. The potterton is also showing it's age and is a slight concern.
I've decided to change both of them to a 'combi' boiler, specifically a Vokera Linea Max, the specs look good, the reviews in this news group are positive, and the company seems to provide good support.
I do however have two questions, 1 important, 1 not so.
Important question: What the heck is the difference between a 'vented' and 'unvented' system?
No so important question: Can anyone recommend any cheap suppliers for the afformentioned boiler? (delivery to South London)
Thanks for any and all help with this.
Seri
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In an earlier contribution to this discussion,

With a vented system, the water going round your radiators is at only just above atmosheric pressure - and has a feed and expansion tank, usually in the attic.
An unvented system is sealed, and has no feed/expansion tank. It runs at between 1 and 2 bar above atmospheric pressure. It is pressurised using mains water with a filling loop (which is disconnected once the pressure is set). Expansion is taken care of by an expansion vessel - a bomb-shaped object containing a diaphragm - with water one side and a compressible gas (air or nitrogen) on the other side.
Most systems which have been around for a long time are vented. Most recently installed systems are unvented. They both have advantages and disadvantages - but the perceived wisdom seems to be that unvented systems are preferable in overall terms.
[I apologise in advance to the purists if any of the above is an over-simplification of the facts!]
HTH.
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wrote:

is
couldn't have described it better myself ! :o) rob owen
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On 24 Jan 2004 13:48:20 -0800, snipped-for-privacy@hotmail.com (Seri) wrote:

Do a search for a post from Ed Sirett, look at his signature and then look at the FAQ on sealed systems.
In the context of central heating, unvented is the same as sealed and vented is also known as open vented. Quite a lot of modern boilers, combi or not require to be run on a sealed/unvented circuit; others will accept either.
In the context of stored domestic hot water (meaning that there is a hot water cylinder, unvented and sealed are also used interchangeably.
There are two issues that I notice with the Vokera Linea Max.
- It does have a small internal store of 10 litres and will give you 18 litres per minute until that is exhausted. After that has run out, the performance drops to 11.5 lpm. Are you happy with that - it is at the low end of combi boilers?. This model is going to cost a little over 1000 inc VAT from somewhere like Discounted Heating, who normally are fairly good on price.
- This boiler is a band D and right at the bottom of the efficiency range at 78%. There is a fair chance that this type of boiler will be off the market in a year or so.
For similar or even slightly less money, you could get a 35kW wall mounted condensing combi which will give you 14lpm flow of water under the same conditions and should fit in the same space of a floor mounted model or even less. It will provide an efficiency of over 90% into the bargain and save energy costs. The Vaillant Ecomax 835, Worcester Bosch and Ideal Isar would be worth considering in this class.
.andy
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This question does not really apply to Combis, unless you want to keep a hot water tank, heated as a zone on the _heating_ circuit. If you intend to ditch hot water cylinder (and cold tank in loft) then you only need to consider the make and model of combi. Don't rule out Vaillant and possibly Glow Worm CXI range (both by Hepworth heating). If you do keep a hot tank then this can be fed from existing cold tank, or if you don't want the latter then you must use a pressurised hot tank - i.e unvented. These are pricey (> 600) and have to be installed by a plumber/CH engineer who is BBA registered. Alternatively there are things called heatbanks which cost even more but are the holy grail of heating. The problem with combis is that even a small one (24 kW) will only provide 10 litres of 'hot' water per minute. This means 10 litres of water that is 35 deg. centigrade higher than the incoming mains. In winter this means luke-warm baths unless you go for a 28Kw or 37Kw boiler. The latter will probably need a new 28mm gas feed from the meter - this is not an insignificant problem, likewise low mains water pressure or flow rule out a combi (and during water or electricity interruptions you get NO HOT WATER, and without a cold tank, can't flush the bog). Another regular (sound expert) is a Sarf Lunnon man and he says this is a regular problem - I can't comment. If it is a condenser, you need to avoid causing a nuisance with the 'plume' of steam coming out of the flue. Checkout the Sealed heating FAQ- google for Ed Sirett for details.
--
Andrew

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writes

28mm will only be in a part of the gas pipe run.

Most of the world do not have storage tanks and they survive well. water ad electricity outages are very rare for most people.
If hot water backup is essential an in-line instant electric heater can be installed in the combi hot draw-off. Leave it off. if the combi is down turn it on. It will only do one tap at a time but give backup and a trickle shower.

This is from the discountedheating. 15.5 litre/min is not bad at all. I would go for this instead of the Vokera
Glow-Worm 38CXi Condensing Combi Boiler . BTU's - 130,300
. kW - 38.2
. DHW Flow Rate - 15.5 L Per Min @ 35C
. Height - 715mm
. Width - 450mm
. Depth - 334mm
. Sedbuk Rating A - 90.6%
. Built In Frost Protection
. Fully Modulating
PRICE INCLUDES DELVIERY
934.13 Including VAT at 17.5%
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writes

If you are paranoid about electric outages and no hot water, then if you have the space fit two multipoints which give approx 9 litres/min. Join the outlets up. Many of these operate without any electrical controls, purely mechanical. So that gives approx 18-19 litres.min for about 325 each. Then buy a B&Q cheap condensing boiler to do the heating only @ approx 400. That is 1050 for the three. You also have the advantage that if one multipoint drops out you have a backup. Just two multipoints could do heating and hot water, but you need to know how to pipe it up. These can be operated by mains pressure using a bronze pump and two Telford typhoon quick recovery cylinders coupled up.
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Why are multipoints suddenly the flavour of your month?
Have you got a job lot of Ascots in your lockup or something?

.andy
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wrote:> >If you are paranoid about electric outages and no hot water, then if you

the
purely
Then
be
quick
Some people here just want simplicity and they provide it. Not that efficient but cheap and simple.
You could tee up the outlet of two of them and take this to the top of the coil in a Telford Typhoon cylinder. The bottom of the coil can then go back to the inlets of the two multipoints. The cold water mains connects onto the two inlets. A bronze pump between the cylinder coil and the two inlets for the CH. The DHW is then off the mains giving approx 18-19 litres/min, and the CH flow to the rads is taken off the cylinder draw-off and the return to the cold feed. A pump is on this leg. The cylinder only heats CH and acts as a buffer to prevent cycling. Simple, easy, no cycling, lots of backup and hot water in a power outage.
The cylinder could be replaced by a plate heat exchanger to save space, but cycling would be introduced.
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Do you happen to know what the efficiency is as a matter of interest? Presumably no better than the typical 78% of the basic CH boiler, possibly less since presumably lack of electrics implies that a pilot is required.
On gross efficiency, the typical Main Medway, Medina etc. comes in at around 75%. Presumably it doesn't have a SEDBUK rating because it is not classified as being for space heating purposes.
However, by using it in the way you describe, I suspect that it in effect becomes one. I think that might raise a question mark with respect to Building Regulations.

.andy
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wrote:

you
Join
each.
400.
can
There are pilotless forced flue versions which do give ~78%: Thames, Worcester-Bosch, Chaffeteax. Even so the control systems are still usually simpler than boilers.

Not sure about that. SEDBUK is only a temp measure anyway. You may find that multipoints will be uprated in efficiencies next year, along with the rest.
The Americans have recently discovered "on-demand" water heaters and use them for all sorts of applications, not just DHW water heating. The Myson is a big seller over there.

the
back
inlets
litres/min,
CH
of
but
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On Sun, 25 Jan 2004 10:01:12 +0000, Andy Hall wrote:

I would expect the efficiency to be typical of Balanced flue boilers with permanent pilot lights. The pilot light is typically 250W.
The propsed jerry rigging would of course rely soley on the cessation of flow to control the gas. Umm... This would of course be interesting and would sooner or later find its way in to the rougues' gallery published each month in the CORGI comic _Gas Installer_. (Along with the home made boiler. The warehouse heater flue gases used as a warm air heater for the office block. The flue liner used to extend a fanned-flue boiler terminal through a conservatory roof...).

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wrote:

you
Join
each.
400.
do
can
typhoon
What Jerry rigging are you on about?
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wrote:

the
purely
Then
be
quick
Ascots are no longer made.
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That's what I was thinking and it could have been the explanation for your needing to offload them quickly.
How many can you fit into the back of your Reliant Robin?

.andy
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wrote:

you
Join
each.
400.
can
You plonker! Boiled beef and carrots, boiled beef and carrots,...
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What exactly are multi-points? Reading through the responses here I'm now wondering again (damn, I originally thought I was all decided).
I chose the boiler as I currently have a Potterton Kingfisher Floor Standing boiler, so I figured the Vokera Linea Max would cause only a little disruption to fit in its place.
I'm now thinking that as the Potterton is doing a fine job on the current heating system, even if it is showing its age, then maybe I should just rip out the faulty water on demand boiler and replace it with something else... there is the additional space issue with this of course.
wrote:

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On 25 Jan 2004 04:40:10 -0800, snipped-for-privacy@hotmail.com (Seri) wrote:

They are an instant gas water heater, normally mechanically operated by a valve which turns on the gas when you turn on the tap. This is the (reasonably) modern version of the old fashioned gas geyser which had a single outlet pipe over the bath or sink. The multipoint aspect is because they connect to the hot water plumbing on the output side. They are not very efficient but functional but suffer from the same issue as any instant gas water heater like a combi which is that the flow rate is limited unless there is some form of storage of the hot water or the energy to produce it. There are for example combi boilers with water storage to give shortish bursts of hot water at reasonable flow.
One thing to be aware of with this, as with any gas water heating appliance is that if fitted in a bathroom, it must now be a room sealed type.

I don't think it would make a huge difference to be honest. You should find that a wall mount will fit the space available easily and there is a lot more choice.

I suppose you could go with a multipoint for now and then replace the heating boiler later......

.andy
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(Seri) wrote:

They are called multipoints because they supply more than one point (tap).

78% can be goyt from some models, which is only the same as most regular boilers.

Two bathroom combi models are around. Multipoints have the flowrate of a low rate combi.

I don't think opened flued multipoints are around any more.

Or later have the multipoint do the CH as well. Multipoints are available from about 325. Very cheap for what they are.
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On Sat, 24 Jan 2004 13:48:20 +0000, Seri wrote:

Take a look at the SealedCH FAQ below for some of your answers. www.discountedheating.co.uk will deliver at not much more than trade prices.
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