I'm about to start the last major upheaval in my house.
Replaceing the ancient back-boiler.
The whole project may involve most of these:
Move or remove Water tank. I don't want it where it is it is in the way of a
subsequent minor loft conversion/over stairs mezzanine.
Move or remove water cylinder from landing to a cupboard in 3rd bedroom or
loft. Mainly for the same reason as above.
Install new boiler in 1/ under stairs, 2/ in loft 3/ somewhere else. Though
exhaust of 1/ may interfere with subsequent garage updgrade at future date.
Finally new gas fire in lounge to replace back boiler.
These are the major considerations, how else do I choose Combi boiler or
Current shower is balanced gravity fed which is newish in a new bathroom
that I don't want to upset - though I would like to improve shower
performance possibly in conjunction with raising water tank since I need to
move/replace it. Alternatively a combi boiler would give the needed extra
pressure or I could add a pump?
As you can see I have so many variables it is hard to pin any of them down?
Where do I start?
OK. This gives you the choice of
a) combi boiler (can be DIY install with notification to Building
b) pressurised sealed HW cylinder (must be professionally installed)
c) heatbank. ((as (a))
These all involve using mains cold water, so the first thing to check
is that the mains flow at the kitchen tap is adequate. Time filling
a bucket and calculate. If it's less than 20 litres/min as a rule
of thumb then you will need to upgrade the pipework if need be to the
street or all of these will be disappointing.
You can get around that. Quite a number of boilers can use pairs of
50mm high temperature plastic waste pipe which can be routed over long
distances to avoid this issue.
If you have the space for a cylinder or a heatbank (similar sizes)
then this will give far superior performance to a combi.
This would all depend on the cold mains and the performance of a
combi. You can get combis that will give 18 litres per minute of
shower temperature water.
You can use a pump together with a gravity tank but not on the mains.
Check the water supply before anything else. THis may dictate what
can and can't be economically done.
To email, substitute .nospam with .gl
Two W-B Juniors can provide excellent flow rates, back up as two boilers is
there and CH zones without any additional zones valves. Installation is
fully DIY and simple. Electrical work consists of a mains to each and one
wire to a clock programmer to each. It is also cost effective costing
around £1100. Cheaper if you negotiate as you are buying two.
Your requirements point to a mains water fed system with no tanks. Check
the mains pressure/flow. The Powermax may be suitable, with all being in
A combi explanation for you....
Firstly, a combi is a "combination" of the heating and water system in one
case, eliminating external tanks and cylinders, and generally supply hot
water at high main pressure. To confuse a little, some can run at very low
pressures and even off tanks. Generally most are fed from the mains. It is
generally a matter of mounting the boiler and connect up the pipes. The
expert designers have done the hard work for you and put all in one case.
There are three types of combi:
1) The Infinitely Continuous Combi -
Heats cold mains water instantly as it runs through the combi. It never runs
out of hot water. This is the most common type of combi, generally having
lower flowrates than Nos 2 & 3 below. The largest flow rate instant combi is
a two bathroom model, 22 litres/min ECO-Hometec. Being a condenser it is
very economical too. http://www.eco-hometec.co.uk
2) Unvented Cylinder Combi -
An unvented cylinder is a similar to a conventional cylinder but run off the
high-pressure cold mains. A combi with an integral unvented cylinder has
approx a 60 litre cylinder heated to approx 80C, with a quick recovery coil
that takes all the boilers output. A fast acting cylinder thermostat ensures
the boiler pumps heat into the cylinder ASAP with a recovery rate from cold
around 5-8 mins (Ariston claim 8 mins). The 80C water is blended down to
about 45-50C. e.g's, Ariston Genus 27 Plus, Glow Worm, Powermax, Alpha
3) Invinately Continuous/Unvented cylinder combi -
The Alpha CB50 is a combination of both having atwo stage flowrate, of high
flowrate when using the stored water with an automatic flow regulator
switching in to reduce flow to an invinately continuous flowrate of approx
4) Heat Bank Combi -
Incoming water is instantly heated running through a plate heat exchanger
(as is most instantaneous combi's) that takes its heat from a "domestic hot
water only" store of water at approx 80C (instantaneous combi's take the
heat from a heat-exchanger heater via the burner). A fast acting thermostat
ensures the boiler pumps all of its heat into the store ASAP with a recovery
rate about 5-8 mins from cold. The 80C water is blended down to about
45-50C. They are generally two stage flow rates, in that when the thermal
store is exhausted it reverts to what the bunrer can produce, which is
approx 11-12 litre/minute. e.g. Vokera & Worcester floor standing models
(standard washing machine sizes).
N.B. The heat bank is a variation of a thermal store, but is "not" a thermal
store in the conventional sense in that a coil carrying cold mains water
runs though a store of hot water kept at about 80C. Heat-banks are far more
efficient and give higher flowrates than conventional coiled thermal stores.
The stainless steel plate heat-exchangers do not scale up so easily.
5) Combined Primary Storage Unit
(Not classed as a combi, but a derivative of a combi, but still a one box
solution, so still in the same family)
These are a combination of a large thermal store, or heat bank, and boiler
in one casing. The units are large (larger than standard washing machine
size) and floor mounted. The heating is taken off the thermal store, which
in many cases the DHW taken off the store using a plate heat-exchanger
(heat-bank). Unlike the Heat-bank in 3) above the thermal store supplies
heating "and" DHW, giving the "combined" to the title. They are available
from 1 to 2.5 bathroom models. Gledhill do an excellent condensing
version, the Gulfsream 2000. http://www.gledhill.net
Nos. 2) 3) 4) & 5) have high flowrates. No. 1 "generally" has low flowrates
but there are always exceptions and some can be high - e.g. the ECO-Hometec
infinitely continuous combi, actually has a very high flowrate. Nos 2), 3)
4) & 5) use stored water, but in different ways. Unlike No. 1 "some"
versions will eventually run cold, but that takes quite a time, hence some
are referred to as "two bathroom" models, having the ability to fill two
baths with very fast recovery rates. As hot water is being drawn off the
high rating burner is also reheating. Very rare do these combi's run out of
hot water in average use. When taking one shower the burner may be
re-heating faster than what can be drawn-off. No. 3) above uses stored
water but will not run out of hot water (high and low flowrates). Most
versions of N. 4) above are two stage flowrate models (high and low
flowrates) and will also not run out of hot water.
There are combi models that give hot water and heating simultaneously as
Combined Primary Storage Units do. Most don't as they are hot water
I'll add a little more detail maybe you two can agree on something ;-)
House is 3-bed semi. 1 bathroom/toilet (ie one room). 6 rads + 1 teeny
So the upshot is that nowadays combis are worth considering?
I'm think that when I had a look 6 years back that wasn't so much the case.
I am going to do all the work myself though get a corgi chap in for final
gas hook up to maintain boiler guarantee - that is the case is it not?
mains flow is 19.8 l/min - I guess that is okay.
One issue that concerns me is reliability of a combi - my back boiler gets
next to no service and is 30 years old - glowworm model I'm finding it
excellent value! okay so it isn't quite the efficient boiler of today. Have
combis got more reliable of late?
If I put the boiler under the stairs (prefered location) available dims are
1240mm high x 770mm wide (width better alot less) depth slim as possible but
no actual restriction.
Hollow floors eases plumbing. All existing pipework is copper. 15mm
throughout except 22mm joins upstairs to downstairs rads. Shower is gravity
fed from same loft tank with very little head of water - perhaps 3ft max
shower head tank water level (full). Had a just tollerable dribble for too
long now :-( With a Trevie ceramic mixer valve.
There is the usual heating top up tank - Do any new real candidate systems
still need this?
Can I put the whole shooting match in the loft?
Loft is too low for a full loft conversion so I might as well use it for
something - I think the one thing that I don't like about that my perception
of heat waste from the cylinder and boiler - I kind of liked the idea of
putting them somewhere I would benefit from any inefficiencies. However I
rather suspect these days those innefficiencies may well be negligble - or
at least unbenefittable from.
If it is ill advised to put the whole lot in the loft - which bits are a
good idea to put up there? House is 1930's with the hefty purlins and very
low since the rafter line is part in 1st floor. So height of any apparatus
is a factor.
Andy - do you see away to ditch the tanks without going combi? Is that
I need to do some reading re: definitions of these systems - any links?
That is quite low. and just about acceptable for a low flowrate combi. Look
at getting the mains from the road updated.
In general they were never unreliable. Some models were crap, but so are
some cars, washing machines, etc.
A heat bank. Look at a Potterton Powermax (boiler and cylinder all in one
box). It can go in the loft.
An Alpha CB50 combi will be fine for you. 19 litres/min and drops to 11
litres/min when internal store exhausted. You will fill a bath reasonably
quickly and have good showers.
http://www.heatweb.com Good explanation.
Look at my recent post, 20/9/2004 on combi boiler explanations.
On this I differ and presented the calculations as to why last week.
Yes, although the claims on some of the products miss out vital
information like the temperature of the cold water used in tests.
It is highly advisable to do sanity checks on what is physically
possible rather than relying on marketware.
To email, substitute .nospam with .gl
I'd defer to Ed Sirett, John Stumbles or John Boilerdoc on that one
since they have regular contact, and raden because his company fixes
boards and supplies spares.
Like anything if you buy a decent one then results should be good.
Generally boilers of German origin or design seem to fare well.
This would give you a fair choice. Most should fit that.
Most boilers will run a pressurised primary.
You can DIY install a combi or a combi with indirect heatbank - i.e.
where the coil is in the primary circuit. This could be interesting
to do if you had a small combi relatively close to the kitchen and
used its directly produced water downstairs and then heated a cylinder
upstairs for the bathroom. In effect, you heat the cylinder by using
the CH part of the combi with the normal motorised valves as though
you were using a conventional boiler.
In a heatbank application you can buy the complete package, and
typically the heatbank cylinder is run vented. They can come with a
feed tank for this integral with the cylinder. You would heat the
heatbank via a coil and then the bulk (tertiary) water in the
heatbank is pumped through a plate heat exchanger to heat the DHW.
This arrangement can be DIYed because there is no large bulk of water
at mains pressure.
You could even have this arrangement with the CH vented and a direct
heatbank, but then the top of the heatbank must be above the highest
radiator. With a pressurised primary you do have more flexibility.
Yes you can. If you go for a decent condensing boiler, they run very
efficiently and you can also have the cylinder foamed to 100mm instead
of the typical 50mm.
You do have to board the relevant part of the loft and provide a rail.
THis is a Health and Safety requirement so that gas fitters can work
safely to service the boiler.
No reason why not if it will fit. Positioning the cylinder where
the roof tank was should be OK, otherwise locate over a loadbearing
Yes you can do that.
One option are a heatbank as described. Albion do them as one
example. You can DIY this because the cylinder water is not
The other is a sealed pressurised cylinder.
This is not a DIY job as the Building Regulations require that a
trained fitter installs them. They are also supposed to have annual
maintenance under the terms of some insurance policies.
To email, substitute .nospam with .gl
On Tue, 21 Sep 2004 00:14:33 +0100, Andy Hall wrote:
Whilst your existing boiler is nigh on indestructible . I think
it is fair to say the combi boiler is no more unreliable that
other modern high-tech boilers. Modern boilers especialy combis
incorporate many of the components which were spread out around the rest of
the house in a conventional system.
Reliability is not a consideration when deciding combi or not.
Modern boilers probably save enough gas to cover their higher repair
 My own did not even
burn out a thermocouple in 14 years. However I had to deal with a scaled
shut float valve at least twice, an over-flowing header tank at least once.
Also 2 pumps.
The usually reason for the death of a back boiler is severe corrosion due
to installation errors or themrostat failure leading to excitement and
consequent loss of customer peace of mind.
They also are some of the most inefficient designs of boilers in use. So
you have been paying for its 'upkeep' with +25% (or more) on your gas
In my own home we have gone from £480 to £350 for the year.
GlowWorm Galaxy BBU to Keston C25 + tank and wall thermostats.
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
I too have followed some of the to-ing and fro-ing over combis versus
traditional boiler and tanks. I can only give you *my experience* in
this matter as a user.
We just replaced a fairly standard boiler/header tanks/immersion - baxi
back boiler with a Halstead Ace High (combi).
There are a lot of figures quoted on here about flow rates, well this is
a bog standard combi quoted at 35 degree rise at 13 l/pm.
It's quite simply the best shower I've had. Lots of water, lots of heat.
The boiler DHW temperature control is set to about two thirds and the
mixer valve on the shower at a little over 50% hot. I have never yet
fully extended the shower lever for maximum flow. I *assume* therefore,
that the 13 l/pm is more than enough for what I expect of a shower.
Because of other threads on here, I specifically asked the girlfriend
how long it takes her to fill the bath. She reckoned 5 or 10 minutes.
Whether it is actually longer or shorter than 5/10 mins, I don't know.
The fact that she was so non-chalant about it makes me assume she's
never thought about it, and so I would guess it is not an issue for her.
(I have never measured the volume of the bath, and for other readers
here - NO I am not going to. It looks like a normal bath to me).
FYI, the house is a 3 bed semi, 1 bathroom & 1 toilet. If I'm in the
shower and the hot tap in the kitchen is turned on, it is noticeable. It
doesn't bother me at all, but it yes, it is noticeable.
No idea, wouldn't know if they ever were/were not reliable to start
You can get the Ace High branded as Wickes own model (identical boiler
from the same factory but with a different badge) for 500 ish quid. At
that price, I don't think reliability is the biggest issue.
I believe most manufacturers publish the physical installation
characteristics, certainly Halstead do.
One of the things I love about the combi is the space we have saved -
and even if I found something which bugged me about it, I doubt it would
ever out-do the advantage of the extra space. We used to have 2 airing
cupboards basically taken over by heating, one for the immersion and one
for the pumps and other various gubbins. (This latter cupboard was not
particularly well done and was actually a source of amusement for any
plumber who ever saw it).
On top of that, the room with the back boiler which *had* to have the
gas fire front + hearth etc etc, and of course the various tanks in the
That is now all gone. The space saved in the loft is quite remarkable
(so much so that I've held off finishing decorating upstairs in order to
have a staircase put into the loft as a starting point for a full loft
conversion) and downstairs, we now have a full height 3ft wide by 3ft
deep cupboard to use as we wish.
This particular combi includes frost protection, pump cycling etc. For
this reason I imagine you could site it virtually anywhere - the main
constraint being the flue routing.
Those are just my thoughts as a user, having in the past three months
gone from a traditional system to a combi. I wouldn't go back, I love
Maybe you should ask again in the middle of winter, when apparently the
incoming water supply is that much colder - personally I don't see this
as a problem, but won't know until the depths of winter. I assume and
expect that the combi will just carry on as normal and I'll still have a
[What now follows is a blow by blow account of what other boiler I could
have had tuppence cheaper, with a higher flow rate, how it is useless in
the event of a power cut, how I could have had two of them instead of
one and how the [Toytown/Tinseltown*] [Combi/Trad*] model [10/40*] is
better because it uses a 4mm metric thread on the case screws instead of
5mm.]. (*Delete as appropriate).
All the best :-)
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