I am thinking of changing from a warmflow bluebird oil boiler to gas but
don't know whether to go for a combi gas boiler or stay with my current
setup of stored hot and cold water. We had a water cut of 3 days in the new
year and stored water was a blessing so what are the main advantages of a
combi. Gas heating is still in the minority in Northern Ireland at the
moment but catchingup.
I'm sure this is a subject on which there will be divided opinions,
but mine is that stored cold and hot water is best, with both at
low pressure of course.
As far as I can see the only disadvantantages of this are that
(1) low pressure means you can't have high pressure showers unless
you either install a special pump or else go for an electrically
heated shower which takes mains pressure water in, and
(2) that you might lose some heat from a poorly insulated hot water
tank to parts of the house which don't benefit from it.
The only advantage of a combi is that it allows you to have high
pressure hot water and does away with the need to have a HW tank.
It takes just as long for water out of the hot tap to get hot,
because water has to travel down the same distance of cold pipe
before it gets from the source (be it the HW tank or the boiler
itself) to the tap.
The disadvantage of a combi is that it costs more, is more complicated,
and there's more in it which can (and does) go wrong, and if you want
a bath it'll probably take longer to fill than from a pre-heated tank,
because the boiler can't heat it up as quickly as you want it to come
out of the tap.
A minor disadvantage is that if your house is already plumbed for
low pressure hot water, and all your pipes, joints, and taps have
been used to it for years/decades, then suddenly changing to higher
pressure may invite latent leaks to read their heads. The risk is
probably low, but not zero.
I was once foolish enough to put one into a flat which previously had
no central heating at all (open fireplaces or gas or electric fires
only, cooking with gas, hot water by immersion heater). The installer
suggested the "obvious" solution was to mount the boiler in the cupboard
which then housed the HW tank, and to remove and discard the HW tank
which would become redundant with a combi.
I've regretted letting the salesman sell me a combi ever since, and
I wouldn't have one now for all the tea in China.
The primary question is: does your current hot water system work well
enough and meet your needs? If the answer is yes, then using a gas
heating only or a system boiler will involve least expense and hassle.
On the plus side combis are cheap and easy to install from scratch (but
that is rarely relevant when retrofitting to an existing system). They
may also save space in some cases. They do very good showers without
need for pumps etc. Ideal for flats designed for couples with no family
On the negative side you need to install a fairly powerful one to get
adequate performance for bath filling (35kW minimum), and they cope less
well with multiple concurrent demands. (for the combi evangelists out
there, is is worth noting that these limitations can be mostly overcome
if you spend enough on storage combis with built in pressurised hot
water cylinders). You also need adequate (flow rate and pressure) and
reliable mains cold water supply.
The final option would be a mains pressure storage system of some sort
which will give the best of both worlds in many cases.
I too am considering the options in case my 25-yo boiler gives up the
ghost. The combi needs good pressure mains, I believe; have you got it?
It looks like, if you are upgrading or replacing (as opposed to
installing fresh), the combi is the most drastic option. I see from the
wiki and from the Vaillant and Worcester websites that there are two
non-combi options: open-vent (the same as what you and I have) and
sealed system (which only does away with the cold-water tank). I am
still not clear of the benfits of each set-up, so I will stop here.
Indeed. Replacing a stored hot water system with a combi often involves
a lot more plumbing changes than installing a replacement conventional
I see from the
If you are referring to a non-vented stored hot water system, this has a
mains pressure cold feed to the hot cylinder rather than having a
gravity feed from a cold storage tank. The advantage is that you get
mains pressure hot water, and you don't have to accommodate a cold tank.
The disadvantage is that, if the mains fails, you have no hot water -
and you need a load of pressure limiting valves and (internal or
external) expansion vessels - and the whole thing can only officially be
installed and maintained by a suitably qualified person.
Question 1 - Why do you want to move away from oil which is a fuel
which can be easily shopped around for and if bought in summer is
Apart from our resident lunatic who simply parrots adverts he has
read, most logical thinking people realise that a store of water gives
you some security of supply if the mains goes off. If the storage tank
is high enough you can operate a quite satisfactory shower and have
the benefit of being able to fill a bath very quickly. If an immersin
heater is fitted you also have an emergency heat alternative in the
event of a failure of the primary heat source and you can (with a
suitable cylinder or additional indirect coil) utilise solar heat for
hot water contribution. There is even a feed in payment for hot water
systems available now subject to meeting certain requirements.
£260 for 500 litres at moment which lasts about 6 weeks in winter. Over
Christmass and new year lasted only 4 weeks.
Another issue is the cold water storage tank in loft. This was replaced with
a plastic/PVC one 20 years ago so how long do they last before needing
replaced. Galvanised one was 25 years old when it needed replace.
Pah, bought 1000l on 14 Dec, bought another 1000l on 20 Jan and for
more than your 52p/l inc VAT. B-( BTW 1000 and 2000l is quite often
a break point in discount, our supplier will normally knock 1p/l off
for 2000l and "negociate" for 1000l.
There are ones in one of our lofts that are probably getting on for
30 years old. Plastic tanks, properly installed, out of sunlight last
basically for ever.
In December with temperatures of minus 15 and below and the heating pipes
comming under the back garden I had the heating timed to come in for 15
minutes every 2 hours through the night to stop the pipes freezing, the oil
in the feed pipe on my brothers boiler froze at point of entry to the
Ah, I thought that modern open-vent systems might pressurise the hot water.
Great analysis, thanks.
How complicated is it to convert from open-vented to sealed? I am
thinking isolating the cold water tank and rerouting mains water, but
there may well be more I haven't thought of. I am very limited by
hardwood flooring and a single hatch near the boiler that, alas, has a
dwarf-wall between it and the hot-water tank.
Also, is it correct that with open-vented I don't have an expansion tank
to inspect for water loss? I am thinking of relocating the boiler
directly above its current location, which would impair access massively
(unfloored loft and awkward, tri-bearing trusses).
Kostas (expecting this is useful to the OP as well)
I fear that you may be confusing your water circuits! A house with a
central heating and stored hot water system has two separate water
circuits. The *primary* circuit connects the boiler to the radiators and
to the indirect coil inside the domestic hot water cylinder. This can be
vented or unvented (pressurised). If vented, there will be a small fill
and expansion tank - usually in the attic. If unvented, there will be a
filling loop with pressure gauge, a pressure relief valve (safety
device) and an expansion vessel. Converting from vented to unvented is
fairly straight-forward and involves removing the F&E tank and adding
the other components mentioned above. Whether vented or unvented, the
primary circuit keeps circulating the *same* water - and usually has
corrosion inhibitor in it.
The *secondary* circuit contains the water which flows to your hot taps,
and is stored in a hot water cylinder. It is heated by water from the
primary circuit which flows through a coil inside the cylinder, but
doesn't mix with the secondary water (you wouldn't want to bath in
inhibitor!). The secondary circuit can be vented or unvented. If vented,
there will be large a cold water header tank which feeds cold water into
the bottom of the hot cylinder to replace the hot water flowing to the
taps. If unvented, the cold water feed to the cylinder comes from the
mains rather than from a header tank - and you then need non-return
valves, pressure limiting valves, an expansion vessel and an
over-pressure relief valve.
Converting from vented to unvented is a major operation because the hot
cylinder has to be replaced with one which is capable of sustaining the
much higher water pressure, in addition to adding the other components.
As I mentioned in my previous post, this officially has to be installed
by someone who is suitably qualified because there are lots of safety
Which circuit were you asking about?
Thanks again, Roger (and harry too).
I was unaware of the details of the two different cold tanks; do I
really have a small F&E tank in the attic? I can't see it, but it's
probably on the unfloored side of the big tank.
I have no idea which circuit I am (they are) talking about, all I wanted
to figure out is what kind of boiler I should be looking at. I am now
guessing this is about the primary circuit, shall we establish? Let me
use Vaillant terminology, as per the following:
am discounting combis.
I like the fact that I know nothing about this F&E tank and have nothing
to maintain. Does this mean that I am looking at open-vent boilers? What
is the benefit of a vented primary circuit?
Re pressurised secondary circuits: am I right that they are possible
with both open-vent and system boilers? Also, I had a look at prices for
the Vaillant tanks and the eyes watered, even without the big job you
and harry mentioned. What does one get in return? You suggested "mains
pressure hot water, and you don't have to accommodate a cold tank." I
don't mind the latter; can one get a similar effect to the former by
installing a wee pump close to a gravity-fed HW tank? Any other benefits?
If you have an oldish traditional style boiler, then quite likely.
I am discounting combis.
Small header tanks would be for an open vented boiler. Many modern
boilers will instead used a sealed system with no tank.
I would argue that the sealed system is the one to go for in most
circumstances. Easier to implement, less space, easier and faster to
fill and bleed, no risk of floods if you get a leak in the heating
system etc. No sucking air into the system and no pump over. On the down
side its a tad harder to add inhibitor since you can't just log it in a
tank, but you will get less corrosion with a sealed system anyway usually.
Note that some system boilers can be run open vent as well. System
boilers are just heating boilers with the other gubbins required to make
the system work (like the pump etc) all one one box.
You get a very long life tank, hot water at mains pressure without need
for pumps etc, and no cold water cistern in the loft. Tanks are commonly
available in large capacities, so you can also store more hot water.
On the downside, they are more pricey to install, need regular
servicing, will be dependant on the flow rate of your cold water
main, and won't give you any water (hot or cold) if your supply is
 if its a fire hose capable 1 inch MDPE pipe, then great, if its an
dribbling scaled 1/2" lead jobbie then bad.
The Vaillant ones you were looking at can also interact with a boiler
that has external temperature control. This means that the boiler can
use low flow temperatures for maximum efficiency when heating the house
via the rads, but then switch to a higher temperature for reheating the
tank so as to make best use of its capacity.
You *might* have a sealed (unvented) primary system - in which case
there wouldn't be an F&E tank. Have you got an expansion vessel anywhere?
Sadly, neither have they!
terminology used by Vaillant in that reference is very confusing and
ambiguous - so I'm not surprised that you are confused!
The primary and secondary circuits can each independently be either
vented or non-venting - giving four possibilities - whereas Vaillant
give the impression that it's all or nothing. The most common current
domestic arrangement is a non-vented primary and vented secondary
circuit. You could almost certainly achieve that with either of
Vaillant's so called Heating Only or System boilers. The only difference
would be that the system boiler would have the pump and pressure vessel
inside the boiler casing whereas the Heating Only boiler would require
these to be installed external to the boiler. The system boiler saves
space - and some plumbing - but the pump and pressure vessel are
probably then proprietary, and more difficult and expensive to replace.
No. A vented primary circuit DOES have an F&E tank. An unvented system
has a pressure vessel and filling loop instead. You would still have to
maintain *that* in terms of checking the pressure periodically and
topping up if necessary - just as you have to check the level in an F&E
Others have already explained the pros and cons in terms of adding
inhibitor, pumping over, etc. Generally, non-vented primary circuits are
Because non-vented cylinders have to sustain mains pressure, they are
built like the proverbial brick s**thouse - and probably last for ever.
But they *do* require expert installation and regular maintenance - and
you get no hot water if there's a mains water supply failure.
You can indeed fit a boost pump on the hot water outlet (NOT the cold
water inlet!) of the hot cylinder, increasing the flow rate to the taps.
This would usually be controlled by a flow switch so that it would only
operate when you open a hot tap. It wouldn't be silent, of course, but
shouldn't be too intrusive.
Are these of interest in my (and the OP's case) where we already have an
old Y-plan system? Isn't it easier and cheaper to just stick a Heating
Only boiler in place of the old one and be done with it?
I don't meddle with these things and I have a local technican servicing
my dinosaur annually anyway, so I guess no problem in my case.
Ah, the Vaillant looks to my untrained eye as a problem in this
department. The weather compensation module requires its matching
compensator and (I think) an additional mixer module.
Thanks for the input!
No idea. Never been told to keep an eye on anything. Does the F&E tank
not fill up as needed with a cistern-type, floating-ball inlet?
The terminology used by Vaillant in that reference is very confusing and
I used ambiguous language above, because I was under the impression I
have nothing to do when an F&E tank is fitted.
Thanks. Modulo some minor points in this and my posting in response to
John Rumm, I am now less scared of the day my boiler inevitably goes. I
think that there is a simple, economical solution in the form of a
heating-only ("open vent" as per Vaillant) boiler available.
On Feb 3, 7:51=A0pm, Kostas Kavoussanakis
I as the original poster was looking for a cheaper alternative to oil
with minimum disturbance. My concerns were the pvc cold water storage
tank in the loft might need replacing as it is 20 years old and if I
needed to do that I may as well go for something different either a
combi or a system with hot water storage cylinder like the vaillant
one. I live in a chalet bungalow with the bathroom down stairs
bedrooms upstairs and cold water storage tank in loft so get very good
pressure in the hot taps so mains pressure hot water is not a required
so vented system would suit me.
The F&E tank *largely* looks after itself - using a ball-valve to top up
when necessary. *However*, because the ball valve opens so infrequently,
it can seize up - and not deliver water when required - with the result
that the F&E tank empties, and air is drawn into the system. That's why
it's a good idea to keep an eye on it periodically.