On Tue, 09 Aug 2005 18:48:09 +0100, Geoffrey
wrote:

On Tue, 9 Aug 2005 18:05:25 +0100, "Jo King"
wrote:

Just some questions about a combi boiler. What is the difference between a
condensing combi boiler and just a plain combi boiler . I have an old back
boiler behind the fire place that runs the C.H. and heats the hot water
cylinder, would a combi do away with the cylinder, do I have to take the
back boiler out completly or can it still sit behind the fire place, would
the flue liner have to come out of the chimney. Thank you for any help.


As I'm going to have to do the same thing eventually (well soon
probably) I'd like to tack on another question. Can I do away with the
cold water tank in the loft if I have a combi or is there no
connection? My instinct tells me that the cold water tank in the loft
has nothing to do with the boiler/heating system but as I know
naff-all about plumbing I'd thought I'd just ask this one stupid
question.

One thing I DO know (I think....) is that the little header tank could
be got rid of should I choose to go the combi route. Is there any way
to get rid of it with a conventional boiler?



The small tank is a feed and expansion tank for the primary heating
system water. It is the water circulated through the radiators, the
boiler and the coil in the HW cylinder.

Most new boilers can work with a sealed system which essentially
replaces this little tank with a pressure vessel for expansion and a
filling arrangement through a flexible hose and non return valve.
There are some advantages as explained by the FAQ in Ed Sirett's
signature. This is not an issue of whether it's a combi or a system
type of boiler which heats a cylinder.

The cold storage tank could potentially be eliminated by the use of a
combi.

However there are issues to consider:

- You would get mains pressure hot water which may be attractive, but
only if the flow rate of the mains water supply is good (at least 20
litres/minute.) You should measure this at the kitchen cold tap to
get a rough idea. Internal plumbing alteration to upgrade pipework
may be needed. In the worst case, an upgrade to the service pipe from
the street may be required and that can be expensive.

- Unless you get a high flow rate combi of at least 35kW or so, HW
flow performance in comparison to the flow rate from a storage tank
system can be disappointing. Be especially careful to look at usage
requirements in terms of number of baths and showers.

- It may be necessary to upgrade the gas pipework from the meter to
the boiler.

None of these are insurmountable issues apart perhaps from an
inadequate water supply at the street main, but it does make sense to
check before deciding.



--

..andy

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