On 2007-04-28 12:36:29 +0100, said:

On Apr 28, 10:22 am, Andy Hall wrote:
On 2007-04-28 10:07:59 +0100, said:

Hi,
I'am deciding on replacing a very old baxi boiler-it uses too much
gas.
I only have one bathroom and I will also replace rads aswell as the
old ones are begining to leak,I will need 11 radiators.For now I just
want to know if the Vaillant eco-tec combi will be a good choice or if
I should avoid it.And what model will I need,,the 19 kw one or the
more powerful type.I'am pretty sure the system wll need to be power
flushed as its over 26 years old.
Thank you.

It isn't possible to determine the size of boiler required from the
number of radiators. They can be small ones of a few hundred watts
up to large panel ones of several kW.

Having said that, you can oversize a boiler quite happily within
reason, because they automatically modulate down to the required output.

The risk comes in undersizing. Assuming that the house has been
heated adequately in the past, you can make a reasonable estimate. Go
round and measure each radiator and not whether it is single or double
panel and with/without fins. Then look at data sheets on radiator
manufacturer web sites for similar products. You can get a good
approximation by size and use the heat output figures. Then add them
up and allow perhaps 10% more.

The more important point becomes provisioning of hot water. If all
of the system is of this age, then it would make sense to replace the
HW cylinder as well and to use a fast recovery type. These are able
to accept 25kW or more, thus providing a fast re-heat.

Alternatively, if you feel that you would rather dispense with the
cylinder and go for a combi, it would make sense to go for a high
power one - e.g. 38kW. However, checks into water supply flow and
gas provisioning are then needed. A 19kW combi is going to be very
disappointing for filling a bath in anything less than geological time
in the winter. In general, pipe re-runs may well need to be done in
order to get sensible flow of hot water to the right places and to
reduce the effect of turning on a tap in one place affecting another.
A combi *can* be OK if requirements are modest and an adequate one
fitted, but if you are used to the flow and filling time from a
cylinder, it will be difficult to match it. Either way, not something
to be undertaken without careful checking.
No Andy I meant 19kw ch output

Then you may need to do the sums on what is needed.

What is the output of the existing boiler? If the heating is currently
adequate and there is no short cycling of the boiler (which would
indicate it's over powered for the application) then all you need to do
is to match or exceed it.