On Wed, 24 Dec 2003 18:28:40 +0000 (UTC), "Michael Chare"
wrote:

The point I'm trying to make is where can I go to get a good,
impartial recommendation about the right kind of system, and what
specification that system should be? It's no good talking to a plumber
because all you will get (in my experience) will be "You want a combi,
gov! Sort you out a treat it will!" before they even know how many
bathrooms I have, or whether I have a washing machine and a
dishwasher, or how many people live there.

I have not read previous posts on the subject (but I have read the faq). So here
are my thoughts:

The size of a boiler needs to be matched to the size of the radiators etc to
which it will be attached.

It needs to be rated to exceed the sum of the radiator outputs plus a
reasonable margin - typically 20% unless the house is exposed, the
ceilings are high or the heating runs for short periods There are
uplift correction factors for this.


The heat loss from each radiator depends on its area. If you cant find this in
a radiator catalogue or else where try a factor of 170 Btu/sq ft /hour

It also depends on the type of radiator, single/double panel, fins or
not and the model. You can't just pluck numbers out of the air -
they will probably be wildly inaccurate, BTUs are really a
deprecated unit. It is far easier to work in watts, metres etc.

The heat losses for each room should be calculated and radiators sized
to match those losses, assuming -3 degrees outside temperature,
choosing the appropriate derating factor to account for the flow and
return temperatures from the boiler.



You can also work out the heat loss from the pipe work. Say 10 Btu/hour per foot
lenght for 15mm piping.

This is generally not a significant factor and if the pipes are within
the building envelope it simply adds to the radiator outputs.
Outside the envelope or in places where heat is not wanted, the pipes
should be lagged with insulating material of at least the same
thickness as trheir diameter.


Do you have an indirect cylinder and will you be keeping it? A ball park figure
for an indirect cylinder is 12000 Btu/hour

That's old practice and at 3.5kW will lead to very poor hot water
recovery performance. Best practice nowadays with an indirect
cylinder is to use one with a fast recovery heat exchanger and then to
use a diverter valve or switched zone arrangement from the boiler.
When heat is required to reheat the water, the boiler is switched over
and runs at full output for a shortish time. The cylinder can absorb
the full output which may be 20-30kW. This way the boiler runs more
efficiently, the hot water is recovered more quickly and the boiler is
off line from the heating for a minimum period.



Calculate the total heat loss and then maybe add on a safety marging say 20% and
this will give you an idea of what size boiler to use. The look to see what
size boilers you can buy and what the difference in price between a small one
and a larger one is. This will give you and idea of what to do. Also how big is
the boiler that you have now? does it run continously on a very cold day or does
it spend time switched off whilst the radiators cool the water/boiler to the
point where the boiler will start gain.


Quit a lot of modern boilers, both condensing technology and not, are
able to modulate their output dynamically to match the heat demand
rather than switching off entirely. This makes matching to heat
loss less critical as long as the boiler is not grossly oversized,




Alternatively get a few quotes and look what size of boiler is specified.

You might also want to consider a high efficiency condensing boiler. Do you
want to dispense with your expanion tank and hot water tank )which I assume that
you have now)

Happy Xmas
Michael Chare


..andy

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