On Fri, 18 Jul 2003 18:41:23 +0000 (UTC), "duncan"
wrote:

Hey guys

What a response.

Its funny that not matter how you pose a question on here about
heating/ho****er it always ends with combis in the red corner and
conventional systems in the green corner, fighting it out. LOL

I feel that I'm a well confident DIYer having now nearly completed the whole
house extension myself. Wouldn't choose a combi for its convenience unless
it fits the bill.

Andy - I've sent away for that book so will take a few evenings reading
that.

It will give you a good appreciation of the steps in radiator and pipe
sizing, radiator derating and so on plus the issues with pump
selection and setting, organisation of pipes to avoid pumping over
problems etc.

You can then get a lot of information from manufacturer's web sites.

With combis its even more important to get the piping sizes correct and
design right to prevent hot water not feeding enough sources (kitchen,
shower etc.)

It does mean that care needs to go into the sizing and branching off
of pipes so that there is reasonably good balance of flow naturally.
Then there are techniques with flow restrictors, blending valves etc.
if you need to go further.


It is still bothering me that having two showers in use in the morning is
still
gonna be a problem with a combi. Plus the missus likes a bath and not sure
that
she has the patients to wait for it.

This is the big advantage of a cylinder. You can make it adequately
sized to be sure that you will always have a plentiful supply,
regardless of the draw rate.

For example, I have a cylinder with multiple tappings on the top which
will take 22mm pipework. It is fed from the roof tank in 28mm pipe.
The effect is that I can run two baths at full tilt or a bath and a
shower or two showers at 25 litres/minute each.

You can go for very large combi boilers and get a flow rate of up to
about 20 litres per minute with a 35 degree temperature rise.
For a lot of the year when the mains water is warmish - say 15 degrees
and above, you will be able to mix the heated water with quite a lot
of cold - e.g. in a shower mixer.

However, if you are concerned about the worst case, that would be in
the winter when the water supply can be well below 10. If you
consider that the user temperature in a shower is around 40 degrees,
you can see that almost all of the boiler hot water will be needed and
little opportunity to add cold. Therefore you end up with 20 litres
per minute between two showers - i.e. 10 each. Then it becomes a
matter of opinion as to what you find acceptable. Personally I feel
that 15 litres/min in a power shower with adjustment between hard jets
and drench mode is about the minimum - others say less or more.
If you already have a shower in place, try measuring the flow rate
with a bucket and stop watch to get a feel of what you are getting.



Thermal stores - any suggestions on them please.

The well known site on these is DPS. www.heatweb.com although the
cylinder manufacturers are making them as well.

The main advantages that you can get out of one of these are that

- the primary circuit can remain vented so that the bulk water in the
cylinder is not pressurised. Sealed cylinder pressurised systems
require a BBA or IoP approved installer to fit them

- The cylinder can store water at higher temperatures than if it is
used for hot water. It can be at 80 degrees rather than 60, so a
third more energy is stored in the same space. The mains cold water
is fed through a plate heat exchanger external to the unit and then
the primary water from the tank is pumped through the exchanger when
there is demand to heat the water. This type of exchanger is highly
effective on heat transfer and so if the water supply is up to it,
much greater flow rates than with a combi can be achieved.

As the store is being used, the boiler will be adding heat back in,
although typically at a lower rate than you are using it.
To give a simple example, you might be drawing hot water at a rate
such that 60kW is required to heat it to the extent required. If the
boiler is able to deliver 30kW into the cylinder, then the net rate of
use is 30kW. Therefore the amount of hot water that you can get at
the 60kW rate is twice as much if the boiler is adding heat in, using
this example. Once the stored energy runs out, the flow rate would
have to be halved to maintain the same temperature rise. Make sense?

The same additive effect occurs in a conventional cylinder of course -
the difference being that the amount of energy stored is 25% less.

If you are going the condensing boiler route, it won't be as efficient
with a thermal store as it can be with a conventional cylinder because
the operating temperature is higher. It's also better to run the
radiators separately from the primary circuit than from the store.



What sort of cylinder should I go for if i go down the convential route?

A fast recovery type. Vendors of these include Albion and Telford
among others. These have various arrangements to increase the
surface area of the coil - e.g. more turns, multiple pipes, etc. The
point is to increase the transfer rate of heat so that the water is
heated more rapidly, and if you have hot water priority arrangement
that the boiler is not taken from heating the house for too long.








duncan
well at least i'll have all weekend to read the response lol

There should be plenty of those. Thrills and spills for all the
family :-)




"duncan" wrote in message
...
hi,

Any recommendations for sources of information on
designing a central heating/ho****er system for please.

have found sites that provide info on rating the boiler/rads
but nothing on the actual design. type of heat storage etc.

duncan





..andy

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