On 6 Nov 2005 04:32:50 -0800, "
wrote:

thanks for this , what's a stack by the way? can you also tell me how
to do the water flow calculation

You're welcome, Neil.


The stack means the soil stack, although obviously you need to look at
the design rules in the PDF file. If this is on the inside of the
house, it may be worth considering, otherwise probably not. I was
simply thinking that generally getting things through a loft space is
a lot easier than under floors, and could be a way to avoid having 4m
of waste pipe on the outside of the house. Obviously it depends on
the arrangement of the house etc.

To calculate the flow, take a container (e.g. bucket) of known
capacity. (e.g. 5 litres). Using the watch, time how long it takes
to fill from the sink cold tap.

Example: If it takes 15 seconds to fill a 5 litre bucket, then that
would be 4 times a minute and the flow rate is 20 litres per minute.

You could do this using a 2 litre measuring jug, but it's not as
accurate as a larger container because of the difficulty in accurately
checking the watch.

It's worth repeating the test a couple of times and averaging the
readings.

Note also that it will vary somewhat during the day.

The water suppliers are only legally required to deliver 9 litres per
minute, and obviously this could be disappointing in comparison with
the existing system.

Generally as long as there is about 20 litres/min or more, then
results are reasonable.

Keep in mind that the quoted combi flow rates are for a 35 degree rise
in temperature, so in effect in the winter this will be the total hot
flow rate for bath/shower temperature (when mains cold is in the
typical 5-8 degree range).

If you want to be scientific, you could measure the current flow rate
into the bath and also time how long it fills to normally used depth.
Then you can calculate how long it would take with the flow (possibly)
limited to what the mains can do, or what the combi can do.

If mains flow rate turned out to be an issue, then options are to get
the pipe from the street upgraded (could be expensive), or stick with
roof tank and use that to supply the combi.

If the combi is felt to be inadequate but the idea of a box in the
airing cupboard appeals, then there are larger combis than 33kW and
also appliances which store some amount of hot water within the boiler
itself (Powermax is an example of this and there are a number of
others). Look for "Combined Primary Storage Unit".

Don't be concerned if the appliance rating appears to become large in
relation to the heating needs of the house. In heating mode, the
boiler will reduce (modulate) its output down. The full power is
required for water heating, and for basic combis which don't store
water, the HW production rate is directly proportional to the
appliance rating.



It may well be that the type/range of combi is entirely suitable -
probably is for a single person who is not time constrained. However,
the whole project is a substantial investment, so an hour spent doing
the homework and checking does make good sense.




--

..andy