"John Rumm" wrote in message
o.uk...
On 29/12/2011 17:47, Doctor Drivel wrote:

"chris French" wrote in message
...
In message o.uk,
Dave Liquorice writes
On Sat, 24 Dec 2011 19:32:09 +0000, geoff wrote:

I think Geoff was commenting on combis and slow water delivery.
You
need a very powerful one to be able to get a reasonable flow of
hot
water out of it.

Not really, mine is a 28kW which is fairly standard nowadays, and it
delivers sufficient flow rate.

28kW is quite a powerful boiler. A modernish house won't need
anything like that to keep it warm.


If I've got the maths right 28kW can only manage 10l/min raising the
temperature from 10 to 50C.

(28/(4.187*40))*60


From memory, (we had something like a 28kw combi in the old house,
maybe 24KW) that sounds about right.

Sure it was sufficient for sinks and showers (good shower in fact, we
had plenty of mains pressure), but as Blair says takes a ges to fill a
bath

35kW is becoming the norm.

Having lived with a 35kW combi in the past, I would count that as about a
realistic minimum if you need bath filling. Great showers, but still
fairly feeble for volume delivery in the winter.

You can get a 50kW plus and the flow rate
belts out.

It will be better but not spectacular.

It will approach what the cold mains can deliver.

You are also getting to the stage where you are going to run out of flow
capacity on a standard domestic meter and governor if it has to share the
supply with much else.

62kW can go down the pipe past the meter.

"John Rumm" wrote in message
o.uk...
On 29/12/2011 11:19, Doctor Drivel wrote:

"John Rumm" wrote in message
o.uk...

I think Geoff was commenting on combis and slow water delivery. You
need a very powerful one to be able to get a reasonable flow of hot
water out of it.

Powerful? A domestic gas meter can cope with approx 62kW. so a 35kW
(approx 15 litres/min) is only using half the available capacity.

Not sure what your point is...

Lots of installers still slap in 24kW combis for moderate sized
households, and their DHW performance is fairly feeble, and hence it does
the reputation of the breed as a whole a disservice.

I would class 35kW and up as a moderately powerful combi, in the sense
that is about the point where it becomes tolerable to live with in a house
that uses the bath reasonably often. Many makers don't do anything much
above 40kW for the domestic market.

But many do.

With the fairly decent cold main feed we have in this place, even a 60kW
combi would be marginal.

That will deliver about 27-28 litres/min.

If you were selecting a PHE for a heatbank fed from a decent mains supply,
you would probably stick a 100kW unit in there to make sure you always got
adequate performance from it.

The idea is to ensure a cool return from the plate so it is over-sized and
to ensure it works well if scaled up inside.

On 30/12/2011 13:53, gremlin_95 wrote:
Won't having a combi with such a high output use a huge amount of gas?!
We have a 37kw storage combi which is perfectly adequate but given the
choice, we would have gone for a heat only boiler + unvented cylinder.
We went for a combi simply because we haven't the room for an unvented
cylinder.

It shouldn't use any more than a smaller one.

If you need to heat 100l for a bath it's not going to take any different
amount of gas whether you do it in 2 minutes or 4. Unless of course the
bath is cold by the time you've filled it!

Andy