On 21/05/2017 11:43, The Natural Philosopher wrote:


To heat 15 l/min (an average single shower) from 5C to say 45C takes 42KW.

Average showers are targeted to deliver typically from 8 lpm these days.
15 lpm would be on the higher end for a normal shower mixer.

To fill a bath with water at 45C in 4 minutes is around 20l/min or 55KW.

For sure you can add a heat store to a combi at EVEN MORE EXPENSE to
give better figures than this, but that is pretty much what a DPHW
system IS.

My point is simple. Heating a house requires a constant low to medium
heat output over a long period of time. A boiler designed to do that
efficiently does not have the capability for a massive output over 4
minutes of the day when you want to fill a bath. Attempts to rectify
this with high power but modulating boilers etc simply make the boiler
more complex and costly.

Most modern (decent) system boilers have modulation anyway. Typically
the higher powered combis don't output the same power to the CH side
either. (my old 35kW Ideal Isar only delivered a maximum of 25kW to the
heating side).

The more complex solution is a thwacking great boiler.

Prices are similar, but even then, if you install the monster combi, you
can STILL only cope with one shower or bath at a time and you cannot
open a hot tap elsewhere.

Even a 35kW combi will feed two showers at once. Not as well as it will
do one, but still each will be better than an electric shower for example.

In short:

Installing a system boiler geared to the CH requirements and a DPHW tank
to provide peak flow of piping hot water gets you the optimal solution
to both problems,

I agree its a good system (its what I installed here). However it has
some of the same limitations as a combi. If you mains supply pressure or
flow rate is inadequate, then neither system will work well.

and is no more expensive than a small combi with a
large heat bank, or a large combi with no heat bank.

Not convinced. If you compare like with like kit there is not much
difference in price. If installing yourself so you don't need to cost
the labour, then there is not much in it. If paying for the
installation, then (depending on the starting situation) there may be a
fair amount of extra work for an unvented system.

And even a large combi can't meet the same peak demands that stored hot
water can.

That rather depends on your peak demand. The last combi I fitted was for
a place with one shower and no bath. (With no ideal location where a
cylinder could go). The 24kW combi met the peak demand with ease.

The only cheap option is a pathetically limited small combi with litle
or no heat bank.

Or a gravity fed cheap water tank with a system boiler.

That's not much cheaper than unvented by the time you add the cost of a
cistern even if the cylinder itself is a bit cheaper. However its better
suited to properties with low mains flow rates.

I have experienced both. They suck, big time,.

I also have experience of a variety of styles of system. However do not
find it so easy to make blanket Dr. Dribble style assertions. One needs
to look at the bigger picture.




--
Cheers,

John.

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