On 5/21/2017 11:43 AM, The Natural Philosopher wrote:
A system boiler of 8-15kW, plus pressurised hot water tank with the
added advantage of an immersion heater if you don't want to run the
boiler, is around £1200 in total.

A 45Kw combi to deliver the same hot water flow rate is similar, or more
costly, and is certainly bigger in the boiler room

(You can site a pressurised cylinder wherever you like)

If you have an unpressurised tank you need pumped showers.

(Interestingly, in the two new builds going up here, with no mains gas,
they are using air source heat pumps, UFH, and a pressurised tank plus
the mandatory immersion heater to bring the stored temp up to 60C to
kill the bugs. Air sourced heat pumps simply can't deliver the peak
output, and indeed the insulation level installed to allow them to even
heat the house at all is phenomenal)

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

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.

The simplest solution is a heat store. Mutatis mutandis a decent sized
heatbank on a combi is no different to a system boiler with a DPHW tank
attached.

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.


The worst solution is to enforce a pathetic rate of hot water flow on
the house-owner. Because you didn't give a **** when you built the poxy
little house you sold him.

Actually no, the worst solution is a gravity fed unpressurised tank. In
a bedroom, so no upstairs hot water pressure exists at all, and you have
to pump.

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


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

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.

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


Agree with all you say, apart from the final point; gravity fed water
tank is OK *as long as you have a shower pump, and (perhaps) one for the
other DHW*. This does add £300 to the cost: don't buy a cheap shower
pump, get a Stuart Turner.