On 21/05/2017 09:41, Scott wrote:
On Sat, 20 May 2017 21:48:49 +0100, John Rumm
Not much value in ECO mode really - it only fires fairly infrequently to
temper its small store of hot water.
It does seem to make a big difference. Or at least if the heating is
on the water arrives more quickly.
Yup, having eco mode turned off makes it far more usable IME - with
faster hot water delivery, especially when all you want is a small
amount. Eco mode on will save some gas, but I would not expect it to
make a significant difference to consumption in most households.
I may not be to do with Eco mode,
I suppose but I think it is as I think it is much the same effect even
if I turn the heating down to 10 degrees.
The bath takes ages to fill. This may however be related to having an
unusual thermostatic mixer tap arrangement that serves the bath and
the shower.
Or an underpowered boiler...
Or the hot water not nearly as hot as it was before?
Often with combi you won't be adding any cold to the mix - i.e. you
select a fill rate that delivers the hot water at final use temperature.
When that is the case, the boiler is running flat out anyway.
Unlike a stored water system, you don't have the option to deliver the
hot water at a much higher temperature than you need so that you can
augment it with extra cold water (or at least there is no advantage in
doing so, since to get it hotter, you need to have it slower).
Its sometimes instructive to do the sums for various inlet and outlet
temperatures to see what rates you can expect. If you assume that a kg
of water needs 4200 joules of energy for every C lift in temperature,
and the combi can provide 24000 x 60 joules per minute, then you can get
some numbers. So for example, if the incoming water is at 10 deg C, and
you want a bath at 45 deg C, then that's a 35 degree lift. 24000 x 60 /
35 / 4200 = just under 10 lpm. If the inlet water is colder, say 5 deg,
and you want water at 50, then it becomes 24000 x 60 / 45 / 4200 = bit
over 7.5 lpm.
The radiators warm up even when the heating is supposed to be off. I
understand this is because the boiler uses the heating as a heat sink
when it is pre-heating the hot water. Again, I assume this means
wasting energy.
That sounds more like a faulty diversion valve.
Is this what used to be called the 'motorised valve' when I was a boy?
Yup mostly. Depending on the boiler it might be motorised, or some have
a kind of solenoid activated one. However the basic principle is common
to many combis; when they detect DHW flow, they fire up the main burner,
and activate the diversion valve. This re-routes the primary flow so
that it only goes through the secondary plate heat exchanger for the hot
water and not the rads. If this valve were not fully diverting the flow,
then you would expect the rads to get warm, and the ability to heat the
incoming cold water would be impaired.
As a complete diversion, if I wanted to replace my boiler with a mains
water filled storage tank is there a market for second-hand boilers or
would economic and regulatory issues consign it direct to the scrap
heap?
The latter usually alas, unless you can flog it to someone who will self
install.
Note however, that you *can* have a hot water cylinder heated with a
combi. Just because it can also do hot water on its own, does not mean
you have to use that capability. There is nothing stopping you from
adding external zone valves and controls to divert the CH water through
a cylinder in the normal way.
In some cases this can be a system worth installing from new. if you
have a conventional gravity fed HW cylinder. You get fast bath filling,
but also have mains pressure (wholesome) hot water available direct from
the combi side for either a shower or kitchen tap. With a mains (i.e.
unvented) cylinder, then there is less benefit in this approach since
the cylinder will also feed a good shower. However if you already have
the boiler, it saves the cost of replacing that - and 24kW is more than
adequate for a fast cylinder recharge.
--
Cheers,
John.
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