On 09/11/2016 15:36, Clive Page wrote:
On 09/11/2016 14:25, Martin Brown wrote:

My instinct is that provided the pump is fast enough to avoid a 13C
temperature drop on the loop the overall efficiency won't be all that
much different. The only thing that might make a difference is if one of
the burner controllers is smart enough to determine the system thermal
inertia and loading to avoid overshooting the 65C target.

Can anyone explain to me why it is desirable to avoid a temperature
difference of more than 13C ?

Its advice that applied to old non modulating non condensing boilers. If
you designed for a delta of about 12 degrees, then you could be safe in
the knowledge that even if you dropped the flow temperature to 65, you
would still keep the return temperature above the point at which you
would get condensation of the flue gasses in the boiler (once it was all
up to temperature of course - you would still get some condensation at
startup - but that water could be boiled off soon after). This at the
time was a good thing since it stopped the boiler with its cast iron
lump HX from rusting away.

That means when it starts up from cold,
with the inflow around say 15C that the outflow should be no more than
28C - which means the pump must send the same water around several times
before the outflow gets to 75C?

With old boilers they just ran at a fixed output - the system balancing
combined with the boiler's internal stat would set the parameters. So it
would condense a bit at startup, and then run normally once up to
temperature.

I don't understand why a boiler with a flame at several hundred Celsius
would be more efficient working like that than it it raised it right
away from say 15 to 75C.

They don't have the power to lift that quantity of water to 75 in one pass.

The efficiency gain in the condenser comes from two factors. Firstly
being able to extract more heat from the combustion in the first place
(bigger more efficient HX), and secondly by condensing the water
produced during the combustion process, you can claw back the energy
tied up in the latent heat of vaporisation of the H2O manufactured when
you burn a hydrocarbon fuel.

--
Cheers,

John.

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