In message , Malcolm Reeves
) wrote:
On Tue, 14 Dec 2004 23:46:12 +0000, Hugo Nebula abuse@localhost
wrote:


Which is best - running a central heating pump at its fastest or
slowest speed setting?

The right speed. Boilers are designed for 80C out, 70C back, a 10C
drop. Radiators are designed for 75C average temperature.

To fast and boiler drop is 10C giving reduced efficiency as the
lower
the water temperature the more energy taken from the hot gases.
Radiator temperature is higher making them give out more that their
rating.

I'm not so sure. Surely if you run the pump faster, as you say the
rads give out a bit more heat, so the average water temp in the boiler
must be _lower_ for the same power input. Yes it returns at higher
temp, but it will also leave the boiler with less temp rise, if the
power input is the same but water speed higher. I'm assuming a non
modulating boiler here. Overall then system efficiency would increase
slightly with faster pumping. The downside of course is increased pump
noise and wear, and some systems might pump over.


To slow and boiler drop is 10C. Efficiency is higher

If water speed drops, the temp rise in the boiler would increases, and
the water must leave at higher temp (despite the return being cooler).
Consequently efficiency would be less.

Is that lot wrong, and if so, why?


Condensers like a low
return temp as that helps them condense. For them 55C return is
good
so you have a lower radiator temperature and lower radiator output
and
hence condensers need larger radiators (if they are to condense all
season).

So whats the difference with condensers that stops them rusting when
conventional boilers die if run condensing? Stainless exchangers?


NT

In an earlier contribution to this discussion,
N. Thornton wrote:


To slow and boiler drop is 10C. Efficiency is higher

If water speed drops, the temp rise in the boiler would increases, and
the water must leave at higher temp (despite the return being cooler).
Consequently efficiency would be less.

Is that lot wrong, and if so, why?


How are you defining efficiency?

The usual definition is something along the lines of the ratio of the amount
of heat which gets into the water compared with the amount of gas energy
burned. For the optimal heat transfer from the hot gases to the water, there
needs to be a certain difference in temperature. If the return water is too
hot, reducing this differential, less heat is transferred - and more goes
out through the flue, heating up the neighbourhood. By most definitions,
this is *less* efficient.
--
Cheers,
Set Square
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On 26 Dec 2004 07:09:13 -0800, (N. Thornton) wrote:

In message , Malcolm Reeves
) wrote:
On Tue, 14 Dec 2004 23:46:12 +0000, Hugo Nebula abuse@localhost
wrote:

Boilers are designed for 80C out, 70C back, a 10C
drop. Radiators are designed for 75C average temperature.

To fast and boiler drop is 10C giving reduced efficiency as the
lower the water temperature the more energy taken from the hot gases.
Radiator temperature is higher making them give out more that their
rating.

I'm not so sure. Surely if you run the pump faster, as you say the
rads give out a bit more heat, so the average water temp in the boiler
must be _lower_ for the same power input. Yes it returns at higher
temp, but it will also leave the boiler with less temp rise, if the
power input is the same but water speed higher. I'm assuming a non
modulating boiler here. Overall then system efficiency would increase
slightly with faster pumping. The downside of course is increased pump
noise and wear, and some systems might pump over.

Think of the water and pump like one of those coal trains that go
round and round feeding coal fired electricity station. The faster
the train goes round the lower the coal level in the wagons. The
amount of coal shifted doesn't change. Same with CH faster pump lower
drop but watts shifted is the same. BUT, the energy taken from the
hot gas is more is the water is cooler, hence a lower return, larger
drop, slower pump, makes the boiler a bit more efficient.

To slow and boiler drop is 10C. Efficiency is higher

If water speed drops, the temp rise in the boiler would increases, and
the water must leave at higher temp (despite the return being cooler).
Consequently efficiency would be less.

Is that lot wrong, and if so, why?

Efficiency is getting the most heat from the hot gas. You get more if
the water is cooler. If the return is cooler then the boiler water is
cooler and you get more energy out of the gas.

Condensers like a low
return temp as that helps them condense. For them 55C return is
good
so you have a lower radiator temperature and lower radiator output
and
hence condensers need larger radiators (if they are to condense all
season).

So whats the difference with condensers that stops them rusting when
conventional boilers die if run condensing? Stainless exchangers?

Yes stainless steel. Condensers have 2 heat exchangers. I guess only
the first is usually SS.


--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
, or ).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

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