In message , Malcolm Reeves
) wrote:
On 26 Dec 2004 07:09:13 -0800, (N. Thornton) 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.

I'm sure thats not correct. Faster flow for same power input will mean
rads run a little hotter, thus will dissipate more heat. Thus
efficiency greater. Faster flow also means lower temp water /out/ of
the boiler.


BUT, the energy taken from the
hot gas is more is the water is cooler,

yes

hence a lower return, larger
drop, slower pump, makes the boiler a bit more efficient.

I think theres a simple logic error there. Slower pumping does mean
lower return temp, but it also means greater temp rise in the boiler,
and hotter water leaving the boiler. What affects efficiency is the
/average/ water temp in the boiler, not simply the return temp.


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.

absolutely.

If the return is cooler then the boiler water is
cooler and you get more energy out of the gas.

No. At least not for the same rate of gas burning.


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.

thanks. I would have thought the 2nd would be ss though, rather than
the first - not that it matters a whole lot to us.


NT

The message
from (N. Thornton) contains these words:

Same with CH faster pump lower
drop but watts shifted is the same.

I'm sure thats not correct. Faster flow for same power input will mean
rads run a little hotter, thus will dissipate more heat. Thus
efficiency greater. Faster flow also means lower temp water /out/ of
the boiler.

Faster flow with the same power output will mean the output temperature
is lower and there will be a lesser temperature drop across the
radiator.

--
Roger

"I would have thought the 2nd would be ss though, rather than
the first - not that it matters a whole lot to us."

Depends what you call the second and the first. The first heat exchanger the heating water sees on it's return to the boiler is the condensing one. This is the second heat exchanger that the products of combustion see and the one where the vapour is formed, and it can be made of alluminium as an alternative to stainless.

Probably the way to go is with the single ss heat exchanger as seen in the Buderus.