CH pump - fast or slow?

radiators

Maybe I'm being thick, but that looks to me as if it implies:

- in parallel rad systems, output per rad proportional to rad size, when that's not necessarily whats wanted, its defniitely not in this house at least.

- in older series plumbed rad setups, very little heat output from rads on the end of the chain in small houses, or no heat at all in large houses.

Maybe someone can explain? :)

NT

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.

The philosophy of having a uniform drop does assume - maybe optimistically(!) - that the system has been designed with the correct size of radiator in each room to balance the heat losses of the room. If you have any seriously over-sized radiators, you need to restrict the flow a bit more so as to reduce the average temperature. [If you've got any seriously under-sized rads, you're stuffed!]

Are you talking about single pipe systems - where the water flows through all the radiators in series rather than parallel? If so, there are hopefully not too many of these still around - and the normal balancing rules wouldn't work.

Here, each radiator is receiving water which has already been through all the upstream radiators, rather than direct from the boiler. The average temperature of each radiator is thus lower than that of its predecessors - so you need progressively larger radiators if you are to get any decent heat out of them.

The message from snipped-for-privacy@meeow.co.uk (N. Thornton) contains these words:

The design output of radiators is quoted with that sort of temperature drop. ISTM that with balanced rads you will get the same proportion of the rated heat output from each rad even when circumstances mean that you won't get the actual rated heat output. Should you have a badly designed system (ie some radiators that don't match the heat demands of the room) you need some other means of regulation as well such as TRVs.

Single pipe systems are little better than ornament. When the heating is needed the most the radiators at the end of the chain have the least chance of any significant temperature drop.

Yes. In practice it is very difficult to achieve perfect balance, BUT, the alternative is one radiator gets very hot and loads are very cold so even a rough balance is better. TRVs will do the final balancing. A rough balance is still needed though since otherwise the morning warm up will not be even.

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.

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.

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

boiler

increase

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.

yes

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.

cooler).

absolutely.

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

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 snipped-for-privacy@meeow.co.uk (N. Thornton) contains these words:

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.

"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 hea exchanger the heating water sees on it's return to the boiler is th condensing one. This is the second heat exchanger that the products o combustion see and the one where the vapour is formed, and it can b made of alluminium as an alternative to stainless.

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

-- Paul Barker