Kilowatts?

[muggles]

2. Balance the system by closing the lock shield valves sufficiently to ensure that there is at least a 10C drop between the flow and return with the thermosatic valves fully open.

It's 20ºC not 10ºC Chris (unless the boiler manufacturer states otherwise). Welcome to the forum

 

[WeatherCompenation]

I'm talking about the temperature drop across EACH radiator being at least 10C. In practice I can usually achieve stable heat distribution with a 15 degree or less drop but I have never managed 20 in a free-flowing situation. Locking down the lockshields any further results in poor warm-up characteristics so a compromise is necessary. HOWEVER, once the system comes up to temp and the thermostatic valves start reducing flow in some radiators before others then the temperature difference between the flow going in to the radiator and the return coming out can be in excess of 30C.

The whole house (4 bedroom, detached no wall insulation) is comfortably heated to 23C at the moment with the boiler flow temp at 63 and the return to the boiler at 48. Temp outside is -5C. Very little visable plume, steady flow of condensate - the boiler is condensing and running at its max efficiency as far as I can tell!

I should have mentioned that, geven a choice, taller radiators (with top feeds!) are preferable to wider ones, again ensuring that water has chance to lose as much heat as possible before exiting the radiator and returning to the boiler.

 

[WeatherCompenation]

Oh yes and thanks for the welcome. I have been lurking for a while and just wanted to share my experiences. So long as heating system designers/operators think about ways of making the return water as cool as possible then the nation's huge investment in Condensing boilers might actually start to reduce fuel consumption and fuel bills!

 

[ajrobb]

I should have mentioned that, geven a choice, taller radiators (with top feeds!) are preferable to wider ones, again ensuring that water has chance to lose as much heat as possible before exiting the radiator and returning to the boiler.

I'm sorry but, apart from top feed, that is nonsense.

Shorter radiators put out more heat per unit area. Water loses heat to the radiator very effectively, it doesn't need a 'chance'.

 

[WeatherCompenation]

You have missed the point or maybe I worded it badly. The hottest water with a top feed will tend to be be at the top of the radiator, the coldest will be at the bottom. However, there is, of course, a flow of water into and out of the radiator and so some mixing occurs meaning that some of the warmer water will disappear out of the radiator. With a taller radiator the water stands a much better chance of achieving a stratified gradation of temperature from top to bottom. It is this "chance" to achieve a more stable temperatire gradient from top to bottom which ensures that only the coolest water returns to the boiler. I know this to be true - I have a mixture of radiator heights and the tallest ones are the ones which return the water at the lowest temps.

Remember too I am not interested in having small radiators going flat out and returning water at relatively high temp to the boiler. This is the conventional wisdom, I am trying to achieve as low a temp for the return for as long as possible so that the boiler is actually able to operate as a condensing boiler. You might say that the radiators in each room are therefore less "effective" which is why they have to be oversized BUT the system taken as a whole is certainly more efficient.

By the way, I didn't have to change my radiators in my home to make them oversized - the installation of double glazing achieved the same effect by cutting the heat losses.

 

[ajrobb]

You have missed the point or maybe I worded it badly. The hottest water with a top feed will tend to be be at the top of the radiator, the coldest will be at the bottom. However, there is, of course, a flow of water into and out of the radiator and so some mixing occurs meaning that some of the warmer water will disappear out of the radiator. With a taller radiator the water stands a much better chance of achieving a stratified gradation of temperature from top to bottom. It is this "chance" to achieve a more stable temperatire gradient from top to bottom which ensures that only the coolest water returns to the boiler. I know this to be true - I have a mixture of radiator heights and the tallest ones are the ones which return the water at the lowest temps.

You canna change the laws of physics. The temperature of water leaving the radiator is governed entirely by the flow rate, flow temperature and the heat lost. Geometry only affects how much heat can be lost and shorter radiators lose more heat per unit area. You've got 'oversized' radiators due to insulation, good. As you say, they allow you to condense more water. It is not sensible to buy oversized radiators as you won't get the money back from condensing savings. On the continent they have to size radiators larger for much colder weather than we are experiencing now. My sister lives in Sweden and they commonly experience temperatures of -25°C and -50°C sometimes further north. During milder weather they can make more savings from condensing boilers. Anyway, she uses a ground source heat pump.

This has got nothing to do with getting a good shower.

 

[PEDANTICVINDICTIVEMAN]

Isn't keeping boilers condensing for longer, going to rot the nuts off the Ali xchangers quicker ?