Boiler or balance?

[Dan Robinson]

I've never trusted that calculator. Best to do it room by room with a couple of online radiator suppliers.

 

[D_Hailsham]

I have tried the boiler size calculator, which told me I need a 48Kw boiler

I got about that figure as well!

I have tried redoing it a few times and keep getting slightly different results, but all in the low thirties Kw.

I haven't been able to get that low.

I suspect that the online calculator I suggested is not really suitable for your type of property. The basement is a problem because calculation of the U-value for the wall varies with depth; and the attic could be a problem if it is within the roof space.

Have you checked the outputs of your rads?

 

[Aloysius]

I have finally got round to measuring up my radiators, using the watt measurements on page 50 of the Stelrad catelogue (these seem to be the biggest of the Elite range). My set of 16 radiators comes out as follows:

Stelrad Elite 50t
Type Height Length Watts
single 700 1000 1597
single 700 700 1118
single 700 700 1118
single 700 700 1118
double 600 1200 2134
double 600 1200 2134
double 600 1400 2489
double 300 1400 1518
double 700 700 1408
double 600 600 1067
double 800 600 1422
double 300 1200 1518
double 300 1400 1518
double 500 1000 1409
double 500 1000 1409
towelrail 500 1200 539

Total Watts 23516

Does that indicate that there will be a capacity issue for my Greenstar System 24i boiler, or does it still point towards balance as being the cause for the three of four highest radiators in my house not getting fully warm? This is the case even when two or three (single) radiators are switched off.

 

[D_Hailsham]

My set of 16 radiators comes out as follows:

Stelrad Elite 50t - Total Watts 23516

Does that indicate that there will be a capacity issue for my Greenstar System 24i boiler, or does it still point towards balance as being the cause for the three of four highest radiators in my house not getting fully warm?

So you have a 24kW boiler feeding 24kW of rads in a large house. That would seem to be OK. But there' are two problems:

1st Problem
The boiler is designed to run with a temperature difference between flow and return of about 20C at max output but the radiator data is based on a 10C difference 75C/65C. If you want the same output with a 20C difference the rads will have to be run with an 80C flow and 60C return.

The problem then is that the boiler will not be condensing (the return temp has to be below 55C for condensing to occur). So we set the flow temp to 75C and return to 55C. Fine, but the radiator output will reduce to 85% of the figures given in the catalogue. SSo you would have to put in rads which are about 20% "oversized" to get the required output

2nd Problem
The boiler has a built in pump, which has an available head of 2m to pump the water round the heating circuit. The available head takes into account the pressure loss through the heat exchanger, which increases as the flow rate increases. If the boiler is run with a 10C difference instead of the expected 20C, the pressure loss through the heat exchanger will be higher so, instead of having 2m of available head you could have only 1m. In a house your size with the number of rads you have the required head could be over 2m. It all depends on how the system was designed and the pipe sizes used.

 

[Aloysius]

My set of 16 radiators comes out as follows:

Stelrad Elite 50t - Total Watts 23516

Does that indicate that there will be a capacity issue for my Greenstar System 24i boiler, or does it still point towards balance as being the cause for the three of four highest radiators in my house not getting fully warm?

So you have a 24kW boiler feeding 24kW of rads in a large house. That would seem to be OK. But there' are two problems:

1st Problem
The boiler is designed to run with a temperature difference between flow and return of about 20C at max output but the radiator data is based on a 10C difference 75C/65C. If you want the same output with a 20C difference the rads will have to be run with an 80C flow and 60C return.

The problem then is that the boiler will not be condensing (the return temp has to be below 55C for condensing to occur). So we set the flow temp to 75C and return to 55C. Fine, but the radiator output will reduce to 85% of the figures given in the catalogue. SSo you would have to put in rads which are about 20% "oversized" to get the required output

2nd Problem
The boiler has a built in pump, which has an available head of 2m to pump the water round the heating circuit. The available head takes into account the pressure loss through the heat exchanger, which increases as the flow rate increases. If the boiler is run with a 10C difference instead of the expected 20C, the pressure loss through the heat exchanger will be higher so, instead of having 2m of available head you could have only 1m. In a house your size with the number of rads you have the required head could be over 2m. It all depends on how the system was designed and the pipe sizes used.

Thanks so much for this detailed and helpful response. I can't really claim to understand much of it, but I'm sure it will come in handy. I assume the first step is still to ensure the radiators are correctly balanced - is that right? And then consider these issues from there.

A silly question probably, but with regard to the first point about perhaps needing oversized radiators, is it relevant at all that the problem top floor radiators get hotter to some degree if some lower floor radiators are turned off/down, and are coldest if all the lower floor radiators are turned on?

 

[Aloysius]

To see if I understand point one, is this saying that even if all the radiators were correctly balanced, if it was operating efficiently I would need to add another couple of radiators in order to get the full 24kw of heat that the boiler is able to produce (as each rad would only output 85% of its potential as long as system was set up to enable the boiler to condense properly)? It may be an option for me to add a rad or two if advisable.

However, is this issue likely to be any kind of cause of the problem with uppermost radiators (only) not heating fully? The way I read it was that a perfectly balanced system would mean than all my radiators heated up, but not to their maximum capacity (the balancing would mean that the total shortfall of c. 15% would be evenly distributed between all the radiators). Is that right?

 

[D_Hailsham]

I've merged your last two posts so they can be answered in one.

I assume the first step is still to ensure the radiators are correctly balanced - is that right? And then consider these issues from there.

Balancing will certainly help.

is it relevant at all that the problem top floor radiators get hotter to some degree if some lower floor radiators are turned off/down, and are coldest if all the lower floor radiators are turned on?

The reason the top rads go cold when the downstairs rads are on is partly a balance problem, but also due to the pump not being able to deliver the water to the upstairs rads. Balancing with a higher differential will help.

To see if I understand point one, is this saying that even if all the radiators were correctly balanced, if it was operating efficiently I would need to add another couple of radiators in order to get the full 24kw of heat that the boiler is able to produce (as each rad would only output 85% of its potential as long as system was set up to enable the boiler to condense properly)? It may be an option for me to add a rad or two if advisable.

The problem is that each rad will be giving off less heat, so each room will be affected. Now this may be OK in some rooms if the rad is already larger than needed for the actual heat loss.

However, is this issue likely to be any kind of cause of the problem with uppermost radiators (only) not heating fully? The way I read it was that a perfectly balanced system would mean than all my radiators heated up, but not to their maximum capacity (the balancing would mean that the total shortfall of c. 15% would be evenly distributed between all the radiators). Is that right?

The basic problem with the upstairs rads is balancing. Yes, when correctly balanced each rad will be giving about 15% less than expected.

Here's a quick way of balancing the system.

1. Remove all TRV heads and set all wheel valves to fully open.
2. Set all lockshield valves to 1/3 of a turn open from closed (fully clockwise).
3. Feel all return pipes (cooler one) to see how warm they are.
4. Select on which is midway between hot and cold and use it as a 'standard'.
5. Any rad with a hotter return than the 'standard' should have the LS valve closed a tiny fraction.
6. Any rad with a cooler return should be open a tiny fraction.
7. Repeat 3 to 6 until you are satisfied with the result.

Do no expect perfection. The adjustment each time is very small and you need to leave time (say 10-15 mins) between step 6 and step 3 for the system to settle down.

When finished replace all TRV heads and set to required temperature. Set all wheel valves to required position.