calculating a combi boiler size

[tjsarge]

HI. just refitting an end of terrace 2 bed house, I plan on putting 7 rads in which dependent on type will be from 29,000 BTU to 38,000BTU.

Was looking at Vaillant 24 or 28 the outputs seem to be is 78k and 96k respectively.

I understand its bad to buy too big a boiler, even the smallest of these two is double what i need...is there any ratio i should use or rule of thumb for radiator BTU sizes to output of boiler.?

the other question is the Litre per minute output of hot water, is it a simple as more is better? as i want a good shower output

sorry not a plumber /engineer, just an enthusiastic amateur

 

[Dan Robinson]

Higher kW means more hot water... depends on your incoming supply really. No point have more capacity than the supply will utilise. Your gas safe installer should be able to advise you further though.

 

[Agile]

Check he knows how to measure the dynamic flow rate!

 

[bell824]

as well as the above points the boiler out put on both these models can be adjusted so you only really need to think about the flow rate you want and if your dynamic pressures and flow can handle it.

 

[tjsarge]

thanks guys..

so if i got this right.. I assume no prob with much higher BTU output on the boiler vs the rads I am going to get.. as it is adjustable..
so i could go for the vaillant 24 or 28

mains pressure is good.. although I have not tested for specific bar yet

But i need to get the boiler guy to help match the mains water supply to lpm potential out of the boiler.,

without googling dynamic water flow, thats my best guess as to what it means

 

[DP]

OP, with kitchen tap running, collect water from another tap for 15 seconds. Multiply that quantity and you have a reference figure, A 24kwatt boiler will give you 9.4l, 12.7l for 31kw (no 28kw I am afraid) or 15.2l per minute from 37kw machine.

If you have only collected 2l of water in the bucket, there is no point in thinking bigger than 24kw boiler.