What size boiler is required please?

[Cab]

Hi

I need to upgrade my old system boiler as the exixting one is under powered following recent extension works to my property.

The house has cavity wall insulation, 300mm loft insulation and plastic framed double glazing. The property is a compact 4 bedroom house with bathrooom and en suite. There is also a conservatory with large radiator. In total there are 13 radiators and when i add up all the outputs of each radiator and allowing 2kw for the unvented cylinder it comes to 14KW.

I want a Worcester Bosch wall mounted system boiler but not sure which model would be best suited to the system requirements. I've had vastly varying recommendations on size and have been left a unsure which model to choose .

I would be very greatful for your help on choosing the correct size?


p.s Our current boiler is rated at 8.8kw and has to run 24-7. It can just about manage to keep the house at about 19 degrees C and the water at 60 degrees C with a very very slow reheat time.

Thanks to all

[stevetheplumber]

answered your own question really if the heating load is 14 kw thats what you need however it may be that your existing rads are to small for the rooms so allways allow a margin so id go with a minimum of 18kw
personally i seldom use system boilers as often the same combi is cheaper to buyand i usually connect the hot water to either one shower or to the kitchen sink

[Cab]

I guess i'm probably going to get shot down in flames now by saying that i was thinking of a '24CDI' or a '30CDI' .

My reason for this choice would be because surely it would be better to have a system that isn't straining to keep up with demand so therefore would have an easy life and is less likely to go wrong. Also 'Worcester' say the boiler will supply varying levels of output to suit.

What do you think ?

[payney1974]

Originally Posted by Cab

I guess i'm probably going to get shot down in flames now by saying that i was thinking of a '24CDI' or a '30CDI' .

My reason for this choice would be because surely it would be better to have a system that isn't straining to keep up with demand so therefore would have an easy life and is less likely to go wrong. Also 'Worcester' say the boiler will supply varying levels of output to suit.

What do you think ?

Alot of people think that but it is best to have it sized for what you need as it is more efficient.

[doitmyself]

Originally Posted by Cab

In total there are 13 radiators and when i add up all the outputs of each radiator and allowing 2kw for the unvented cylinder it comes to 14KW.

Presumably the radiators were in place before the cavity and roof insulation were installed and possible even the double glazing. If so they are probably now oversized and no reliable guide to the size boiler required. You should use one of these calculators to find out what size boiler you need:

Sedbuk Boiler Calculator or EST online boiler calculator.

The results will be the same, some people find one easier to use than the other.

If these calculator come up with a result less than the 14kW of your rads, that does not mean you will have to change the rads. It does mean that your boiler will be able to run at lower temperatures, which is more efficient and will save you money.

[grahamgas]

What boiler do you have at the moment?

It could be possible to uprate this boiler, if it is possible.

[derrybongo]

how much kw do you add for running a shower or a bath after using the calc

[Cab]

Originally Posted by doitmyself

Presumably the radiators were in place before the cavity and roof insulation were installed and possible even the double glazing. If so they are probably now oversized and no reliable guide to the size boiler required. You should use one of these calculators to find out what size boiler you need:

Sedbuk Boiler Calculator or EST online boiler calculator.

The results will be the same, some people find one easier to use than the other.

If these calculator come up with a result less than the 14kW of your rads, that does not mean you will have to change the rads. It does mean that your boiler will be able to run at lower temperatures, which is more efficient and will save you money.

Done the calculators as you suggested. They come out at 13KW

The existing boiler is a Glow worm EP30B.


Thanks

Originally Posted by derrybongo

how much kw do you add for running a shower or a bath after using the calc

I think 2KW is added by the calculators for supplying enough hot water to re-heat the cylinder. My cylinder is a Telford Tempest 170ltr unvented which supplies one bath and two showers.

Cheers

[derrybongo]

if there is no cylinder with only the boiler to fill the bath (and heating)are we looking at a 30kw just so we can supply hot water at a decent rate even though for heating we only require 14kw

[Blackcatgas]

Originally Posted by derrybongo

if there is no cylinder with only the boiler to fill the bath (and heating)are we looking at a 30kw just so we can supply hot water at a decent rate even though for heating we only require 14kw

Yes, It takes the big Kw to instantly heat the hot water, combis are modulating so if you require 14kw for your heating they will modulate down and then run full rate say 30kw for the hot water demand.