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]

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]

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 [DLMURL="http://www.energysavingtrust.org.uk/business/Business/Building-Professionals/Helpful-Tools/Whole-house-boiler-sizing-wizard"]EST online boiler calculator[/DLMURL].

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]

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 [DLMURL="http://www.energysavingtrust.org.uk/business/Business/Building-Professionals/Helpful-Tools/Whole-house-boiler-sizing-wizard"]EST online boiler calculator[/DLMURL].

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

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]

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.

 

[REDSAW]

exactly as myles says.

 

[Cab]

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.

I have a 'regular' boiler at present which i want changing to a 'system' boiler. I have a cylinder (unvented 170ltr) and don't require a combi .

My dilema is what would be the best power output to go for to have fast re-heat times on the cylider and central heating. At present if i let the temp of my house drop right down to about 12 deg C it takes a good 12 hours or more to get back up to 19 deg C. Also heating the cylinder takes ages too if i've got both the heating and the hot water on.

Thanks

 

[doitmyself]

I have a 'regular' boiler at present which i want changing to a 'system' boiler. I have a cylinder (unvented 170ltr) and don't require a combi .

Thanks for repeating that, even though it was quite clear from your original question. Some contributors seem, somehow, to never read the original question properly and automatically assume that you want/need a combi boiler.

My dilema is what would be the best power output to go for to have fast re-heat times on the cylider and central heating.

The two calculators came up with 13kW, i.e 11kW for heating and 2kW for HW. There is no point going over the top and installing a 30kW boiler as the rads will not be able to dissipate that amount of heat. I suggest you install a 15kW boiler. That leaves 4kW for the hot water, which should be sufficient for the cylinder.

Does your house cool down quickly when the heating is turned off? If it doesn't, one possible solution is to install a system with hot water priority. This means that, while the HW is being heated, the Central Heating is temporarily turned off. The full output of the boiler is then allocated to heating the HW, resulting in reduced heating up times. Obviously the cylinder must be capable of handling this amount of heat. The other advantage is that you do not have to allow any extra in the boiler size for HW so you would only need an 11 or 12kW boiler.

Having 14kW of rads when you only need 11kW is a good thing. The boiler will be able to run at lower temperatures. This means that weather compensation should be considered.

I know you say you want a WB boiler, but you should have a look at the Broag-Remeha Avanta Range - 18s. This boiler can provide both weather compensation and HW priority. It might seem to be oversized, but the CH output can be set to a lower amount if necessary, without affecting the output to the HW cylinder.

 

[Blackcatgas]

I have a 'regular' boiler at present which i want changing to a 'system' boiler. I have a cylinder (unvented 170ltr) and don't require a combi .

My dilema is what would be the best power output to go for to have fast re-heat times on the cylider and central heating. At present if i let the temp of my house drop right down to about 12 deg C it takes a good 12 hours or more to get back up to 19 deg C. Also heating the cylinder takes ages too if i've got both the heating and the hot water on.

Thanks

Sorry cab I was replying to Derrybongos post not yours, even though he hijacked the thread.

Thanks for repeating that, even though it was quite clear from your original question. Some contributors seem, somehow, to never read the original question properly and automatically assume that you want/need a combi boiler

.


My reply was to Derybongos question

 

[Cab]

Just a quick note to say thanks to all for your input/advice/suggestions. It is very much appreciated

Thanks