What size of gas boiler

Pythagorous

Hi y'all,

We are in the process of changing from oil to gas and therefore deciding what size of boiler is appropriate.

The Worcester Bosch seems to be the model of choice, but I'm unsure if a 30 CDi or 40CDi would be best (not combi as we are keeping the current hot water tank).

http://www.worcester-bosch.co.uk/homeowner/boilers/gas-boilers/greenstar-40cdi-classic-regular

The house is about 1900 sq ft, detached with 13 radiators (2 verticals, 2 bathroom ladder types, 4 singles and 5 doubles).

Does anyone have any personal experience to help decide if the extra £500 for the 40 CDi would be worthwhile?

Thanks

captainhindsight_2

It is common for many to just oversize their system.

What you need is a full heat loss calculation done on your property, any competent heating engineer should do this and from this work out what size boiler you need.

Too big is better than too small, but you want the correct size and you will only work this out from a full heat loss calculation.

I would suggest vaillant to worcester-bosch, with the worcester you are just paying extra to see you boiler on tv adverts every day.

Edit: are you changing from oil to lpg or mains gas? If to lpg, biomass or heat pumps would be better as you could claim the RHI which could put you in profit rather than your heating being an expense (it is also worth considering even if you are switching to mains gas).

Pythagorous

Thanks, that's very useful. Changing to mains gas from oil.

Smiley_Dan

I'd ask in the energy forum. This forum's for off (gas) grid-ers.

Robwiz

We have a similar sized house and the same number of radiators. Our boiler is rated at 32 kW but it's non-condensing so only 80% efficient. With condensing boilers, to achieve the energy savings you need to have the return temperature no higher than 55º C – oversizing the boiler makes it harder to achieve this. If you are using the boiler to heat your DHW, you need a flow temperature of 70º C to achieve 60º in the cylinder.

Silent_Dancer

You can do a whole house heat loss calculation here.

http://www.idhee.org.uk/calculator.html.

captainhindsight_2

Silent_Dancer wrote: »

You can do a whole house heat loss calculation here.

http://www.idhee.org.uk/calculator.html.

This is just a rough guide, what the op wants is a full heat loss calculation.

Silent_Dancer

captainhindsight wrote: »

This is just a rough guide, what the op wants is a full heat loss calculation.

This is the model that a heating engineer should be using.

It is provided online by the Institute of Domestic Heating and Energy Engineers so members of the public can do their own calculation. The model has been devised by the Building Research Establishment.

Oversizing a boiler means that it will never operate in condensing mode and therefore not saving energy. The boiler will instead just cycle on and off increasing wear and tear therefore leading to premature failure.

Regarding the OPs situation it is difficulty to comment without knowing the full details of the house. A well insulated house of that size built to current building regs may only require a 12kw heat input whereas a draughty poorly insulated property may require 40kw. Hence my suggestion that the OP should use the heat loss calculator.

captainhindsight_2

Silent_Dancer wrote: »

This is the model that a heating engineer should be using.

It is provided online by the Institute of Domestic Heating and Energy Engineers so members of the public can do their own calculation. The model has been devised by the Building Research Establishment.

Oversizing a boiler means that it will never operate in condensing mode and therefore not saving energy. The boiler will instead just cycle on and off increasing wear and tear therefore leading to premature failure.

Regarding the OPs situation it is difficulty to comment without knowing the full details of the house. A well insulated house of that size built to current building regs may only require a 12kw heat input whereas a draughty poorly insulated property may require 40kw. Hence my suggestion that the OP should use the heat loss calculator.

Wrong again

It is a basic version, a full heat loss calculation will take in to account window sizes the age of the property known u values of insulation, type of construction, individual room sizes, orientation of the building, location of the building etc etc etc.

The number of inputs needed for a full heat loss calculation is almost 10 pages long so it is not possible that that link provided is as accurate it is just a basic version of a full heat loss calculation

Silent_Dancer

captainhindsight wrote: »

Wrong again

It is a basic version, a full heat loss calculation will take in to account window sizes the age of the property known u values of insulation, type of construction, individual room sizes, orientation of the building, location of the building etc etc etc.

The number of inputs needed for a full heat loss calculation is almost 10 pages long so it is not possible that that link provided is as accurate it is just a basic version of a full heat loss calculation

From the Institute of Domestic Heating and Energy Engineers website.

Welcome to the IDHEE on-line calculator for the Crown Copyright document "WHOLE HOUSE BOILER SIZING METHOD".

It is offered by the IDHEE as a service to members, and for consumers to check the boiler size offered by a non-member.

Whilst the IDHEE can accept no responsibility for the factors used in the calculation, it recommends its use when changing a boiler to ensure unnecessary over-sizing does not occur.

As the resulting boiler size may be significantly smaller than the unit to be replaced, it is important to realise that the recommended boiler will operate far more efficiently than the over-sized boiler being replaced. (This will be in addition to the higher efficiency of the newer boiler).
http://www.idhee.org.uk/calculator.html

So the professional body for domestic heating engineers uses this calculator. Note that the IDHEE calculator is the Building Research Establishment one that is the basis of SAP calculations for building regs and EPCs.

Of course other heat loss calculators are available, such as the HHIC one. Which one do you use, captainhindsight?

captainhindsight_2

Silent_Dancer wrote: »

From the Institute of Domestic Heating and Energy Engineers website.
So the professional body for domestic heating engineers uses this calculator. Note that the IDHEE calculator is the Building Research Establishment one that is the basis of SAP calculations for building regs and EPCs.

Of course other heat loss calculators are available, such as the HHIC one. Which one do you use, captainhindsight?

I personally don't use any but my staff use full sap stroma software

This clearly highlights your lack of knowledge of the modern heating sector.

The link you providid is just for guidance purposes any enginer that uses that is clearly unqualified !