Is it worth replacing my standard system boiler for a combi

Thanks all for the advice.

My current boiler is a Potterton Suprima 100, which is rated at 100 BTU (29kw). I was thinking to replace it with a Vaillant ecoTEC Plus 30kW. Is that a decent boiler?

It doesn't say it is a condensing boiler though.

The other one I like is the Worcester Bosch Greenstar CDi Classic Regular 30kW
I'd rather have an Intergas. Why do you like those two?
 
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Most here would prefer the Vaillant. Or even another brand.

All current boilers are condensing.

But do you really need 30 kW ?

Have you calculated the heat loss? Or even tell us what kind of house and the insulation level.

Tony
 
All current boilers are condensing.

All current boilers are designed to be condensing but they can only operate as a condensing boiler if fitted to a heating system which allows them to operate in condensing mode. If the temperature of the water returning to the boiler from the radiators is too high then the boiler cannot condense efficiently if at all.

It is possible to buy boilers that are not condensing but these are only permitted ( by Building Regulations ) in a very few situations where a suitable flue and / or condensate drainage cannot be installed. Very unlikely in a domsetic situation.
 
I've used the online calculator at http://www.idhee.org.uk/calculator.html. It came out at 24.6KW. But I thought it may be better to have a bit extra rather than be short? A different estimator said I'd need 40kW!

It's a semi-detached Edwardian house, over 3 floors. 1900 square feet in total. 12 radiators, of which 4 are large old cast iron ones. Solid brick with render on the outside but no interior or exterior insulation. Original wooden single glazed windows.

We have one shower fed from the hw tank, and one electric shower.

Do you think 30kW boiler is too much?
 
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I'd rather have an Intergas. Why do you like those two?

Only because those two brands are the ones my plumber recommended and is most used to. I'll ask him about the Intergas ones you suggested before. They look good - and are a bit cheaper too
 
You did not mention the loft insulation. That can lose a lot of heat if not properly insulated and is very cheap and easy to bring up to standard.

With solid walls the heat loss probably is about 24 kW.

A simple way to check the results of these calculators is to use the floor area and use 100 W/m2 for a badly insulated house with solid walls down to about 30-40 W/m2 for a well insulated property. But that does require judgement on the figure to use!

But I am against over sizing boilers and would normally expect to use exactly the power which equals the heat loss. But ideally taking action to reduce heat loss by draught proofing, insulating the loft and probably ground floors as they are often a severe loss of heat in that style of house.

Tony
 
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Thanks Tony. The loft has a large room in it, which was installed when the house was built 100 years ago, even though the other half of the semi doesnt. The loft room has a sloping ceiling which is entirely uninsulated, so that room gets very very cold in winter. I am planning to do it, but it's not easy as there is no access.

My plumber said something about oversizing a bit and then dialling back the CH heat output. Does that increase efficiency?

I can't decide between 24kW and 30kW. Probably 24 would be enough as we don't heat all rooms in the house. But if it's under I guess the house will just take much longer to get warm?
 
Over sizing REDUCES efficiency caused by short cycling as well as potentially some other problems.

Is the house really two stories with a loft room?

The output of the boiler can of course be reduced in the settings. But little point in buying a 30 kW and reducing the output to 24 kW.

Tony
 
Yes - It is two stories plus a loft room + lofty bathroom, so 3 in total.

I want it to be efficient, but I'm worried about undersizing and then wishing I'd gone bigger. I did that with the log burner and it never gets the room comfortably warm.
 
Another reason for inefficiency / poor heating is the selection of radiators that are too small ( in terms of heat output ) to heat the room when the temperature of the water circulating through the radiators is low enough to allow the boiler to be condensing. Reusing existing radiators that were sized to heat the room with a delta of 40°C will not provide enough heat when the delta is only 20°C

delta ( roughly speaking ) is the difference between room temperature and average temperature of the water in the radiator
 
I've just received the quote from my plumber. He's a good guy that I've used before, and I know he'd do a good job. However the price did seem on the high side.

Vaillant EcoTec Plus 430 boiler, fitted to replace existing boiler, including a new Honeywell 3 port valve, and the installation of a Grundfoss condensate pump (because boiler is in the cellar).

Total cost £2300 + VAT.

Does that sound OK? I'm based in Croydon (South London)
 
Oh OK. So it's a good price. Great. I'll go for it then.

The pump is newly fitted and is the top Grundfoss one apparently. With auto power levels etc.
 

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