Worcester Bosch - Greenstar 27 Cdi


29 Jul 2008
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United Kingdom
We are having an extension built (or will soon be!), which includes the replacement of our ageing boiler and my architect has specified a Worcester Bosch - Greenstar 27 Cdi.

My plan is to have the main contractor install the new boiler (assuming they are corgi and are capable/willing to do so) and I haven't as yet got a contractor to ask if this boiler is right/up for the job?

Whats the general thoughts?

It will be relocated to a location in the garage and will serve a stone built house with 4 bedrooms, 1 bathroom (electric shower) - in total 12 radiators and possibly a towel radiator. As I have said, electric shower, but two young kids and a wife that likes to have a bath regulary!

Any other suggestions? Its a combi we will be replacing so had thought of a condensing combi as these are most effecient?!
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I would (as a heating engineer) suggest unvented cylinder and open vented heat only boiler. Would steer well clear of a Worcester. Essential that the system is chemically cleaned and power flushed for trouble free operation. Simple draindown is NOT a system cleanse, neither can the system be cleaned in couple of hours.

12 radiators means zilch. If each radiator output is 1kw, even the smallest combi will have no trouble carrying the heat load. What you need to know is what the total heat load is, and then go for the boiler. Alternatively, get a heating engineer to take burner pressure reading on present boiler and be advised what the projected heat load will be with towel radiator connected.

Assuming you have open vented system at present, unless valves and fittings are checked for soundness, fitting a sealed system boiler might not be a good idea.
All boilers are condensing now.

If you dont have a combi now be VERY careful and make sure you understand the short comings.

Has the Architect measured the water pressure and flow? He cannot specify a combi unless he has checked thats adequate.

My strong advice is to get a local gas registered engineer and NOT through the builder as they screw the gas man down and he does a bad job.

I would never work for a builder!

Agile - thats why I am questioning my architect - I can't knock his ability to draw but I suspect he knows F all about central heating! - hes certainly never even looked at our existing boiler, never mind measured water pressure and flow.

Not sure if the current system is open vented or not - is there an easy to tell, for a complete novice?

DP - I get what you mean by 12 radiators meaning nothing in terms of demand/output, will check sizes and confirm approx output.

I was thinking that I may be better off getting a heating engineer to carry out the boiler installation and associated works as a separate to the main builder work
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Never trust a builder or architect on heating issues; the most dangerous people are those with incomplete knowledge.

Combi or cylinder system is largely a matter of preference. Unvented cylinder is better performing in theory, but only if installed properly and the incoming water is adequate all the way form the street valve to the boiler. 1 bit of it in 15 mm, and the whole advantage is out the window.

Open vent system is very outdated and has several drawbacks; tanks really are a thing of the past, whatever you choose, make sure it is a pressurised system.
Open vented hot water systems are sill a very good solution and work properly if there is a poor mains water supply.

Good pressures can be obtained using shower pumps.

They are still the only economic system to use where the mains water supply is poor.

They can also supply more outlets by installing more cisterns and cylinders regardless of the mains water supply.


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