I have a three bedroom semi, built circa 1975 and need a new boiler. I want to have a combi boiler and am looking at Worcester Bosch systems. A Greenstar 24i junior has been suggested. Would this be powerful enough to heat eight radiators and one bath/shower? The house is fully double glazed and has some insulation in the loft, although not up to current recommendations. I have been quoted £1200 to replace the aged Baxi Barcelona boiler, which would include removal of the cistern etc. Does this seem reasonable?
24kW will heat your house with some to spare. The real gas guzzler is the bath/shower. The CH and the DHW are never on together so they don't interact as such, if you select DHW the CH goes off whilst you're using the DHW, nobody ever notices unless you're lining people up to have umpteen showers one after the other.
As for the bath/shower, be prepared for 15 minutes to run off a bath and for the shower to not be a veritable 40C torrent, especially in winter. Perfectly acceptable unless you're a power shower connossieur(sp?). Always better than an electric shower in any case. I have a 28kW and am happy with that but have to back off the ( powerful ) flow of the shower a little bit in midwinter to maintain the temperature: still a good shower though. You'd need 30+kW to avoid that.
I advise putting in a pressure regulator in line with the shower to avoid getting slugs of cold water coming through when some numbnut in the kitchen turns the hot water on when you're using the shower. With a pressure regulator valve thingy the flow will decrease but I hear that the temperature will remain sensibly constant ( NB: A thermostatic mixer can't cope with the speed of temperature change when someone sucks all the HW out of another tap so don't rely simply on the thermostatic mixer alone if you want frustration-free showers!).
Mixers designed for combis/multipoints cope with this OK. They handle very wide pressure variation on the hot, and rapid temperature changes (using spiral bi-metalic strips which look like large pieces of swarf). The cheap wax pellet types won't cope though.
Peak output on a combi is more related to how fast you want to fill a bath/shower etc than house heating. In almost all cases you won't need a high peak output for house heating: Just a high efficiency at low to moderate powers.
Thats where I will stop. I am not a connoisseur of model types, but I don;t think you will need much more than 10KW to heat the house in the coldest conditions.
Interesting, I happen to have the cheap wax pellet type so that explains a lot! I should have bit the bullet and shelled out for a decent thermostatic mixer in the first place....
The £1200 pounds is about right for the fitting fee. If it's the bottom line not counting the boiler cost including all the necessary work to standards and the paperwork and the sundries and VAT.
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