Worcester Bosch 42 CDI or Valiant 837 Ecotec

Boiler sizing tools, heatloss tables or mears calculators are about as useful for combi's, as french doors on a submarine.
Both the heating requirement and the hot water requirement have to be taken into account when sizing a boiler.

If you have a heat-only boiler the heating requirement is the main factor. You just add 2kw or 3kw for the cylinder and select the appropriate cylinder size for the number of baths and showers.

If you have a combi the main criteria is the hot water requirement and the flow rate. Installing a 40kw boiler when the flow rate is only 5 l/s is a waste of money.

The problem comes when the mains has the necessary flow to feed a 40kw boiler but the heating requirement is very low, say 10kw. If you buy a WB 42CDi you will easily meet the HW requirement. But the CH requirement is a different matter. The 42CDi modulates between 9.4kW and 30kW. That means the boiler will be on the lower limit of modulation when the temperature is lowest. As the weather improves and heat requirement decreases, the boiler will no be able to modulate any lower. So it reverts to on/off mode and the efficiency drops.

There is a danger that, even in the cold weather, the boiler will run in on/off mode, as has happened with others on this site with a boiler which was considerably oversized for the heating requirement.
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If the heating requirement is very small but the HW needs are genuinely very high then the solution is a Rinnai instant water heater with the CH supplied by a small boiler like the Worcester 12Ri which modulates down to about 3-4 kW.

I see many powerful combis fitted when the cold water supply does not justify them.

Its often forgotten that a loft conversion for occasional use only ie better fitted with an electric shower. Far cheaper.

If you must fit a high power combi then you will suffer from inefficient and less well controled heating.

Ascots were made ( or sold ) from Radiation House at 615 North Circular Road, London !

Rinnai are made in Japan !

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Ascots were made ( or sold ) from Radiation House at 615 North Circular Road, London !

Rinnai are made in Japan !
I was referring to the fact that any instantanous water heater was referred to as "the ascot"; rather like any vacuum cleaner was called "the hoover" and you "hoovered" the floor.

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