What value to use for sizing boiler?

[AlanRob]

Hi all,

I am in the process of costing / ordering for having an old oil boiler & gravity fed hot water cylinder replaced, potentially by a Glow Worm 30HXi boiler & Megaflo unvented cylinder 125 litre. (good prices been given)

Regarding the megaflo cylinder, it has a rating of 18.3 kW, is this the value that is used for sizing the boiler when performing a whole house calculation or do I use the std 2kW?

Using 2kw gives a boiler size of approx 10kw & 18.3kW gives a boiler size of 25kW.

Any help appreciated as I don't want to drastically over size or worse still, have an under powered boiler that couldn't cope.

Rgds,

Alan

 

[chrishutt]

If you want the quickest possible reheat time for the Megaflo, you will need the boiler to have an output of at least 18kW. Anything less will tend to extend the reheat time, but this is unlikely to be a problem in practice unless your hot water (DHW) demand is really pushing at the performance limits.

Obviously an 18kW or less boiler will not meet CH and full DHW demand simultaneously, so again you need to consider how much that matters. However adding the full loads together to get 25kW would be considered excessive.

On the other hand, the smaller the boiler, the more efficiently it will operate (and cheaper to buy). You could size it at around 10kW for maximum efficiency. But be aware that in extreme circumstances the boiler could be struggling to meat demand (but not the end of the world).

In short there is no easy answer. You have to consider likely peak demands on DHW (how many in household, etc.) and users' tolerance for occasional under performance.

 

[AlanRob]

Chris,

Thanks for the response, much appreciated.

In essence then, I'm looking to get as close to the Heat Ouput in condensing mode value for max efficiency but when demand for HW is satisified, the boiler would operate normally.

The difference in price between the 30Hxi & 15Hxi is around £100 - cost saving here, has half the max gas rate and gives 16kW in condensing mode. Happy medium and room for expansion when extension built & garage converted.

As for user tolerances.... got a wife & 2 daughters...who like their showers, washing m/c on frequently...