Sizing a boiler, any point in going bigger on a combi?

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I have measured my cold tap which has a flow rate of 11.8 litres a minute. The Baxi I am looking at comes with 12.2 Maximum DHW flow rate (Delta T @ 35'C rise) or 15 if I go for the bigger boiler.

Is there any advantage in going for the higher capacity boiler as I assume it is not working flat out. Also does a boiler reduce the flow rate so 11.8L input does not necessarily give me 11.8L output if the boiler is at near capacity but an over capacity boiler with reserves might give me 11.8L output for 11.8L input?

Many thanks to the experts here who always provide great answers.
 
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Is there any advantage in going for the higher capacity boiler as I assume it is not working flat out.

You also have to consider the minimum heat output from the boiler compared to the heat load when heating the house. It can be that the heat load required to keep the house warm is less than the minimum the boiler can modulate down to.

If the boiler's minimum output is greater than the heat needed to keep the house warm then the boiler will have to cycle ON and OFF. This cycling is wasteful of gas and the extra wear and tear on the boiler can shorten it's life.
 
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Thanks for that. The house is 4 bedroom. I have calculated the rad output as 12Kw
yes but unlikely that all the rads will be on max all the time, , the difference between a boiler at max and min is called the turn down ratio, some boilers have reall good turn down ratios, some not so good
 
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You need to know your total incoming flow rate, not just the flow from one tap which may have internal restrictions.

Baxi boilers tend to have fairly poor modulation ratios - the 830 only has a ratio of about 3.5-1. Many boilers now offer around 10:1
 
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Captain Nemesis

I have measured my cold tap which has a flow rate of 11.8 litres a minute. The Baxi I am looking at comes with 12.2 Maximum DHW flow rate (Delta T @ 35'C rise) or 15 if I go for the bigger boiler.
What do you want to do with the HW?

Fill sinks/handbasins, or have showers?
 
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Time filling a sink is important as is the shower. I will be downsizing the boiler as I have overkill at the moment. I presently have a 13 year old Worcester Bosch Greenstar 42CDI which seems far to big for a house with 11.8 litre per minute flow rate from the cold kitchen sink tap, which is right by the stop cock and incoming feed and 12kW max radiator output.

I am now thinking about the Ideal Vogue 32. That version has DHW of 13.1 l/min while the heating is 4.6-26kw. As per the opening question I don't want a slower flow rate from the hot taps because I have reduced the boiler size so does the output from a boiler match the input to the boiler regarding flow rates? i.e. The boiler receives 11.8 l/m so kicks out 11.8 l/m or does a smaller capacity boiler reduce the flow rate due to restrictions within the boiler.
 
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The hot water flow rate is say 12 l/m with a 35 degree rise.

If cold incoming is around 10 degrees that gives you a 45 max delivery temp@12 l/m.

If your flow rate stays the same but you have a more powerful boiler then the temp rise would be greater and you would be able to achieve delivery temps at 12 l/m up to the max on the hot water temperature selector on the boiler which can't be adjusted above 60 degrees on the safer boilers.
 
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Great answer and something I hadn’t considered. My understanding was that there was no point in getting a boiler much bigger than the the flow rate of the cold feed but by getting an oversized boiler allows that 11.8 l/m to output hotter water. The Ideal Vogue 32 has an output of 13.1 L/m which is less than the 15 of the existing Greenstar 42cdi but seems a good compromise.
 
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