How to size a boiler?

Over Clocker

Over Clocker

Over Clocker

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Hi guys,

We'll be looking at system boilers when we do our home extension next year, and was wondering if anyone could help in sizing the boiler?

We'll be getting a 'megaflow' style system.

We'll be going for wet underfloor heating for the downstairs, which will be around 100 sq/m or so.
Upstairs will be radiators, likely 6 of them, and should be a total of 3 bathrooms.

Will a 27 be sufficient for the above or should we be looking at something larger?

Thanks,
Mal
 
The right way to size a boiler is to use a whole house heat loss calculator. There used to be online versions, but now it seems you need to calculate it yourself (there may be an excel you can download):

https://www.energysavingtrust.org.u.../CE54 Domestic Heating sizing method 2011.pdf

The point being that the boiler should be matched to the heat loss of the house in the depths of winter (usually -1 degrees outside). It takes into account the construction of the house, the roof, the size of windows, the floor etc etc. Remember, an oversized boiler will be inefficient. An undersized boiler may work adequately in the summer but wont heat the house up properly in the winter.

It used to be the case that you would add in an additional 2KW for hot water, though the unvented cylinders these days (like Megaflo) have 20+ KW coils in them. The best way to heat them is on PDHW (Priority Domestic Hot Water). Couple this with weather compensation and oversized radiators and you'll have a pretty efficient system.
 
Over Clocker said:
Thanks a lot.

Nope, it's not a system boiler currently, just a conventional one.

Thanks a lot for those links.

Cheers,
Mal
Click to expand...

Something to consider is if you’re moving from a regular boiler to a system boiler is your pipework may not cope. System boiler is a high pressure system in comparison.
 
Last edited:
Roughly 1.5KW for each rad and 3 KW for a cylinder coil

Thats 12kw

Leaving 13kw for underfloor downstairs

That boiler is easily large enough for what you want but you'd be best off with a 24kw
 
fezster said:
The right way to size a boiler is to use a whole house heat loss calculator. There used to be online versions, but now it seems you need to calculate it yourself (there may be an excel you can download):

https://www.energysavingtrust.org.uk/sites/default/files/reports/CE54 Domestic Heating sizing method 2011.pdf

The point being that the boiler should be matched to the heat loss of the house in the depths of winter (usually -1 degrees outside). It takes into account the construction of the house, the roof, the size of windows, the floor etc etc. Remember, an oversized boiler will be inefficient. An undersized boiler may work adequately in the summer but wont heat the house up properly in the winter.

It used to be the case that you would add in an additional 2KW for hot water, though the unvented cylinders these days (like Megaflo) have 20+ KW coils in them. The best way to heat them is on PDHW (Priority Domestic Hot Water). Couple this with weather compensation and oversized radiators and you'll have a pretty efficient system.
Click to expand...
I'd be surprised if all modern boilers couldn't be oversized without issue. Modern boilers have much better modulation rates than they used to.

You need to consider the lower end of the modulation rates as the minimum, this allowing a decent overhead of power which means with a decent setup you can have your home heat up faster then modulate low enough to maintain the heat.
For instance my combi is a 32kw and can modulate down low enough to stay on continually to maintain the temperatures this achieving high efficiency.
 
The larger kw output boilers (35+) are unlikely to be able to modulate down as well as smaller boilers. This is usually a limitation of the gas valve itself, rather than any software restriction. However, newer boilers are definitely getting better in this regard.

A combi is usually always a compromise because homeowners want the increased hot water flow rates from massive kw output boilers, but it's highly unlikely anything but the biggest 5/6 bed house OR a ridiculously poorly insulated house would require anything like 40kw.

The biggest mistake, often made by a lot of gas engineers also (!), is to add up the kw output of radiators to size a boiler. This wrongly assumes the radiators have been sized precisely to the heat loss of the house and this is almost never true.
 
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