Boiler sizing & efficiency ?

[stooley]

I'm looking to replace a gas-fired 100k BTU AGA-type range cooker / boiler with a modern gas boiler (and an electric fan oven !) but am uncertain which size to go for. Ignoring the oven, the radiators around the house total to around 60k BTU (each with a thermostatic valve) and the house warms up quickly whenever the heating comes on, so allowing for the water cylinder I assume around 80-90k BTU is what I'll need. However the heat output range quoted for condensing boilers seems very wide : for example, the Worcester Bosch Greenstar 30CDI quotes 26 - 102k BTU, which I assumed would be fine. However the WB website describes this as for small - medium sized houses and recommends the 50% more expensive 40CDI (output range 32k - 140k BTU) for a 4-5 bed house like ours.

Is it really advisable to go for the bigger boiler, even if it's rarely going to be called upon to run at much more than half its maximum output ? Or put another way, is there an obvious downside (in terms of efficiency, life expectancy, ...) of specifying a boiler to run at closer to its maximum output ?

Any guidance much appreciated.

 

[BigBurner]

They are trying to offer a one size fits all - to an extent. They modulate down. Best get one that modulates down very low. The top end rating doesn't matter as long as it can heat the house and is cost effective. A higher top end means a faster warm up. Get one with an integral weather compensator.

 

[gas4you]

If you only need 60,000 btus (18Kw) then that is the size you need. If you add 3 Kw on for DHW then you could go for a 24Kw.

I would fit a Vaillant Ecotec Plus 624 and range rate it upon commissioning to 21Kw max.

 

[Agile]

My energy efficiency course only told me to add 2 kW for HW.

However, the heat source should be based on the whole house heat loss rather than the radiator heat load.

Tony

 

[tonybhoy]

They are trying to offer a one size fits all - to an extent. They modulate down. Best get one that modulates down very low. The top end rating doesn't matter as long as it can heat the house and is cost effective. A higher top end means a faster warm up. Get one with an integral weather compensator.

All of the CDIs can be adjusted to modulate lower than the max boiler output set by WB when the boiler first fires (by entering the service mode) to have the boiler modulate on less power at the start of the cycle down to whatever you want it to actually.

 

[joshgroeny]

They are trying to offer a one size fits all - to an extent. They modulate down. Best get one that modulates down very low. The top end rating doesn't matter as long as it can heat the house and is cost effective. A higher top end means a faster warm up. Get one with an integral weather compensator.

This sounds efficient. Could you please reccommend a particular machine that matches these criteria?

Cheers,
Josh

 

[mogget]

They are trying to offer a one size fits all - to an extent. They modulate down. Best get one that modulates down very low. The top end rating doesn't matter as long as it can heat the house and is cost effective. A higher top end means a faster warm up. Get one with an integral weather compensator.

This sounds efficient. Could you please reccommend a particular machine that matches these criteria?

Cheers,
Josh

Let's guess...

hmm...

Probably a Broag 39c

 

[D_Hailsham]

the radiators around the house total to around 60k BTU (each with a thermostatic valve) and the house warms up quickly whenever the heating comes on,

There is no guarantee that the radiators are the correct size; they may be oversized, which is why your house heats up quickly.

Use the Sedbuk Boiler Calculator to find out what size boiler you need.