Oversizing radiators - how much?

[wizbongre]

I've read lots on here about oversizing radiators so the system can run at a lower temp and keep the boiler condensing more of the time.

Is there a rough target factor for oversizing?

(I'm guessing it probably depends on fuller heat calc's but a rough rule of thumb would be good.)

 

[muggles]

At the risk of appearing pedantic, I'd call it "correctly sizing radiators" rather than "oversizing radiators". Most heat loss calculators give you figures for high flow temperatures which results in undersized radiators for a modern system. As a rule of thumb, go for radiators of double the output a calculator tells you you need, so if the calculator says you need a radiator of 1000w output at Delta 50, choose one that states 2000w at Delta 50, which will give you 1000w at Delta 30.

Delta 50 is 50°C difference between the average radiator temperature and the room target temperature. If you're aiming for 20°C in the room, that gives you an 80°C flow temperature and a 60°C return for an average rad temp of 70. 70-20=50, Delta 50. Delta 30 for the same situation means a 60°C flow, 40°C return, and a nice happy boiler that condenses almost all of the time.

 

[JohnD]

I sized mine at about 50% over the builders' sizes, adjusted for convenience and fitting the rooms. Several I just changed from single to double with fins.

At the time, I was away on business a lot, and this warmed the house up quickly when I returned.

With more modern controls that's not such a problem.

The whole-house calculator (with upgraded insulation) says 12kW with a 20C difference. But you need more to heat it from cold, and when nightime temperatures are -10C you need more. If I'd left the housebuilders rads in, the house would never have been warm in cold weather.

My boiler is 24kW, and seldom runs at full power, except when heating the system from cold.

 

[wizbongre]

At the risk of appearing pedantic, I'd call it "correctly sizing radiators" rather than "oversizing radiators". Most heat loss calculators give you figures for high flow temperatures which results in undersized radiators for a modern system. As a rule of thumb, go for radiators of double the output a calculator tells you you need, so if the calculator says you need a radiator of 1000w output at Delta 50, choose one that states 2000w at Delta 50, which will give you 1000w at Delta 30.

Delta 50 is 50°C difference between the average radiator temperature and the room target temperature. If you're aiming for 20°C in the room, that gives you an 80°C flow temperature and a 60°C return for an average rad temp of 70. 70-20=50, Delta 50. Delta 30 for the same situation means a 60°C flow, 40°C return, and a nice happy boiler that condenses almost all of the time.

Not pedantic at all - this is a great answer, thanks. Explains something I've wondered about clearly and succinctly, and helps me a hell of a lot to size properly going forward.