Radiator Sizing

[Tricky Dicky]

Whilst remodelling the bathroom I need to move the existing radiator. The current 600 X 500 double panel convector has always made the bathroom really cosy especially first thing in the morning however according to an online calculator I only need a single panel 600 X 500 to give me 1643 BTU or 481W. The radiator will not have a TRV as it is the by pass radiator for the boiler so no temperature control.

SWAMBO really likes the comfort of the existing radiator and is questioning if going down to half size will keep the bathroom as cosy. Is there any merit in oversizing radiators or should I stick to one that produces the calculated output?

Richard

[The Natural Philosopher]

TBH I am with SWMBO

The hotter and drier a bathroom and its towels are, the better I like it

And the less mould an condensation there will be

[Theo]

Oversizing is useful for a condensing boiler as it allows it to run at a lower flow temp, making it more efficient.

You can always fit an oversized rad and then throttle it down via the lockshield if the heat output is too high. Although in this case you'd need to ensure you didn't violate the boiler's minimum flow rate (does it have a flow meter to check?).

Theo

[Brian Gaff]

Well, you can go too far though. If mine is really hot then I can feel quite faint. I guess it depends on your definition of cosy. I have to say that I only have three storage heaters in my house and some of those bar infra red heaters, including one in the bathroom and as long as its on ten mins before you want a bath or shower, it seems fine and I've not got mould. I do however only have one outside wall. Brian

[Alan Lee]

Radiators are spec'd to give their output at a DT of 50, so what they say on the label may not relate to your boiler. The DT is the difference between the flow and return temperatures at the boiler. DT50 was fine for old cast iron non-condensing boilers which ran at 75-80 deg. C. For a modern boiler that runs at 50 deg C, your DT would be around 25 or less, so the output of the radiator will be half of what they quote on the sales pitch. That's why HPs require larger radiators, not because they run at lower temperatures, but because they have a much smaller DT, anything from 5 to 10 is typical. A DT10 heat source will require a radiator 10x bigger than its DT50 rating.