Radiator size calculator

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Radiator size calculator

Watts for a room, and what the same radiator gives at a heat pump's flow.

It runs entirely in this browser tab: nothing you type is uploaded, and it makes no request of any kind.

Settings

Insulation standard
A rule of thumb: the room next door with the same volume wants a different radiator. Choices: Poorly insulated, Average, Well insulated, To passive standard.
The room
A bathroom is heated warmer than a bedroom, and by about a quarter. Choices: Living room, Bedroom, Bathroom, Kitchen.
Flow temperature
A radiator is rated at 75; at 45 it gives about a third of that. A number from 20 to 90.
Work out the catalogue output
The whole question when a heat pump goes in.

How to use it

  1. Paste your text into the box above, or open a file.
  2. Adjust the settings beside it until the result is what you wanted.
  3. Copy the result, or save it as a file. The result updates as you type.

Questions

Where do the coverage rates and densities come from?
They are the published typical figures — what a tin of emulsion claims, what a cubic metre of dry sand weighs, what a bag of screed covers. They are printed on the result so you can compare them with the bag in front of you, and the bag wins.
Is the ten per cent reserve part of the calculation?
No, and that is why it is a setting. It is a trade convention for breakage and offcuts, and the quantity without it is shown on the same result, so you can order either.
Can I use this instead of a heating engineer or a structural check?
No. A heat loss from six fields is an order of magnitude, not a design: a standard calculation takes every room, orientation and junction separately. The snow tool weighs snow; whether your roof carries it is on its drawings.
Does anything I type here leave the page?
No. There is no account, no server and no network request. What you type stays in this tab, and in the link if you choose to share one.

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