How To: Calculate Radiator Sizes

A local plumber came to the site today (Sunday) and gave me a quick price for installing all the plumbing and heating - excluding the sanitary ware and the showers . He said he would install rads that were 1.5 times that needed - "because the rooms will heat up quicker". I suppose it makes sense in one way - the thermostat will hopefully turn off the heating when it gets hot enough - bit a bit more investigation is needed. No written quote or anything like that - just a price!

Here's a guide to sizing radiators for a house

The size of radiator required for each room can vary. The first factor to consider is the temperature that you want to maintain in the room, a rough guide of the ideal temperatures is listed below.

• Lounge - 21°C
• Dining Room - 21°C
• Kitchen – 16°C
• Bedrooms - 15°C
• Bathroom -23°C
• Stairs/ Hallways - 18°C

The second thing that needs to be considered is the heat that you are going to loose from each room, the calculations for this are quite complicated. If you get this wrong and choose a radiator that is too big then it will be costly and it will overshoot its temperature. If the radiator is too small then it won’t reach its desired temperature.

An easy way to calculate the size of radiator required for each room is to use a BTU calculator which will give you the exact size radiator by amount of heat required. However there is an even easier way to do get the right size radiators and this is by measuring the room in cubic feet and then doing the following:

• In lounges and dining rooms- Multiply cubic feet by 5
• In Bedrooms – Multiply cubic feet by 4
• In common areas and kitchens – Multiply cubic feet by 3
• For rooms facing North – Add 15%
• For rooms with French windows – Add 20%
• For rooms with double glazing – Take away 10%

This should give you the output of any radiator in BTU’s (British Thermal Units). Adding all the totals should give you the total BTU’s for the whole house. If the house contains a hot water tank then add 20% and a further 10% for general losses. This will then give you the approximate size of the boiler you need for your home, it is however advised to get the calculations double checked by a qualified installer.