Hot water radiators

[Matt L.]

I’m re-doing my upstairs and need to figure out radiator sizes. I don’t want to freeze or overheat. I’ve found a place that sells old ones at reasonable prices but am unsure of what size to buy. Does any one have a simple formula to compute the size or BTUs? P.S. I am not a mathmatician.

[lg]

Radiators

Sizing & Positioning

A Sizing Radiators

The size of radiator required for a room depends upon two factors. Firstly, the temperature

that you want it to be able to maintain which is a relatively straightforward task and you

can use Table 1 below as a guide.

Table 1 Ideal Room Temperatures

IDEAL ROOM TEMPERATURES (oC)

Lounge 21

Dining Room 21

Kitchen 16

Bedrooms 16

Bathroom 23

Stairs 18

Secondly, the heat loss from your room. The calculations for this are quite complex since

they depend upon the size of windows, number of doors and, in particular, the

construction materials used to build the house. The best calculations take every

measurement of every room and especially the window sizes and outside wall sizes. They

can work on a specific temperature required and even take into consideration the type of

materials used in the building (to calculate “U” values) to get an accurate radiator size. Too

big and the system will overshoot its temperature and be less economical to run, too small

and it won’t reach its desired temperature. (Some of the poorer installers get round this

complex step by putting in radiators that are to big, and then fitting thermostatic radiator

valves to every radiator to cut the heat down.) This calculation produces a heat loss figure

in watts, of how much heat you need to warm that room up to the design temperature

from -3 deg C in one hour. (Ref. Website: http://www.gasman.fsbusiness.co.uk/radiators.htm) The

calculation can be done by using one of the radiator manufacturer’s heat loss calculations

(Ref. Website: http://www.cityplumbing.co.uk/heatloss.htm or http://www.radcalcs.com) and usually your

installer will do this when quoting.

A quick and easy way to calculate the size of the radiator required for any room in your

house is by measuring the room in cubic feet and then applying the following factors to

the figure arrived at:

Lounges and dining rooms Multiply cubic feet by 5

Bedrooms Multiply cubic feet by 4

Common areas and kitchens Multiply cubic feet by 3

For rooms facing north Add 15%

For French windows Add 20%

For double glazing Deduct 10%

This will give you the output of any radiator in BTUs (British Thermal Units). Adding the

total for all the rooms in your house or flat will give you the demand in BTUs for the whole

house. Add 20% to the total for a hot water circulating tank and 10% for general losses.

This will give you the boiler size you need for your house. (Source:www.muswell-

hill.com/foxandco/pages/calculating_radiator_sizes.htm). It is very unlikely that any radiator

will match the exact heat required, so select the first size of radiator above the heat

requirement. With rooms greater than 6 metres (18 ft) in any one direction, it is worth

considering distributing a number of radiators to minimise the thermal gradient within the

room.

B Positioning Radiators

Generally speaking it’s best to fit radiators beneath the windows of a room. The reason for

this is two fold. Firstly, cold air falling by the window will be balanced out to some extent

by the warm air rising from the radiator. Fitting radiators on the opposite wall can actually

cause a noticeable draught, since warm air rising on one side of the room and cold air

falling at the other exaggerates the flow around the room.

Secondly, and purely a practical consideration, is that for the most part, the wall area

beneath a window doesn’t tend to have furniture in front of it. You therefore make use of

unused wall space and, at the same time, do not end up shielding the radiator with a chair

or the like. If you have double glazed windows, the air will not fall by the windows to

anything like the same extent. Therefore you have more flexibility in positioning the

radiators. This can be a big help especially if you wish to have full length curtains, which

would cover the radiators if they were under the window.

On a similar point, avoid placing objects directly above radiators, as it will reduce their

effectiveness. Shelves positioned immediately above can have an adverse effect so make

sure they are at least a couple of inches above the radiator. If a shelf is set in this position

and is reasonably narrow, the air will flow around it. Radiator covers without enough

allowance for air circulation can also be a problem. Make sure air can pass freely in at the

base, and out through the front at the top.

Source: http://www.technosolution.co.uk/diy/centralheating/CHPositionRads/PositionRads.htm

[Andrew Kissel]

A simple calculation that I have been using for years sucessfully. Take the square footage of the room and multiply by 35. That will give you the BTUH requirement. Contact a local heating wholesale supply house and ask them to convert the BTUH to a HW radiator size. Make sure you at least air test the radiator for leaks prior to installing !

[scj]

is it possible to covert a hot water radiator to radiant floor heat in certain areas of a house/ie mud room, sunporch, bathroom and not the entire house?

[Author]

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