How do I work out the radiator output needed for a particular room?

If you are replacing an existing radiator and are happy that the old one produced enough heat for the room, then it should be reasonably straight forward to identify the output of the radiator by finding the nearest size and design to your old one. From the published output of the near equivalent you can then search for a new radiator of the style that you want and of the appropriate output.

If you are starting from square one, things are slightly more involved. In the absence of existing radiators, you need to work out the rooms heat loss. You can either do this by working it out long hand down to the nearest watt, or the quick 'close enough' method is to use our radiator calculator. Once you have you have a heat loss figure in watts, you then need to select as the output size of radiator, ensuring that you correct any published radiator output data to the conditions prevalent in your application. Once you know what output you need and you know what the output of a particular radiator is in your application, then you can choose your radiator. See below for the detail behind the method.

How do I work out the heat loss in a room?

Heat is lost from a given room through two principle ways. Firstly it is lost via the surfaces, i.e. through walls, floor ceiling, windows etc. and secondly through ventilation in the room. The detailed and rigorous method is to do a heat loss calculation which considers these two aspects in turn. The quick and easy way is to use our calculator tool, which may not be as rigorous and accurate as doing a full heat loss calculation, however it provides a ball park heat loss figure that is sufficient for the needs of most radiator buyers.

To work out the heat loss to a room you need to have some understanding of the construction of the various surfaces. The rate of heat loss through different surfaces is given by U values. For any one surface type, the heat loss in watts is calculated by multiplying the surface area (in m2) by the U value of the type of surface, by the temperature difference of between the preferred temperature in the room and the temperature of the air on the other side of the wall, floor, window or whatever. With U values remember that the higher the value the higher the conductivity to heat (i.e. higher heat loss)

i.e

It is assumed that rooms self ventilate so that heat will be lost from the room through doors, windows, and floor boards etc. The rate of ventilation is expressed as 'air changes per hour'. For the ventilation heat loss consider the minimum design temperature and air change rates given in BS5449. These are a good guide to use for heat loss calculations if your building is modern. Older houses will in reality have slightly higher air change rates due to poorer fitting windows and doors.

Where rooms are ventilated by an extract fan e.g. a bathroom or a kitchen, then find out the minimum air flow of the fan in m3/hr and divide it by the room volume, to get an air change rate. If the the air change rate is higher than that marked in the table above then use it in your heat loss calculations. The ventilation heat loss for you room, in watts, is given by multiplying the volume of the room by the air change rate, by the difference in temperature between the desired room temperature and the temperature outside (normally taken for design purposes as -1°C), by the ventilation factor constant,  thus

Notes.

1.     The outside temperature is normally taken for design purposes as being -1°C
2.     The ventilation factor is always taken as being 0.33 w/m3 °C

Each different surface is considered in turn and the heat loss values are added together and added to the heat loss value for the ventilation.

Example

Using a table as in the following example makes the process easy.

Is there a quick way to work out the radiator size for a room?

For most people working out a room's heat loss to the exact watt, is too much process for the straight forward requirement of sizing a radiator. So for a close enough figure which will help to work out what radiator output to get we offer the following calculator.

Is there a quick way to work out the heat loss in a conservatory?

Because there is so much glass in a conservatory they loose heat at a far higher rate than other rooms. The 'close enough' method used by many in the trade to work out heat loss in a conservatory is to work out the floor area in square metres and multiply it by 200 to get a heat loss value in watts. Its not a scientifically robust method however for working out the heating requirements of a conservatory it is perfectly sufficient for the needs of most.

What are U values and how are they used?

U-Value is the measure of the rate of heat loss through a material. It is measured in units watts per m2 per degree centigrade (w/m²°C). Therefore if a material has a U value of 1 w/m²°C, it means that for each degree centigrade difference of temperature between the two sides of 1m² of the material, 1 watt will transfer through the material. They are complex to work out and we recommend that for detailed and accurate U-value assessment, you consult a building services engineer or wade through the following via this link Conservation of Fuel and Power - Appendix A. Tables of U-values (pdf document). Once you have found the U values that are appropriate to the materials used in the various walls, windows, ceilings and floor of the room then you can use them as shown in the heat loss calculation table above

How do I calculate the actual heat output in a radiator?

Radiator outputs are expressed as being at a particular delta T, i.e. at a particular operating condition. It is unlikely that the delta T in literature will exactly match the delta T of your particular application, therefore, it is important to correct the published output data to match your application to deduce suitability.  Follow the example to see how its done

Problem.   A model of radiator has an output of 1500 watts at a Delta T of 50°C, but for my application the delta T would be 60°C so what is the radiator output in this instance.

Solution.  Take the output as stated for delta T of 50°C, i.e. 1500 watts and multiply by the factor for delta T for 60°C from the table below, 1.2675 to get the corrected output of 1901 watts.

What is delta T?

Delta T is the value of the temperature differential that the radiator will operate at. Follow the example to see how it is worked out.

Problem. Need to work out the delta T where the supply water temperature is 70°C, the return water temperature is 55°C, and the required room temperature is 20°C

Solution. 

What is MWT?

This is the Mean Water Temperature. That is the radiator inlet water temperature + the outlet water temperature divided by 2. In the example above the MWT is 62.5°C

How do you convert from BTUs to watts and visa versa?

Our radiators are generally rated in watts (w) or kilwatts (kw), but you may have a British Thermal Units (BTU) rating in mind. The following should help;

What are the different radiator types?

Leaving aside the designer radiators and towel rails, generally the standard type radiators tend to come in 4 configurations as shown below;

Description	Heat outputs (watts) @ delta T 60°C for a radiator of 520 mm high by 1000mm long	Plan view
Single panel has one panel and no convector fins	550
Single convector has a single panel with convector fins	808
Double plus has two parallel panels with convector fins on one of the tanks	1224
Double convector has two parallel panels with convector fins on both tanks	1551

These configurations either come in round top (often known as roll top) format or compact radiators (e.g. the warmastyle range). Compact radiators usually have grills and side panels. Roll tops and compacts are generally the lower price end of the market.

Some radiators are termed as wet. This means that they are the type that are connected to a central heating system and need to be plumbed in. In contrast other radiators are termed electric, which means that they do not require plumbing to install. Hard wired radiators are electric ones that are designed for wall mounting with a permanent connection to a electric circuit. Plug in radiators, in contrast are usually free standing and equipped with an electric plug for connection to conventional socket.

Storage radiators are more often termed storage heaters and are permanently wired to a low tariff electric circuit, and are designed to charge up with heat energy during the night and to discharge heat through the day and evening.

Designer radiators is a catch all term for the more expensive radiators. The variety is almost endless and includes floor and wall mounted, cast iron and aluminium, tubular, high wall, stone and marble, stainless steel, electric and wet. Towel rails are simply radiators designed for bath and shower rooms and come in all shapes and sizes.

Where should the different radiator types be used?

For utility areas, bedrooms, and kitchens it is usual to install roll top or warmastyle radiators. Where they are used in sitting rooms, lounges, dining rooms and hallways, radiator cabinets are usually installed to make the installation more decorative. Often customers are keen on a more decorative look and feel for their reception rooms and hallways and install a designer type radiators. It is worth considering that many of the more decorative radiators have modest heat outputs for their size when compared with standard radiators, particularly those with convector fins; so if you are short on floor space consider how much area a designer radiator would take up. Aluminium radiators have a particularly high output for their size and are ideal if you value a clean crisp appearance, and have limitations on your wall space.

Having worked out the out put of radiator that you require, consider the room and the type of radiator that would suit that room. Where it goes within the room depend on your preference, however it is normal to install them under windows, although if you prefer full length curtains this may not be a workable solution. Do not obscure your radiators with furniture. Not only will this impact their performance, it may damage you furniture. Also consider the depth of the radiator, particularly in halls and corridors. Choose shallow ones that intrude less into the walkway.

Bathrooms can have conventional wet radiators or can have towel rails. Hard wired electric towel rails or radiators can be used, however plug in heaters of any kind, can not be used in bathrooms, or shower rooms.

What considerations are there installing a radiator?

Standard radiators are designed to be light enough to be mounted on walls using the standard brackets and suitable expanding fixings. Designer radiators are usually heavier and may also require mounting feet, so that the weight is mostly taken by the floor. Look out for feet being offered as an added option, for heavier radiators. When mounting radiators to plaster board walls it is sensible to fix the mounting brackets through to the internal timber studs (vertical timbers) or to attach to a spreader board which is in turn fixed to the vertical timbers as this helps prevent undue stress on the plaster board. Fixings in plaster board are vulnerable to damage not only to the plaster board, but more importantly with wet radiators, if the bracket comes away from the wall it can lead to a leak in a  connection or pipe to the radiator. 

When mounting on solid walls ensure that the fixings are suitably secure and strong, and that there is sufficient gap between the skirting board and the base of the radiator (usually recommended to be 2")

We would draw your attention to the following:  

• Steel radiators are for use with indirect systems only and in a heating system properly designed and installed.
• Corrosion may occur due to faulty system design or installation, thus causing radiators to leak

The recommendations of the manufacturer's Association of Radiators and Convectors are: 

• The system should be designed to follow the recommendations of BS EN 12828 (BS 5449). Special attention should be given to the correct location of the pump in relation to the cold feed and vent positions to avoid air entry or water discharge through the open vent 
• The Installation work should be done in accordance with recognised good practise. When making soldered joints, it is important to choose a type of flux which is ready soluble in water and to use the minimum amount of flux and solder
• Care should be taken when cutting copper pipe to avoid small particles of copper entering the system as these can give rise to electrolytic corrosion if they come into contact with a radiator
• Use of a corrosion inhibitor (in accordance with BS 7593) is strongly recommended and the manufacturers instructions must be strictly followed

Failure to observe these recommendations would inevitably invalidate the manufacturer's warranty.