A radiator is installed properly when it is sized to the room’s heat loss at the water temperature your boiler actually runs at, positioned where air can circulate, fitted with a thermostatic valve and a lockshield valve on the right pipes, and the whole system is balanced, bled and protected with inhibitor afterwards. Getting any one of those wrong is why some rooms never warm up while others overheat.

Why installation matters as much as the radiator

A radiator is a simple thing: a steel or aluminium panel that hot water passes through. What makes the difference is everything around it. An oversized radiator on a weak pump will not heat evenly. An undersized one will run flat out and never get the room warm. A good radiator on a system full of sludge will be cold at the bottom within a year.

Whether you are adding a radiator, replacing old ones or fitting a whole new system, the same principles apply. If you are planning a larger job, our central heating installation page explains how we survey and design a full system.

Step 1: size the radiator to the room

Every room loses heat through its walls, windows, floor and ceiling, and through draughts. The radiator has to put back at least that much heat on a cold day. The right way to work that out is a room-by-room heat loss calculation, which takes account of:

• Room dimensions.
• Wall construction and insulation.
• Window size and glazing type.
• Whether the room is above or below heated rooms.
• The temperature the room should reach (a living room is usually heated warmer than a bedroom).
• How exposed the wall is.

The answer is a heat output figure, in watts or BTU per hour. The radiator you choose needs to deliver at least that much at your system’s running temperature.

The Delta T trap

This is where many radiator swaps go wrong. Manufacturers quote radiator output at Delta T 50, tested under the European standard BS EN 442. Delta T is the difference between the average water temperature in the radiator and the room temperature. Delta T 50 roughly means water averaging around 70°C in a 20°C room.

A modern condensing boiler is most efficient when it runs cooler, so that the water coming back to it is cool enough to condense. At lower temperatures a radiator gives out much less heat. At Delta T 30, for example, a radiator gives out only around half of its Delta T 50 figure. So if your system is set up to run efficiently, the radiator may need to be noticeably larger than the old one to heat the same room.

Situation	What it means for sizing
Older boiler, high flow temperature	Catalogue Delta T 50 output is roughly right
Condensing boiler set up to run cooler	Choose a larger radiator or a double panel, using the lower Delta T output
Planning a heat pump in future	Size for low temperatures now, which often means much larger emitters
Well insulated room, new windows	Heat loss is lower, so a smaller radiator may do

Step 2: choose the right position

Under a window is still the best default for most rooms. The radiator warms the cold air falling from the glass and sets up a circulation of warm air across the room. With modern double glazing this matters less than it used to, so an internal wall can work if the layout demands it.

Good practice for position:

• Leave a gap above and below the radiator so air can flow through and over it. Do not box it in tightly or push a sofa hard against it.
• Keep long curtains from covering it, or the heat goes behind the curtain and up against the glass.
• Fix the brackets to something solid. A double panel radiator full of water is heavy, and plasterboard alone is not enough without suitable fixings.
• Make sure the valves can be reached and the bleed point is at the top where you can get a radiator key to it.
• Keep it level, or very slightly raised at the bleed end, so air collects where it can be bled.

Step 3: fit the right valves

A radiator normally has two valves, one at each end.

• Thermostatic radiator valve (TRV). Senses the air temperature around it and closes as the room warms up, so each room holds its own temperature. Traditionally TRVs are fitted on the flow pipe (the hot pipe coming from the boiler). Many modern TRVs are bidirectional and can be fitted on either pipe, but check the valve’s instructions.
• Lockshield valve. Fitted on the other end and set once by the installer to control how much water flows through that radiator. It is used to balance the system and is not meant to be adjusted day to day.

Current Building Regulations guidance expects individual room temperature control where practical, which in most homes means TRVs. The exception is the room with the main room thermostat. A TRV there can fight the thermostat, so that radiator usually has a manual valve or a TRV left fully open. Adding a smart thermostat or smart TRVs can give you finer control room by room.

Step 4: fill, bleed, balance and protect

Once the radiators are on the wall and connected, the job is not finished.

1. Flush. On a new installation or where the system is dirty, the system should be cleaned so debris and old sludge do not collect in the new radiators. BS 7593, the code of practice for treating water in heating systems, sets out how.
2. Fill and bleed. Refill the system, then bleed each radiator from the lowest floor upwards until only water comes out. Our guide on how to bleed a radiator covers the details.
3. Repressurise. On a sealed system, bring the pressure back up to the figure in the boiler’s instructions.
4. Leak check. Check every joint with the system hot and cold.
5. Balance. With all TRVs fully open, adjust each lockshield so the radiators nearest the boiler do not take all the flow. The engineer checks the temperature drop across each radiator and adjusts until they are even.
6. Protect. Add inhibitor at the concentration the manufacturer specifies and, ideally, fit a magnetic system filter to catch any sludge that forms.

Skipping balancing is the most common reason a house has some radiators too hot and others lukewarm. Skipping inhibitor is the most common reason new radiators go cold at the bottom within a few years.

Common radiator installation mistakes

• Swapping like for like without checking the room’s heat loss or the system’s running temperature.
• Fitting a TRV in the same room as the room thermostat.
• Not balancing the system after adding or changing radiators.
• Fitting new radiators to a dirty system without cleaning it first.
• No inhibitor, or topping up with plain water after bleeding repeatedly.
• Hanging a heavy radiator on plasterboard without suitable fixings.
• Hiding the radiator behind furniture or a tight cover.

If you already have cold spots or radiators that will not heat evenly, our guide to a radiator not heating up explains what to check, and power flushing may be needed before new radiators go on.

How we price it

Installations are quoted as a fixed price after a free survey, so the price you agree is the price you pay. For a new or extended system we work out the heat loss and radiator sizes as part of the survey. Smaller jobs, such as moving or replacing a single radiator, are charged at our published rates, which you can see on our prices page.

Frequently asked questions

Do radiators need to be under windows?

No, but under a window is still usually the best place because the radiator counters the cold air coming off the glass. With good double glazing, an internal wall can work well if the room layout needs it. What matters most is free air flow around the radiator.

Should a TRV go on the flow or the return?

Traditionally the TRV goes on the flow pipe. Many modern TRVs are bidirectional and can be fitted on either pipe, which the valve’s instructions will state. If an older one-way TRV is on the wrong pipe, it can bang or hum as it closes.

How do I know what size radiator I need?

Work out the room’s heat loss, then choose a radiator whose output meets it at your system’s real water temperature. Using the catalogue Delta T 50 figure on a system that runs cooler will leave you with an undersized radiator.

Why do new radiators need inhibitor?

Inhibitor slows the corrosion that forms sludge inside radiators and pipes. Without it, new radiators on an older system can develop cold spots within a few years. Boiler manufacturers usually expect the system to be treated for their warranty to apply.

Can I just replace a radiator with the same size?

You can, and on a system with a high flow temperature it may be fine. But if the room was always cold, the boiler has been changed, or you want to run the heating cooler for efficiency, it is worth checking the heat loss before choosing a like-for-like replacement.

Radiator installation across Hertfordshire, Bedfordshire and Cambridgeshire

We install radiators and complete heating systems from our base in Stevenage across Hertfordshire, Bedfordshire and Cambridgeshire, with a 12 month labour guarantee on our work. Find out more about our central heating installation service or call 01462 506374 to arrange a survey.