Choose new radiators by the heat each room needs first, then by type and size to suit the wall, and choose an installer who sizes them properly, fits thermostatic radiator valves, cleans and treats the system water and balances the radiators when they are done. Whether you live in Harpenden or anywhere else in Hertfordshire, a well-sized radiator on a clean, balanced system heats the room evenly and lets your boiler run at its most efficient.

Start with how much heat each room needs

Every room loses heat through its walls, windows, floor and roof, and through draughts. The radiator has to replace that heat on a cold day. An installer works this out with a room-by-room heat loss calculation that takes in the room size, the construction, the window area and whether it has outside walls. The answer is given in watts or BTU per hour (one watt is about 3.41 BTU per hour).

This matters more now than it used to. Older rules of thumb assumed the boiler would run very hot. Modern condensing boilers are most efficient when the water coming back from the radiators is cool, which means running at a lower flow temperature. Since June 2022, building regulations in England say a full new heating system (boiler, radiators and pipework) should be sized to run at a maximum flow temperature of 55°C.

Why the published output can mislead

Radiator outputs are tested to BS EN 442 and published at Delta T 50, which means the average water temperature is 50°C above the room temperature. At the lower temperatures a condensing boiler or a heat pump runs at, the output drops sharply. One major manufacturer’s correction factor shows a radiator at Delta T 30 gives only about half its Delta T 50 output. So a radiator that looks right on the label may be too small if you want your boiler to run cooler. A good installer takes this into account, and our central heating installation surveys size every radiator for the temperature your system will actually run at.

Radiator types compared

Type	What it is	Best for	Trade-offs
Single panel (type 11)	One panel with fins behind	Small rooms, tight spaces	Lowest output for its size
Double panel (type 21 and 22)	Two panels, with one or two sets of fins	Most rooms in most homes	Deeper off the wall
Column	Vertical steel columns, traditional look	Period homes, feature walls	Heavier, and often more expensive for the same heat
Vertical or designer	Tall, narrow panels or styled shapes	Rooms with little free wall under windows	Check the actual output, which can be modest
Aluminium	Light metal with low water content	Rooms that need quick heat, lower temperature systems	Needs compatible water treatment
Cast iron	Heavy traditional sections	Period homes	Very heavy, so floor and wall fixings must suit
Towel radiator	Ladder-style rails	Bathrooms and en suites	Lower output than a panel radiator of similar size, and chrome usually gives out less heat than a painted finish

For most rooms a double panel radiator is the practical choice, because it gives the most heat for the wall space. Column and cast iron radiators suit older homes and many people choose them for looks, but check the output carefully.

Valves: small parts that make a big difference

Every radiator needs two valves.

• A thermostatic radiator valve (TRV) on one end. It senses the room temperature and throttles the flow when the room is warm enough. Since June 2022, when a boiler is replaced in England, each room should have its own temperature control, which for most homes means a TRV on every radiator except in the room with the main thermostat.
• A lockshield valve on the other end. It is set once by the installer to balance the system, so every radiator gets its fair share of hot water.

Valves come in angled, straight and corner patterns to suit how the pipes come out of the wall or floor. If you want to control rooms from your phone, smart TRVs are an option, and our smart thermostat installation page explains how they work with your boiler.

Where to put radiators

• Under windows is still a good place. The radiator warms the cold air falling off the glass, and it keeps the rest of the wall free.
• Keep them clear. Long curtains, sofas pushed against them and deep shelves above them all trap heat.
• Check the wall. Heavy column and cast iron radiators need a solid wall or suitable fixings, and sometimes floor-standing feet.
• Think about the pipes. Moving a radiator to a new wall means new pipework. On a timber floor that is straightforward. On a solid floor it can mean chasing pipes into the wall or running them on the surface.

What a good radiator installer should do

When you compare installers, look for one who does all of the following. We work to this list on every radiator job.

1. Sizes each radiator from a heat loss check for the room, not by copying the old one.
2. Checks the pipework. Some older homes have microbore pipes (8mm or 10mm), which can limit how much heat reaches a bigger radiator.
3. Cleans the system. Fitting new radiators to dirty water shortens their life. A proper clean to BS 7593, the British Standard for heating system water, and in some cases a power flush, removes sludge and debris.
4. Adds inhibitor to protect against corrosion, and checks there is a magnetic filter to catch debris.
5. Fits TRVs and lockshields, then balances the system so the rooms furthest from the boiler heat up as well as the nearest.
6. Tests for leaks and bleeds every radiator after refilling.
7. Checks the boiler settings and explains how to set the TRVs and thermostat. Any work on a gas boiler itself must be done by a Gas Safe registered engineer.
8. Gives a clear, fixed price before starting, with what is and is not included.

Red flags

• Quoting for radiators without seeing the rooms.
• Refilling the system without cleaning or adding inhibitor.
• No mention of balancing.
• Leaving the old sludge-filled water in a system with a new boiler.

Replacing like for like or changing size

If you are replacing a radiator with one the same width between the pipes, the job is quick, with little disruption. If the new radiator is a different width, the installer either adjusts the pipework or uses extension pieces, and may need to make good a small area of wall or floor. Changing to a larger radiator is worth doing if the room has always been cold, but check the cause first. A radiator that is cold at the bottom or in the middle usually has sludge, and one that is cold at the top usually needs bleeding. Our guides to a radiator not heating up and how to bleed a radiator cover both.

Mistakes to avoid

• Buying radiators online by look alone, without checking the output.
• Choosing a designer radiator for a large, cold room where it cannot provide enough heat.
• Replacing the boiler but leaving undersized radiators and dirty system water.
• Fitting TRVs and never balancing the system.
• Putting a heavy radiator on a plasterboard wall without proper fixings.

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. The survey covers the heat each room needs, the state of the system water and the pipework, so the quote includes everything the job actually requires. Our work comes with a 12 month labour guarantee, and our prices page explains how we charge for all our work.

Frequently asked questions

How do I work out what size radiator I need?

The accurate way is a heat loss calculation for the room, which takes in its size, walls, windows and insulation. Online calculators give a rough figure, but remember that published outputs are at Delta T 50, so allow for a lower running temperature if your boiler is set to run efficiently.

Should I replace my radiators when I get a new boiler?

Not always. Radiators in good condition can stay, but the system should be cleaned and treated, and rooms that were always cold may need a larger radiator. Fitting TRVs is expected when the boiler is replaced.

Do I need to flush the system when fitting new radiators?

Yes, at least a thorough clean. Old system water carries sludge and debris that will settle in new radiators and valves. A power flush is worth it where the system is heavily sludged, followed by inhibitor and a magnetic filter.

Are aluminium radiators better than steel?

Aluminium radiators heat up quickly and are light, which makes them easy to hang. Steel panel radiators give a good output for the price and suit most homes. The right choice depends on the room, the look you want and the system they are going onto.

How long does it take to replace a radiator?

A like-for-like swap in one room is usually part of a day, including draining down, refilling and bleeding. Replacing radiators throughout a house, with new valves and a system clean, takes longer and is best planned as one job.

Radiator installation in Harpenden and across Hertfordshire

If you are replacing radiators in Harpenden or anywhere nearby, we can size them properly, fit and balance them, and leave the system clean and protected. See our central heating installation service, find out more about our work in Harpenden, or call 01462 506374 to book a free survey.