A radiator should be sized from the room’s heat loss and the flow temperature the system runs at. Adding or moving one has implications for the rest of the system – flow, balancing and sometimes pipe sizing – which is why it is worth doing properly rather than teeing into the nearest pipe.
Sizing, and the number that gets misread
Every radiator has a quoted output in watts, and that figure is quoted at a specific temperature difference between the radiator and the room – conventionally written as delta-T. The trap is that different manufacturers and retailers quote at different deltas, and a radiator quoted at a high delta will look far more powerful than the identical radiator quoted at a lower one.
In practice: if your system runs at a modest flow temperature, which any efficient modern condensing system should, a radiator quoted at high-temperature conditions will deliver considerably less than its headline figure in your house. This is the single most common reason a newly fitted radiator disappoints, and it is a specification error rather than a fault.
The correct approach is to work out the room’s heat loss, decide the flow temperature the system runs at, and pick a radiator whose output at *that* delta meets the loss. It is arithmetic rather than judgement, and it takes minutes.
Types, and what each is actually good at
| Type | Strengths | Trade-offs |
|---|---|---|
| Steel panel, single/double, with convector fins | Cheap, high output for size, universally available, responds quickly | Plain-looking. Fins collect dust and need cleaning to keep output up |
| Cast iron column | Beautiful in period property, huge thermal mass, very even gentle heat | Heavy – floors sometimes need checking. Slow to warm and slow to cool. Expensive |
| Steel or aluminium column | The column look without the weight, faster response | Lower thermal mass than cast iron, so it behaves differently |
| Towel rail | Right for bathrooms, dries towels | Low heat output. A bathroom usually needs another emitter as well |
| Low surface temperature | Safe where vulnerable people or children are at risk | Bulkier for the same output |
| Vertical / designer | Fits where wall space is limited | Output per pound is poor, and some are genuinely inefficient shapes |
The traditional position exists for a reason: the coldest surface in the room is the glazing, and warm air rising in front of it counteracts the downdraught that otherwise pools cold air at floor level. Modern glazing reduces the effect but does not remove it. Moving a radiator to a different wall for aesthetic reasons is entirely legitimate – just do it knowing what the trade is.
Adding a radiator to an existing system
Adding a radiator is rarely just a radiator. Three things follow from it:
- The boiler now has more emitter than it was sized for. Usually fine, because most systems are oversized to start with – but worth checking rather than assuming.
- The system needs rebalancing. A new radiator teed into a convenient pipe will take flow from somewhere else, and the somewhere else is usually the far end of the house. This is how adding a radiator makes another room colder.
- The pipework may be undersized for the additional flow, particularly if several radiators are being added or if the existing run is already at its limit.
None of these are reasons not to add one. They are reasons to rebalance afterwards, which takes an hour and is the difference between an improvement and a redistribution.
Moving and removing radiators
Moving a radiator means new pipework, and the cost is almost entirely determined by what is under the floor. A suspended timber floor with accessible voids is straightforward. A solid concrete floor means either chasing, running in the screed, or going round at skirting level – each with a different cost and a different look.
Removing a radiator for decorating is a small job we are happy to do, and it is worth having done properly rather than opening the valves and hoping. A radiator taken off without isolating and draining correctly means dirty system water on a carpet and air introduced into the system, which then needs bleeding and rebalancing. Both valves closed, the radiator drained into a tray, capped, and the same in reverse afterwards.
If you are removing a radiator permanently – to free a wall, say – be aware that you are reducing the system’s total emitter surface, and the room will need its heat from somewhere. Capping a radiator off and being surprised the room is cold is more common than it should be.
Valves, and the cheap upgrade people overlook
Every radiator has two valves: a control valve, which is where a thermostatic radiator valve goes, and a lockshield, which is the one used for balancing and is usually hidden under a cap.
- Thermostatic radiator valves let each room hold its own temperature and stop bedrooms being heated to living room temperature. They are inexpensive and one of the better value improvements available.
- TRVs seize. After a few years, particularly if left in one position all summer, the pin sticks and the valve stops responding – held open, so the room overheats, or held shut, so it never warms. Exercising them each autumn helps; replacing a seized one is a small job.
- One room must not have a TRV, normally the one with the room thermostat, so the system always has somewhere for the flow to go.
- Lockshields are for balancing and should be set deliberately, not left fully open. Most systems have never had this done.
One room always cold? Before buying a bigger radiator, it is worth checking whether the system has ever been balanced.
Questions we get asked about this
Why is one radiator always cold?
Most often balancing – the flow is going round a shorter circuit instead. Also check whether the TRV has seized, whether the radiator needs bleeding, and whether sludge has settled in the bottom. Only after those is it a sizing problem.
My radiator is hot at the top and cold at the bottom.
Sludge – magnetite settled in the bottom of the radiator, blocking circulation through the lower part. Cold at the top and hot at the bottom is the opposite problem: air, which just needs bleeding. See radiators cold at the bottom.
Can I add a radiator without changing the boiler?
Usually yes. Most existing systems have spare capacity. What it does need is rebalancing afterwards, or you will simply have moved the cold room somewhere else.
Are designer vertical radiators less efficient?
Not inherently, but output per pound and per square metre of wall is generally poorer, and some shapes convect badly. If you want one, size it from the actual output figure at your flow temperature rather than from how substantial it looks.
Do cast iron radiators need special pipework?
Not special pipework, but they are heavy – a large column radiator full of water is a serious weight, and on a suspended timber floor it is worth checking the joists. They also hold a lot of water, so the system takes longer to warm up and stays warm longer after it shuts off.
Should every radiator have a TRV?
All but one. The room containing the room thermostat should have a plain valve, so that when every other TRV closes there is still a path for the water. Fitting TRVs to every radiator including that one causes the boiler to short-cycle against closed valves.
What happens when you call
Call and describe it
You speak to an engineer, not a call centre. Most jobs can be narrowed down on the phone.
We look at it properly
A real diagnosis before a price, and a straight answer if it is not worth doing.
A written quote
Itemised, so you can see what you are paying for.
The job, and the paperwork
Certificates where the work needs them, and what will need attention next.
Where we do this work
Across Norwich and the whole of Norfolk. If you are on the edge of the county, call and ask — we will tell you honestly whether we are the right people.
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Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.