Why some rooms never get warm

The heating goes back on in October and the house splits in two. The lounge is warm within twenty minutes. The back bedroom, the one over the garage or at the far end of the extension, stays cold all evening, with a radiator that’s only ever lukewarm.

It’s easy to blame the boiler. But the boiler is heating the lounge perfectly well, so it clearly isn’t short of heat. The problem is usually where that hot water goes once it leaves the boiler.

When some rooms heat and others don’t, the cause is usually the system rather than the boiler. Hot water takes the easiest route, so the radiators nearest the boiler take more than their share and the far ones are left short. Balancing fixes that by adjusting the small valve under each radiator. The usual guide is a difference of about 12°C between the pipe going into a radiator and the pipe coming out. The other common causes are a radiator valve sitting behind a curtain or sofa, which reads the trapped warm air and switches off early, and a radiator that’s simply too small for the room.

Is it one radiator, or the whole house?

Worth settling first, because the two have different causes.

One radiator cold at the top while the rest of the house is fine is almost always trapped air, and it’s a five-minute job with a radiator key. One radiator cold at the bottom points to sludge. Both are covered in our guide to cold radiators, and this post doesn’t repeat it.

This post is about a pattern across the house. Radiators that are hot all over in some rooms and only warm all over in others. Rooms near the boiler fine, rooms far from it struggling. Upstairs warm and downstairs not, or the other way round. A radiator that bleeds fine and still never gets properly hot. If that sounds like your house, the radiators themselves are probably working. They just aren’t all getting a fair share of the hot water.

Why do the radiators nearest the boiler get all the heat?

Water takes the easiest route, like electricity. Your boiler pumps hot water out along a loop of pipework, and every radiator is a branch off that loop. The first branch is short, with very little pipe to push through. The last one is at the end of a long run, round corners and up a floor. Given the choice, most of the water goes through the short branches near the boiler, and less and less reaches the end of the line.

So the nearest radiators run hot, and the water comes back to the boiler still hot because it barely had to go anywhere. The radiators at the far end of the loop only ever get what’s left.

Balancing fixes this. Every radiator has two valves. One is the one you turn, the thermostatic valve with the numbered head. The other, on the opposite end, usually has a plain plastic cap. That’s the lockshield valve, and it’s there to set how much water that radiator is allowed to take. Close the lockshields down a little on the greedy radiators near the boiler, leave the far ones more open, and the water spreads out across the house.

The way you judge it is temperature. Measure the pipe going into each radiator and the pipe coming out. The common guide is a difference of about 12°C, adjusting each lockshield until it’s close. Too small a difference and that radiator is taking more water than it needs. Too big and it’s starved.

There’s a second benefit that’s easy to miss. A balanced system sends cooler water back to the boiler. As our post on boiler flow temperature explains, that’s what lets a condensing boiler actually condense and recover heat it would otherwise lose up the flue. An unbalanced system, with its near radiators pushing hot water straight back, can stop that happening whatever the dial is set to. So the cold back bedroom and a gas bill higher than it should be can come from the same cause.

Can I balance my radiators myself?

You can, and plenty of people do. A pair of cheap clip-on pipe thermometers does the measuring, and the lockshield usually turns with a spanner once the cap is off. What it really needs is patience. The system has to cool right down before you start, each adjustment takes a few minutes to show, and opening or closing one radiator changes the flow to all the others. Working through a three-bedroom house is an afternoon rather than a quick job.

Where it gets harder is when balancing on its own doesn’t fix it. A pump set too slow can’t get enough water to the end of the line however carefully you set the valves. Sludge partly blocking the far radiators will show up as a cold room that balancing won’t touch. And some houses never had an even system to begin with, which is the next question.

Why does my radiator valve switch off when the room’s still cold?

This one catches people out, because the valve looks like it’s working.

The numbered head on a thermostatic radiator valve, or TRV, measures the temperature of the air right around itself. It doesn’t measure the room. Most of the time those are close enough. But put the valve behind a long curtain, under a deep windowsill shelf, or behind a sofa pushed up against the radiator, and it’s sitting in a pocket of warm air that the radiator itself is heating. It reads that pocket, decides the room is warm, and shuts off. Meanwhile the rest of the room never got there.

The manufacturers are clear about this. Danfoss, one of the biggest TRV makers, says the sensor should never be hidden behind thick curtains or furniture. Where it can’t be avoided, the fix is a remote sensor, a TRV head with its sensor on a short lead that can be fixed to the wall where it reads the room properly.

It works the other way round too. A valve in a cold draught by a door reads low and stays open, so that room ends up warmer than the setting.

There’s also the room with the wall thermostat. That room is the one that tells the boiler when the house is warm enough, so the radiator in there normally shouldn’t have its own TRV fighting it, or the TRV should be left fully open. If both are trying to control the same room, the boiler can switch off when the rest of the house still needs heat.

And after a summer off, the small pin inside a TRV sometimes sticks closed. If one radiator stays cold whatever number you set, take the head off and check the pin moves. Our cold radiators guide covers sticking valves too.

Is the radiator just too small for the room?

Sometimes, yes. Every room needs a certain amount of heat to stay warm, and that depends on its size, how many outside walls it has, the windows, and what’s above and below it. A bedroom over an unheated garage, or a kitchen extension with a lot of glass, needs much more than an inside room the same size.

Plenty of houses round Colchester have had a loft conversion, an extension or a knocked-through room added since the heating went in. Sometimes the radiator was carried over, or a new one was picked to fit the wall rather than the room. Balancing can’t fix that. A radiator that’s too small for the room will be hot all over and the room still won’t warm.

It matters more if you’ve turned your flow temperature down, as the August post suggests. Radiators give off less heat when the water is cooler, so a radiator that only just coped at 80°C may fall short at 60°C. The fix there is a properly sized radiator, which starts with a heat loss calculation for the room.

What’s causing my cold rooms?

What you notice	Most likely cause	Who fixes it
Rooms near the boiler hot, far rooms only warm	System out of balance	You can, with patience. Or an engineer
Radiator hot all over, room still cold	Radiator too small for the room, or a TRV reading the wrong air	Move the furniture first. Then an engineer
Radiator switches off early, room never warms	TRV behind a curtain or furniture	You. Clear the space, or fit a remote sensor
One radiator cold whatever the setting	TRV pin stuck after the summer	Often you. Free the pin
Far radiators cold at the bottom	Sludge	An engineer. A power flush
Whole house slow, all radiators lukewarm	Boiler, pump or flow temperature	An engineer

What does an engineer actually do about it?

Balancing it properly, mostly, and going round the house twice, because the first pass always shifts something. The pump setting gets checked against the size of the system while we’re at it. And sometimes the honest answer is a bigger radiator, not another quarter turn on a valve.

If sludge is part of the problem, a power flush clears it out of the radiators and pipework, and rebalancing the system afterwards is part of that job. That’s the order it has to happen in, because balancing a system with half-blocked radiators means doing it again once they’re cleared.

An annual boiler service is about the boiler rather than your radiators. But it’s the one visit a year when someone qualified is already in the house, so it’s worth mentioning the cold back bedroom while they’re there. A service plan means that visit happens every year without you having to remember to book it.

If you’d rather just have someone tell you what’s going on, get in touch. It’s usually quicker to diagnose than people expect.

Common questions

Should I start balancing from the radiator nearest the boiler?

The usual method is to put the heating on from cold and note the order the radiators warm up. Start with the first one, which is usually the nearest, close its lockshield right down, then open it gradually until the in and out pipes are about 12°C apart. Then move to the next in order.

Does balancing save money?

It can. Rooms that heat evenly mean you’re not turning the thermostat up to get the far rooms warm, and a cooler return lets a condensing boiler work as it should. How much depends on how far out the system was.

My TRVs are on 5 everywhere and it’s still uneven. Why?

A TRV only limits a radiator. It can’t push more water to a radiator that isn’t getting enough. If the far rooms are starved, turning their TRVs up won’t help. Balancing will.

Do I need to rebalance after a new radiator or a power flush?

Yes. Anything that changes how water flows round the system changes the balance. A properly done flush includes it.

What number should my TRV be on?

It depends on the make, but 3 is usually somewhere around 20°C. Lower for bedrooms, higher for the room you sit in. Remember it reads the air around the valve, not the middle of the room.

Hot lounge, cold back bedroom. It’s rarely the boiler.