Flow & Return — how our heating works

Two separate kinds of water

The single most useful thing to know: there are two water circuits in the house, and they never mix.

The heating water (the “primary” circuit). A closed loop: the same water goes round and round, boiler → radiators and cylinder coil → back to the boiler. It's usually dark and has inhibitor in it. It's drawn as the thick pipes here.
The water you use at the taps (“domestic” hot water). It sits in the cylinder upstairs. The boiler never heats it directly. Instead, hot heating water runs through a copper coil inside the cylinder and warms it, like a kettle element. It's drawn as the thin pipes.

That's why your boiler isn't a combi. A combi heats tap water instantly and has no cylinder. Because you have a tank on the first floor, you have either a regular or a system boiler. Use the switch at the top to see each one:

Regular (heat-only, “conventional”). There's a big cold-water tank in the loft that feeds the cylinder, and a small feed & expansion tank that keeps the heating circuit topped up. The pump and valves sit in the airing cupboard. The boiler has no pressure gauge.
System boiler. It's sealed: there are no loft tanks, the pump and expansion vessel are inside the boiler, and there's a pressure gauge on the front (normally 1 to 1.5 bar when cold). The cylinder is fed straight from the mains.

Quick check: is there a pressure gauge on the boiler? Then it's a system boiler. Are there tanks in the loft? Then it's a regular one. Either way, the heating works the same way.

What happens when a room gets cold

The programmer allows it. The heating channel is on for this time of day.
The thermostat calls. It only measures the air where it's mounted. When that air drops below the setting, it switches on.
The zone valve opens. A motorised valve in the airing cupboard opens the pipe to that zone. This takes 10 to 20 seconds. When it's fully open, a small switch inside it tells the boiler and pump to start.
The boiler fires and the pump pushes. About 15 to 20 litres a minute leave the boiler on the flow pipe. The water splits between every open radiator in the zone.
It comes back cooler. Each radiator gives off heat, and the water returns through the return pipe 10 to 20 °C cooler, to be heated again.
The room warms and the thermostat clicks off. The zone valve closes. If no other zone or the cylinder needs heat, the boiler and pump stop.

Three motorised valves, one per zone plus one for hot water, is called an S-plan. With two heating thermostats and a cylinder, that's almost certainly what you have. If you only find one valve with three pipes, it's a Y-plan, but that can't give you two separate heating zones. You may have one of those and a second 2-port valve.

The valves on each radiator

Every radiator has a valve at each end, and water only flows through when both are open. One lets water in and the other lets it out, so shutting either one stops the flow.

● The thermostatic valve (TRV) is the numbered head (*, 1 to 5) on the way in. The number is a target temperature, roughly 1 ≈ 12 °C, 3 ≈ 20 °C and 5 ≈ 28 °C, not a fixed opening. A wax capsule in the head expands as the air around it warms and slowly pushes the valve shut. It's fully open when the room is well below the number, partly open as the room gets close, and shut once it's there. So turning it down does mean less water and a cooler radiator, but only once the room is near the new number. In a cold room, 2 and 5 both start fully open. At * it stays shut, apart from frost protection.
■ The control valve (lockshield) is on the way out, usually under a plastic cap. It doesn't react to temperature. The installer sets it once to balance the system, so radiators near the boiler don't take all the flow. You can also shut it fully to take a radiator off the wall.

In the simulator, the knob colour shows how open each valve is (green open, red shut). A TRV turns red by itself when its room warms up.

Close one radiator in the simulator and watch the others in that zone: they get a bit more flow. Close all of them while the thermostat is still calling, and the water has nowhere to go but the bypass valve. The boiler then heats up fast and switches itself off and on (“cycling”).

Don't put a TRV on the radiators in the room with the wall thermostat. If the TRV closes first, the thermostat never warms up and keeps the boiler running for nothing.

Why one zone can feel wrong

Thermostat 2 is on the landing, but it controls radiators on three floors. Heat rises up the stairwell, so the landing is often warm before the bedrooms are. When the landing hits its setting, all zone-2 radiators turn off, even if the bedroom upstairs is still cold. Try “Warm landing, cold bedrooms”.

Common fixes:

Set Thermostat 2 a little higher, and put TRVs on the bedroom radiators so each room stops at its own temperature.
Turn the landing radiator down, so the landing doesn't warm up first.
Balance the lockshields, so the far radiators (top floor) get their share.

Radiator cold? A quick checklist

Whole radiator cold: one of its two valves is shut, a TRV pin is stuck, or that zone isn't calling (check its thermostat).
Cold at the top, warm at the bottom: there's air in it. Bleed it with a radiator key. On a system boiler, top the pressure back up afterwards.
Warm at the top, cold at the bottom: sludge has built up. It usually needs a power flush.
Far radiators lukewarm, near ones hot: the system needs balancing at the lockshields.
Nothing at all: check the programmer, then the boiler display for a fault code. On a system boiler, check the pressure is above 1 bar.

Flow temperature and your gas bill

Modern boilers are condensing boilers. They recover extra heat from the exhaust, but only when the water coming back is below about 55 °C. With the flow at 70 to 80 °C, the return is often too hot, and you lose around 10% efficiency.