Y Plan and S Plan Heating Systems: Which One You Have

Why a system with a cylinder needs these valves at all

A combi boiler heats tap water directly and has its own internal valve to switch between heating and hot water, so none of this applies to a combi. With a system or regular boiler and a hot water cylinder, the boiler heats water that circulates round a closed loop. Some of that loop runs through the radiators, and some runs through a coil inside the cylinder, where it heats your stored bath and tap water. If you are not sure which boiler you have, our guide on what is a system boiler shows how to tell a system boiler from a regular one.

Something has to decide where the boiler water goes. When the programmer asks for heating, the radiators need it. When the cylinder thermostat says the stored water is cold, the coil needs it. Motorised valves do that job, and the Y plan and S plan are the two standard layouts for them. Both use the same other parts: a programmer, a room thermostat, a cylinder thermostat strapped to the cylinder, a pump and usually a wiring centre that ties them together. Our guide on how a hot water cylinder works explains the coil side in more detail.

How a Y plan works

The Y plan is built around one mid position valve. It has three ports: one inlet from the boiler and pump, usually marked AB, and two outlets, marked A for the heating and B for the hot water cylinder. A motor in the plastic head on top turns the valve to send water to A, to B, or halfway between so both get some.

On the common Honeywell style mid position valve, a spring returns the valve to the hot water port when the motor is not powered. So with nothing calling, or with a dead motor, the valve sits open to the cylinder. When the heating calls, the motor drives the valve across. That is why a failed motor on a Y plan often shows up as hot water that still works and heating that does not, although a spindle that has seized with scale can stick in any position, and a fault in the valve's internal switch can leave the boiler not firing at all.

Y plans were fitted in huge numbers in the 1980s and 1990s because one valve is cheaper and quicker to fit than two. Plenty of London houses and conversions still have one.

How an S plan works

The S plan uses two two port zone valves: one on the pipe to the radiators and one on the pipe to the cylinder coil. Each has its own motor head and is closed when unpowered. When the room thermostat calls, the heating valve opens. When the cylinder thermostat calls, the hot water valve opens. They work independently, so heating and hot water never have to share one moving part.

Inside each head is a small switch that only closes once the valve is fully open. That switch is what tells the boiler and pump to run. It is a neat safety feature, because the boiler cannot fire against a closed circuit, but it also means a valve that fails to open leaves its zone cold with the boiler never even lighting.

An S plan plus is the same idea with more zones, typically a third valve so upstairs and downstairs heating are controlled separately. It is the usual choice in larger houses and is what most installers fit on a new system with a cylinder today.

How to tell which one you have

The valves are usually in the airing cupboard near the cylinder, sometimes near the boiler, under a floor or in the loft. Look for a brass valve body in the pipework with a white or grey plastic box, about the size of a fist, clamped on top. There is normally a small lever on the side of the box and a cable coming out of it.

• One valve with three pipes coming out of the brass body in a T or Y shape: a Y plan.
• Two valves, each on a straight run of pipe with one pipe in and one out: an S plan.
• Three or more of those straight valves: an S plan plus with extra heating zones.
• No motorised valve at all, the cylinder heated only when the heating is on: often an older gravity hot water system, sometimes called a C plan.
• One valve with three pipes on a system where the hot water always takes priority over the heating: possibly a W plan, which uses a diverter rather than a mid position valve.

What the manual lever does, and a quick check you can make

The lever on the side of the head is there for filling and draining the system. On most two port valves you can slide it across to a manual position and latch it, which holds the valve open, and it unlatches itself once the motor is powered again. A Y plan lever does a similar job, holding the valve in its mid position.

With the power on and the zone calling, the lever should move across by itself and stay there. If a zone is cold and the lever has not moved, the head is not driving the valve. If the lever feels unusually slack, with none of the usual spring resistance, the motor or gearing inside the head may have failed. If it will not move at all, even gently, the spindle in the brass body may have seized. Do not force it, and do not take the cover off: the head runs on mains voltage.

Valves that sit in one position all summer are more likely to stick when the heating is first turned on in autumn. If a zone stays cold on that first cold evening, it is worth checking the valve before assuming the boiler has failed.

Common faults on each system

Most motorised valve faults fall into a few patterns, and the layout you have tells you where to look first. If your radiators get hot when only the hot water is on, our page on radiators hot when only the hot water is on goes through the valve, gravity circulation and towel rail causes in order.

• Heating works, no hot water, Y plan: valve not reaching the hot water port, a seized spindle, the internal switch, or the cylinder thermostat not calling. Our no hot water but heating works guide goes through it.
• Hot water works, no heating, Y plan: often the motor or its synchronous drive in the head, which leaves the spring holding the valve on the cylinder side.
• Heating works, no hot water, S plan: the hot water valve not opening, or its switch not telling the boiler to fire.
• Radiators warm when only the hot water is on, S plan: the heating valve stuck open, or letting water past.
• Hot water only when the heating is on: a hot water valve or its switch failing, with the heating valve's switch doing the work of firing the boiler. Our guide on hot water only working when the heating is on covers it.
• Everything silent: check the programmer, the thermostats and the fused switch before the valves, because a dead wiring centre stops both zones at once.

What the repair usually involves

Very often only the head is at fault. On most common valves the head unclips or unscrews from the brass body and a new one can be fitted without draining the system, so the job is short. Where the spindle or the body itself has seized, the valve has to be replaced in the pipework, which means isolating and part draining the heating circuit, then refilling and bleeding it. On a sealed system the pressure is then topped back up.

An engineer will normally test the call from the programmer and thermostats, check the voltage at the valve, and confirm the switch in the head is sending the demand to the boiler before replacing anything. It is quicker than guessing, and it avoids fitting a new valve to a system whose real fault is a cylinder thermostat.

If you have hot water or heating missing and you think it is a valve, call us on 020 4652 5111 or send an enquiry with a photo of the valves and the cylinder. Our 20 engineers across London see these every week, especially in the first cold spell of the year. Our rate is on the pricing page and agreed before we attend.