Low Pressure From Taps on an Unvented Cylinder

Is it the cylinder or the whole house?

An unvented cylinder has no loft tank. Hot water leaves it at whatever pressure the mains supplies, less what the combination valve takes off. That makes it an unusually good test of your incoming supply: if the hot taps are weak, the cold taps are usually weak too, and the cylinder is only where you noticed it.

Start by running the cold tap at the kitchen sink, which on most homes is fed straight from the mains. Then run the hot tap at the same sink and compare the two. Do the same upstairs. The pattern tells you where to look.

• Cold and hot both weak everywhere: the problem is on the mains side, before the cylinder.
• Cold strong, hot weak everywhere: look at the cylinder inlet group, the strainer and the pressure reducing valve.
• Weak at one tap or one shower only: the fault is local to that outlet, not the cylinder.
• Hot pressure drops only when another tap or the washing machine is running: the supply pipe is too small or restricted for the demand.

Start at the stopcock

The most common cause of weak pressure after any plumbing work, or after a long period of no work, is the main stopcock not being fully open. Stopcocks are often turned back a quarter turn to stop them seizing, or left partly closed after an earlier repair. Turn it fully open, then back off a small amount only if it feels hard against its stop.

Look for a second isolating valve. Many homes have one just inside the property and another at the cylinder or under the kitchen sink. Any one of them partly closed will throttle the whole house. In the unvented cylinder cupboard you will usually see tagged isolation valves on the cold feed. Both handles should sit in line with the pipe when open.

Old stopcocks are a repeat offender. A stopcock that has been in place for decades can have a corroded or scaled seat that never opens fully even with the handle at the stop. That is something an engineer confirms by measuring flow with the valve open, not by feel.

How much flow does an unvented cylinder need?

Pressure and flow are different things. Pressure is the force in the pipe when nothing is running. Flow is how many litres a minute you get when a tap is fully open. A supply can show a healthy static pressure on a gauge and still collapse to a trickle the moment you open a tap, because the pipe feeding the property is too narrow, partly blocked, or a long run of old pipework.

Manufacturers of unvented cylinders publish a minimum inlet pressure and a minimum flow rate for each model, and the figures are in the installation manual rather than on the cylinder itself. Your cylinder will not perform to its rated output if the supply sits below them. Which figures apply depends on the model, so the manual for your cylinder, not a rule of thumb, is the reference. Part G guidance treats unvented installation as notifiable work for installers with the necessary skills, and a commissioning record that shows the supply conditions on the day is worth asking for.

A simple householder test is a timed fill. Open the kitchen cold tap fully and time how long a ten litre bucket takes to fill. It is a rough figure, not a measurement the engineer will rely on, but if a bucket takes well over a minute you have a supply problem before the cylinder is involved. Write the time down. It is useful evidence if the water company or an engineer needs to look.

The inlet strainer and the pressure reducing valve

If the mains side is fine and the cold taps run well, the next suspect is the cylinder’s inlet group. This is the set of valves on the cold supply into the cylinder, and a typical group contains a pressure reducing valve, a strainer, a check valve and an expansion relief. The pressure reducing valve (PRV) lowers incoming pressure to a set working pressure. That pressure differs between models, so the figure for your cylinder is in your manual and on the valve or data plate.

The strainer is a small mesh screen that stops grit and scale reaching the valve seats. After pipework changes, a mains repair or a burst in the street, debris can pack the screen and cut the flow. A blocked strainer produces a drop in hot flow that gets worse over weeks, and cold taps stay fine because they do not pass through the group.

A worn or failing PRV does the opposite of what people expect: it can sit stuck at a low setting and starve the cylinder, or creep upward and push pressure past the set point. Both show up as a change in the pressure reading on a gauge fitted downstream. Cleaning a strainer or replacing a PRV cartridge means isolating the supply and working inside the valve group. That is an engineer’s job, and nobody should adjust the sealed safety valves, which means the expansion relief valve on the group and the temperature and pressure relief valve on the cylinder.

The tundish and discharge pipe are not part of the pressure path, but they tell you something. If the discharge is dripping and the pressure at the taps is low, a failing valve group is a more likely cause than a mains problem. See tundish dripping: causes and checks for what that points to.

When it is only one tap, mixer or shower

If only a single outlet is weak, the cylinder is probably innocent. Unscrew the tap aerator and look for grit and limescale. Check the isolation valves under the basin or behind the bath panel: they are often only half open after a tap change. Mixer taps and thermostatic showers have small filters on their inlets, and these clog first in hard water areas.

Showers deserve a note. A thermostatic mixer shower fed by an unvented cylinder expects balanced hot and cold pressures. If the cold supply has dropped but the hot has not, the shower can go cool or cut out, and it is easy to blame the cylinder for something the cold supply started. Test hot and cold separately at the basin before drawing a conclusion.

Pumped showers are a separate matter. A booster pump on the hot supply of an unvented cylinder can starve the cylinder by taking more than the supply can give. If a pump was added after the cylinder was fitted, mention that when you describe the fault.

What the engineer checks, and repair or replace

The engineer’s first reading is the static pressure at the main stopcock, followed by a flow reading with a tap fully open. A gauge reading that collapses when the tap opens points at the supply pipe or a partly closed valve. A healthy reading at the stopcock but a poor one past the inlet group points at the strainer, the PRV or the check valve. Those two readings alone separate a mains fault from a cylinder fault in most cases.

The inlet group is a replaceable assembly, and a worn PRV or a scaled check valve is a repair that leaves the cylinder in place. The cylinder only becomes a replacement question when the fault is the vessel itself, such as a failed shell, or when a replacement group cannot be sourced for an old model. Low tap pressure on its own is not a reason to replace a cylinder, and anybody who says so before measuring is guessing.

When the fault is on the supply side, the cylinder cannot fix it. A supply pipe that is too small, lead, or heavily scaled is a plumbing or water company matter, and no cylinder swap will improve it. A properly sized unvented cylinder with a restricted supply will still give weak hot water.

For a fault that sits on the cylinder side, unvented cylinder fault repair covers the valve group work, and the hot water cylinder diagnosis page sets out the order the tests are done in. If the cylinder is reaching temperature but the water runs out too fast, that is a different problem, covered in why hot water runs out fast.