Low Hot Water Pressure From an Unvented Cylinder: Causes

An Unvented Cylinder Passes Pressure On, It Does Not Create It

This is the point that catches people out. A vented system with a loft tank makes its pressure from height, so a pump can be added to raise it. An unvented cylinder is fed directly from the cold main and delivers hot water at whatever the main gives, reduced by the pressure reducing valve on its inlet and by every restriction between the cylinder and the tap. When the hot water is weak, something is either limiting what arrives at the property or restricting the path out of the cylinder. There is no setting inside the cylinder that turns the pressure up.

The Comparison That Splits the Diagnosis in Two

With everything else in the house turned off, run the cold tap at the kitchen sink. In most properties that tap is fed directly from the incoming main and gives a fair impression of what is arriving. Then run a hot tap on its own and compare the two.

• Cold is strong, hot is weak. The supply into the property is fine, and the restriction sits on the hot side: the cylinder's inlet controls, the cylinder, or the hot pipework.
• Hot and cold are both weak. The problem is upstream of the cylinder, in the incoming supply, the stopcock or the supply pipe itself.
• Hot is fine at some outlets and weak at others. The cylinder is doing its job and the issue is local to those outlets, such as a scaled aerator, a collapsed flexible tail or a thermostatic mixing valve.

That one comparison saves a lot of guesswork, and it is the first thing an engineer will ask about.

When Only the Hot Side Is Weak

The pressure reducing valve on the cold feed to the cylinder is the usual culprit. It contains a strainer, and in a hard water area, which covers much of London, that strainer collects scale and grit over the years. A partly blocked strainer or a cartridge that has begun to fail chokes everything downstream of it, so every hot outlet suffers while the cold outlets carry on unaffected. Cleaning or replacing that valve is routine work for an engineer holding the unvented qualification.

Isolating valves are the next thing to look at. A service valve on the cold feed that has been knocked, or that was never reopened fully after earlier work, produces exactly the same symptom and costs nothing to correct. It is worth ruling out before anything is dismantled.

Scale is the slow version of the same problem. Hard water deposits build inside the cylinder outlet, in older pipework and inside thermostatic mixing valves, which reduces hot flow gradually over years rather than overnight. If the decline has been so slow that you only noticed after a holiday, scale is a strong candidate.

Finally, check whether the weakness is really flow or really temperature. A cylinder that has run out of stored hot water part way through a shower feels like failing pressure to the person standing in it, but the flow is unchanged. That is a sizing or reheat problem, not a pressure one.

When Hot and Cold Are Both Weak

If the cold tap is weak too, the cylinder is not the problem and nothing done to it will help. The common causes are the incoming supply pipe, which in older London properties is often lead or small bore and sometimes shared between flats created in a conversion; a partly closed internal stopcock or external stop tap; or genuinely low supply pressure at the boundary.

It is worth knowing how low the guaranteed floor is. Water companies work to a minimum static pressure standard at the boundary that is commonly expressed as seven metres of static head, which is far below what an unvented cylinder needs to perform well. Meeting the regulatory minimum and delivering a good shower are not the same thing, so "the water company says the pressure is fine" and "the cylinder has enough to work with" can both be true and still leave you with weak hot water.

Where the Expansion Vessel Shows Up

The expansion vessel absorbs the increase in volume as the stored water heats. When it stops doing that, the symptoms are distinctive and worth recognising: water dripping from the tundish or running out of the external discharge pipe when the cylinder heats, knocking or banging when a tap is opened or closed, and pressure that noticeably surges then settles. Weak flow on its own is rarely the vessel; weak flow alongside any of those signs is a different conversation.

The correct pre-charge for the vessel depends on the model and on the pressure reducing valve setting for that installation, and checking or restoring it means isolating the cylinder and draining down to remove the water pressure from the vessel first. That is work for an engineer holding the unvented qualification, not a homeowner job, and it should be done with the manufacturer's figures rather than a general rule.

Can You Increase the Pressure?

Not by adjusting the cylinder. The realistic options all involve either removing a restriction or adding storage upstream:

• Remove the restrictions you have. Clean or replace the pressure reducing valve and its strainer, descale mixing valves and outlets, and confirm every isolating valve is fully open. This is where most of the recoverable flow is.
• Upgrade the incoming supply pipe. Replacing an old lead or small-bore supply between the boundary and the property with modern larger-bore pipe often makes the biggest single difference in older London houses. It involves excavation and coordination with the water supplier.
• Fit an accumulator. This is a pressurised store that fills slowly at mains pressure and releases quickly, so short high-demand draw-offs are met even where the mains flow rate is modest. It needs space and proper sizing.
• Break tank and booster set. Where the mains genuinely cannot support the property, water is stored in a tank and pumped. This is a design exercise, not an accessory.

One thing to be clear about: you cannot simply fit a pump on the incoming main and turn the pressure up. Under the Water Supply (Water Fittings) Regulations 1999, a pump drawing more than 12 litres per minute directly from the mains requires the water undertaker's consent, which is why accumulators and break tanks exist as the compliant answers.

When to Stop Investigating and Call an Engineer

Some symptoms are not diagnostic puzzles and should be dealt with promptly:

• Water discharging from the tundish or the external discharge pipe, continuously or repeatedly.
• Water noticeably hotter than usual at the taps, or a thermal cut-out that has tripped.
• Any leak from the cylinder body rather than from a fitting.
• Hot water that has failed completely while the boiler runs normally.

The relief valves on an unvented cylinder are safety devices, and one that is operating is telling you something. If the picture is broader than hot water alone, our articles on why a heating system will not hold pressure and what different pipework noises mean cover the system side of the same house.