Why my central heating keeps losing pressure

What this calculator tells you

How fast your system is losing pressure, how serious that is, and what it is costing you in water. It does not tell you how many litres the pressure drop represents, and that is the most important thing on this page.

• The rate of pressure loss in bar per day, when the change is large enough to read
• A severity classification, taken from the worse of two signals rather than from the gauge alone
• What that pattern usually indicates and what to check
• An annual volume and cost, derived from how often you top up rather than from the pressure

Two of the six standard attributes are missing here, and they are missing on purpose. There is no rate of loss in litres per hour and no daily volume, because a gauge reading cannot produce either without three quantities you do not have. Every other calculator on this site reports all six. This one reports four and says why.

How to take the measurement

You need two gauge readings with a known gap between them, and the gap matters more than most people expect.

• Find the pressure gauge. It is usually on the front of the boiler or on the filling loop pipework near it, and on a sealed system it reads somewhere between about 1 and 2 bar when cold.
• Take the first reading cold, meaning the heating has been off for a few hours and the pipes are at room temperature.
• Note the date. The rate depends on the interval as much as on the change, and a remembered "a while ago" makes the answer meaningless.
• Take the second reading cold as well, at the same sort of time of day, and read the dial square on rather than at an angle.

Both readings cold is the single rule that matters most. A hot system reads higher than a cold one, and on most systems that difference is larger than the change a real slow loss produces over several days. Mix one hot reading with one cold reading and you have not measured a small loss, you have measured the temperature.

How long to leave between them follows from what the gauge can see. A dial marked in steps of about a tenth of a bar cannot show you anything smaller, so the interval has to be long enough for the loss to add up to at least one step. A day is almost never long enough. A week usually is, and for a system that only needs topping up once or twice a year, nothing short of a month will show anything at all.

Getting the cold reading right, and reading a dial without introducing more error than you are measuring, is the whole of the input here. how to read the system pressure gauge covers it properly.

Using the calculator

What your result means

There are three shapes of answer, and only one of them is a rate. The other two are worth as much, because each rules something out.

The severity is not taken from the gauge alone. It is the worse of two signals: how fast the pressure is falling, and how often you have to top the system up. Visible damp or staining anywhere near pipework raises it one band, which is a stated judgement rather than a measurement, and the result says so.

Top-up frequency carries at least as much weight as the pressure reading, and that is deliberate rather than a compromise. A frequency is a behaviour you have watched over months. A pressure rate is a coarse dial read by eye twice. The input that looks vaguer is the better evidence, because of what each one is a measurement of.

A system that only loses pressure while the heating is running is a separate case with a different answer, and it is common enough to be worth checking before anything else. why pressure drops only when the heating is on sets out what that pattern points at.

How the calculation works

Two separate calculations that never touch each other. One classifies the pressure, one counts the water, and no number crosses between them.

• The rate = the fall in pressure ÷ the days between the two readings, in bar per day
• The band = which of four rate ranges that falls in, then taken as the worse of that and the top-up frequency
• The annual volume = top-ups per year × litres per top-up
• The cost = that volume in cubic metres × your volumetric rate

Why this calculator does not give you a litres figure

Because it cannot be done honestly, and every tool that appears to do it has invented the missing parts.

To convert a pressure drop into a volume of water you need three quantities, and a gauge on the wall gives you none of them.

A calculator that hands you litres from a bar reading has quietly assumed all three. It has picked a system volume, picked a vessel charge, and assumed both your readings were at the same temperature. None of those assumptions is shown to you, and the answer moves enormously depending on what they were.

So the annual volume on this page comes from somewhere else entirely: how often you top the system up, multiplied by how much water goes in each time. Both are things you can actually observe, the second is an adjustable assumption you can change, and neither is derived from the pressure reading. That figure is honest about being an estimate. A litres figure from the gauge would not be an estimate, it would be a guess dressed as a measurement.

It works as a measure of the loss because a sealed system has no legitimate consumption. Nothing draws water off it for use. So over a year in which the pressure ends roughly where it started, every litre you put in replaced a litre that left, which is exactly the quantity you wanted.

Assumptions and sources

Two of the figures here are ours rather than anybody else’s, and they are the two that most affect the answer, so they are named plainly.

• The water added per top-up, taken as 1.5 litres. No published measurement of this was found. It is derived from the gas law that governs an expansion vessel, for a typical 8 litre vessel pre-charged to 1 bar and topped up from 0.5 to 1.5 bar, and across the usual vessel sizes and pre-charges the realistic range is about 0.9 to 2.5 litres. It ships as an adjustable input for that reason.
• The four pressure loss bands. These are this project’s own thresholds and no published classification has been found to cite for them. They are stated as a working scale rather than as a standard, and if a sourced one turns up they will be replaced by it.

The refusal above is enforced rather than promised, and it can be checked. The one function on this site that returns a volume for a heating system has no pressure field on its input type at all, so it is structurally incapable of deriving a volume from a pressure. A test moves both pressure readings across their entire range and asserts that the litres figure does not change. An assurance in prose would decay quietly. That will not.

The gauge resolution of a tenth of a bar is also an assumption rather than a specification. It describes a typical domestic dial marked in steps of about that size and read by eye. If your gauge is digital, or finer, it can resolve a smaller change than this calculator will report.

What to do next

Four things, and the first two cost nothing and rule out the two commonest causes.

• Look at the outside end of the pressure relief discharge pipe, which terminates where you can see it, usually on an external wall. Water or staining there means the system has been discharging through the valve, which is water genuinely leaving with nothing wrong with any pipe.
• Take two cold readings a week apart and write both down with their dates. Most people compare a hot reading with a cold one and measure the temperature instead of the loss.
• Count how often you have topped it up over the last year. That number carries more weight than the gauge and it is the one you already have.
• Look for damp, staining or a tide mark near pipework, under floors and in airing cupboards. Water leaving a heating circuit is hot, so it evaporates and leaves a mark where a cold leak would leave a puddle.

If none of that finds it, the loss is somewhere you cannot see, and there is no household method that locates a buried circuit. how a loss from a heating circuit is traced explains what the available methods can and cannot establish.

One case does reach your water meter, and it is worth knowing which. a system topped up automatically shows the loss at the meter, because an automatic filling device replaces the water continuously instead of in one lump when you open a filling loop by hand.

Common questions