Measure the loss before you diagnose it
Everything useful follows from one number: how much pressure the system loses, and over how long. Guessing without it produces the classic outcome, which is a floor opened in the wrong room.
- Switch the boiler off and let it cool fully. Hot water expands and reads high.
- Fill to 1.2 bar cold through the filling loop, then close both valves and disconnect the loop if yours detaches.
- Photograph the gauge. The timestamp does the record-keeping for you.
- Read it cold every morning for a week, before the heating fires, and do not touch the filling loop in between.
Now you have a rate, and the rate maps onto a cause:
| Drop from 1.2 bar cold | Most likely cause |
|---|
| No measurable change in a week | Not losing water. If you still get fault codes, the fault is at the boiler. |
| A tenth of a bar over a few weeks | A weep at a valve, tail nut or towel rail. Findable, worth fixing before it grows. |
| Noticeable drop each week | An active leak, usually hidden under a floor or in a wall, or a relief valve passing. |
| Into the red inside a day or two | A significant leak, or the system venting to the discharge pipe outside every time it fires. |
Vaillant's own guidance is that low pressure means there is not enough water in the system, that leaks are the most common cause, and that if refilling does not hold you should close the filling loop and call a registered engineer. Their installation instructions add the numbers a Vaillant applies: below 0.8 bar the display flashes the pressure as a warning, and below 0.5 bar the boiler shuts down and shows F22.
Rule out the two faults that mimic a leak
The expansion vessel
The signature is distinctive once you know it. Pressure that climbs well above normal when the heating runs, then falls below where it started once everything cools, and repeats every day. The vessel has lost its air charge, so the water expanding in the circuit has nowhere to go, the relief valve lifts to protect the system, and the volume that escaped is gone for good. Vaillant's maintenance procedure has the engineer close the service valves, drain the product, measure the pre-charge at the vessel valve, and top it up where it reads below 0.75 bar, replacing the vessel entirely if water comes out of the valve.
No detection equipment will find this, because nothing is leaking into your building. It is a heating engineer's visit.
The pressure relief valve
Go outside and find the small discharge pipe that exits the wall near the boiler, usually 15mm copper or white plastic, pointing down. Wipe it dry, run the heating, and look again. If it drips or runs, the system is emptying itself to the outside. That is either the valve passing because grit is sitting on the seat, or the vessel fault above pushing it open. Either way, it explains the missing water completely, and it is a boiler repair.
Check both of these before booking anyone to look for a leak. Between them they account for a large share of the systems we get called to, and we would rather you spent the money on the right trade.
Where a real heating leak hides
If the vessel and the valve are ruled out, the water is going into the building. Look at the accessible things first, because half of these are found without any equipment at all.
- Radiator valve spindles and tail nuts. Look for a white or green crust, or a rust bloom on the pipe below the nut. A weep that evaporates as fast as it appears never makes a puddle but will empty a system over months.
- Towel rails. The blanking plug and the element boss are the usual points, and a warm rail dries the evidence between inspections. A common find in flats.
- The underside of old radiators. Corrosion pinholes start as a brown tear stain on the paint.
- Under and behind the boiler. Staining on the wall or the floor under the case. If the source is inside the appliance, that is a Gas Safe engineer's job, not ours.
- Pipework under a screed or timber floor. The hard case. A pinhole in 15mm copper under screed can drop the gauge weekly for months with no visible damp, because the screed absorbs the water and the heating dries the surface. By the time a board lifts or a ceiling below stains, it has been running a long time.
- Underfloor heating circuits. A warm stripe or a patch that stays warm after everything else has cooled, one zone that never reaches temperature, or a manifold that needs constant topping up. Covered under underfloor heating leak repair.
There is a cost to leaving it, beyond the damage. Every refill introduces fresh oxygenated mains water to a circuit designed to hold the same deaerated water for years, which drives internal corrosion and produces the next pinhole. A system topped up weekly is being actively degraded by the topping up.
What a detection visit does that guessing does not
The circuit is isolated and pressure-tested first. That confirms the loss is real, sizes it, and frequently ends the job there, because a circuit that holds on a test gauge for an hour is not leaking and the fault is at the boiler. We will tell you that and stop.
Where the loss is genuine, the pipe run is traced with thermal imaging through screed and tile, acoustic listening on pressurised pipework, moisture mapping across the floor, and hydrogen tracer gas on a drained circuit where the covering is too thick for thermal to read. The purpose of all of it is to open one small area instead of searching, which is the difference between lifting a board and lifting a floor.
Then the pipework is repaired or re-run in copper, and the floor, screed, plaster or ceiling is reinstated by our own people. Most heating losses are located inside two hours. Our rate is £150 per hour, agreed before we attend, and if we do not find the leak you do not pay for the time we spent looking. The rate is fixed and cannot rise; the total depends on how long the job takes, and we will tell you where we are as we go. Full detail on pricing.
Where the leak has damaged the building, the cost of tracing it and getting access to it is often recoverable under the trace and access provision of a buildings policy, and we write the report with that in mind. That is set out under insurance claims and trace and access.
When not to call us
- You have topped up once this year. That is normal for a sealed system and needs nothing.
- The pressure only fell after radiators were bled or a valve was changed. Refill and watch it.
- The discharge pipe outside drips when the boiler fires. Heating engineer, for the relief valve or the expansion vessel.
- Pressure spikes hot and drops low cold on a daily cycle. Expansion vessel. Same answer.
- The leak is inside the boiler case. Gas Safe registered engineer, or Vaillant if the appliance is in warranty.
Call us when the system is genuinely losing water into the building and nobody can see where. Related reading on the boiler and heating pressure hub, and the repair itself under boiler and heating leak repair.
