When to stop and get urgent help
Keep away from wet electrical equipment or water escaping from the boiler. Repeated lockouts or rapid pressure loss warrant prompt professional assessment, not repeated attempts to force the appliance to run. If you smell gas, hear a carbon monoxide alarm or suspect a gas or CO leak, leave the property and call 0800 111 999 from a safe place. Avoid flames and electrical switches; call 999 for a medical emergency. Gas work needs a Gas Safe registered engineer qualified for the appliance.
Why does it drop when the heating is not even on?
That was the question a new homeowner asked in a 2022 UK discussion after looking around the radiators and finding no obvious water. The thread did not establish a final cause. The useful distinction is between heating being switched off and the heating circuit being unpressurised: they are not the same thing. Record whether successive cold readings fall, rather than assuming that an unused boiler rules out a system problem. Do not turn isolation valves or open the casing to experiment.
First make sure you are describing the boiler's heating-system pressure, not weak flow from a tap or shower. A gauge on the appliance and the force of water arriving at an outlet are different observations. If both have changed, report both, but do not assume one explains the other. Photograph the accessible gauge or display and identify the appliance model.
An overnight fall can be useful evidence because you can describe a period when the heating was not being used. It does not, by itself, locate a leak or identify a faulty component. Keep the start and end readings with their approximate times and note whether the system had cooled before the first reading.
If you have only recently moved in, ask for the boiler and heating-system records. You may not know whether the previous occupier regularly refilled it. “This is the first week we have used the heating” is useful context and more accurate than saying the problem definitely began after you moved.
Source: Public homeowner account: pressure loss with heating off, August 2022; unresolved
Compare like with like before describing the drop
A hot reading and a cold reading are different reference points. Worcester Bosch distinguishes normal temperature-related movement from a large rise that may need expansion-vessel attention by an engineer. Use the correct manual for your model, rather than a universal pressure target. Without changing settings, record the date, reading and whether the system is cold or heating. Add the last refill and any fault code. Do not run an appliance suspected to be unsafe just to complete this record.
Build a short record around normal use rather than constantly watching the gauge. For example, note the cold reading before the normal heating period, any reading you happened to see during operation and the next cold reading. Label each clearly. Do not create additional heating cycles just to fill a table, particularly if the boiler is locking out or behaving unusually.
Include events that change the interpretation: radiators were bled, a radiator was removed, work took place on the boiler or the system was refilled. These events should be listed in time order. They are not proof of a cause, but they prevent the engineer comparing readings as though nothing happened between them.
Use the appliance manual to interpret its display and fault messages. Copy the exact code rather than translating it into a guessed fault. If the gauge is hard to read or the display intermittently goes blank, say so. Uncertain readings should remain uncertain; rounded guesses can make a small change look like a much larger loss.
Source: Worcester Bosch: pressure changes and frequency of topping up
Why pressure can rise hot and fall too far when cold
Heating water expands as its temperature rises. An expansion vessel accommodates that change; the pressure relief valve protects the system if pressure becomes excessive. Vaillant's explanation distinguishes these two functions. If expansion control is ineffective, pressure can rise excessively during heating and water may discharge. The later cold reading can then be low. That sequence is a reason to assess both the expansion arrangement and discharge behaviour, not simply refill again.
A dripping relief outlet is not automatically proof that the valve alone caused the original problem. Replacing a valve without understanding why it was discharging can leave a wider issue unresolved. An engineer needs to identify the actual sequence and whether a component needs attention or replacement. Neither recharging the vessel nor operating the relief valve is a homeowner step in this guide.
Compare that pattern with a steady fall between successive cold readings. Both deserve investigation, but the observations are different. A fall immediately after radiator bleeding has different context again: releasing air or water can change the reading. Vaillant also identifies appliance and system leaks as causes of pressure loss, including leaks that are not readily visible.
Keep any recent installation or service record with those observations. If the system was recently worked on, contact the original installer and check the actual warranty or cover terms. The mechanism explains why the pattern matters; it does not assign responsibility or identify a failed part without assessment.
| Pattern | What it can indicate | What distinguishes it |
|---|---|---|
| Falls once after bleeding or drain-down | System water or air was released | Whether it then remains stable under the correct manual guidance |
| Rises markedly during heating, then low when cold | Expansion or discharge behaviour needs assessment | An engineer checks the complete pressure cycle |
| Falls repeatedly between cold readings | Ongoing loss or another fault remains possible | Accessible joints, appliance and concealed system require appropriate checks |
| Outside pipe drips | Pipe identity and discharge cause need establishing | A drip alone does not identify the defective component |
Sources: Vaillant: pressure loss, radiator bleeding and concealed leaks
Source: Vaillant: expansion vessel and pressure relief valve explained (PDF)
It was serviced, so why is the symptom still there?
In a January 2025 UK thread, the poster described changing readings and noise after a service, while heating and hot water still worked. No confirmed fault or repair followed in the visible discussion. For your own visit, describe the unresolved symptom rather than asking for another routine service. Bring the previous report and ask what was checked, whether the behaviour occurred during the appointment and what further evidence would help. A short video of an accessible display can document an intermittent symptom without opening anything.
A maintenance appointment and an investigation of recurrent pressure loss can have different scopes. Check the service record rather than assuming every radiator, concealed pipe and operating condition was assessed. Ask whether the pressure concern was included in the original booking and whether it was reproduced during the visit.
For a second enquiry, lead with what remains unresolved. State how often you have refilled, whether the frequency has changed and whether the appliance now stops operating. If there is noise as well as pressure movement, describe when it occurs and whether it was present before the earlier visit. Avoid opening the casing to obtain a recording.
A useful response from the repairer explains what was checked and what the next assessment is intended to establish. It may be reasonable for a fault not to show itself during a short visit, but that limitation should be explicit. Ask what ordinary-use information would help and what would trigger a return, rather than accepting another unexplained refill as the complete answer.
Source: Public homeowner account: pressure concerns after a service, January 2025; unresolved
No puddle does not mean there is nothing to investigate
Worcester Bosch identifies visible joints, concealed heating pipework and relief-valve discharge as areas to consider when pressure repeatedly falls. Look only where you can safely see: a radiator connection, staining or the location of a drip. Report the pipe and where it leads if known; do not assume every outside drip is the same discharge. Never cap an outlet or attach a bag to test it. Ask the engineer which observation supports the next check instead of ordering a valve or a new boiler from the symptom alone.
Describe visible evidence without extending it into a diagnosis. A wet radiator valve, a stain below pipework and a drip outside the property are separate observations. Include their locations and whether the moisture appears fresh during ordinary use. You do not need to touch hot fittings, climb outside or remove floorboards to check them.
If the outside pipe is inaccessible, say that. A safe distant photograph showing where it leaves the building may be more useful than an uncertain description of its material. Do not attach a bag, bottle, cap or hose to any discharge outlet. The engineer needs to identify the pipe and interpret its behaviour.
A dry room narrows what you have observed, not what could exist. Equally, an old ceiling stain is not proof of a current heating leak. Tell the assessor whether the mark is new, changing or already present before the pressure problem. Separating those facts helps avoid opening a surface just because it is the only visible mark in the house.
Source: Worcester Bosch: investigating repeated pressure loss
Agree the investigation before opening floors or walls
A landlord on MoneySavingExpert reported an inconclusive visit after a new boiler installation, followed by uncertainty about invasive access. That account remained unresolved in the follow-up. It illustrates a useful booking question: what will the next visit establish before surfaces are opened? Ask for the reason for proposed access, who authorises it and whether making good is included. If the engineer suspects a concealed heating leak, agree the investigation separately from pipe repair and decoration.
Ask for a staged proposal. What does the engineer think the next check can establish? What result would justify tracing concealed pipework? What alternative explanation remains if that check is inconclusive? You are not asking for a guarantee; you are asking for a reasoned sequence before disruption.
If leak detection is recommended, clarify whether the visit covers locating a suspect area, exposing it, repairing it or reporting findings. These are not interchangeable services. Equipment may help narrow an area without identifying a specific joint until access is opened. Ask who approves opening floors or boxing and who is responsible for making good afterwards.
In a flat, mention whether pipes enter a communal riser or run through an area you cannot access. Permissions and keys can matter as much as the equipment used. If insurance is involved, ask the insurer about authorisation and report requirements before commissioning non-urgent invasive work. Do not assume that detection, pipe repair and decoration are all covered together.
Source: Public landlord follow-up: no visible leak and uncertainty about access, January 2017
Topping up is not an ongoing repair plan
Worcester Bosch advises contacting the installer when refilling is needed substantially more frequently than its expected occasional top-up. Follow your own appliance instructions if replenishing pressure is appropriate; stop if you cannot identify the filling arrangement. Report recurrence rather than treating each refill as a separate event. Make the call specific: when the system was last cold and holding pressure, how long the fall took, what still works and what has already been tried.
A useful way to describe recurrence is the interval between refills and the resulting interruption. “It worked for a week, then for two days, and now stops overnight” conveys a change that one isolated gauge photograph cannot show. Include the approximate dates even if you do not have every reading.
Do not convert that record into permission to keep replenishing indefinitely. If instructions are unclear, the filling arrangement is unfamiliar or pressure does not behave as expected, stop and ask for advice. A generic video may show a different valve arrangement from yours. Repeatedly trying different controls can obscure what happened and create another problem.
Ask what the repair plan is intended to change. If the immediate action restores operation but the cause is still uncertain, that should be explained as an interim result. Agree how ongoing pressure behaviour will be reviewed. A record of stable readings during normal use can support the follow-up, but it should not require you to carry out professional pressure testing or interfere with the boiler.
Source: Worcester Bosch: pressure changes and frequency of topping up
What to ask about the diagnostic and repair charge
Pressure loss can require more than a component swap, so confirm what you are authorising at each stage. Ask whether the first work is appliance fault diagnosis, checking accessible heating joints or specialist leak tracing. If extra investigation is proposed, ask what evidence supports it and how further time will be agreed.
AK Plumbing publishes a £60 attendance charge and £160 per hour for plumbing work, with the hourly quote agreed before work. The attendance fee covers getting the engineer to the property; it does not include an assumed diagnosis or repair. AK is not VAT-registered and adds no VAT. Use the pricing page below and confirm the scope of the proposed work before accepting it.
For a possible concealed leak, request separate clarity on access, pipe repair, materials and reinstatement. For a boiler component, ask which finding supports replacement and whether a return visit is needed. Comparing two quotations is only meaningful when both cover the same work. A lower attendance fee alone does not tell you the eventual investigation or repair cost.
What counts as a useful result after the visit?
The result should relate to the recurring pressure complaint. Ask what was found, what changed and how the appliance or system was checked. If the visit established a likely area rather than the exact source, that limitation should be recorded. If a component was replaced, retain its description and the reason for replacement.
Agree what you should observe afterwards using normal operation. Comparing later cold readings with an agreed reference is more useful than repeatedly checking a hot gauge against a cold starting point. Ask whom to contact if the pressure falls again, what information to provide and whether further attendance has a charge. Do not assume a new part proves that every possible source of pressure loss has been excluded.
AK is based in North London, covers Greater London and provides a 24/7 phone line with emergency dispatch. When enquiring, mention how quickly pressure falls and whether heating or hot water is unavailable. The public experiences linked in this article explain the questions people encounter; they are not AK case studies or a prediction of what your investigation will find.