Boiler losing pressure in a rented home: what letting agents should do
The tenant keeps topping the boiler up. That can come from visible pipework, a leak hidden under a floor or behind a wall, or a fault on the boiler itself. Find out what has already been checked before anyone assumes the leak is under the floor.
1. Why a boiler keeps losing pressure
Worcester Bosch's guidance on constant pressure loss names two likely causes: a leak on the heating system or a faulty pressure relief valve. It suggests checking visible joints, including radiator valves and connections, considering possible leaks on pipework under the floor, and looking for drips from the relief valve's discharge pipe, which usually ends outside. It then points you to your installer.
Ask the heating engineer who attended to write down what they inspected and what they found. A note that the boiler has been serviced is a different thing from a finding that the heating circuit is leaking. Do not ask the tenant to dismantle the boiler or run diagnostic tests.
2. Record the history before you call
A short, honest record helps the office understand the job. Include what is known and label anything that has not been checked.
- How often the pressure falls, and how long it has been happening.
- Any readings already taken, with the date and whether the heating was on or off.
- Visible damp, staining or drips around radiators and the pipes you can see.
- Previous boiler visits, plumbing repairs or changes to the system.
- Whether the home has radiators, water-fed underfloor heating, or both.
- Whether leak sealant has been added and, if known, the product and date.
This is a record for the engineer. It is not an instruction to keep refilling a faulty system; ask the heating engineer or the boiler manufacturer how the boiler should be run in the meantime.
3. How central heating leak detection finds the leak
Each test answers a different question, and the engineer chooses the combination that suits the system.
| Test | What it tells you |
|---|---|
| Pressure testing | Whether the heating circuit itself is losing water, and later whether a repair holds |
| Thermal imaging | The route of the hot pipes under floors and behind walls, and warm or wet patches |
| Tracer gas | Where gas put into the circuit escapes through the floor or wall |
| Acoustic listening | The sound of water escaping under pressure, which can help narrow down the spot |
A temperature pattern or a signal needs to be read alongside the other findings. What each detection method does is worth reading before you discuss the survey.
4. Prepare the heating for the survey
Radiator systems and underfloor heating are prepared differently. ADI's booking terms ask for the heating to be turned on on the morning of the appointment and, if the system can still heat up, to be running for a couple of hours before the engineer arrives, because warm pipes show up on the thermal camera. Underfloor heating is the exception: its circuits should be switched off from early that morning, as they give off so much heat that thermal imaging stops working. If leak sealant has ever been put into the system, the terms make it your job to tell the office when you book, because sealant can seriously hold up the detection. Confirm the instructions for your own booking with the office. The list of what the engineer needs covers access and parking.
5. Have someone reachable on the day
Arrange for a contact who can make decisions during the visit, especially if the next step needs more access or the landlord's approval. After the survey, the findings and repair guide helps you agree what is repaired and what is still outstanding.
Falling pressure often turns up alongside other symptoms, and each needs testing on its own. In one ADI job, a combination boiler had been losing pressure for two weeks while water pooled under the stairs. The engineer pressure tested the supply and hot water pipework separately, then used tracer gas and thermal imaging to find a damaged hot water pipe under the hallway floor, plus a separate leaking WC valve. Read that case.
What not to do
- Do not add leak sealer to the system on the strength of a guess.
- Do not have floors lifted to look for the leak before the circuit has been tested.
- Do not treat regular topping up by the tenant as a fix.
When to call
Call when the heating engineer has ruled out the boiler and the pressure still falls, when there is damp with no obvious source, or when the landlord's insurer is involved. Tell the office what has been checked and by whom.