Boiler Losing Pressure Overnight in Manchester? Here's the Cause | Next Gen Boilers
Quick Answer
Why does my boiler keep losing pressure overnight?
A boiler that consistently loses pressure overnight has a water leak somewhere in the system — either internally in the boiler or externally in the pipework or radiators. Even a very small (pinhole) leak can cause the pressure to drop significantly over hours.
If your boiler pressure is fine when you go to bed but low in the morning — or you're topping it up every few days just to keep the heating running — there is a leak somewhere in your central heating system. The leak might be obvious (visible water stains, damp patches) or entirely hidden (a pinhole leak in a buried pipe). In this guide, Next Gen Boilers Ltd explains the most common causes for Manchester homeowners and what needs to be done.
How Fast Should Pressure Drop?
A sealed central heating system should hold its pressure almost indefinitely. A very small drop over several months (0.1–0.2 bar) is within normal range due to tiny amounts of dissolved gas coming out of solution. If your pressure drops noticeably overnight — say from 1.5 bar to under 1 bar — you definitely have a leak.
The faster the pressure drops, the more significant the leak. A system losing 0.5 bar or more overnight needs prompt attention — left long enough, the boiler will lock out on low pressure, leaving you without heating or hot water.
Pressure Relief Valve (PRV) Weeping
The most common internal cause of a boiler losing pressure slowly is a weeping pressure relief valve. The PRV is designed to open and release water if the system pressure gets dangerously high. Over time, these valves can begin to weep slightly — allowing small amounts of water to escape continuously.
Signs of a weeping PRV include water dripping from the copper PRV discharge pipe (usually running through an external wall), a damp patch under the boiler, or a slight mineral deposit around the valve body. PRV replacement is a straightforward Gas Safe repair costing £100–£200 including parts and labour.
Pinhole Leaks in Pipework
Pinhole leaks in copper pipework are extremely common in older Manchester properties — caused by corrosion accelerated by system sludge, aggressive water chemistry, or electrolytic action where different metals meet. A pinhole leak can be too small to cause visible water damage but large enough to drop system pressure by half a bar overnight.
Finding a pinhole leak sometimes requires a pressure test — our engineers can pressurise the system and isolate sections to locate the source. Leaks in accessible pipework are repaired with solder or push-fit fittings. Leaks in buried or concealed pipework are more complex to access.
Internal Boiler Leaks
Seals, O-rings and pump glands inside the boiler can develop leaks over time, particularly on boilers over 8–10 years old. An internal leak is often visible as a water stain on the boiler casing, rust marks around fittings, or a damp interior when the boiler casing is removed. Internal leaks need a Gas Safe engineer to diagnose and repair.
If the heat exchanger has developed a crack or pinhole (often due to limescale-related stress), this is one of the more expensive repairs — costing £500–£900+. On older boilers, the cost of heat exchanger replacement sometimes approaches the cost of a new boiler installation.
Frequently Asked Questions
Is it safe to keep topping up my boiler pressure?
Topping up occasionally to compensate for seasonal pressure variation is fine. Topping up more than once every few months means you have a leak that needs to be found and fixed. Ignoring it risks water damage and system corrosion.
How do I top up my boiler pressure?
Use the filling loop — a braided or rigid connector between the cold water supply and the heating system. Open both valves slowly until the pressure gauge reads 1–1.5 bar, then close both valves. Some boilers have an internal filling loop, others have an external one under the boiler.
How much does it cost to find and fix a boiler pressure leak in Manchester?
A diagnostic visit starts from £89. Simple repairs like PRV replacement are £100–£200. Finding and repairing hidden pipework leaks varies significantly depending on access — from £150 for an exposed pipe repair to several hundred for concealed pipework.
Can a combi boiler lose pressure without any visible leak?
Yes — pinhole leaks and slowly weeping PRV valves often leave no obvious visible signs. The pressure drop is the only symptom. A Gas Safe engineer can find these through pressure testing and visual inspection.
My boiler keeps losing pressure but I can't see any water anywhere — what should I do?
Call a Gas Safe engineer for a proper leak detection visit. Leaks can be hidden behind walls, under floors or inside the boiler casing. Continuing to top up without finding the leak will eventually cause more significant damage.
Need emergency boiler help in Manchester or Salford? Call: 07305 687 331