Boiler Losing Pressure? Heres What To Do

Since the introduction of combi boilers and the sealed systems they work on, there has been one big problem that has driven people mad. Boilers losing pressure! Possibly, one of the most frustrating problems that can occur on a boiler. So, what causes boilers to lose pressure and what can be done to stop it?

Why boilers lose pressure

To understand why boilers lose pressure we need to understand what a sealed system is.

Unlike older gravity-fed systems that are open to the air, sealed systems are completely shut off. No water should be able to get in, or out. Which, in turn, allows them to be pressurised. So instead of a cold feed tank feeding into the system, sealed systems have a filling loop so water can be entered manually. If you’re reading this post, chances are you’ve had to add water via the filling loop before.

There are four reasons why a boiler will lose pressure.

  1. If any radiators have been bled recently or the boiler has automatically let the air out.
  2. Water is escaping through a leak on pipework or a radiator.
  3. The pressure relief valve is passing and is letting water out.
  4. A fault with the expansion vessel is causing the pressure to get too high and active the pressure relief valve.

Bleeding the radiators

Whenever you look at the reading of the pressure gauge, the reading is showing you the pressure inside the system. Which technically doesn’t just mean the amount of water. If the pressure is 1bar it may well consist of 70% water and 30% air. Or 90% water and 10% air. It should be 100% water but sometimes air can get into the system if there is ever a leak, or if the system has been drained. You’ll know there’s air inside because the radiators won’t get hot at the top.

Most people know this can be released through the bleed with a bleed key. But, a lot of people forget that this affects the overall pressure. So, when a radiator is bled, the pressures inside the boiler drops. The auto air vent inside the boiler will also release air automatically. This stops the air blocking parts inside the boiler. Wherever the air gets out the pressure will need to be replaced via the filling loops.

Leaks on pipework or radiators

The most common and frustrating cause of a drop in pressure are leaks on radiators and pipework. It is a problem that frustrates customers, engineers and plumbers. As well as causing the pressure to drop regularly, leaks also let air into the system. Air in the system will stop radiators from working correctly and can even accelerate corrosion in the system.

Even the smallest leak can affect the pressure, and, its the small ones that are the hardest ones to find! Bigger leaks make themselves obvious, wet patches on carpets, ceilings or even water dripping through the ceiling. Anyone can find a big leak with ease.

It’s the small ones that drip almost unnoticeably, that are a nightmare to find. Unfortunately, they can be expensive to find too! It can take hours to find a small leak, and gas engineers don’t work for free! Before you go paying someone to find the leak there are a few things you can do yourself.

How to find the leak
  1. Walk around the house and check all the ceilings for watermarks or discolouring. Small leaks may never let out enough water to drip through the ceiling but they may turn it a yellowish colour.
  2. Thoroughly check all radiators, valves and visible pipework. Look for any discolouring, leaks can sometimes leave brown or black stains on pipework and valves. Radiators can also develop pinhole leaks. If your radiator looks like its rusty, check if the rust marks are slightly damp. Sometimes, you can see visible marks on pipework but it feels dry to touch. A good way to check is to tie a piece of kitchen or toilet roll around the pipe. There might be a small leak if the tissue is damp or slightly stained a few days later. Sometimes, a leak can be so small the water will evaporate before you can even see it. This is a great way to find these awkward leaks.
  3. Check under your boiler for any obvious leaks, check the pipework and feel underneath the case.
What if you can’t find it?

If you can’t find anything, chances are it’s under the floor. Some houses have a void space, if yours does, your plumber or gas engineer should be able to crawl under and visually check the pipework. Unfortunately, if the leak is upstairs, floorboards and carpets will have to be lifted.

Leak sealer

Sometimes with very small leaks, leak sealer can be added to the system. Leak sealers travel around the system and seal any minor leaks or weeps. This can be a great temporary measure, but shouldn’t be used as a permanent solution. The best solution is to find the leak and repair it permanently.

Faulty pressure relief valves

Required on all sealed systems, pressure relief valves are a safety device. They prevent the pressure in the system from getting too high and doing damage to the pipework, radiators or boiler.

When the pressure gets above 3bar, the pressure relief valve opens and releases water out of a pipe that goes out of the wall. Once activated, pressure relief valves may not close properly resulting in a constant drip or trickle from the copper blow-off pipe outside.

Watermarks may be visible on the wall underneath, if you’re not sure, tie a freezer bag around the pipe (make sure rain can’t get in) and leave it for a few days. After a few days if there is water in the bag you know the pressure relief valve is passing.

Faulty expansion vessel

Designed to allow water to expand into, expansion vessels are a vital part of any central heating system. Inside is a diaphragm that contains air. When the water expands it pushes against the diaphragm and compresses the air. Over time, the air can dissipate, or the diaphragm can puncture. Leaving more room for the water to expand into.

When this happens the pressure of the boiler will get very high when hot and activate the pressure relief valve, generally resulting in a constant trickle of water.

If your boiler loses its pressure after being on for a while this is probably why. Generally, this is an easy fix for any gas engineer, if the air has dissipated, the expansion vessel can be pumped up again. But, if the diaphragm has punctured a new one will be required. External ones can be fitted where there is enough space, sometimes this can be easier than replacing the internal one.

Summary: Why boilers lose pressure

Now you should know all the tricks of the trade for finding and diagnosing pressure drops on central heating systems. But remember, if you do find a leak, it’s always best to find a professional to repair it. However easy a leak may seem to repair, small problems can escalate quickly if you don’t know what you’re doing! If you’ve used these tips to find a leak we would love to hear about it, get in touch –


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