Central Heating Loss of Pressure

Boiler pressure gauge showing operational pressure range

A drop in pressure, below a certain level, in your central heating system will cause your central heating to stop functioning.

This article tells you:

  • How to top up the pressure yourself.
  • What the causes of a loss of pressure might be.
  • How to book a heating engineer to trace and repair the fault.
  • What costs might be involved

If you’d just like to go ahead and get an engineer organised now then call HomeForce on 0131 315 0000 or complete our tradesperson booking form.

Note that, over time, there will be natural seepage of water from a central heating system that causes a loss of pressure. Water evaporates or “leaks” out (in tiny amounts) over time no matter how expertly fitted.

How to top up central heating pressure

  1. Boilers / central heating systems will usually have a pressure gauge either:
    • on the front of the boiler
    • beside the other controls (maybe under a flap / door)
    • on the pipe work leading to / from the boiler.
  2. When the system is cold the pressure should be around 1 Bar. (Check the specific instructions for the boiler you have)
  3. When hot the pressure may rise to around 3 Bar.
  4. The pressure can be topped up by a small tap on a valve leading to the boiler. This tap is similar to the image shown but usually black.
  5. To top up the pressure turn the tap (usually anti clockwise) by a small amount until you can hear water going into the system and see the pressure rising.
  6. Once the pressure reaches 1 Bar close the tap / valve once more.
  7. You may now need to reset your boiler to make it fire up again.

Important Notes

  • If you don’t feel comfortable trying to top up the pressure yourself then ask HomeForce to arrange a visit from a qualified engineer.
  • On some systems there may be two taps to be opened. One that diverts water from the cold mains water and one that allows it to flow into the central heating system. This is called a “fill loop”
  • Some systems require the “Fill Loop” to be disconnected when not being used.
Valve Taps on pipes
Valve Taps on Pipes

What causes a loss of Pressure?

As noted in the introduction to this article some water leaches out of a central heating system naturally – think of it as “The Angel’s Share”!

Therefore some loss of pressure, over time, is bound to occur.

If the pressure loss is happening quickly / you are frequently topping up the system it is likely that there may be another fault.

Finding the fault can be trial and error / educated guesses even for an experienced, qualified and Gas Safe registered engineer.

The most common problems are boiler related:

  • A leaking valve, particularly a Pressure Release Valve
    • The water expands in the system when heated. This valve stops the pressure building too high but if it is faulty it may allow water to escape through the overflow pipe.
  • A faulty “expansion vessel” which can mean:
    • The membrane inside the vessel separating air from water has perished,
    • A loss of air pressure inside
    • A blockage in a pipe connected to the expansion vessel

What is an expansion vessel?
An Expansion Vessel maintains an even pressure throughout a central heating system.

When the water expands or contracts as it heats or cools the expansion vessel regulates the overall pressure in the system.

A non-Boiler related issue might be:

  • A leaking radiator or valve on a radiator.
    • These leaks can be almost microscopic so a large accumulation of water does not materialise.
    • A good trick is to place tissue paper in places you suspect there may be a leak and then see if any water stain marks appear.
  • A pipe leak
    • The most dreaded leak in a system, both by homeowners and heating engineers, is perhaps a pipe leaking somewhere under the floor.
    • If you are in anything other than a ground floor flat then your downstairs neighbours may notify you of the leak before you know of it yourself.
    • These leaks are often very small as it may take some time for water to be noticeable in the form of water stains on the ceiling of the flat below.
    • THE GOOD NEWS: – Is that they can often be fixed by putting a “Leak Sealer” additive in to the system (though some gas engineers are skeptical of the effectiveness of these sealers)

Tracking down pipe leaks under floors

To track down a leak in pipes hidden under the floor often the only way to get to it is to lift the floor coverings (Carpets, Laminate, Tiles etc.) and inspect the pipes.

Few plumbers or heating engineers, in our experience, relish tackling this sort of task. This is partly because of the pressure they will feel to find the leak as the customer worries about escalating costs and partly the complications of lifting then replacing flooring.

A trawl of the internet will show some companies offering “Thermal Imaging” to find leaks. However we would suggest exercising caution particularly if companies offer “No Find, No Fee”. Read the small print before you sign anything.

What are the costs?

The cost to make a repair will depend on what’s wrong. A heating engineer will usually try the simple things first. For which allow around £60 plus VAT (when applicable), materials and parking.

  • Leak additive
  • Check radiators and valves for signs of leaks (NB: – Repairing may take longer see below)
  • Re-pressurise the expansion vessel

If radiator valves do need to be replaced then this could take an hour or a ½ day depending on circumstances.

If these repair attempts don’t work then an engineer will start looking at valves that may need replaced. This could take anything from 2 – 4 hours (£120 – £240) depending on how much “stripping down” of a boiler is needed to access valves.

Note: – Engineers won’t usually go straight to replacing a valve unless there are obvious signs of a leak.

If replacing an expansion vessel (internal to a boiler) then this is potentially an expensive (Circa £1,000 + ) task. It would be worth discussing with the gas engineer if replacing the boiler all together (although more expensive) might represent better use of money.

Book a Central Heating Engineer now