Are you having trouble with a faulty combi boiler?

It’s never a good sign when your boiler starts playing up, and you don’t know whether the fault is serious enough to call an engineer, or whether you’ll just be wasting their time.

As an experienced engineer in the Edinburgh area, I’ve become extremely familiar with the workings of combi boilers and their faults over the years, which is why I’ve put this blog post together to try and help you diagnose the problem and get an idea of what needs to be done to fix it.

NOTE: Please DO NOT try to fix the problem yourself, unless it’s a very simple fix such as topping up water pressure or defrosting a frozen condensate pipe – for everything else, always call a Gas Safe registered engineer.

About combi boilers

The combi boiler was first invented by Vaillant in the 1960s, although it was not until the late ’70s that the imports first came to the UK. Nobody thought they’d end up becoming so popular, and many plumbers and heating engineers thought they would be a passing trend.

Modern combi boilers are just perfect for not taking up a lot of space in the home, whilst giving you instant access to hot water and heating day or night – at a moment’s notice. There are no hot and cold storage tanks or expansion tanks to worry about, so you can reclaim space in your attic or cupboard.

Combi boilers in general are particularly reliable, and a lot of call-outs I seem to get are for very simple things that homeowners could really do themselves with a little knowledge and know-how. Sometimes it can be as simple as replacing the batteries in the programmable room thermostat.

I always feel guilty having to charge customers for a call-out when it’s such a simple fix, which is why I write blogs to give the best possible advice for my clients.

Fault 1: Boiler is indicating low pressure

If your boiler’s pressure has fallen to around 0.4 bar or lower, there’s a good chance your boiler will refuse to start up and will most likely go into boiler lock-out. In most cases, this is a fairly simple fix, but you might also want to check there are no leaks in your radiators, pipework and safety valve first.

The Fix: Locate the filling loop (usually underneath your boiler) and turn the small tap counter-clockwise. The boiler’s pressure should start to rise, and you should turn the tap clockwise again when it reaches around 1.5 bar. Check out our detailed guide on what to do if your combi boiler loses pressure for more advice.

Fault 2: Blocked or frozen condensate

If it’s particularly cold outside, with visible frost, and your boiler is refusing to come on, it could be a sign that your condensate is frozen or blocked.

The Fix: First, go outside locate the condensate. If possible, try using a kettle of hot water to thaw it out. Next, remove the condensate pipe below the boiler and drain to a bucket. That should fix the problem, but you should consider asking an engineer if he/she can retro-fit a condensate trace heater for future.

Fault 3: No heating

If your hot water is working, but your radiators aren’t getting hot despite the thermostat telling you the heating is on, check the thermostat is not flashing up with a battery symbol or displaying a flashing light on the receiver unit at the boiler.

The Fix: Replace the batteries; these thermostats usually require two AAs. If you remove and replace within one minute, your time and temperature settings should be saved. If not, you will have to reset your settings. This isn’t really a boiler issue, but most people blame their combis first when the heating fails to come on.

Fault 4: Lukewarm water/only hot when the heating is on

If your hot water is staying lukewarm when you run the tap, or you’re only getting hot water when the heating is on, the chances are it could be a faulty diverter valve or faulty hot water thermistor. We’ve actually got an entire blog dedicated to helping you diagnose this problem.

The Fix: First, ask an engineer to replace the thermistor, as this is small relatively cheap part. However, they may also need to replace the diverter valve, or strip down and replace the diagram washer and spindle.

Fault 5: Boiler cutting out, ‘chugging’ sounds

If your boiler is cutting out, or going into lock-out whilst making sounds like it’s chugging or gasping for air, it may simply need a service. There could be an issue with build-up in the flue pipe, causing a blocked air intake, a corroded flue, or dirt to build-up on the burners.

The Fix: Arrange for your boiler to be serviced with the burners and jets cleaned, and gas analysis done. If the problem persists, an engineer should check your flue and disassemble to look for blockages and corrosion, as well as to clean the Venturi. Replace any parts necessary.

Consider a replacement

Combi boilers – and basically all gas boilers – are now a lot more reliable than they were 10 years ago. 10 – 15 years ago, combis were only just incorporating condensing technology and went through a period of change and fine tuning.

So, if you’re having trouble with a 15-year-old combi, ask yourself whether you would be better off spending a lot on expensive repairs, rather than just having it replaced with a modern, high efficiency boiler that comes with a 10-year guarantee.

TIP: If your combi is newer, but still out of guarantee, it might also be worth asking the company carrying out the repairs if you can get it on a service plan – this way you’ll be covered for any further call-outs for a small monthly charge.


I hope this blog post has given you some possible insight into what could be wrong with your combi boiler, and the possible solutions you can try either yourself or with a Gas Registered engineer.

However, if you’re unsure of anything, or are still having trouble, please don’t hesitate to get in touch or leave a comment below – I’d be glad to help out.