Are you experiencing problems with a faulty system boiler?

Knowing which action to take when a boiler breaks down can be tough, as usually there’s little way of knowing how serious the problem really is and how difficult it’ll be for an engineer to fix – if it’s even worth fixing at all.

As an experienced Edinburgh gas engineer, I’ve had plenty of dealings with system boilers in the past, and know some of the biggest faults they tend to encounter.

I’ve decided to use my own expertise to list the top five common faults with a system boiler, as well as some of the fixes involved, to help you diagnose the problem and have an idea of the type of work involved.

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

About system boilers

Just like regular boilers, system boilers heat the radiators or under floor heating and a hot water cylinder. This can be a high pressure unvented cylinder, or a traditional vented cylinder which is fed from a cold water storage tank in the attic.

System boilers come with an expansion vessel and pump built into the boiler. This makes them better value, as there’s not much difference in price between regular boilers and system boilers. Also, with the parts being internal to the boiler, they are usually also covered by an 8-10 year guarantee.

A lot of faults with system boiler set-ups are not actually due to the boiler itself, but rather the external valves fitted for heating and hot water selection, along with separate hot water cylinders and thermostats – so it’s usually a good idea to check these first before assuming it’s a problem with the boiler.

The good news about system boilers is that you have a separate hot water tank and usually an electric immersion heater to heat the water as a standby – so, a boiler breakdown doesn’t mean you’re left with no facilities.

Fault 1: Either no heating, or no hot water

If you’re getting only heating, or only hot water through (in other words, one is working, the other is not), this is usually a faulty 2-port or 3-port motorised valve.

The Fix: An engineer will usually replace the motorised valve head, if it’s a detachable modern version. The other option is to drain the system and replace the valve before refilling the system and adding inhibitor. Sometimes, it’s also possible to replace only the synchron motor on the valve if it has burnt out.

Fault 2: Hot water is far too hot

If your hot water used to be fine, but now it’s coming out of the tap far too hot (positively scorching), it could be an issue with the cylinder thermostat. Another tell-tale sign of this is if your central heating is working fine with no temperature issues.

The Fix: The best bet is to replace the cylinder thermostat; this is a straight swap if you have a standard vented (indirect) cylinder. However, if the thermostat is built into a high pressure unvented cylinder, call in the professionals.

Fault 3: Pump running constantly or boiler switching on by itself

If you can hear your boiler’s pump continually running, or your boiler is switching on by itself for no good reason, this is usually due to a faulty auxiliary switch on one of the motorised valves. This causes a live feed continually to the boiler, and would explain the trouble you’re having.

I’ve previously been called to a job like this and found another engineer had already changed the time clock and room thermostat because the boiler was coming on by itself. However, after some tests, we actually discovered it was actually faulty auxiliary switch on one of the valves as I mentioned above.

The Fix: Get an engineer to replace the faulty motorised valve.

Fault 4: Low boiler pressure

With pressurised systems such as a combi or system boiler, the boiler will simply refuse to come on if the boiler’s pressure drops below 1 bar. This can happen if you’ve bled or aired radiators, or taken a radiator off the wall for some reason. It’s also wise to check for leaks to ensure there’s nothing more serious going on.

The Fix: Locate the filling loop, check the pressure gauge, and open the two taps before allowing the pressure to rise back up to 1.5 bar. Close the the taps again. There is more information to help you out in this blog post.

Fault 5: Heating won’t come on, low temperature outside

If it’s particularly cold outside and you can see frost, whilst a fault code flashes up on your boiler and your heating refuses to come on, you’ll probably find when you check the manual that everything points to a frozen condensate pipe.

The Fix: Locate the condensate outside, and if possible, try thawing it out with a kettle of hot water. Remove the condensate pipe below and drain to a bucket. The boiler should now fire up. It’s best to get this problem checked by an engineer to see if you can retro-fit a condensate trace heater to avoid this issue in future.

Conclusion

I hope you’ve found this blog helpful when dealing with your faulty system boiler. Checking some of the tips above first can be helpful and act as great little pointers when it’s time to call in an engineer. However, if none of these fit, it could mean there’s a more serious issue with the boiler.

If you have any specific questions about system boiler faults, or your boiler is experiencing an issue I haven’t mentioned above, feel free to leave a comment below or get in touch with me directly – I’ll be more than happy to help!