Ignition lockout — frozen condensate pipe

At the end of February 2018 and the beginning of March we had very cold weather in the UK. Many areas that rarely experience hard frosts were frozen in for several days. Roads became impassable and many central heating systems failed. We were swamped with phone calls from customers whose boilers had locked out. Most of these were caused by frozen condensate pipes.

All high efficiency boilers (condensing boilers) recover additional heat from the combustion gases by cooling the gases, within the boiler, until the water vapour condenses out. This releases the latent heat which had kept the water in a vapour state (as a gas, steam). The water which has been condensed out has to be disposed of. It passes into a condensate trap and from there into the condensate waste pipe. It then passes out of the building, very often along an external plastic pipe. Good practice in installation means that larger plastic pipes are fitted, often up to 1½” (43mm). They are also fitted as near vertical as possible, though horizontal runs are often inevitable.

In extremely cold weather the condensate passing along these pipes may freeze. This is a particular problem when the temperatures outside stay below zero for several days. If the condensate freezes it will block the pipe. The condensate will back up into the boiler and the boiler may make loud gurgling or rumbling noises. The boiler will lock out and may show flashing lights or flashing error codes. If the condensate pipe stays frozen, resetting the boiler will not solve the problem. It will simply lock out again.

Fault codes vary from manufacturer to manufacturer so you may need to read the User Instructions for the boiler. If the fault says it is an ignition fault, or that the boiler fails to stay alight, and it is freezing outside, there’s a good chance it’s caused by a frozen condensate pipe.

Worcester Bosch use a blue flashing light on some of their boilers, a slow flash which is long on and short off. Some Worcester boilers also us an EA fault code. Baxi may use LF or E133 fault codes. Potterton may use E133 and E28. Both Baxi and Potterton, among others, use a combination of lights on some boilers to indicate different errors. Ideal use an LF fault code and Vaillant use F28. Ferroli use an F1 fault code, as do Glow-worm. Glow-worm also use F4. It seems daft and there’s very little logic to it. If you have the User Manual, and it says your fault code is an ignition or gas fault, it may also be a frozen condensate pipe.

It’s worth having a go at defrosting the external pipe if you can reach it safely. Obviously, climbing up ladders in icy weather is risky. You have to decide whether you can work safely or not. Using very hot water is risky too. If we decide we can work safely we use hot water. We would always start defrosting the pipe at the point furthest from the boiler. This is the point nearest the drain where the pipe empties out. If you start there, the melted ice water has somewhere to run away to. If you start near the boiler the pipe will still be blocked further along and the ice water water will have nowhere to run to, so it may re-freeze. Starting at the end furthest from the boiler, we thoroughly warm the drain pipe, working progressively back towards the boiler.

When the pipe has defrosted, and the water has run away, we reset the boiler. This may be turning a knob back to a reset position before turning it the other way or it may be pressing a reset button. Sometimes the reset button is tiny and has to be pressed with a pencil point. Different manufacturers use different methods. If a frozen condensate pipe was the cause of the problem the boiler will run again if a program cycle is on and the room thermostat is turned up.

There are additional pieces of kit which could be worth trying.
Heat trace kits are available which warm the external condensate pipe if the kit thermostat senses temperatures approaching freezing. The heat trace wire is fitted round the condensate pipe and cable tied to it. The pipe plus wire must then be insulated with weatherproof pipe lagging. This kit requires electrical installation.

Worcester Bosch make a CondenseSure condensate syphon. This is a clever piece of kit and requires no electrical power. It is a large, 500ml auto-syphon, fitted to the condensate pipe below the boiler, before the condensate pipe passes out of the building. Most boiler condensate traps are auto-syphons. These help to limit the risk of condensate freezing, by releasing a small cupful of warm condensate at a time rather than a slow dribble.

The CondenseSure syphon is clever because it holds a much larger volume of warm condensate (500ml) before releasing it all at once. Any frozen layer of condensate in the external pipe should be melted as the larger warm volume of condensate passes by. Any residual condensate which then freezes should be melted by the next pass of condensate. The CondenseSure syphon is designed to be fitted, where possible, round the 22mm Flow pipe below the boiler. It will pick up a small amount of additional heat from the Flow pipe. The syphon also has an insulating cover to limit heat losses.

Boiler condensate traps can become blocked with sediment and debris from within the boiler. This is more likely if the condensate trap is not checked during servicing. A condensate trap blocked with sediment will cause boiler lockout in the same way as a condensate trap with a frozen outlet pipe. The sediment may be casting sand, left over from the heat exchanger manufacturing process. It can also be aluminium oxides in a boiler with an aluminium heat exchanger.

Most condensate traps are made of relatively transparent or translucent plastic. This makes it easier to check for debris during servicing. The older Ideal Icos and Isar boilers had tiny black plastic condensate traps. These need to be removed to check for blockages and they can be a pain to work with. Thankfully, no manufacturer seems to be using black plastic now.

Where the boiler condensate pipe connects to internal waste water pipework, freezing is less likely to be a problem. However, where the condensate pipe connects below a kitchen sink, fatty deposits passing along the sink waste pipe can block the end of the condensate pipe where it tees into the sink waste pipe. This causes the condensate to back up into the boiler and causes the same problems as a frozen condensate pipe. We’ve seen this happen on a few occasions so, if you think the condensate pipe may be blocked, check to see if it tees in under the sink.