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Updated 01 November 2020   Boiler Services Ltd 2009-2020

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Condensate drain problems – Frozen condensate pipework?

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If you have a condensing boiler (also called a high efficiency boiler) which stops working in very cold weather, and particularly if you have been hearing new strange gurgling noises, you may have a frozen condensate pipe.

Fault codes (error codes) vary from manufacturer to manufacturer but if your Potterton HE boiler is showing E133 (which may be displayed as E1.....33) it may have a frozen condensate pipe. Baxi is part of the same group as Potterton and uses the same code on some of their boilers so if your Baxi HE boiler is showing code E133 you should be checking the condensate pipe. Some Baxi boilers use fault code LF and some Potterton boilers use E28. Some boilers use a series of lights; you need to refer to the User Instructions to find which combination of lights might indicate a frozen condensate pipe.

Ideal also use an LF fault code on some of their boilers. Ferroli and Glow-worm use an F1 fault code on some of theirs. Glow-worm may also use fault code F4 for the same problem.

Worcester Bosch Greenstar boilers, including the CDi combi boilers, may show codes EA or D5 which indicate a possibly frozen condensate drain pipe, especially if you get gurgling noises and a blue flashing light. (Apparently on the Worcester 26CDi the flashing light may be red.)

If you have a blue flashing light on your Worcester Greenstar boiler it may be a frozen condensate pipe. The flash is a slow flash, long on and short off.

On Vaillant ecoTEC plus VU and VUW boilers fault code F29 may indicate a blocked condensate duct and fault code F68 can be a blocked condensate route. It’s possible that either of these fault codes could show up if the condensate pipe was blocked with debris or frozen. Fault code F77 can indicate a fault with a condensate pump if one is fitted. On some Vaillant boilers the fault code may also be F28.

Unfortunately, there’s very little consistency in the fault code used. Some manufacturers use different codes on different models of their boilers. If the instructions say your fault code is an ignition problem, boiler fails to light or to stay alight, it could be a frozen condensate pipe causing it.

A little basic science or a short explanation about condensing boilers

If you boil a kettle of water you put in energy to raise its temperature up to boiling point. The rise in temperature is directly proportional to the amount of energy you put in, until it starts to boil. If  you prevent your kettle from switching off you continue to put in energy but the temperature doesn't rise any further; it stays stubbornly around 100C until all the water has been boiled away.

All that extra energy is carried away with the steam being boiled off. Steam is water in a vapour state but it's still water. There is a reverse side to this process. If you could condense the steam back to water it would release that extra energy again but you'd end up with lots of water, outside the kettle.

When you burn gas in a boiler you are changing methane gas, and oxygen from the air, into carbon dioxide gas (the stuff we all breathe out) and water but the water is in the vapour state, and loaded with extra heat energy. Regular (non-condensing) boilers pass this water vapour out through the flue and all that extra heat energy, keeping the water in a vapour state, is lost. With non-condensing boilers about one fifth of all the energy from the gas we've burnt ends up where we don't want it, costing us money, unnecessarily warming up the external environment.

Condensing boilers are clever (condensing boilers and high efficiency boilers are the same thing). They cool down the water vapour produced when the gas is burnt and it changes back to the liquid water state inside the boiler, releasing the extra heat energy inside the boiler, from where it can be sent round the house. The problem is that the boiler now needs to get rid of the warm waste water (the condensate).

The trap is full of condensate and as more dribbles in, the excess simply dribbles out to the drain. If the condensate pipe passes outside the building to the drain it will be subject to freezing in very cold weather. Though the condensate starts off warm it is only a slow trickle and cools in the chilled pipe. In severe weather it freezes and the pipe quickly becomes blocked.

Most manufacturers now use a much cleverer, but still very simple, trap called an auto-syphon.  It collects about a small cupful of warm condensate and then dumps nearly the complete cupful in one go, every few minutes. The last cupful may have left a thin layer of ice in the condensate drain pipe but the new cupful of warm condensate melts the ice. Because there is no constant dribble it is very much less likely to freeze solid.

Please note: Boilers must never be run with condensate traps empty!

Whether they use simple “u” bends or cleverer auto-syphons, boilers must never be run with condensate traps empty. These traps form a seal which prevents poisonous combustion gases passing out of the boiler along the condensate drain pipe. If the trap is removed for cleaning it should always be refilled with water when it is fitted, before running the boiler.

Condensate drain pipe locations and sizes

Where possible, condensate pipes should be run to a drain point inside a building. It makes them much less likely to freeze. If they are run outside the building they should be as near vertical as possible, as short as possible and, unless they are large-diameter pipes, insulated with waterproof lagging.

If the boiler uses a simple trap, not an auto-syphon, the external condensate pipework should be increased in size to at least 40mm (1") and lagged. If a sharp fall cannot be achieved on the pipe you may need to increase it to 50mm (2").  The lagging must be a closed cell foam type and waterproof. If you use lagging that absorbs water and becomes wet you will make the situation worse, not better.

Even if the boiler uses the much cleverer auto-syphon system you may still need to increase the pipe size and/or lag unless the external run is short and near vertical. Most of the 5 or 6 winters up to January 2013 had prolonged, very cold spells. Same again in Feb/Mar 20108.  Increasing the pipe size and lagging the pipe make sense!

In a loft, terminating your condensate pipe in the plastic vent pipe of the soil stack works well. The vent pipe will have an open top or an air admittance valve so it should not "pull" the condensate trap even if you seal the condensate pipe to the vent pipe. Even if the soil stack should become blocked low down, it can't back up to the loft and threaten your boiler without spilling over the the level of the loo in the bathroom first.

Remember, lofts can become very cold. Increasing the pipe size and lagging the pipe may be necessary in the loft too.

So if the condensate pipe has frozen?

Defrost it if you are sure you can do it safely. You already know that boiling water is dangerous, and so is falling off ladders. You can use hair dryers and hot water bottles but they may take a long time. Hot water is quicker. The pipework can deal with very hot water but you would need to take care. If you must work above ground level, make sure you have help and can work securely.

To defrost a frozen condensate pipe, we would start at the end furthest from the boiler, the end nearest the drain where the condensate pipe empties out. That way, the melted ice water has somewhere to run away to. If you start near the boiler, the ice water can’t run away and may re-freeze before the whole pipe is defrosted. We would aim to get the pipe warm enough to be sure we’ve melted the ice.

If the condensate pipe is in a loft and has frozen you'll have to use a fan heater or hair dryer and wait a little longer.

Once the ice blocking the pipe is melted you will need to re-set the boiler.

Finally, a Scandinavian joke

Q – Why do the British put their waste water pipes on the outside of their buildings?

A – So they can get to them easily if they freeze.

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Beware of connecting a condensate drain pipe directly into a rainwater down pipe. If the lower end of the rainwater down pipe blocks, even a short, sharp shower can produce enough water to fill the down pipe, back up along the condensate pipe and fill the boiler combustion chamber, gas valve and fan.

Much better to put a branch into the down pipe and terminate your condensate pipe so it drops condensate into the open mouth of the branch pipe. Keep it above  the top of the branch pipe and the air break will ensure a blocked down pipe can't back fill your boiler in a rain shower.

If you can successfully melt the ice in the condensate pipe you will need to follow the boiler manufacturers' instructions to re-set the boiler. Remember, some boilers go through an electronic checking cycle after being re-set. This can take several minutes so if your boiler shows strange symbols and makes clicking, switching noises for a few minutes after being re-set, wait for it. Give it 15 minutes and it may start working normally.

If anyone knows the error codes (fault codes) that indicate a frozen condensate pipe on Glow-worm, Ferroli, Alpha, Biasi, Halstead, Ideal, Vokera, Keston or any of the other manufacturers' boilers, please let us know and we'll try to add them here. You can email me. If you could send me links to the information on the pages of their web sites that would be particularly useful. Thanks.

Condensate trap design

The condensate drains into a sump in the bottom of the boiler combustion chamber and drains out of the boiler along a condensate drain pipe. Some of the carbon dioxide from the combustion process dissolves in the condensate and makes it weakly acidic so the condensate drain pipe is always made of plastic as the mildly acid condensate would eat away at a metal pipe.

There is always a condensate trap between the boiler sump and the condensate drain pipe and this trap is one of two types. The first is a simple "U" bend trap and its purpose is to prevent any combustion gases from passing out along the condensate drain pipe.