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Tips for preventing frozen condensate in pipes
Published:  23 January, 2018

Neil Macdonald provides tips for dealing with frozen condensate in pipework.

As winter approaches and temperatures drop, homeowners crank up the heating. The result? A significant increase in the number of calls to boiler manufacturers and heating engineers! So let’s take a look at one of the common, avoidable, domestic heating issues reported at this time of year – frozen condensate.

Frozen condensate occurs when the condensate drainage pipe on condensing (high efficiency) boilers freezes and becomes blocked with ice, causing the boiler to shut down. In the vast majority of cases, such problems occur where the condensate drainage pipe is located externally to the building for some part of its length.

All boilers currently supplied for sale on the UK market are required by UK Building Regulations Part L and European ErP legislation to be of a minimum efficiency. This effectively mandates a condensing boiler to be installed when the appliance is replaced, in almost all cases.

A correctly installed and commissioned condensing boiler will extract latent heat from the flue gases and produce condensate, which must then be removed from the boiler.

Regulations currently allow condensate drainage pipes to be run either internally or externally, or a combination of these. The standards documents also give guidance on how to install the pipes in order to reduce the possibility of freezing.

And, while there have been historical instances of condensate drainage pipes freezing due to poor installation practice (i.e. non-compliance with British Standards, such as uninsulated external pipe-runs and inadequate fall), it is important for all installers to be vigilant to minimise the risk of a ‘compliant’ installation freezing.

To help minimise this, in 2014, BS 6798 was revised, with several notable changes relating to boiler condensate disposal. However, it is important to note that the appliance manufacturers’ instructions take precedence over the standard and should be adhered to, helping ensure correct appliance operation, and validity of any warranty.

Here are the Heating and Hot Water Industry Council’s (HHIC) ‘rules of thumb’ for avoiding frozen condensate:

Disposal method

An internal condensate disposal method (e.g. internal soil-stack connection) should always be the preferred option, keeping pipework within the thermal envelope of the building.

Be careful when installing in unheated locations (e.g. loft, garage) as condensate pipework should be considered as ‘external’ in these instances, and installed appropriately.

If a condensate pump is utilised then the manufacturers’ guidance on siting, pipe-size etc. should be consulted.

Keep it short

External runs should be as short as possible, preferably no longer than 3m in length. Any external boiler condensate pipe must be a minimum internal diameter (ID) of 30mm, and insulated, using suitable waterproof and weatherproof insulation.

Any internal condensate pipe should be a minimum of 19mm ID and upsized to 30mm ID minimum before passing through the building structure to outside. All pipework should fall a minimum of 2.5° with bends and fittings being kept to a minimum.

Above the line

Where condensate pipework terminates externally to an outside gulley or drain (foul-water only), the pipe must terminate above the water line, but below the grate by at least 25mm, with the end cut at a 45° angle.

A drain-guard should also be fitted. These steps help to minimise the risk of blockage or freezing through wind chill.

Bespoke options

Bespoke methods to ensure condensate pipework does not freeze may also be acceptable, whether integral to the appliance, or standalone products. These can take many forms, such as larger internal appliance siphons, or pre-insulated pipe products.

Checking it twice

The Benchmark Commissioning Checklist should be completed as required, to record details of the condensate drainage pipe installation.

Trace heating

Where an external condensate drainage pipe is installed, the customer should be made aware of the risks and consequences of its freezing and offered the option to fit trace heating (or other measures approved by the boiler manufacturer or service organisation).

If such a system is used, then the installation instructions of the trace heating manufacturer and any specific recommendations regarding pipe diameter, insulation, etc. should be followed.

All other relevant guidance on condensate drainage pipe installation should also be followed.

Neil Macdonald is Technical Manager at Heating and Hot Water Industry Council