Preparing Plumbing for Winter: Lagging, Condensate and Standing Water

Four runs freeze before anything else in a London property’s plumbing: loft pipework laid above the insulation, garage runs, the outside tap, and the boiler condensate. The condensate is the one that strands people, because it is small-bore, usually external, and it locks the boiler out on the coldest morning. All four are a morning’s work in October.

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We attend 16 London boroughs: Islington, Camden, Westminster, Hackney, Kensington and Chelsea, Hammersmith and Fulham, Wandsworth, Lambeth, Southwark, Lewisham, Tower Hamlets, Greenwich, Croydon, Bromley, Barnet, Ealing.

What should I do before the first frost?

Lag every exposed run in the loft, garage and outbuildings, insulate the boiler condensate, isolate and drain the outside tap, and test the stopcock closes. Four jobs, one morning, done in October rather than discovered in January.

Lag every exposed run in the loft, the garage and any outbuilding, keeping the insulation continuous across bends, tees and valve bodies.
Insulate the condensate pipe along its whole external length, in weatherproof insulation rather than indoor foam.
Close the internal isolation valve serving the outside tap, then open the tap and leave it open through the winter.
Close the internal stopcock fully, prove it with a running cold tap, and reopen it a quarter turn off its stop.

The order matters less than the completeness. A run lagged everywhere except at the bends freezes at the bends, because a bend has more surface area exposed to cold air than the straight either side of it. The same applies at a valve body, which is thicker metal and sheds heat faster than the pipe.

Which pipes freeze first?

Four runs freeze before the rest: loft pipework laid above the insulation, garage and outbuilding runs, external taps, and the boiler condensate. All four sit outside the heated envelope, and the condensate is the narrowest.

Loft pipework above the insulation line. Insulation laid between the ceiling joists keeps heat in the rooms and leaves the loft at outside temperature.
Garage and outbuilding runs. Unheated space, and often an outside tap feed left charged all winter.
External taps and the feed through the wall. The coldest point is inside the structure, where nobody can see it.
The condensate pipe. Frequently 21.5mm and frequently external, so it holds the least water and loses it fastest.

Why does the boiler stop on the coldest morning?

The condensate pipe carries acidic water away from a condensing boiler. It is small-bore, frequently run outside, and freezes solid at the first hard frost. The boiler detects the blockage and locks out to protect itself.

A condensing appliance recovers heat from its own flue gases, and the water vapour in those gases condenses to liquid as it cools. That liquid is mildly acidic and it leaves the appliance through a plastic waste pipe rather than through any part of the gas installation. Small bore plus external routing plus a slow trickle is the combination that produces an ice plug at the first hard frost.

The lock-out is the appliance protecting itself rather than a fault in it, and it clears when the pipe clears. Rillwater Plumbing & Heating works on the wet side — pipework, valves, cylinders and waste runs — and does not carry out work on a gas appliance, its casing or its flue. Insulating or re-routing an external condensate run is plumbing work; the appliance is not.

How should the outside tap be dealt with?

Close the internal isolation valve serving it, then open the tap itself and leave it open through winter. Isolating alone leaves water sitting in the run between valve and tap, which is the section that splits.

Find the isolation valve on the feed, usually a small screwdriver-slotted valve inside the wall the tap passes through.
Close it, then open the outside tap fully and let the standing water drain out.
Leave the tap open all winter, so any water that does reach the run has somewhere to expand into.
Disconnect and drain any hose left attached, because a full hose freezes back into the tap body.

What temperature should an empty property be left at?

Enough to keep water moving rather than to be comfortable. Many insurers state a minimum in the policy wording for unoccupied periods, so check the schedule — the figure they name is the one that protects the cover.

No single temperature applies to every property, because the answer depends on the insulation, the exposure and where the pipework runs. The figure that matters is the one in the policy wording, and it is stated as a condition of cover during an unoccupancy period rather than as advice. A property held above the insurer’s stated minimum satisfies the condition; a property held at a comfortable temperature that nobody wrote down does not.

Two conditions travel with it in most wordings: the water isolated at the stopcock, or the heating maintained, for a stated number of consecutive days. Read both in the policy wording rather than assuming which one applies.

What is worth checking now rather than in January?

Three things: that the stopcock closes, that the loft lagging is continuous at bends and valves, and that the condensate run is insulated along its whole external length. Each takes minutes in October and hours in a cold snap.

The stopcock closes. Close it fully, prove it with a cold tap, and reopen it. A valve that will not move in October is replaced calmly; the same valve in January is replaced with water running.
The loft lagging is continuous. Walk the run and look at the bends, the tees and the valve bodies, which is where insulation stops and freezing starts.
The condensate run is insulated end to end. Any length of bare plastic outside the building is the length that will plug with ice.