Freeze prevention measures are most tested when the air temperature outside a building does not rise above freezing, 32 degrees F, even during daylight hours. In such conditions, water pipes and appliances in rooms without heating are likely to freeze even with good insulation. The main or well service into the property is unlikely to freeze; building regulations should mandate it be buried below the level to which frost penetrates in your location.
Why Toilets are Vulnerable
Toilets are almost invariably fitted to outside walls, for stability, to make pipework and sewer lines easier to run and to preserve as much free space as possible in the room. External walls transmit cold from the outside into rooms; if the ceramic of the toilet is bolted to or touching an exterior wall the sub-freezing temperatures can be communicated to interior fittings. If a draft space has been inadequately sealed or insulated around either a pipe or a sewer line, the freezing air from outside can blow in; the toilet will be first to come into contact with the freezing air. Further, it is conventional to insulate pipes and storage tanks. Few people insulate toilets.
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If Water Freezes in the Toilet, But Not in the Pipes
Water in a toilet, both in the cistern and in the commode bowl, is static. Static water freezes more readily than moving water. If the water in the toilet is frozen but the water in the pipes is not, it is probable that the water in the pipes is flowing, that an appliance is in use or a tap is switched on. If the pipes are insulated they may resist freezing for longer than the exposed water in the toilet. If pipe heating tape or cable is in use, this should protect them from freezing even if the temperature in the room is below freezing for a prolonged period.
How to Minimize Damage
Freezing water expands. If the toilet is not shattered by the frozen water, it is probable that only the surface of the water is frozen. Do not try to shatter the ice; the concussion could damage the structural material. To avoid damage to the cistern and the commode bowl, pour in warm water sufficient to free the ice from the sides; then lift out the slabs. Using a hair dryer also works. Do not use any sort of naked flame or a hot air gun; the temperature variation is so great that it could shatter ceramics and will almost certainly melt PVC. If the top of the ice is frozen solidly around mechanisms in the cistern, flush the toilet once the bowl has been cleared and leave the ice in the cistern alone. Hot water and hot air can damage the seals in the mechanism. When the rim of ice eventually melts it will leave the mechanisms undamaged.
Thawing the Sewer Line
Ifm after thawing the water in the commode bowl, the toilet still does not flush, it is likely that the water trapped in the "U"-bend to prevent odors backing up is also frozen. The "U"-bend can also shatter from the expansion of the ice, so this should be rectified. Again, a hair dryer should melt the ice inside the pipe. Replacing bowls of steaming hot water under the "U"-bend when they cool may also work, but it will be a long process.