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How does one stop water in the holding tank from freezing,is this a problem?

Asked by: Andy Durler  | 10.06am, Saturday 4 January

WW says:

We have a 40 gallon polypropylene holding tank used with a Mansfield Traveller. I empty the tank as much as the pump-out will allow at the end of every season. The boat is now twenty years old and (without wishing to tempt providence) we have not had a problem in that time.
I assume that, if the amount of water in the tank is small, any expansion due to freezing will be resisted by the side walls which are stiffer nearer the base. The surface might rise in the centre like an ice cube in the fridge but no damage is done.

Graham Booth  | 10.58AM, Saturday 4 January

Do you mean the main water tank or the toilet holding tank?

Rupert Smedley  | 4.31PM, Sunday 5 January

If you have any holding tank treatment added, it should reduce the freezing point to well below -5C and also normal waste has a lower freezing point than pure water. I also agree with Graham, a holding tank freezing is unlikely to ever be a problem, and, as for any tank, is only an issue if completely full with no room for expansion.

Mark Langley  | 6.57PM, Sunday 5 January

Also you can use a food-safe antifreeze (such as Freezeban) to protect the water system, but would prohibitively expensive to treat the main water tank. It also has to be flushed through fully on de winterising. Never be tempted to use any domestic or engine antifreeze solutions as these are harmful or toxic. Depending on the formulation! Adding salt to the tank may, if it is galvanised or stainless steel, increase the rate of corrosion as well and should be avoided.

Mark Langley  | 11.07PM, Sunday 5 January

Readers say:

dilute 2kg of salt in warm water and flush in to a near empty tank

kevin m kinirons  | 3.18PM, Saturday 4 January

so you fill up again and get slightly salty water but that is enough to stop the water freezing ,mmm it,ll save salting the veg. ok thanks for that Kevin

Andy Durler  | 3.55PM, Saturday 4 January

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