water problem!

The 'Low coolant level warning light' came on this morning, on my 2003 Peugeot Boxer. To bring the level up to normal in the header tank, took 8 pints of water!  This is rather worrying as it seems a lot of water to lose so suddenly.
There doesn't seem to be any water leaking, with the engine running.  The warning light is now out.  Help please anyone!

Does it run OK, not just sitting on tick-over but out on the road.
If its not leaking then it must be going into the cylinders.
You get more responses if you do an introduction, I notice you never have.

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If you haven't already done so - check the oil level. If it is high, do not run the engine! If that 8 pints has gone into the sump then you will have an emulsion of oil and water instead of pure engine oil, and that will do no good at all for engine wear.

I don't know that engine specifically, but possible causes are head gasket failure (obviously) - and on some engines failure at the water pump can result in coolant escaping into the oil ways.

Also, I understand by "water" that you may mean water & antifreeze mix. But if you really do mean water then you should correct that ASAP - as well as risk of damage by frost, the rate of internal corrosion in the engine will be high without the correct coolant mixture.