Corsair H50 Problem - Coolant Freezing?

I have a PC (as in my profile) that has it's processor (i5 2500k) cooled with a Cosair H50 watercooler. It is the sealed unit type where the fluid is already in the cooler and no maintenance is needed: http://www.corsair.com/hydro-series-h50-cpu-cooler.html

This has worked fine for over a year now, but recently I have found that if I turn on the PC after a particularly cold night it hangs on the BIOS screen with CPU fan failure, press F1 to continue message and makes a beeping sound. If I turn off the PC and switch it on again it works ok. The problem only seems to happen if the room is particularly cold and never happens if the room has been heated beforehand.

Therefore, could it be that the coolant inside the CPU cooler is freezing and hence the pump cannot move it?

I am quite worried as if this is happening it probably isn't doing the CPU or the H50 any good at all. :(:

Storage Temp: -20 to 70 C

Ambient Operating Temp: 5 to 50 C (In chassis)

 

I don't think the coolant is freezing, unless your ambient is 0C.

 

It does sound you CPU is overheating due to a pump failing. Go to your BIOS and check your pump's rpm. Better yet, run Prime and see where your temp is under load.

Actually I wouldn't be that surprised if the ambient has been as low as 0C on occasion as the room doesn't have any central heating, there is only an electric heater which I can only afford to run when I'm actually in there and if the temps outside are getting down to -3 or 4 on a really cold night...

I'll try Prime, though I can't check the pumps RPM as the pump header is actually connected to the PSU (with a molex to fan connector) as I heard that the pump needs to have full voltage at all times so if I connected it to a motherboard header I would have to run all the case fans at full pelt too (there isn't any option to control each fan port separately)