Hi" Terry, You will find that the slotted screws are for letting out the air that some times gets into the rads, and when you top up with water air gets inside and can give small problems with the heating of the rads I control my rads about every 3-4 months and top up with water.( you can do this your self its not hard) Some where in the system you should have a gauge that tells you how much pressure is in the pipes there will be a green mark, and a red mark keep the pressure between them and you should have no problems.... John 2000
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