In a 44 year old house with a two pipe central heating system installed in the concrete floor, i will have to drain the system to install new radiators in two bathrooms i am renovating. Is it true that draining the system encourages corrosion of the steel pipes and if so what is a safe period of time for a system to be left empty of water? Also, if copper pipes are to be connected to the existing steel ones what is the proper way to do so?
There isn't really a time limit on how long the system should/shouldn't be left empty.
If you're living there or even if the house is empty in this weather I'd guess you'd want it re-filling asap, for warmth and also if the house is empty then the tank, hot water pipes, cold mains etc could freeze.
It would be best to have the heating on for a few hours during the night.
Compression fittings should fit copper and steel pipes