Going to be moving in a few weeks, the house I am moving into has been empty for a couple of months and one of the stipulations of the vendor's building insurance was to drain down the central heating system - perfectly reasonable, but what do I need to look out for when refilling, and more importantly how do I go about refilling it? It's a combi system so no cold water tank, so how do I go about putting Fernox or something in as I presume you don't just fill it with water?
Yes stick some inhibitor in first. Easiest way is if you have a towel rail; just unscrew one of the plugs at the top and pour it in. Otherwise you'll probably have to remove an upstairs radiator and undo a valve, to pour the stuff down one of the pipes. You can also buy it in expensive syringes (ie very concentrated) to inject into one of the top plugs on a conventional radiator.
For topping up, there should be a 'filling loop' close to the boiler - a short braided metal hose which connects the water main to your CH system. If it's not connected up already (which by rights it shouldn't be unless it's actually being used) and having checked all bleed and drain valves are closed, open the valve. You'll need to go round iteratively opening all the radiator bleed valves in turn and topping the system up, to get the air out as obviously it has nowhere to go but be compressed inside the system. The boiler will have a gauge showing how much water to put in.
I think the main issue is to avoid getting an airlock in the system; it will mean either your boiler won't light or areas of the system will stay cold. It's happened several times to me when refilling, and it's an absolute bugger to shift it - I still don't know how!!
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