New to Oil boiler and underfloor heating

ridethelightning

Hi All,

Im looking for a little advice from anyone who knows about underfloor heating systems.
I recently bought a house which has underfloor heating downstairs and CH upstairs, all heated from an oil burner. This is my first experience with underfloor heating.

Here is the boiler and Ive marked:
V1: Water filling valve
G1: Water pressure gauge
C1: Controller 1


Here is the UFH manifold:


The heating is working ok. The UFL seems a bit patchy (hot and cold areas) which I assume to be some air in the system. I have been trying to determine exactly how I can flush the system but am finding it hard to get all the info I need. I think this has not been done since it was installed as the previous owner has had the annual service done every year for the boiler but knows nothing about the UFH system. Also the CH upstairs was terrible until I bled the radiators as they were all about half full.

When I bled the CH radiators this also dropped the pressure in the UFH system, then both pressured up together when I opened valve V1 (photo 1) so the 2 systems must be connected (also the hot water is coming from the oil burner).

First I have a few questions:
1- Does it seem reasonable I should flush or bleed the UFH system?
2- Do I need to fill it with antifreeze or can I just flush with water?
3- What is a reasonable pressure to set it at? I increased the pressure to 2 bar on the boiler (max in the manual is 3 bar)
4- What is valve (regulator) V5 for? I cant seem to turn it, without worrying it is going to break... Also Im not sure if it is set to Min or Max (as it is fully turned in one direction).
5- Can you please help with the process for bleeding/flushing the UFH.

I read that I should flush 1 zone at a time. So whats the best way to isolate the zones?
What are the black caps at the top of each zone inlet pipe (they have a valve operated by a key attached at the side). Is this to isolate the zone or to bleed them?
There are 2 hose connection points - 1 under gauge G2 and 1 marked V4. Do I need to isolate all but 1 zone and use a hose connected to V4 and a drain under G2 to flush? Can I just open the valve V1 on the boiler instead of connecting a hose to V4??

As you can see Im not quite sure how to do this so appreciate any clear instructions for help. I can supply further photos if needed.