Losing pressure in CH when hot tap goes on

[Anthony Mayfield]

Thanks for the feedback, the PRV seems to be the only new thing in the entire system, it stands out like a sore thumb so hopefully it will last until the boiler gets replaced.

I've taken the pump off and given it a full clean, included impellor, manifold and air release valve. A blockage in the pipes is the only thing I can think of right now, it's just surprising how quickly the flow out pipe heats up and it manages to get all the way to the first radiator so there must be at least a little flow. The rest of the piping is behind cupboard backing so I'm going to pull that off tonight, someone in another thread recommended trying the flow in and out valves although I've been told these will almost certainly leak if I play with them. The way I see it the CH isn't working either way so no point not having a go.

I'll check out your expansion vessel comment in the manual as I'm not sure what you mean. I've filled the system and it's working again although I did have to bleed the pump for ages.

 

[Anthony Mayfield]

So just a quick update to those that might be seeing a similar issue. When I got home I started drilling holes into the back of the cupboard that was hiding the pipework. It lead me to a tap that was controlling the flow out pipe (this is why the flow out pipe valve was sheared off). This was open as hot water was flowing through but there was a T-junction further down where the hot water pretty much stopped in one direction and was slightly warm heading off to the closet radiator.

I've order some citric acid and sentinel x800 online and I'm going to put that into the system to see if it helps to clear whatever is stopping the flow. If not it seems I'm going to have to drain the system (I'll be looking for the correct drain valve today!) and then cut out that section and replace it.

 

[Anthony Mayfield]

Managed to find the drain valve (under one of the kitchen cabinets). It wasn't draining at first but I turned up the pressure and the water started coming out. I didn't wait long enough really but put some of the ds40 in and waited a while. Already started to get flow through the system at that point but noticed that the water coming out was completely blank.

With the DS40 it turns from red to green when the acidity runs out and I quickly realised that I could use a truck load before it cleaned out the system in the way I was doing it. So I got a flexi hose from the store, closed all radiator valves, removed a radiator, and attached the flexi hose to one of the radiator valves and the other end going into a bucket. I then opened that valve and turned on the mains and watched as a load of black crap came out of the system. I tried that on each side and on a few different radiators and then retested the system and finally got some flow through the radiators! (although it quickly slowed again)

I wish now that I had filled the system up and drained a few times, instead I put the chemicals in again and got loads more crap coming out of the system which I think would have come out with a drain and the chemicals could have got the harder to reach bits. By this point though the drain valve was working fully which was a big help.

I also fitted a magnaclean which I would heavily recommend, took a while to fix because I had to get to the pipe and the pipe was so close to the wall it was hard to fit but if I had a clear run of pipe it would only have taken maybe 20-30 mins to fit without knowing anything about it. And you immediately get results if you have an old system you can see all of the metal stuck the magnet within minutes, it also makes it easier to add small amounts of chemicals into the system.