Sorry if I missed anything but I think pump over introduces oxygen into the water increasing corrosion. Much like a bubbler in a fish tank.

Pump over requires water to be drawn down the feed from the header and back up the overflow to the header. This needs either a pressure or temperature difference between these two pipes. 

 

So I would be looking at connecting them both to the same T in the CH loop (if that's allowed) to keep the temperature and pressure the same (at least at that point). Perhaps run them up to the tank adjacent to each other wrapped in same insulation to try and keep them the same temperature as much as possible.

 

 

 

 

Big thanks to @marshian for sharing the details of the old pump that I sent him. It really does explain a lot.

To recap, I’ve not had any involvement with my parents’ heating until recently. So I’ve no idea if any inhibitors have been added over the years.

 

It was another family member who noticed that the rads weren’t getting warm enough and as I live closest, it was me that went to look. That was after a plumber had been and suggested a failing pump and possible  ‘air-lock’.

 

Venting over wasn’t happing as far as I know, until I replaced the pump and even then it wasn’t noticed immediately. It had been drained so that a couple of trvs could be changed. Refilling was slow as we couldn’t remove the air easily.

 

Only when plumber #2 and I decided to try a flush did we then see the vent-over happening. He didn’t comment on the short height of the U though.

 

As we had enough heat for Christmas and NY I waited and called him back to open the H pipe.

 

That was after plumber #3 quoted £5k to replace the whole system with a combi “the only way to fix it”.

 

So to conclude, a bad design that somehow had never had vent-over and poor maintenance of the system which slowly filled with crud until a better pump highlighted the issues.

I would never have got here without the help of you on here.

 

Thanks to all. 

Will read back the posts sometime but was that H actually opened up?.

 

A relatively small system leak could also cause these problems with sludge etc but would require tying up the ballcock on the F&E cistern for a day or two and then see if there is makeup when released. I have a isolating valve on the make up and only open it every few months for a min or two to check for leaks.

Yes, I posted a pic of the inlet side of the pump but said that I didn’t have one of the H. The pipe was cut out and the crud cleaned out and then it was connected back in. Plumber #2 did that and I was cleaning the header tank.