I've searched around the forum and didn't see a similar situation, but I've tried to read through the overlapping components that are relevant. I hope you don't mind me trying to get some more specific advice.
We have had a Navien combi (the 240 size) for six years for our radiators and for domestic hot water. the House is in the Toronto, Ontario area and was built in the 50s with dual brick/no insulation. We haven't done any changes other than changing the boiler, the radiators and pipes were all in place when we bought the home, more than 15 years ago.
We have two types of radiators, which I see on the site is not great, but they were in place before we got there. We have 2 cast iron and 10 that I think are brass tube/copper fin? (they are covered in steel boxes with the vents on the front side, near the top, that remind me of my old elementary school).
Our upstairs is generally uncomfortably cold, even though the fins on the radiators get hot.
That's the background, but not the main reason I'm writing. The issue is that in six years, we've had a number of problems with the domestic hot water and a leaking pressure release valve. Every 6-9 months, it seems that the domestic hot water starts to oscillate (shower gets hot and cold while you're in it), without any error codes on the boiler; we're pretty sure that it is the mixing valve that needs to be replaced for the second time. And in two years, we've had to replace the pressure release valve twice due to leaking.
Our latest service person said that this is all probably due to gunk in the radiator systems (rust, etc.) and it causes the pressure relieve valve to be damaged and stuck open/dripping. So they suggested removing all of the radiators and pressure washing them to clean them out. But this would have nothing to do with domestic/mixing valve, so not sure if that makes sense.
They also suggested that the cold upstairs is likely due to the radiators being bipassed (again, the gunk in the system), and they showed me that the returning water pipe is almost as hot as the outgoing pipe. But I actually think this might be due to the inefficiency of the brass tube radiators.
One place said try Fernox, but the others said that won't help. . .
And finally, as I've spoken to potential companies about "cleaning the rads" most of them have said it would be better to replace the brass tube radiators with stelrads instead of trying to clean them and put them back, but I've heard different advice about having stelrads and cast iron on the same loop, too.
So, in short:
1) why does my mixing valve keep failing?
2) why does my pressure relieve valve keep failing?
3) are they in any way related?
4) should I try Fernox (it was an expensive service, but less than replacing rads)?
5) would it make sense to change some of my radiators to stelrads (in the coldest rooms)?
6) does changing any rads only make sense if I change all the brass radiators at once?
7) does changing radiators only make sense if I change all of the cast iron and brass tube ones to a new type?
Thanks for your help, and I hope most of this makes sense