Background: 1920s house, hydronic cast iron radiator heat. Peerless mod/con boiler with indirect water heater installed by local pro in 2010.

We have been quite happy with the boiler, water heater, operating efficiency, etc. however, I am afraid that there is an issue with the water supply equipment.

First, as of two years ago, the back flow preventer on the fill line started dripping. Since it is a condensing boiler, I routed the drain line to the condensate pump (it was a very slow drip), and shut the two valves around the fill regulator and the back flow preventer. I don't like to leave those open anyway, since a leak while were not home could make a huge mess.

Recently, as part of a kitchen renovation, we moved a radiator, and so had to drain the system. The system was always at 12-13 psi before, and the auto fill regulator label says it stops at 13psi. However, after refilling, the watts auto fill regulator seems to be filling the system to 20 psi and then slowing the flow. I drained down the system a bit to lower the pressure, and tried again with the same result. I would guess its a weak spring on the regulator (looks like it is set by a threaded rod with a locknut), but this stuff isn't old.

So I'd appreciate thoughts on how to proceed. These items can be isolated from both sides via valves, so replacement should be straightforward. Should we replace with watts products? Other brands?

As mentioned, I'm not a fan of leaving the auto fill hooked up with the valves open anyway, so is manual fill really best?

Would all this actually be an indication of a need to put air in the expansion tank (Honeywell, set to 12 psi per label), which also is not old (installed when the boiler went in), or something else?

We have a whole house filter, but I think the boiler fill is the lowest/last point in the piping, so I would imagine junk from the lines could get caught there.

Thanks for your thoughts!