What Is This (answered WATTS ISOTHERM) How do I fix this?

First off I am starting a new thread, hope that is OK here.

So many thanks last week to all who answered. As it ends up, I’m surprised that Google lens, and no one got this. I got to dad‘s yesterday, and of course he had the manuals from 30 years ago. It is a Watts Isotherm “control module” and after reading the summary below, I have a bit better understanding of what was intended and why I didn’t get it. Mostly, the three-way mixing valve here is running backwards with pilot flow, and as someone identified, the box with orange knob is an aquastat, however it actually was for HIGH temp protection, this thing does nothing for boiler return protection other than luck. Further the knobs on the isolation valves were correct because unknown to me dad”s zone valves are on his return manifold which leads to some issues in my mind since this things supply side manfold connects directly to the high temp return manifold.

So now for an as built which might explain why it never may have worked properly. In his defense dad installed it the way his 5 page manual described. But looking online at a spec sheet on a Hungarian site, it is stated that the supply and return are supposed to be onto a primary loop, with this a secondary loop. Pardon the poor sketch, but this is what is there largely. All major components shown. Of note there are no differential pressure reliefs between either manifold set. The remote expansion tank and air scoop are also on the outlet side of the system/boiler circulator. The Isotherm circulator does run only with it’s loops, the system/boiler circulator with any call for heat. Both pumps are Grundfoss 15-58 both set to middle speed. The system is full of rust and sludge but is 30 years old. I am replacing the boiler like for like, Utica/Dunkirk cast iron, power vented, the original was a Sears branded Dunkirk power vented.

The thermostatic mixing valve on the Isotherm is a rock. Even after soaking for two days in CLR, judicious application of heat and penetrating oil, it is a paperweight. I see that it is easily available in the UK for 144 quid, but it does not exist in the US. Further, the circulator is probably questionable and the piping between them is severely corroded anyway. Plus again I don’t know if this truly worked as built anyway. .I have 10 days and the following things here at my disposal that I have collected over the years.

2 Honeywell AM101R-US-1 mixing valves (Cv 3.9)

A Taco I Series-S in 3/4 sweat (Cv 3.4)

A couple Taco 3196-1 differential pressure valves

Lots of new Grundfoss 15-58 pumps and a Taco 007-f5

A Tekmar 362 (which does power on, and was good 20 years ago when I took it out of service)

3 thermistors that should work more or less for either the I Series or the 362.

Fos context, the low temp is 4 loops of 1/2 Pex, 150-250 ft per loop.

Absolutely ANY suggestions for mixing and boiler return protection with what I got MUCH appreciated. I would very much prefer to not mess with either manifold, or the zone valves, though I could reconfigure them a bit. I have plenty of 1 1/4, 1, and 3/4 copper, a good assortment of pump flanges, etc.

Finally, I really don’t want to use the Tekmar 362 unless absolutely necessary, as there is no guarantee it still works, and I would hate to blow three or four days trying to figure that out. If somebody tells me hands-down, it is the smart choice, I’m receptive to it, it just may doom me.