Solaroll, new boiler and furnox

Where did you hear that about not adding additives?
I can imagine that some additives may be deleterious to some epdm tubing....
Not the major players though.
A call to the MFG of either Fernox or Rohmar would be a good plan though.
Dwight at Rhomar is pretty well in knowledge in the odd tubing I imagine...since there has been more odd stuff on this side of the pond.
Where is this tubing? Slab or Staple up?

Mainly you need to keep all ferrous components out of the loop. Even those copper tube boilers has some metal in them.

Make sure you get every loop flushed clean, that in itself can be a job as those manifolds didn't have isolation for the loops.

Make sure the system holds pressure.

If you isolate all the ferrous metal from the hose, there will be no corrosion induced sludge forming.

The hydronic additives do offer other good benefits to the system, make sure they are blended with good quality, low TDS water.

It is a small diameter tube, high pressure drop. If you add glycol the pumps may need to upsize. It is an EPDM tube and should get along fine with glycol, but O2 does migrate through any non barrier tube. So the glycol additives will need to be boosted occasionally. Good hydronic glycols contain oxygen scavengers, that is the component that gets depleted, then corrosion is possible.

It was one of the most comfortable radiant systems. The loops were short, spacing was tight and it was a counterflow layout, so the floor surface temperature was amazingly consistent.

You should have a plan B in mind if or when the loops fail, especially if you are spending $$ to replace the boiler and components.

Often the tube would go bad right at the manifolds, maybe the UV would weaken it over time, you could rip through it with a finger nail! Try not to disturb the manifolds, it was a funky, hard to duplicate fitting there.

FWIW...
I would in no way add any glycol/ antifreeze to the system
Just stick w/ the Fernox or Rhomar.

Looking for some follow up as I am going to work on a system with solaroll. Replacing a header but am going to do a system flush as the lines feel quite crusty inside. Would you hot or cold flush the fernox?

That is a tough question to answer. SolaRoll was a straight EPDM tube. As such the system operating temperature had a lot to do with the amount of corrosion and sludge build up. Also the amount of new water that was added over the life of the system.

I think any soap or detergent, like Fernox dissolves the sludge better if it is warmer, check with Fernox. An air nozzle with a 1/4" barb hose connector is a good way to use air pressure to blast out sludge.

The biggest challange can be thw connection at the manifold. It's hard to find the original connection pieces, so if you have to rebuild one, some 1/4 barb hose adapters can work. Auto parts or welding supply places have 1/4" barb adapters and couplings. Actually 1/4" soft copper tube can work as a splice withn a nylon ratchet clamp.

Hose that has been exposed to UV tends to get hard or split horizontally down the hose. The less you mess with it the better. If you can just add some cleaner and flush at 10- 15 psi, to start.