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Greetings!

We have a Burnham IN-7, installed by the previous owner about ten years ago, which supplies a circa-1920 one-pipe counterflow steam system. Last summer, we had a hot water baseboard loop piped from the boiler to heat the basement. The general contractor said it was a common and efficient way to heat a basement, and we had no reason to doubt him at the time.

The hot water baseboard loop runs directly from one of the boiler's two bottom return tappings, over an aquastat, and through a pump. Water from the baseboard loop returns into the Hartford loop. All of the tubing, aside from what goes directly into the boiler, is red PEX. I've recently read on several sites that PEX is only rated for 180 degrees Fahrenheit. The pump for the baseboard loop does not shut off when the boiler is making steam for the rest of the house, and so parts of the PEX regularly see temperatures of 212 degrees or more.

Is it safe to subject PEX to such temperatures? If not, what would be the best way of fixing the temperature problem without ripping out all of the PEX and replacing it with silicone tubing or similar?

According to How to run a hot-water zone off a steam boiler by Dan Holohan, it's possible to regulate maximum temperature in a hot water zone by redirecting the zone's return water through a bypass and mixing it with water coming out of the boiler. He recommends using at least a 3/4" bypass. Our hot water zone has two garden hose fittings -- one just before the pump, and one just before the return water enters the Hartford loop. Neither seems useful for anything, and our current plumber does not know why the installer put them in.

Would running a hose between the two fittings work for regulating temperature in the zone?

For reference, attached photos of the supply and return for the baseboard loop.