Wouldn't it be better to make the heat trap itself with 45 cm or less of copper, then hook up the PEX to its output? Copper will last centuries in continuous contact with hot water, but I'm not sure PEX will.
My parents house is hot water heated. Originally was coal fired, then oil, then a replacement oil heater, now electric. All the pipes and radiators are original. It's 115 years old.
The whole house is built (and maintained) with a different mindset.
(I think they removed the last knob-and-tube light socket a couple years ago)
There are a lot of cases where piping problems have totaled houses. A substantial fraction of houses built with polybutylene piping have already been demolished because of it.
My concern is not so much that a sample of 100 PEX heat traps installed today will all burst on the same day in 2059 or 2159. It's more that if the average lifetime of PEX under those circumstances is only, say, 50 years, maybe there will be a few percent that burst in 5 or 10 years, depending on how variable the corrosion rate is. So if you can protect yourself from that with 45 centimeters of copper, which will cost a heck of a lot less than the water heater does, maybe you should.
