Seems like something trivial to make with PEX these days. I have no idea if it's regular practice or not. The only downside I can see is if there was recent hot water, it might take slightly longer to reach the faucet.
My water heater has one made from PEX. From memory, I would guess it adds about 18".
With 3/4" PEX at a relatively slow 1.5 GPM flow rate, that adds about 1.4 seconds to 'hot water delivery time', though that's not entirely accurate, because the loop itself has some heated water trapped in it.
I actually don't think so, because you end up with only a gradient of lukewarm-to-ambient temperature water, which you still have to purge in order to get actual hot water. Even if you're going for just warm water, you still have to purge some of it, then after a few seconds start continually adjusting cold until the hot water from the tank gets to you.
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.
