@eric maclean i looked at high temp rated 2.5 and 5 gallon models as well in that price range. They don't specify the head at which they rate the circulation so I was being conservative. I'm buying a 2.5 gallon for $25 (!) as a circulator for a block heater on another truck so i will report what these 20 mm threads really are. it's easier to just buy one than try to discuss thread standards with limited english speakers (no disrespect intended, i'm glad these are available. I'd gladly pay twice or even 3 times as much to get an american one, but not 8 times as much or more.
Now that I find that the hydrohot, and perhaps the aquahot, are located far from the heater compartment, there could be a second pump but it would be similar if not as perfectly accessible. although you could consider the piping and access when you install a new one apparently you reach the old one to tap it. that 's better than some of the most nefariously located components :-)