Step down/up from 3/4 PEX to 3/8 copper?

OcrRzr

I've got two hydronic unit heaters with the hot water supply running in series thru both from a single circulating pump. The pump piping supply/return is 3/4 copper. The inlet/outlet on the unit heaters is 3/8" copper. My plan was to run 3/4 PEX to unit 1, between unit 1 & 2, and from unit 2 (i.e. all supply/return) and step down to 3/8 and back up to 3/4 at each unit heater. Will that be ok? Total friction/pressure loss from the tubing should still be lower than if I ran 3/8 the whole way, yes?

GGross

Yes this would be best, 3/8" supply/return would be much more restrictive than 3/4". If you are concerned about pumping requirements most hydronic unit heater manufacturers will post the pressure drop through their heat exchangers somewhere, whether on their website, spec sheets, or install manuals.

I would suggest posting a quick sketch of your proposed piping here before you start so the folks here can make sure you are following best practices.

hot_rod

How many gpm are you going to be pushing? Enter that number into the calculator at PPI and see where you velocity goes.

Here is 4 gpm of 180F water at 4 gpm, 20’ of 3/8 Pex. 4fps velocity is the suggested velocity in hydronic piping.

The gpm is the number you need to answer your question accurately.

https://www.plasticpipecalculator.com/PressureDropHeadLoss.aspx