PEX Diameters for Radiant heating

RLYable Handyguy

Good Morning
Again I come to you narrowing down the items of choice for use in my 920sqft Radiant Heating system.
Due to local availability AND Feedback/reputation, Munchkin 80m and Megastore or Maxitherm are at the top of my purchase list... Thank you!

I've heard and read the differences in 3/8 and 1/2 dia Radiant tubing... and that one does not carry anymore heat than the other.... As a rule, I tend to make comparisons by using EXTREME opposites... ie .. given 2 different Volumes of water... a BUCKET and a CUP of hot water... there is more thermal mass in the BUCKET since it has more heat... the Cup will be cold long before the bucket

Knowing that a smaller diameter tube will hold less water than a larger diameter tube... limited by smaller heating loops... higher head... etc
It seems that the return water temp would be lower from a 3/8 tube than a 1/2".. thus affecting the amount of heat passing into the area directly above the tubing towards the end of the loop.

The Radiant zone installation will consist of insulation,heat transfer plates/tubing attached between the floorjoists from below one full loop between each joist. Equal length loops of no more than 300' to eliminate the need of Flow regulators... but plan to balance the loops using the return water temps.

Aside from easier handling with the 3/8", What are your opinions between the 2 sizes?

Thanks
Russ

Unknown

You need shorter (and more) loops with the 3/8" to get the same Delta T/flow rate requirements met. This may or may not be a problem depending on the manifold location, and joist type (no problem in web trusses, for example, but more penetrations for more loops in other joists).

It is more important to isolate rooms than it is to have equal loop lengths. If this is an open floorplan, go ahead and do the equal loops, but individual balancing valves can still help with the fill/purging.

Depending on your load and fluid mix (glycol?) 300' may even be long for 1/2" if you want to avoid enlarging your pump.

ALH_4

3/8

My only complaint about 3/8" tubing is the relatively small selection of crimp fittings when compared to 1/2". Thermal mass is not really relevant. Flow rates and head loss are important. As Rob said, the loops are shorter, and therefore there are more of them. If you are home-running the loops, which with 920sf you probably are, 1/2" will likely save you manifold ports and/or piping.

-Andrew