Debate me Bro-3/8 inch pex 150 foot lenths AOK for high delta T radiant

@hot_rod haven't been offered one of those fancy slide rules at the caleffi booth (although I did win some prize there so I guess I can't complain!)

while I think it could handle 1 gpm in 150' leg of 3/8 i only need about the .5 you mention to deliver 20,000 btus in 4 legs. so the pressure drop is manageable. I have in mind precisely the approach you did, i.e. 3/8 on 4" spacing instead of 1/2" on 6".

3 passes of 3/8" actually provides more pipe surface area than 2 passes of 1/2". and I was thinking to myself that in a shop environment with the feed temps, i.e. high side of delta T, along the outside outside walls and the return winding toward the inside that an even higher delta T would be acceptable and this could favor even lower water temps and/or lower circulation rate.

there is less water volume to that surface area which suggested to me that for a given flow velocity you would get more transfer from water to pipe although given the conductivity of water maybe this isn't a choke point, not sure as to velocity. @Rich_49 , I kind of misstate what will go on at similar flow rate which would be higher velocity in smaller pipe but in going for higher delta T i'm thinking lower flow rate. that is the general approach i'm trying to take by which i meant the flow rate for 20 or 25 delta T is lower than for 10 or 15 delta T assuming the lower flow rate can still carry the required btus in total. but to flow a given volume per minute the velocity would be just a bit higher in 3 passes of 3/8 (.3 sq. inch cross section) to 2 passes of 1/2 (.36 inch cross section) so at the same flow rate the velocity would not be slower but for high delta T you can use lower velocity and I thought, instinctively, the additional surface area to water volume ratio would help with the heat transfer to pull that higher delta T. if there is anything I have observed about radiant floors it is that 6" spaced 1/2 pipe in a 4" slab you can readily identify the pipe location by feeling the floor so anything that makes the transfer to the entire floor more complete and even is better in terms of using the whole mass, albeit I mean even as between adjacent pipes more so than same floor temp at center of room as outside. From my seat of the pants engineering perspective getting more heat to more of the floor rather than warm spots where the pipe is would take better advantage of the radiant approach.

and the reason I titled this "debate me bro", homage to current events aside, is that the industry has pretty much rejected 3/8 in most applications which does as @hot_rod suggests make it hard to get, and i find it so much easier to work with but when the unit price of the 1/2" is virtually the same that means that the convenience I find in 3/8 is offset by 50% higher materials cost and the ease of running it is offset in labor if you are running more length. I use a lot of 3/8 in plumbing as well as radiant and I am continually dissappointed at how difficult it is to get fittings as well as tubing. you can get 3/8 adapters for manablock style and maybe i'm jsut going to have to stick to that although i like the sioux chief style copper manifolds which can't be had in 3/8 to my knowledge.

If more people were using 3/8, I don't think it would be that much of a penalty. so i'm kind of using the occasion to jibe at the issue but also for folks to knock my seat of the pants engineering off its pedestal ;- ).

@mattmia2 don't think I haven't thought about making the manifolds as you describe. so far i just bite the bullet and do a 6" piece of half inch to a 1/2x3/8 reducing fitting.

although i have to admit i used a manabloc for a water system recently where i could choose 1/2 or 3/8 outlets and i did like that. not sure what the oxygen permeation of that material would be and whether to worry about a small portion of a circuit being that way. although I often hit the used market for an old domestic hot water tank and separate the radiant water from the boiler and baseboard feeds anyway rather than a proportioning valve.

absolutely understand that your comparison was equal lengths and I didn't initially specify clearly that i was thinking of more pipe. but as i mentioned that isn't more economical because even though the 3/8 pipe has 2/3 the plastic in it that the 1/2 does, it costs about the same unlike some products that commodities, e.g. spray foam insulation. if to get 4" costs you twice as much as to get 2". that also holds true for rubber roofing. twice the size is twice as much. but with pex 50% more plastic doesn't cost any more. I put that down to 3/8 being so obscure that stocking costs and number of units that move add to the price from manufacturing and all along the supply chain.

one more thing besides flammable refrigerants that i'm for and the industry, writ large, is more or less against ;- ).

fair question. dont have the tools on my cell but IIRC the pressure loss in 150' of 3/8 is still more than 300' of 1/2. so i was anticipating that i wouldnt amp up the circulator and the flow rate would be lower than target but i would be counting on high delta T to get the btus out and i have always 0referred that strategy even with 1/2 inch.

i can see from post below you share my affection dor 3/8. and they aint going broke selling it dor essentially the same price as 1/2.

and , to be fair, it is the pressure delivery at flow numbers on the plumbing side that warn folks off but i shower with 3/8 pex and im happy. i have to admit i chickened out running it on hot side to showers in 4 story building although i wanted to to cut the time to get hot to the shower head. if i would have had easier access to the chase i would have tried it .

so my occams razor take on the 'industry' is not even that they tell folks to do qhat is profitable to them but that they have given up on thinking about the best ways to so things and go the easy way for them. that includes the industry letting us down with regards to speaking out against bad regulation instead of just going along .

brian

@hot_rod
is this PPI calculator on line tool. whats the URL.

I like that it gives velocity. given the shorter length of tubing you might need a higher velocity to get the btus through 3/8 but I am willing to deal with a wider delta T spread so don't have to pump like crazy.