Btu 3/8 vs, 7/8???

Dave_22

I read a website last night that said on an underfloor project, 7/8" Pex carries more heat to the floor than 3/8". I think I read a post on here that said this is not true. It sounds reasonable to me- can anyone explain? Thanks.

tim smith1

3/8 vs 7/8 tubing

The heat carrying capacity is a little different but the main factor is pressure drop in the tube. Due to the smaller tube having a greater pressure drop per ft, the maximum length of each run is less, causing more circuits per area being heated.

Einsiedler

correct

larger pipes will "carry" more heat. same reason larger boilers have larger mains (to carry more heat)

EIN

Dave_22

Then, what makes more sense - 1 run of 7/8" per joist bay or 2 runs of 3/8"???

doug_10

Dave,

> Then, what makes more sense - 1 run of 7/8" per

> joist bay or 2 runs of 3/8"???

doug_10

Dave,

> Then, what makes more sense - 1 run of 7/8" per

> joist bay or 2 runs of 3/8"???

doug_10

Dave,

Sounds to me like you've been looking at a company or two from Vermont.

If I am right, please read this; I am not a contractor and am simply giving advice...

Two runs of 3/8" (or 1/2 may be better) will be better than 7/8" in a single run. Yes, from a technical point of view 7/8" tubing will carry more BTU's because it carries more flow...but will not ACTUALLY work as well...here's why;

1) You will have cold and warm striping in your floors because you cannot get plates to cover the entire joist bay...let alone provide warmth across them anyway.
While a single 7/8" tube may carry even more than two tubes of 3/8", the actual surface area around the two tubes - despite being smaller diameter - provide two separate places to give off heat instead of one.

2) The plates these companies will sell you are not as good as most extruded plates...not as thick, not as consistently good at staying in full contact with the tube and floor...but they are much cheaper.

3) The 7/8" tube is measured as outside diameter (3/4" inside)...to make it sound better. That tubing is also without an oxygen barrier, but that's a separate issue (but another reason not to get it).

Back to the point...use two runs of 3/8" or 1/2" per bay with plates. While you're at it, get ahold of a good contractor near you...this site is a great start. Why? It's not so easy to do it yourself - even though certain companies say it is. Do you want comfort or do you want cheap?

Just my opinion.

Take Care, PJO

Dave_22

PJO,

Yes- you got me!!! I received some literature in the mail and was reading it last night. I have a friend who had radiant installed and will be be in contact with his contractor. Don't think I can afford to do it this year, but am trying to understand all the ins and outs. It's has now turned in to an obsession of mine for the last 6 months. I have learned a great deal on this site and appreciate all the good info I have received. I am an Electrical Controls Engineer who deals with a lot of pumping control issues, so I guess this stuff just fascinates me. Thanks again.