Help sizing pex-al-pex for 2 pipe reverse return loop

joshua_3

So I had to do basement renovations which led to removing the majority of my existing hydronic heating line feeds(amongst lots of other things). It was fairly haphazardly run, so this wasn't the worst thing. Previous setup was direct return 1" main with mostly 1/2 feeds to the radiators. There are 6 radiators total, 2 on each floor/3 floors. The total BTU output of the rads is just under 38k.

My plan was to run a reverse return loop, and I am planning to use Pex-al-pex. My question is what size the main loops should be - 3/4" or 1". Previously 1" was used. I have attempted to research this, and it sounds like 3/4 pap should carry this? Plan was 3/4 main loops with 1/2 line to the rads off that. Thanks for any insight you can provide.


Joshua

Ironman

Generally speaking, a 3/4" line will carry about 4 gpm which = 40k btus. That's based on an estimated head loss of 4ft per 100' of piping.
There are a lot of variables, so there's no pat answer without knowing all of them.

joshua_3

Thanks for the quick reply. Is there any other info I could add to reduce the major variables? I've generally seen around the 40k number being the high side as well. I'd rather not spend my money oversizing the piping to 1" if it is unnecessary, and I don't foresee needing to add heat anywhere in the future from this loop.

For what it's worth I'm also having estimates done, and was planning to hand this job off to the pros. The first number came in quite a bit higher than I expected, so I thought I'd at least sketch it out myself. All the runs are pretty exposed and easy to get to, so I'm not afraid of the 'putting it together' part. I do realize though the design is the most important part, and that led me here. Thanks again for any insight.

Joshua

Ironman

Mainly, length of pipe, fittings, and any devices (like mixing valves) that add resistance to flow.

Again, if your total run is not more than 100', then you'll probably be okay with 3/4".

joshua_3

Well, if I'm adding both the supply and return loops in that number, I'm probably closer to 200'. Not a huge house, but 3 floors with rads(2 each floor), and some rads are on opposites sides. Basically running a loop around the ceiling/outside of the basement with feeds popping up or down to the rads from there. While it's all disconnected I've moved a couple around to consolidate the runs, but it's not feasible to place them all on the same side of the house for a shorter run.

Sounds like fittings are more or less added in as additional feet of tubing for purposes of calculating? If so, I guess that number could be even higher? There are a couple of T's for each rad, and probably a few 90's in spots that might be tight for a bend. Does this mean larger pipe to help lower the resistance? Thanks.

Joshua

Ironman

"Does this mean larger pipe to help lower the resistance? Thanks."

Probably not. 3/4" Pex-al-pex has a slightly larger internal diameter than copper. You can actually move up to 6 gpm and still keep the head and velocity manageable.

You'll probably end up with about 8 - 10' of total head. A Grundfos ups15-58 or Taco 007 can easily move 4 gpm a that head.

Mbyrnes92

What’s going on, I am doing a similar project was wondering if anyone can give me a hand, I want to run a 2 pipe reverse return system I have 5 rads on my first and 5 on my second, here’s a pic let me know if this is a good layout or if I should change anything..thanks 

Ironman

Pic?

hot_rod

If you have adequate sized radiators it is quite possible you could run a larger delta T. it's quite common to see 30- 35 or higher∆ on radiator systems.

500 (f) ∆T
500 X 4gpm X 20∆= 40,000 as @ ironman mentioned
Or put another way
500 X 3gpm x 30∆ = 45,000
If you have a lot of fittings and tube length, lowering the gpm lowers the head.

Running a wider delta returns cooler water to the boiler and increases efficiency, also.

Mbyrnes92

Repiping my house in pex want to run a 2 pipe reverse return system, would this diagram be accurate as to what I want to run? Thanks

EdTheHeaterMan

joshua_3 said:

Well, if I'm adding both the supply and return loops in that number, I'm probably closer to 200'. Not a huge house, but 3 floors with rads(2 each floor), and some rads are on opposites sides. Basically running a loop around the ceiling/outside of the basement with feeds popping up or down to the rads from there. While it's all disconnected I've moved a couple around to consolidate the runs, but it's not feasible to place them all on the same side of the house for a shorter run.

Sounds like fittings are more or less added in as additional feet of tubing for purposes of calculating? If so, I guess that number could be even higher? There are a couple of T's for each rad, and probably a few 90's in spots that might be tight for a bend. Does this mean larger pipe to help lower the resistance? Thanks.

Joshua

200 ft total Pex piping for the complete system? That is not needed to compute the pump head. To calculate the head loss you only need to know the longest run. Since you are using a reverse return design all the rads will be close to the same length. but you need only measure the piping in the longest run and forget about all the pipes that go to the other radiators.

So if the third floor is 20 ft above the basement and the pipe in the basement goes around a 15 x 25 basement, the longest run would be from the boiler around the basement to the riser then back down to the basement and back to the boiler.

15+25+15 around the basement +20 ft up +20ft back down then another +25 ft back to the boiler. That's a total of 120 for the. longest run.

120 * 1.5 for fittings = 180 total length adjusted for fittings.
multiply by .04 (this is 4 ft head per hundred ft pipe) 180*.04=7.2 ft head

As long as you have a pump that can deliver 4 gpm at 7.2 ft head, you are golden with 3/4" main piping

The Taco 007F that comes with many boilers is more than adequate. you should be fine unless your numbers are much, much larger than my example.

EdTheHeaterMan

@Mbyrnes92

You should start your own discussion. This can get very confusing trying to keep two conversations going at once.

But to answer your query, it looks fine. For more info on this, like pipe sizing and the like, please start your own discussion.

Thanks in advance

Ed