I have an in slab hot water radiant system that I recently had to change pumps on. Basic Details below. 7 loops @250ft 1/2" pex-al-pex Taco 007 pump. I was only getting about 3.5gpm total throughout with that pump. It was enough to heat the space but the temp differential was 35f in vs out. I had to replace the pump due to pump failure so going by flow charts I was sitting at about 9 head feet with the 007. With that in mind I upgraded to the 0015 and used speed 3.with that speed I should of been seeing 8-10 gpm but it is only flowing 5 gpm. I thought head feet was non changing unless you change flow characteristics? Or am I wrong?

The head loss goes up with the square of the gpm. Ie, going from 3.5 gpm to 5 gpm will double the head loss. You have other restrictions? I ask because seven 250', 1/2" loops in parallel flowing .5 gpm each should have been about 2.25 ft of head (plus some other losses).

That was my thought to.......I have 7 loops coming off a manifold in parallel. I honestly don't know why I'm only getting 5 gpm. I've checked all loops independently, and all flow well on there own.

Here's a pic. Simple system.

It is made out of three-quarter inch copper pipe on the main section. The supply line is three-quarter inch pex. The supply line itself is maybe 12 feet on the hot side and 12 feet on the Outlet side.

Maybe +1 ft head. The boiler?

Right now I'm just using a conventional water heater as the source. Just until I get a good boiler.

tank or tankless? If the former, then where the additional resistance to flow is coming from is a good question. Low static pressure (reducing pump effectiveness)?

It's a tank. I just checked every loop separately and on there own each loop is doing about 1.3gpm on speed 3 on the pump. When all 7 are open the system is at 5 gpm. I thought the 0015 was a big enough pump but something seems off. There's no air in the system either. How much head loss would a couple 3/4" pex tees add to the system?

I have a similar set up, I couldn't get more then 4 gpm with a Grundfos Alpha. 4 @ 290' loops.

I think it's all the 3/4" pipe and the tanks 3/4" connections. I even removed the heat trap nipples.

I was getting 30* deltaT but since it is a basement slap it still works fine. Cement does suck up the BTU's

Would changing my 3/4" pex supply and return lines into 1 inch pex make that much of a difference? It's only about 24 feet total.

No. The direction is correct according to this?

http://www.industrialcontrolsonline.com/training/online/how-avoid-problems-your-hydronic-system-pumps

Yes, the pump is pulling FROM the heater and expansion tank. I also verified that the air scoop is also in the right direction.

Posted By jonr on 01 Dec 2016 10:10 AM
No. The direction is correct according to this?

http://www.industrialcontrolsonline.com/training/online/how-avoid-problems-your-hydronic-system-pumps

Reading that article though, I'm wondering if I should put an air release valve on the top left corner of the copper piping in my picture. Might be an air bubble stuck there. I am getting decent heat so it's not a heat lock but It might be causing some flow issues if it has to push past a huge air bubble to get into the system. Seeing as the circulator pump cannot push air down a vertical pipe.

Anyone else? I suppose that ideally, one would be able to measure pressure at every point in the system. That would make it clear where the unaccounted for pressure drop(s) are occurring. Let us know if you figure it out.

So I added an air relief valve at the top of the vertical pipe and verified I have NO air in the system. Water flow still low........Crap