Middle cast iron rad poor heat output

kensheets2

Recently I installed a hydronic system using recycled cast iron rads.  I used two pipe reverse return with 5 rads.
The problem child is the middle unit there are two upstream and two downstream of this rad and all except for the middle heat up nicely, top to bottom.
I have TRV on all with a turn valve on the returns.  Bled all rads, even replaced trv and video scoped inside of bottom channel and all is well there.
TRV is hot to the touch return valve is warm at best.
 I would say the rad itself is about 2/3 or possibly 1/2 as hot as others.  I have all rad valves opened all the way.
Is this still a balancing problem perhaps?

neilc

is it possible the pex is kinked somewhere on that rad?
how much of it can you see? back to your mains , , ,

kensheets2

Not likely, down stream rads working great

neilc

I'm referring to the short risers being kinked between the bad rad and the supply and return mains, not the mains themselves, ( or a blockage? )
or are you saying you home run to manifolds?
that rad has all the air out?

neilc

Or,
try this,
since you have those TRVs,
shut all of them but the bad rad,
heat now?

Ironman

Are these five rads and their piping something that's been added to an existing system or are they the total extent of the system?

mattmia2

Are all the radiators the same? Are you sure that is a radiator where the top sections are connected?

mattmia2

I had to think about this a while, but the middle radiator is the worst case flow, especially if the piping to the radiators can provide more flow than is needed. The radiators close to the supply have more pressure from the circulator although the have little suction on the return, the radiators near the return have more suction on the return but little pressure on the supply. The greater pressure/suction at the ends means that the end radiators will get more flow if there is no restriction to even it out. I think what you need to do is close down the radiator valves progressively from the ends toward the middle to balance the flow in the system.

DanHolohan

Is the problem radiator heating at the bottom and not the top, or is it heating the first sections fully top and bottom?

I ask this because systems that ran on gravity (before circulators showed up) connected bottom-and-bottom, as you have now. The slow gravity flow allowed the entering water to rise to the top of the radiator, displacing the cold water, which then fell out the outlet.

Add a circulator to those jobs and the pumped flow now scoots directly across the radiator's lower push nipples. The bottom gets hot; the top stays cold. The classic solution to this problem is to throttle the valve at that radiator. The flow slows to a trickle and begins to mimic what went on in the days before circulators.

mattmia2

Now that i think about this more, i cant decide of the distribution balances out at each radiator or not.

I think this is a new install so it doesn't have oversized mains like a gravity conversion would.

I think this is the physical manifestation of this post:
https://forum.heatinghelp.com/discussion/166481/single-pipe-or-2-pipe#latest

kensheets2

Hello
The system is piped with 3/4 pex and plastic fittings, about 100ft for supply and same for return.  Circs are grundfos ups15-58FC 1/25HP MAX 19.5 head.

Mattmia2, I will try ur idea.  Dan Holohan, the problem rad, when warm heats top but bottom much cooler.

The part that confuses me is the TRV is blazing hot but the return side is barely warm.  All other rads u can only touch for about 5 secs before u have to take ur hand away.
What do u think of the circs?, i was really pushing the head I think.
Forgot to mention if it makes any difference, I havent put the heads onto the TRVs yet so they are all full blast open as is the turn valves on the return.

DanHolohan

Please put the heads on the TRVs. Let’s see what happens. 

kensheets2

Dan, will do.