Non-automotive heating question

[RonD]

I have a hot water heating system, hydronic heating.
Using radiators, a boiler and one pump for circulation.

There are 4 Zones, 3 zones are working fine.
1 zone, the second largest, has limited flow.
It has 5 radiators and 1/2" copper lines between them but.......the lines are all in/below concrete slab.

The first 2 radiators in the direction of the flow will heat up hot but slowly, 3rd even slower and never hot, last 2 just barely warm.
So I concluded something is partially blocking the flow so the hot water is cooling off faster than it can flow thru.
The return line is almost cold.
I can duplicate the symptom on the 6 radiator zone by partially closing the out or return valve on that zone

System has between 15-20psi of pressure, I can close the 6 valves on the other 3 zones and pressure holds so there is no leak in this zone.

Valves "feel" OK, but I guess you never know.
Or could it be in a radiator, don't see how since they are literally just a copper pipe with fins, no bends, just a 90 at each end going down into the slab.

On each rad there is an air bleeder, pressure is of course, the same on all since this is a flow problem, and the hole for the bleeder is not really large enough to test flow.

Brain is drawing a blank on how to test where the blockage might be?

 

[alwaysFlOoReD]

Thermal imaging? I don't know fluid dynamics. Can you try pressure and blow out the obstruction? Perhaps pressurize the system and take readings from the bleeders. As in open the outlet and pressurize the inlet with air, then take readings at each bleeder. Kind of like a leakdown test.

 

[RonD]

Not sure thermal would help, flow is slow, blockage could be the inlet valve or the outlet valve, thermal loss from first to last radiator would be the same.

Yes, when I start to unsolder the lines I may try pressure on the in and then the out lines, line with the blockage should be slower to react to pressure increase.
Might try that now, I can increase pressure in the system using expansion tank, then open the two valves, one at a time, and see what pressure changes I get at each radiator.

 

[alwaysFlOoReD]

The way I was thinking is that both ends are open. Pressurize one end and take readings at each rad. When the pressure drops the obstruction is on the side you have already checked. I realize there will be pressure drops from line length but it shouldn't throw the readings off too much. You probably already figured that out but it makes it clearer in my own mind if I write it out.

 

[Denisefwd93]

Ron I know this an older thread.. did you ever fix the prob?
Seems to me with 1/2" copper and that many rads you will need a High head circulator like a taco 0010 or the grunfos equivalent. most home systems only use the taco 007 Are the rads on a loop? or do they have supply and return piping under the floor? I'm guessing loop. Do you hear water gurgling in the rads? the means air bound. sometimes bleeders don't get all the air out and the loop needs to be purged