Flow Issue? One Cast Iron Radiator Won't get Hot.

shortaznkid4

Hello All,

In the Spring, I added a new heating zone and circulator pump for my basement (zone to my hydronic heating system. While I was at it, I relocated the circulator pump for the pre-existing zone (zone A) from the return side of the boiler to the supply side, so that it is located upstream to the expansion tank. When I fired up the heating system, every radiator in the house is hot and toasty except one, located on the 3rd floor.

I have purged the system of air and I have water coming out of the bleeder. If I bleed this cold radiator long enough, the radiator will get hot. The supply pipe of the radiator will get hot but the return side will not.

Today I disconnected the radiator and flushed it with a garden hose. Black particulates were flushed out and the radiator had steady flow from the inlet to the outlet. The radiator shutoff valve looked fine, but I replaced it with a new one anyways (one with a larger opening). I then reconnected the radiator and purged the system of air. When I fired up the heating system, this radiator is still the only cold one. If I leave the bleeder valve open, the supply side will get hot but the return side stays cold.

I have isolated the supply side from the system, and with the circulator pump off and the radiator valve (on the supply side of radiator) in the shut position, opened the bleeder valve of the cold radiator. When I do this, I still get a steady steam of water, which tells me there isn't a blockage in the return line.

It's a three-story house built in 1909. Zone A is the pre-existing zone and is circulated by a Bell & Gossett Pump (runs very quiet). Last winter, all radiators were functioning perfectly. I have isolated zone B from the rest of the system via shut off valves, so the system is essentially what is was before.

The cold radiator worked before the circulator pump was moved. It seems that the flow on the radiator's return pipe isn't being pulled adequate enough to pull the hot water on the supply side, but what would cause this? What can I do or try next? Did I screw something up? All other 17 cast iron radiator get piping hot. What I missing something?

HVACNUT

You moved one circulator to the supply, yet left the indirect circulator on the return. Not the cause of your issue, just an observation. 

Bleeding rads on a monoflo system can be a real pain. 
If you have another person and a couple phones, have someone at the boiler keeping pressure steady at 25 psi while the other person bleeds the rad. It could take some time so be patient. 
Have the boiler off while doing this.
This is 3 stories with the boiler in the basement so cold pressure will need to be higher than the factory settings of the pressure reducing valve and extrol. If the gauge is correct, set cold water and tank pressures to 15- 18 psi.

shortaznkid4

Nice catch. It never occurred to me to also relocate the indirect circulator to the return side.

So I should have someone lifting up on the pressure reducing valve's purge lever until the pressure gauge on the boiler reads 25 psi. Then while they maintain that pressure, bleed all radiators starting with the 1st floor, working my way upstairs?

I installed a pressure reducing valve that is set to 15 psi. I actually recently replaced the expansion tank and I pre-charged the extrol tank to 15psi (factory pre-charge was 12psi) before installing it. The boiler's pressure gauge reads 16 psi when not operating, and peaks at 20psi when on.

HVACNUT

shortaznkid4 said:

Nice catch. It never occurred to me to also relocate the indirect circulator to the return side. So I should have someone lifting up on the pressure reducing valve's purge lever until the pressure gauge on the boiler reads 25 psi. Then while they maintain that pressure, bleed all radiators starting with the 1st floor, working my way upstairs? I installed a pressure reducing valve that is set to 15 psi. I actually recently replaced the expansion tank and I pre-charged the extrol tank to 15psi (factory pre-charge was 12psi) before installing it. The boiler's pressure gauge reads 16 psi when not operating, and peaks at 20psi when on.

You only need to bleed the 3rd floor rad that's giving you fits.

EBEBRATT-Ed

Where is the pump for zone "A" or am I missing something. You pump on the supply looks downstream of the tee feeding zone A

shortaznkid4

I tried what you recommended but it didn't do the trick.

Then tried to purged the entire system using a garden hose on the "air purge station" valve, and draining the system via that valve while lifting the fill valve lever. Didn't do the trick either.

So I combined your recommendation with the purge I did. I lifted the fill valve lever and let the pressure build up to 25 psi on the boiler's pressure gauge before I opened the purge valve the garden hose was hooked onto and drained the system. Did the trick. The issue must have been a very stubborn air pocket/bubble that needed a higher velocity flow to remove.


Anyways, thank you HVACNUT for your replies!