One Cold Radiator

Shingo

I have a problem with a burnham cast iron radiator that won't get hot.
I have a A.O. Smith hot water boiler and the house is set up with three zones.
A eight foot cast iron baseboard radiator is the only one not heating up in the lower level zone.
I had to drain the system for repairs, (new zone valve and circulating pump) and when I refilled the
system no heat in this radiator.
When I refilled the last time, this same radiator didn't work and then days later it did.
Of course I tried bleeding it. Two days ago I bled it until it got hot, but it wouldn't stay hot.
I have an old house and the pipes are buried but I know the return pipe coming out of this radiator goes directly into two three foot finned radiators (no bleeders)down stream and then down and back to the boiler.
I have three grundfus circulating pumps, one for each zone, and no way to purge zones except through bleeders on the radiators.
I tried raising the temperature in the boiler which raised the pressure in the system and bled and still
no good. Pumps and zone valves are working.
What else can I do?

Alan (California Radiant) Forbes

Do the two, three-foot finned radiators that are piped after the cast iron radiator get hot?
Raise the pressure to 25 psi and turn on the thermostat with the other two thermostats off. Raising the pressure makes the air bubbles smaller and easier to move around.
Add a couple of tablespoons of dishwashing liquid to the system and then do as above. The soapy liquid breaks the surface tension and releases the bubbles.
If none of those work, wait..............like you did the last time.

Shingo

Do the two, three-foot finned radiators that are piped after the cast iron radiator get hot?
No, they do not.

Raise the pressure to 25 psi and turn on the thermostat with the other two thermostats off. Raising the pressure makes the air bubbles smaller and easier to move around.
I have tried this already.

Has anyone used the dish soap idea?

Shingo

I tried adding dawn dish soap and I still have the one cold radiator. anything else I could try?

Alan (California Radiant) Forbes

You will have to add a purge assembly back at the boiler. Where the pipe from this zone returns to the boiler, cut in a hose bibb followed by a ball valve. Re-fill the system, close the ball valve and purge the air with a hose on the hose bibb until the water is free of air.

You will never have the problem again. It's a permanent solution that can be easily taken care of whenever the system is drained.

Shingo

Thanks Alan, I will deal with the radiator this winter and get a new system in the fall, WITH PURGE VALVES.

Shingo

I tried the dish soap idea and after three weeks, during which we had a polar vortex panic, the radiator now gets hot. Thanks to Alan for his advice.