[Mozart609]

Guys,
I'm getting good heat throughout the house expect for the attic. The shut off valve is wide open...little water came out the radiator when I disconnected it, valve is brand new as well. Sorry, it's a steam gas unit.

 

[Ron6519]

Do you mean you have a steam boiler with radiators(cast iron?)? The steam radiators should have a vent on it to allow air to be pushed out ahead of the steam. Is the vent working properly? Is it the correct size? The vent holes get bigger, the further you are away from the boiler.

 

[throrope]

An air logged radiator won't provide heat.

My old house originally had steam heat complete with heavy cast iron radiators in each room that was converted to hot water. Each radiator had a square drive screw near the top on one end, a manual air vent. This vent releases air from the top of the radiator that otherwise displaces steam or in my case water and prevents heating. Every year before heating season, I'd loosen and vent the air until water dribbled out. If you have many homes in the area with these radiators, the local hardware store likely carries the "key" as a regular item.

Because we used the third floor for storage, I never vented those radiators to save on heating bills. The naturally rising heat from the rest of the house kept it above freezing.

 

[roughneck]

One pipe system, or two?

 

[Mozart609]

One pipe

 

[Mozart609]

Do you mean you have a steam boiler with radiators(cast iron?)? The steam radiators should have a vent on it to allow air to be pushed out ahead of the steam. Is the vent working properly? Is it the correct size? The vent holes get bigger, the further you are away from the boiler.[/QUOTE



Cast iron...holes are the same on all radiators

 

[beenthere]

Does any vapor come out of the vent when the boiler is running. The vent may be corroded internally, and not venting the air.

Is there a main steam vent on the main pipe just before or after this rads pipe. If so, it may not be working, and this rad's vent may be acting as a main vent now. Which will cause it to have little if any heat.

Are all the other rads valves fully open. If not, they may be the problem. Steam pressure may be building too quickly, and causing the vent to close before steam reaches it. What pressure is your boiler set to shut off at, and come back on.

 

[Ron6519]

Do you mean you have a steam boiler with radiators(cast iron?)? The steam radiators should have a vent on it to allow air to be pushed out ahead of the steam. Is the vent working properly? Is it the correct size? The vent holes get bigger, the further you are away from the boiler.[/QUOTE



Cast iron...holes are the same on all radiators

The vent holes should vary dependent of it's location in relation to the boiler. Small holes closer to the boiler and get larger further away. This way all the radiators heat up at the same time.
To check if the vent is working on the attic radiator, remove it, and see if it heats up. You will need to stay by the radiator and be ready to use the shutoff when steam starts pushing out the vent hole.
If you replace the vent, get the largest vent size you can, so the radiator will heat up properly.
You can look up, "Balancing a steam radiator system" to understand, in detail, why the size variances exist.

 

[throrope]

The vent holes should vary dependent of it's location in relation to the boiler. Small holes closer to the boiler and get larger further away. This way all the radiators heat up at the same time.
To check if the vent is working on the attic radiator, remove it, and see if it heats up. You will need to stay by the radiator and be ready to use the shutoff when steam starts pushing out the vent hole.
If you replace the vent, get the largest vent size you can, so the radiator will heat up properly.
You can look up, "Balancing a steam radiator system" to understand, in detail, why the size variances exist.

Ron - Good explanation.

Thinking more, in a steam system these vents are functioning devices (i.e. moving parts). For small steam systems, the thermostat cycles the boiler from a centrally located thermostat. The system fills with steam which displaces the air in the radiators. The vents release air at the radiators until filled with steam. I recall the vents use the heat in steam to expand a heat sensitive device and close. For one pipe, the steam condenses in the radiators and by gravity the condensate travels back through the single pipe to the boiler. The single pipe is sized to both carry the steam and condensate in the opposite direction. When the thermostat is satisfied, the boiler cycles off and as the remaining steam condenses, a vacuum is created that pulls air back into the radiators through the vent via an integral vacuum break or other means. The process then repeats itself. This explanation is text book without any knowledge of your system.

A small advancement is the integration of a thermostat with the vent. This vents the air until the room temperature is satisfied. Even with this, the entire system must cycle to maintain comfort.

In a very large turn of century single pipe system I worked with three decades ago, the inlet valve to the radiator was a pneumatic two position control valve that a local thermostat completely opened and closed. The valves originally had leather pneumatic bellows and the building mechanic could still get parts.

The valves upstream of the radiator only provide shut-off, not control.

Again like most have indicated, appears your difficulty is at the radiator.