Effect of putting air vents on 2-pipe radiators

danFromNJ

I was checking out a steam heat system in my co-workers home and discussing it with him. He has a Broomell vapor system that seems mostly intact. The boiler has been updated and has a vaportstat set to 12 ounces cut-out (I believe the Broomell system was designed with a 10 ounce cutout). I couldn't find the condensing radiator that is supposed to be on the ceiling of the basement. There is a large ceiling radiator in the garage, next to boiler room, but it has a supply from the main and a return to the dry return, so it seemed like a normal radiator to me. I guess it's possible someone removed the condensing radiator. What the system still work without this?

Some of the radiators stopped heating. In an attempt to get heat to the radiators, my co-worker removed the plug at the top of the radiator opposite the valve and installed steam air vents. I referenced my copy the "Lost Art" and saw that the problem was most likely a clog in the "p-trap" at the return of the radiator. When I mentioned to him that he should try opening that trap and clearing the hole to see if this fixes the heat issue, he asked what the difference would be between clearing the trap and continuing to use the air vent. I was stumped.

This leads to my main question. At this point, what is the advantage of continuing to use the system as intended vs how it was originally designed? I feel like I understand some of the theories of vapor heating, but with the move away from coal, do these systems truly benefit from the "vapor" heating? Or, is the vacuum lost with the short firing times that the boiler experiences? If the system no longer really goes into a vacuum, would an air vent really effect the system that much?

Thanks in advance to anyone who responds.
Dan

Jamie Hall

The vapour systems do not have to go into vacuum to work properly, although in some situations it may be a benefit -- there has been some debate on that point!

This first question I would ask about your friend's "fix" is... where is the inlet to the radiator? If it is at the top -- as most vapour systems are -- and the outlet trap is stopped up, how does he propose to get the condensate back out of the radiator?

And if the outlet trap isn't stopped up, the problem won't be solved by adding a vent -- particularly at the top of the radiator (steam vents go half way to two thirds of the way up, not at the top).

Indeed -- there are a number of things not advised about the "fix" of the air vent. Quite a number. Which is not to say that it might not work, after a fashion; steam is amazingly forgiving. But why not get it to work right?

The Broomell trap arrangement is about as simple as they come, and the best thing to do is to take it off, clean it out, and put it back. While it's off, pour water down the return to make sure it's clear, too -- although it's not common, they can sometimes clog and since you have it apart, why not?

Then dial the vapourstat down to 8 to 10 ounces.

If there still is a problem with no heat at a radiator, or poor heat, check first to make sure the inlet valve really is open... sounds obvious, but stranger things have happened. Then work back along the inlet pipe and risers to see if there is a low spot which is trapping water and keeping the steam from getting through (that would usually cause water hammer, but you never know...).

Check the main venting and make sure it is working; if this really was Broomell, and someone repurposed the condenser, there has to be a main vent for the dry returns.

danFromNJ

Thanks Jamie

I was thinking about the problem with the vent being that the steam could short circuit across the top of the radiator and not heat the radiator fully.

You mentioned that if the ptrap is clogged, then condensate can't return. However, I thought the broomell trap has a hole above the water that allows air to pass out of the radiator and into the dry return. I thought this hole was the spot that clogged and needed to be cleaned. Am I right about this? If so, I would think the condensate l could continue to make it back into the return via gravity.

The system does have a main vent on the dry return right near the boiler. I am going to look around tomorrow and see if I can find the condenser. I also noticed that the radiator on the ceiling in the garage has a different looking broomell fitting on the return side. It is mounted sideways and looks like a cross. I will post a picture tomorrow to see if anyone recognizes it.

PMJ

The effect of putting vents on individual rads in a 2 pipe is that they will heat less not more. I inherited a situation like that and I know first hand.

While it is possible that a vent on an individual rad in a 2 pipe may let some more steam in when steam is coming up, it totally wipes out that benefit because that rad will now be totally empty of steam faster than those without vents when the boiler cycles off. The problem is not that it lets air out, it's that it lets it in.

If you use pressure(even low pressure) to get steam to go places then you must fill some rads to get it to go to others and you use vents and valves to help steer the flow. But conditions change. The sun is here, the sun is there. The wind is east, the wind is west. Even the rate at which heat is lost from where in the house changes with how cold it is out. Pressure, vents, and valves don't know anything about those things. They are always set the same. Most of the time the burner is off. I have found that where steam goes during that time is very important to how even the heat is - very.

I continually had more success with comfort and efficiency the less air I let back into my pipes with burner off. Without air steam flows to rads with the highest delta T and evens itself out automatically. This is not at all a new concept in one or 2 pipe. I'm struggling to understand why it disappeared.

@danFromNJ, make sure the valves are open and the ptraps clear. Take the rad vents off. Down the road you and your friend might consider vacuum.

danFromNJ

@PMJ Thanks for the response.

Let me see if I have this right in my head. When the boiler stops, in both 1-pipe and 2-pipe, the steam in the radiator will condense creating a slight vacuum in the radiator. In a 1-pipe system, the vacuum will allow air to refill the radiator through the air vent. But, in a 2-pipe system, the air can not return through the vent causing the radiator to continue to pull steam from the boiler which then condenses and creates another vacuum and so on. Is this correct?

By adding a air vent to the the 2-pipe radiator, we are allowing air into the radiator and thereby less steam when the boiler shuts off. This will allow the radiator to cool faster losing some efficiency gains that come from 2-pipe systems.

On Broomell systems, the p-trap has a hole above the trap that allows air to pass out of the radiator and into the dry return. When the boiler shuts off, won't air flow back into the radiator through this same hole. Or is this hole so small that when the steam in the radiator condenses, it is easier for steam to pull from the supply to fill the vacuum left by the condensing steam.

PMJ

You are on the right road with your thinking. One pipe is just as you say. 2 pipe air will flow back into the rad from the dry return through that small hole in the ptrap - just not as fast as it will if it has a vent right on it.

There is a vent somewhere on your dry return. When the boiler shuts off the vacuum from the condensing steam pulls air thru this vent into the dry return and eventually to all the rads. Turning that vent into a vacuum vent or check valve is what is required to stop all air from returning to the system and send the whole system sub atmospheric. Then rads pull steam from the mains always, boiler on or off.