Dry return venting

rorylane

I was looking at my system venting today and realized that the main vents in my dry return will never close unless I have a bad trap. I have a two pipe system with a perimeter main that travels down the center of the house, splits in opposite directions and loops all the way back to the boiler before it is dripped to the wet return. The return is dry all the way back to the boiler as well so my wet part of the return is only about 36'' long. No crossover trap between the main and the return, just the water seal. I've got two Gorton 2s and a BigMouth on the main, with the same setup on the dry return. My question is thus: Is there any reason I cannot just vent the dry return with an open nipple and save the thermostatic vents for the main? Any insight would be greatly appreciated!

nicholas bonham-carter

What about putting all your vents on the dry return(s). That would keep your back pressure of venting low, and give you protection in the event of a trap sticking open.
You really want the air to get out of the radiators as fast as possible, with as little restriction as possible. Right now only have of your total venting capacity is handling the rads.--NBC

PMJ

I recommend also that two pipes work best only vented on the dry return. And if you are interested in even better performance, use a check valve instead of a vent and don't let the air back in between firings.

In two pipe systems vents on the steam mains don't get you much if anything. Even if they allow air to escape slightly faster on each firing, they let 70 degree air back in when the fire goes off which immediately cools the main some amount for no reason. They also insert lots of air in between the boiler and the rads which has to be removed again on every cycle. Both of these things are working against what you are trying to accomplish. You are moving air in and out every cycle that doesn't even need to be there at all.

The most efficient system delivers steam to the rads the fastest after the burner fires. The point of insulating the mains is so they stay hot. Letting cool room air (that just needs to be removed again) back into them is counterproductive and wasteful.

Consider an approach that opens to the atmosphere only on the dry return. Then consider not letting air back in on the off cycle at all. I have tweaked this for many years and I assure you the result is a very quiet very efficient system that provides the most even heat.

Jamie Hall

Umm. Well, normally I'd agree completely with @nicholas bonham-carter and @PMJ -- but I believe you said you had a water seal between the steam mains and the dry returns, but no crossover traps. In which case you still need the main vents on the steam mains, to ensure that they fill with steam as evenly and as rapidly as possible.

I'd be inclined to keep the main vents on the dry returns, rather than an open atmospheric vent (unless you routed that into the chimney) so that in the odd event that a steam trap elsewhere in the system failed you would at least not have steam in the basement!

PMJ

Jamie Hall said:

Umm. Well, normally I'd agree completely with @nicholas bonham-carter and @PMJ -- but I believe you said you had a water seal between the steam mains and the dry returns, but no crossover traps. In which case you still need the main vents on the steam mains, to ensure that they fill with steam as evenly and as rapidly as possible.

I'd be inclined to keep the main vents on the dry returns, rather than an open atmospheric vent (unless you routed that into the chimney) so that in the odd event that a steam trap elsewhere in the system failed you would at least not have steam in the basement!

Jamie, I too have only a water seal between the main and dry return. All the air pushes easily through the rads. I have no issues with filling mains evenly. I have found that with only a vent in the dry return the path with the least resistance to steam flow is the least filled rad and things got more even rather than less when I eliminated the main vents.

Jamie Hall

Interesting. I'll have to think about that. The air would go easily through the radiators -- after all, those traps when they are open can move a lot of air!

nicholas bonham-carter

Maybe a layout drawing of the piping would make it easier to understand what is best.--NBC

rorylane

I see what you mean, @PMJ. Originally there was a pipe plug installed in the main vent location and it all vented through the return. Problem was that the first rads on the line would heat long before the ones on the end. While I think on a smaller system this would work for my application I found that I had more even heating when I vented the main as quickly as possible so that the system can then start venting out through the rads to the return vent. Reason I was thinking that an open pipe could do the job there was that my return condensate is lukewarm -flash steam is not an issue. @Jamie Hall I think steam in the basement might enable me to catch bad traps sooner than I might otherwise, kind of a 'bad trap' alarm...

rorylane

Here's my very best back-of-envelope drawing NBC. Black is main, green is return, blue is wet return, and pink is where I am venting.

PMJ

rorylane said:

I see what you mean, @PMJ. Originally there was a pipe plug installed in the main vent location and it all vented through the return. Problem was that the first rads on the line would heat long before the ones on the end. While I think on a smaller system this would work for my application I found that I had more even heating when I vented the main as quickly as possible so that the system can then start venting out through the rads to the return vent. Reason I was thinking that an open pipe could do the job there was that my return condensate is lukewarm -flash steam is not an issue. @Jamie Hall I think steam in the basement might enable me to catch bad traps sooner than I might otherwise, kind of a 'bad trap' alarm...

Mine is a Mouat system and has two ways that handle the "first rads on the line" issue. First, the rad inlet valves on those are set to a lower max open position to limit how fast they can fill. Secondly, the Mouat traps are two speed - they pass air slower once they fill with condensate so the first rads to get steam no matter where they are slow up automatically letting the empty ones catch up. Between these two things and a boiler that is some oversized, I am able to fill everything quite evenly.

Beyond this, I promise you would be surprised how even small amounts of vacuum speed the steam out to the most remote radiators on each firing. It happens way faster and more evenly than it will pushing a lot of air out of the main each time. It took me a number of years to find this out.

PMJ

Jamie Hall said:

Interesting. I'll have to think about that. The air would go easily through the radiators -- after all, those traps when they are open can move a lot of air!

The truth is that the only time any radiator needs to be full is on design day. So even needing a trap to close is evidence of a radiator that is more full than is needed for the conditions which means overshooting. My Mouat trap elbows can't "close" actually anyway. The slower speed is just a small orifice - but they never can shut off completely.

PMJ

rorylane said:

Here's my very best back-of-envelope drawing NBC. Black is main, green is return, blue is wet return, and pink is where I am venting.

The run down the middle the full length of the building first seems odd. My main is a loop around the outside too but the boiler feeds both ways at the end by the boiler and both supply and return drop into the drip at the far end from the boiler with the wet return running all the way back.

Are any radiators fed from the main run down the middle?

JUGHNE

I have worked on a system with that piping design. I thought of it as a 3" "Horizontal Express" to the end of the house. (boiler at the other end). Then 2" around the perimeter with separate drops into wet return at boiler.
The returns used to have some form of air vent into the chimney.
With a boiler replacement 15 years ago the 2 returns were bullheaded and dropped into the wet return at the boiler. No vents anywhere, traps had no guts in them. Simplest fix was supply valve orifices.
Tapped air vents into each steam main drip.
Tapped air vents into the dry returns.
My logic, however flawed, was that venting the entire system thru valve orifices would be pretty slow. Pressure is under 8 ounces.

Same situation as you have with cool condensate returns.
Probably could use open pipe venting but if someone raised pressure to blow thru the rads the returns would have steam in them.
However in your case you are probably in your basement every day. If you went for the open pipe I would put up as tall riser as possible and then double 90 ell pointing down. Maybe less water/steam damage with dripping on floor rather than steaming the floor boards.

rorylane

No, PMJ, there are no take offs on the run down the middle, about a 40' run. 3 1/2'' main that drops to 3'' and eventually 2 1/2''. 1000 EDR system with Hoffman traps. Runs at 10 oz currently. May have originally had the return vent to the chimney like JUGHNE said, I have a 1/2'' pipe sticking out of the chimney behind the boiler. Interesting using the flue draft to induce some vaccuum...

JUGHNE

As we discuss all of these older systems we have to remember that originally they were mostly coal burners. Constant fire simmering, no need to vent air quickly for on-off firing we have today. Maybe system always steaming or in vacuum.

If the old air vent into the chimney was passing steam would anyone have known, with a coal fire smoldering and smoking up the flue?

PMJ

JUGHNE said:

As we discuss all of these older systems we have to remember that originally they were mostly coal burners. Constant fire simmering, no need to vent air quickly for on-off firing we have today. Maybe system always steaming or in vacuum.

If the old air vent into the chimney was passing steam would anyone have known, with a coal fire smoldering and smoking up the flue?

That's right - almost no need for venting as originally designed. That's how I ended up doing shorter burns with natural vacuum in between. This more closely resembles the original operation. Air never comes back in, mains always hot, new steam travels shockingly fast with no air to push out of the way on each new firing.

PMJ

rorylane said:

No, PMJ, there are no take offs on the run down the middle, about a 40' run. 3 1/2'' main that drops to 3'' and eventually 2 1/2''. 1000 EDR system with Hoffman traps. Runs at 10 oz currently. May have originally had the return vent to the chimney like JUGHNE said, I have a 1/2'' pipe sticking out of the chimney behind the boiler. Interesting using the flue draft to induce some vaccuum...

My system is 1000EDR also. I never see more than 1-1/2 ounces of pressure at the boiler and I have only one 1/2" open pipe solenoid valve on the dry return.

JUGHNE

What controls the solenoid valve....when does it open and when does it close?

PMJ

JUGHNE said:

What controls the solenoid valve....when does it open and when does it close?

When it is cold out I run 10 minute burns with 10 minute waits so 3 cycles/hour. The natural vacuum generated each cycle lasts about 2-3 minutes into each new burn(I have an old leaky system). I have a very sensitive pressure switch on the dry return which opens the solenoid valve when the pressure in the dry return just goes positive relative to atmospheric. It closes again immediately when the burner goes off and the vacuum builds again. I am using a PLC for this control and other features but the plain switch in series with the valve coil would work just fine.

rorylane

I hadn't ever thought of using a solenoid this way, very cool.

PMJ

rorylane said:

I hadn't ever thought of using a solenoid this way, very cool.

Think about it a while. And think about that the dead men never designed for all the breathing in and out. That does work, but not doing it works better.

nicholas bonham-carter

If you have the venting spread out like that, it will contribute to imbalance, as the resistance to the escaping air increases as each set of separate vents close.
When they are at the end of the dry return, the main, and radiators will fill quickly with steam, with all the main vents open. Since the radiators are hooked up in parallel, they should all fill at the same time.--NBC

rorylane

I hadn't before thought about the breathing in side of it, though I suppose once it's 10 below out the main will likely stay warm enough between cycles for those vents to stay closed the majority of the time. This may lead to some more experimentation down the road, thanks guys.