Why steam heat worked in morning but didnt after. Understanding air vent drop away pressure

RayWohlfarth · December 12

I had the hardest time understanding air vent drop away pressure. I read @DanHolohan writings about it but it didnt click. Then it did. I hope this video helps you understand how air vents work on a steam system and the drop away pressure.

https://www.youtube.com/watch?v=dr7sU9SrIds

Jamie Hall · December 12

Nicely done, @RayWohlfarth .

RayWohlfarth · December 12

thanks @Jamie Hall

EBEBRATT-Ed · December 12

Same for me Ray. I will watch the video and see if it penetrated my brain.

Tx,

Ed

EBEBRATT-Ed · December 12

Also, if I am not mistaken a vent rated for a higher pressure like the 75H versus the 75 the vent rated at the higher pressure usually has less venting capacity and a smaller orifice.

RayWohlfarth · December 12

@EBEBRATT-Ed I will have to look into that. I just wanted to explain the drop away pressure as I understood it

CLamb · December 12

In the case you mentioned how does air get into the main with the new vent during the day? Doesn't the pressure at a vent somewhere in the system have to drop below atmospheric pressure for air to flow into it?

EBEBRATT-Ed · December 12

@CLamb it does exactly that. When water changes to steam the water to steam volume increase is 1700 times. The reverse is also true. When the boiler shuts off the steam condenses back to water and reduces in volume 1700 times. As the steam collapses air is pulled in to take its place.

CLamb · December 13

https://forum.heatinghelp.com/discussion/comment/1876919#Comment_1876919

Yes, but for air to be pulled in the pressure in the system has to be below 0psig. 0psig is below 3psig so the vent in question should open, right?

RayWohlfarth · December 16

I guess I could have misspoke as long as there is 5 psi on the inlet the vent, it will never open. In this building, they had 5 psi all the time from early morning to evening

CLamb · December 16

But how, then, did air get into the system? I seem to be missing some important point.

I'm thinking of it as follows. The system starts cold and pushes air out through the vents. All vents are closed by the heat. The system remains at 5psi but somehow air gets into the feed to that vent, can't get out even though it isn't hot, resulting in steam never getting into that feed. Where did that air come from?

gattu_marrudu · December 18

Isn't the 5 psi pressure the main problem with this system? I understand that the focus is on fixing that one vent, but if the pressure was below 3psi, the Hoffman 75 would have worked fine. Wouldn't lowering the pressure be the first thing to try?

Pumpguy · December 18

Wouldn't an easier solution have been to reset the pressuretrol, or preferably a vaporstat, to a max of 2 PSI steam pressure?

Most of what I read here on the wall recommends operating these systems at nice low steam pressures.

RayWohlfarth · December 19

Great points brought up Every steam system is filled with a gas, either air or steam. On a cold start, the pipes and radiators are filled with air. As steam is generated, the air is gently pushed out of the system through the air vents or traps. When the steam condenses, air rushes back into the system to take the place of the steam.

@CLamb Air rushes in to take the place of the steam that condenses. When the system senses a low pressure area from condensing steam, either air or steam rushes into fill the void. You asked where the steam comes from, let's imagine a steam zone valve shuts because the zone is satisfied. With the closed valve, a vacuum forms and air is pulled in by the vent

@gattu_marrudu The 5 psi is the problem but the owner thought he had to maintain 5 psi all the time. I tried explain it didnt have to be that high but he was stubborn and insisted on 5 psi. Sometimes, you just shrug your shoulders and nod. He was upset because it didnt work the way it did before.

@Pumpguy It would have been easier to allow the steam pressure to drop but like I said, the owner was stuck on 5 psi.

Pumpguy · December 19

https://forum.heatinghelp.com/discussion/comment/1878074#Comment_1878074

Understood. In the end, its the customer that's paying, so he gets what he wants.

CLamb · December 19

So, @RayWohlfarth, the air from one zone can migrate to the zone with the wrong vent?

RayWohlfarth · December 19

@CLamb Inside a steam system is chaotic and crazy. Not sure about this project and the purpose of the video was to show that the vent will not reopen until the pressure on the inlet is below the drop away pressure as well as not steam

Why steam heat worked in morning but didnt after. Understanding air vent drop away pressure

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