Radiator vent loudly sucking air INWARD after boiler turns off

CoachBoilermaker

Boiler starts. All rads hot. Room is warm. Boiler stops. All is well.

A few minutes later, the living room Radiator vent loudly sucking air INWARD after boiler turns off. Lasts about 2-3 minutes, then stops. The only noise is inward sucking after the boiler shuts off, no hissing, no spitting, no weird behavior during the heating part.

I do not recall this happening last winter. And I don't think it does it every heating cycle this year.

EBEBRATT-Ed

The volume change between steam and water is tremendous.

If you took a box that measures 28" x 28" x28" and filled it with water, when that water turns to steam the size of the box to hold that steam would be 28'x 28' x28'.

The same volume change holds when the steam changes back into water.

So in your house when the boiler is steaming all the pipe and radiation is full of steam. When the steam is generated it pushes air out of the vents. When the boiler shuts down the steam collapses and turns to condensate. The air expelled previously has to get back in the system and the system goes into a vacuum until the air come back in.

So sucking air in through the vents is normal when the boiler shuts down

Jamie Hall

All true. However if it is doing it this year but not last, or only does it now and then — some other vent somewhere (likely the main vent) is sticking closed when it shouldn't or didn't.

ARobertson13

Sucking in air loudly for 2 or 3 minutes is not normal operation for a properly functioning steam system. This really should not be happening if your pressure is below 2 psi and or the vents are functioning properly.

gfrbrookline

Try smacking the vent if it isn't closing. Sometimes they get out of alignment. It has worked for me for years, especially with Gordons.

Kayden_1015

photos of the vent would be great

neilc

how are the main vents?

and what pressure does the gage show before the burner shuts off?

ethicalpaul

https://forum.heatinghelp.com/discussion/comment/1873762#Comment_1873762

It is normal and it really doesn't depend on the operating pressure. At the end of the cycle, all the steam turns into a very small amount of water, regardless of the pressure of the steam. All the airspace in all the pipes and radiators must be "inhaled" by one or more vents. If it's just one small one that opens first, then it can take some minutes to fulfill the vacuum.

mattmich

The loudest noises my system makes are some hard ticks or bangs early in the cycle as the risers heat up and expand. That lasts a minute or three and won't wake me up when I'm asleep but I can hear it anywhere in the house when I'm awake.

The next loudest sound is when the steam collapses after the boiler shuts off and the vents suck in air. We have Maid-o-Mists, and only the #5s are audible. I have three of those. The rest are all #4s and are so small that I can't hear them taking in air, which of course they are doing.

This hissing sound is normal. Changes over time would indicate a change in the functioning of the vents. But as has already been stated: some vent somewhere is going to be the first to open.

cheers -matt

ARobertson13

I am a retired engineer who has worked for defense contractors on far more complex systems than steam and I have stated on a number of post how vents should operate. Sucking in air loudly for 2 or 3 minutes is not normal for a properly functioning system. I will define properly functioning I mean: 1] Pressure is below 2 psi 2] Vents both mains and radiators will close when steam contacts them or reach some lower temperature. 3] Vents remain closed when they are hot. 4] The steam quality is good, under %2 water droplets. I have operated and modified several steam systems over the past 4 years and have noticed that many veterans of steam heating do not understand what is going on with vents in steam systems. Loud hissing or sucking is an indication that something is wrong in your system. The first system I worked on [29 unit single pipe] had that problem. The psi was close to 4 and the vents mains and radiator where would not close [damaged] or stay closed. I had to spend $3700 to replace them. When I added controls the pressure was lowered and the hissing and sucking sounds stopped. I have noticed this in a number of other buildings. You should not have hissing of steam [which means the vent is not closing] or loud sucking sounds in a properly operating steam system.

EBEBRATT-Ed

@ARobertson13

When water changes to steam or vise versa the volume changes 1700 times. This has nothing to do with pressure. When the steam collapses air is going to get in the pipe somehow unless it is a vacuum system.

If people complain about noisy vents they can put a reverse check valve or a vacuum breaker on the boiler where the noise is not as objectional.

Unless you have some magical way of changing physics to keep the noise down.

ethicalpaul

https://forum.heatinghelp.com/discussion/comment/1877734#Comment_1877734

I respect you based on your cool vent testing posts, but here you give no reasons for your argument. You just say "it isn't properly functioning". I'm sorry, but appeals to authority are not a strong argument for me. I have heard "I've been doing this for 30 years" way too many times from people who shouldn't have been LOL

Here are my reasons in case there are any you find fault with:

• when a system is running for a reasonable amount of time, it is pretty much full of steam with some small amount of air remaining (in a not-completely-steam-filled radiator for example.)
• When the steam is no longer produced, regardless of the pressure during running time, the pressure will reduce as the steam collapses. Until it gets to 0 gauge pressure there will be no sucking sound (I think we agree to this point), no matter the initial pressure
• OK now the relevant part: The remaining steam will collapse and there will be a significant vacuum that must eventually return back to atmospheric pressure (0 gauge)
• So air must enter the system
• It can only enter via the main vent(s) and/or some number of radiator vents (I'm discounting the vacuum breaker since that is very rare on a residential system in my experience)
• Some of those vents might open sooner than others and it's not always possible to know which will open first
• Some of those vents might be more noisy or be in a more noticeable area of the house, resulting in the noise which is the topic of this thread
• None of the above indicates any problem with the system (I'm not counting running pressure of the system, which we all agree should be low, but I'm not counting it because it is not relevant to the amount of air that must be inhaled into the system every time the boiler stops making steam)

dabrakeman

Also worth mentioning that on many normal operation cycles none if the radiator vents actually close but of course the main vents do. Not suprising that upon initial collapse of steam at cycle end the larger of tge radiator vents would be a favorable air entry point.

ARobertson13

To all, I have had many duties in my career. One of them was instructor. If you looked at my post on vents, testing and how they operate, you would understand. I do not have that problem with vents hissing steam or loud sucking of air on systems that I worked on because I understand what is going on in the systems. I have worked with both large and recently a small 4 unit building and I know what to do to resolve the problem. There should be no loud noises hissing or otherwise on any steam system that is properly operated. This what I tell my Coop members and the clients I have. It has nothing to do with experience. It has to do with do you understand what is going on and what methods to correct it. If the pressure is high, lower it. If the vent is not operating properly, test it and replace it with one that was tested. If you need a steam control device to operate the boiler instead of a thermostat, get one. Basic problem solving:

gfrbrookline

I agree with @Jamie Hall sounds like a stuck closed main vent. What do you have for main venting, send pictures.

EBEBRATT-Ed

@ARobertson13

You explanation IMHO is flawed. It's a matter of Physics that when the steam collapses the air has to get back in the system.

Do you agree?

Nothing in the statement you posted:

"It has to do with do you understand what is going on and what methods to correct it. If the pressure is high, lower it. If the vent is not operating properly, test it and replace it with one that was tested. If you need a steam control device to operate the boiler instead of a thermostat, get one. Basic problem solving:"

gives any information or tells anyone how to fix the problem we are talking about.

"It has to do with do you understand what is going on?

Yes, I do, the change in volume from steam to condensate is 1700x

"If the pressure is high lower it"

I agree low pressure is better in most cases. But as @ethicalpaul mentioned this has 0 to do with steam collapsing and the resulting vacuum.

"If the vent is not operating properly, test it and replace it with one that was tested."

Vents vent air and let air in. They close when they sense steam. They should not leak water. They should not pass excessive steam. That's all there is to it.

There is no mystery here. Air has to get back in the system when the boiler cycles off.

dabrakeman

You say if the following is met one should not experience the issue:

"1] Pressure is below 2 psi 2] Vents both mains and radiators will close when steam contacts them or reach some lower temperature. 3] Vents remain closed when they are hot. 4] The steam quality is good, under %2 water droplets."

Not sure how the 4th point is actually verified to a particular % but I can say with reasonable confidence that the criteria are met at least in my case. I do know I could minimize the issue by reducing venting rate on two of my rads but that creates a tradeoff I have decided against for my particular situation.

I'd never assume I know or have experienced everything with regards to my area of expertise.

ARobertson13

I was busy with contractors so I could not respond. If your vents Mains and Radiators are closed when they are hot. They should remain closed for several minutes after the steam has ended. So you should not hear anything loud. Also, there are leaks in every steam system. Vents that appear to be closed are leaking air slightly. Supply valves are also leaking air. In addition, after the steam has turned to water, the reduced temperature of the system [mains, risers and radiators] cause a further reduction in pressure. The real problem is that many of the vents manufactured are not operating properly out of the box [OOB]. Most do not close with a case temperature of below 150F. As I indicated before, I had these problems with balloon air sounds because the vents would open when they cooled of at high psi. It is important to determine if the noise is escaping air or suction. I was in defense manufacturing in long Island so I was able to open up these vents clean them out [they were filthy OOB and boiling will not help] and adjust them so that they would close at or below 140F. Then reseal and test them. The Steam heating world is the only business that I know of that would tolerate this level of product OOB failure. Most of the contractors that I came in contact with and building supers, just put them in and do not check them. Could you imagine electricians installing circuit breakers or Auto mechanics installing spark plugs with this level of OOB failure.

ethicalpaul

I agree vents are a problem but regardless of how great the vent is, eventually when it or one of its friends opens, air will enter the system and it's always the exact same amount of air regardless of the operating pressure during the previous call for heat. I'm repeating myself now and promise to stop!

mattmich

ARobertson13:

» It has to do with do you understand what is going on

Agreed.
And both Ed and Paul have told us what's going on.

As far as the "I am an engineer" thing, I suppose we can all go digging out our diplomas and such and play mine-is-bigger-than-yours, but I'd rather skip it.

The first thing we need to agree on is that the "operating pressure" of the boiler is irrelevant. What you describe, and some of us have observed, happens after the boiler shuts off.

And I didn't even think about it, but as dabrakeman said — not all vents close during normal operation. I have at least one big radiator that won't heat all the way across unless it's really cold. So there's an obvious entry point.

Are we agreed so far?

cheers -matt

RTW

For what its worth: I've lived with my single residential pipe steam system for almost 30 years and will say routine venting maintenance often works. You could try boiling the offending vent - and the main vent referenced by Mr Hall - in white vinegar for about 5 minutes. Debris can accumulate over time. And, you could swap out the offending vent from another radiator as an experiment

Regards

RTW

ARobertson13

Why not do what I did? I tested my system at various operating temperatures. Then shut the boiler off. When I got to .75 psi difference in pressure, the noise subsided. I wonder why?

ARobertson13

That will be last i have to say about this matter. I think the administrator may just end this discussion.

mattmich

Well, you're welcome.

» I tested my system at various operating temperatures.

[EDITED because I misread: You mean "temperatures" or "pressures"??? I read "pressure", which is adjustable. "Temperature" is adjustable in terms of the thermostat, but only affects how long the boiler runs, not how it goes about it." Steam temperature is of course fixed. Let's assume "pressure," then.]

Probably not. Residential steam has no "set pressure" — the pressure is what it is when the steam comes up. The regulation comes in only if the pressure, for whatever reason, goes above the Pressuretroll's cut-out point.

On my system, the pressure jumps to ~ 8 inches water (0.2 PSI) as soon as the steam comes up, and never goes over half a PSI. It doesn't really matter where I set the Pressuretrol. Others have reported pressures even lower than that. I believe it comes down to steam production vs. pipe diameter & fall-out rate as the steam cools.

nellc wrote:
» and what pressure does the gage show before the burner shuts off?

That would be a good question to answer, along with Jamie Hall's suggestion that the main vent might be sticking. Is it?

If your system will build pressure beyond 2 PSI, something is wrong.

cheers -matt

neilc

oh, don't drag me back into this,

only reason I asked was, that if the pressure was going nuclear, then it could have damaged the main vent(s), locked it shut, and that would put excess vacuum relief load on any rad vent, and go hiss, hiss,