Steam venting -- a slightly contrarian point of view

Jamie Hall

I admit to getting somewhat bemused by the discussions of how to vent steam heating systems which turn up from time to time -- often involving detailed calculations of pipe size and volume and the like, and often using (usually somewhat erratically) @gerry gill 's invaluable research and measurements on vents.

The first thing to recognize is that there really are two aspects to venting. The first is to ensure that the steam mains -- including any associated longer risers -- are filled with steam as evenly and as quickly as possible. The question here is usually posed as something along the lines of "how big a vent do I need". Wrong question. The real question is, "how small a vent can I get away with". The velocity (not really the right word but it will do) at which a steam main will fill with steam and begin to deliver steam to the radiation is governed by three things: how long it takes the main itself to warm up and stop condensing the steam, what the boiler power is to generate steam to fill the main, and how fast air can get out of it. While it may be entertaining to calculate the volume of air in the main and what size vent will allow that air to escape at a certain rate and pressure, that ignores the effect of the other two aspects -- which are at least as important. It may make economic sense to skimp on main vent size (big ones don't come cheap) but, on the other hand, it may also be worth remembering that in the bad old days some two pipe systems vented through open pipes to the chimney or even the basement -- and many, but not all, two pipe systems vented into the dry returns with crossover traps, which have a huge airflow capacity (the Barnes & Jones Big Mouth is just a standard trap minus the float, after all).

So for mains... one can say that in general bigger is better.

Risers and runouts are a slightly more complicated situation, and here the consideration is different for one pipe systems and two pipe systems. First, in two pipe systems, the riser or runout will likely be adequately vented by the radiation, unless it is very long and the radiation at the end is relatively small. On one pipe systems, however, a very good case can be made for providing "main" vents on the risers or runouts, unless they are very short. Why? Because in those systems, the vents on the radiators serve a very different function than the traps or other devices on two pipe systems.

On two pipe systems, the heat output of the radiator is controlled by two factors: the size of the radiator and the rate at which steam can enter the radiator (there are a few systems where the steam entry is controlled by limiting the air exit with an orifice at the outlet -- but these behave like one pipe radiators, not two pipe radiators). Venting is not a consideration here, save only that the dry returns are adequately vented (and, like mains, bigger is better here). Rather, the power output may be controlled for the heating demand of the space by adjusting the inlet valve or, if so fitted, by changing the inlet orifice, provided only that the radiator is big enough to begin with.

One pipe is another matter entirely. For a one pipe system once the radiator is filled with steam and the vent is closed, there is no further control of power output possible; the radiator will put out its full rated power until the boiler stops supplying steam (which, incidentally, is why -- besides controlling pressure -- it is essential that a boiler on a one pipe steam system is cycled off at reasonably regular intervals; to regain control of heat in various spaces). What the radiator vent does here is control how long it takes for the radiator to fill with steam from the time that steam reaches it, and thus how long it takes to ramp up to full power -- and therefore the total energy output, averaged over the boiler cycle length, it will produce. Now you can slow this down with a smaller vent, but once the vent size is enlarged to be more or less equivalent to an open pipe of the vent attachment, you can't speed it up. It can be seen, though, that the more evenly steam reaches all the radiators the better one's control can be -- which is an argument as noted above for providing main type venting on longer or more heavily loaded risers and runouts.

Miscellaneous rambling. For what they're worth.

ScottSecor

@Jamie Hall I could not agree with you more with regard to one pipe radiator vents. I especially agree with ""how small a vent can I get away with." I have tried to explain this concept unsuccessfully to a few on this site. Hopefully your explanation resonates with many that visit HH.

ChrisJ

@Jamie Hall
I concur.

ethicalpaul

The real question is, "how small a vent can I get away with".

I use this for main venting too, not sure if you meant that or not.

holograham

I think I follow. I am in the process of balancing my steam heat system in a fairly large house.

Just added main venting (a ton - probably overkill). So now steam reaches my radiators very fast. I plan to put Gorton 4's on every radiator (20) and then balancing from there in single size increments.

Do you have any other balancing suggestions in this vein of slower as possible?

Steamhead

holograham said:

I think I follow. I am in the process of balancing my steam heat system in a fairly large house.

Just added main venting (a ton - probably overkill). So now steam reaches my radiators very fast. I plan to put Gorton 4's on every radiator (20) and then balancing from there in single size increments.

Do you have any other balancing suggestions in this vein of slower as possible?

The Hoffman #40 in my experience has a venting rate roughly halfway between the Gorton #4 and #5. Also the Vent-Rite #11 is roughly between the Hoffman #40 and the Gorton #5. This will let you really fine-tune things.

I like the Vent-Rite #1 adjustable vent for bedrooms, since some folks like their bedrooms cooler. This lets them dial it in.

JShep

To use the same lead in as another commenter above, and ask a different question; I think I follow.  I am in the process of balancing the two pipe steam system in my very large house (6,000 sqft, 5 levels).

- Each level of the house is roughly 23’x50’
- The basement (not included in the 5 above ground levels) is where the boiler is, and it has 9’ ceilings
- The boiler is ~15’ in from the front of the house
- There is one really long continuous steam main; ~25’ of 3”, ~60’ of 2.5”, ~10’ of 1.5” (I’m about to fork it where the 2.5” drops to 1.5” with about 5’ of 1.25” to bring heat into a small back building connected to the house.
- The return main runs parallel to the steam main the whole way.  Both form a large S shape, from the boiler (N side) to the front of the house (East), across the front to the side of the house opposite the boiler (South), half way toward the back of the house (west), then back over towards the other side where the boiler is (north), then all the way to the back of the house (west) then heading back opposite the boiler side (south).
- There are two (remaining… two small ones previously removed) main risers that head up about 60’ (to floor level on the 5th floor).  One on the SE corner of the building, one on the NW corner.  Each steam riser is 1.25” (with matching 1” returns).
The riser in the front of the house feeds 1.5x the number of radiator heating surface as the riser in the back of the house.

I attached a picture of the original plans which somewhat detail this better than words.

The SE riser comes off the 3” section of the steam main, maybe 20-25 pipe feet from the boiler.  The NW riser comes off the 1.5” section of steam main at the end of the single main’s ~90-100 feet of horizontal travel.

I’ve got 5-10 minutes lag (warm start vs cold start) between the front of the house rads (the SE riser) and the back of the house rads (NE riser) seeing their first bit of steam from the boiler.  That’s challenging, because the bull of the heating surface from the radiators is in the front of the house, so it starts getting hot quicker, and it’s got more heat output… so there’s a notable temp difference.

I see two options; reduce the number of radiators (or close more of them) in the front of the house… or get the steam to the back of the house faster.

the latter option seems more logical… which to me has two paths forward.

The lengthy S pattern mains were laid out logically for enabling radiators on each of the 4 sides of the house… but more efficient boilers coupled with each floor basically just being a front room and a back room made the radiators on the two side walls of the house overkill, so someone (or multiple someones) removed them over time such that I now have 60’ of pipe travel horizontally in the basement without a single radiator fed along the long that run until it gets to the back of the house where the riser at the end of the run is.  Or more succinctly, I’ve got 60’ of pipe connecting the boiler and the rear main which is only 30’ away from the boiler.  Moreover… that 60’ of pipe is sized for what once was a greater number of branches off the main which no longer exist.

So the two logical options are;

- cut the vast majority of the 2.5” run of the steam main out and run the steam main in two directions off the header instead of 1 long snaking S shape… toward the riser up front and the riser in the back.  That’s a metric **** load of pipe to remove and an SAE equivalent of a decent amount of pipe to replace… I’m 100+ hours in this winter of getting a new boiler in and … enough … money worth of parts and pieces and pipe and tools to get things running.  Best case I’d tackle this late next summer.

- add vents (on the riser vs at the end of the main where it meets the riser?) in the back of the house to draw the steam there faster.  That’s if my understanding of your logic above and everything else I’ve read is correct… which it may not be.  That’s a $50-200 additional investment ($50 of which I already made with the Gorton #1 that arrives today) and minimal work (as I already removed a 115 year old extension at the end of the main I’m using to route my new 1.25” leg I noted above, wherein I replaced the old 90 with a reducing tee to give me a 3/4” vertical facing connection to drop the Gorton in).

so I suppose the very long way around asking my question is; can I compensate for unnecessarily large pipe volume needing to be traversed by increasing vent capacity at that end of the main?

SteamingatMohawk

@Jamie Hall I don't think your dissertation is all that contrarian. Two additional points to ponder are "the vent the mains fast and the radiators slow" mantra and the addendum to the Gill report on too much venting with the specific example.

Peace to all!

Jamie Hall

To answer @JShep 's question... only partly. Steam travel time is a main is partly dependent on venting the air, but also is dependent on just simple length of pipe to be heated up. If you are seeing a pressure rise when steaming starts of only 1 to 2 ounces or so and then levelling off, additional main venting isn't going to help much. Be sure the pipe is insulated -- that can make a remarkable difference.

JShep

Jamie Hall said:

To answer @JShep 's question... only partly. Steam travel time is a main is partly dependent on venting the air, but also is dependent on just simple length of pipe to be heated up. If you are seeing a pressure rise when steaming starts of only 1 to 2 ounces or so and then levelling off, additional main venting isn't going to help much. Be sure the pipe is insulated -- that can make a remarkable difference.

Makes sense.  Physics is still physics regardless of the system…

I’ve admittedly not paid enough attention to the pressure gauge to beyond the initial values (by way of having swapped my gauge for the only one I could get quickly, which runs 1-5 pounds instead of the factory installed one which was up to 25psi if not higher).  After I installed it (and cleaned the pressuretrol pigtail, low water sensor, etc.) I fired it up and it was 20-40% of 1 pound on initial steaming.  Assuming it’s the same as other pound/ounce equivalents so ~3-6 ounces… I’ll give it a closer look in a bit and see if it levels off or continues going up.

looks like either way though, spring or summer is going to bring about some repipe work to cut the 30+ unnecessary feet of 2.5” pipe out of the basement to, thus bringing the two risers much closer in distance to the boiler to even things out.

jumper

I agree but if we're going contrarian then there are mechanical alternatives to venting. Don't allow air in. In the fifties there were buildings in Toronto that used water powered eductor to pull respectable vacuum. Worked well except when it didn't.

Jamie Hall

I was kind of avoiding vacuum assisted or natural vacuum systems, as they bring up a whole different group of physics and thermodynamic considerations... which are not always quite self-evident.

SteamingatMohawk

@jumper Worked well except when it didn't.

One of my many sayings: "It's not a problem until it is, then it's too late."

jumper

@Mohawk When the wrong guy does the wrong thing then it is indeed a problem.

Who is the correct guy to look after an unusual (and unorthodox) system? When too much air leaks into a "tight" vapor installation .....

SteamingatMohawk

It's exactly the challenge I have had to deal with...even finding someone credible by using HH didn't yield someone close by that would come look at my situation. I practically begged one HHer from an hour and a half away and he declined my "invitation". He did help a lot by text/email.

Fortunately, the advice I've gotten from HH has been immensely valuable and helped me improve my system. My issues this year are a real dilemma, because of the problem free history for the last 4 years. I haven't positively proved there is a problem. The shoulder season weather here is "worse" making diagnostics that much harder to actually find the root cause(s).

Peace to all this weekend and safe travels for all.

CKNJ

@Jamie Hall @ScottSecor Agree with your conclusions on one pipe completely, but for those of us that have one pipe "steam only" radiators (whereby only a single connection across the bottom) there is something else to consider that took me a while to realize when balancing my system. I am not an expert, but below is what I believe occurs inside the radiator during a heating cycle. If my assumptions are wrong, please be kind?

It seems that each section of a steam only radiator is like an inverted water glass. When steam reaches each section, there is no mechanism to displace the air from the top as it is a hollow sealed chamber, correct? So that would mean each section creates it's own vacuum chamber as the steam condenses, thereby pulling steam into itself until satisfied. Too large a vent means each section's vacuum will pull air from the vent, defeating the design. A properly sized "small as possible" vent will allow each section to pull steam from the riser to fill itself as designed.

I figured this out when I had a Gorton 5 on a large 12 section steam only radiator in my living room. On a 30 degree day, the radiator got hot all the way across but took a long time to satisfy the thermostat located in the same room. I downsized to a Ventrite #11 and now only 6 sections get hot, but they seem "hotter" and satisfy the thermostat in 1/2 the time. That is when the above occurred to me, if I am correct. If I am incorrect, apologies.

Jamie Hall

I think you probably are correct. Those radiators connected only across the bottom are kind of a fringe case.

ethicalpaul

It's hard to know exactly what happens in those, but I suspect the incoming steam does push the air around and circulate in there and eventually pushes most of the air out the vent, at least on a long call for heat.

I will say the Gorton #5 is hardly a large capacity vent so I'm not sure what that had to do with it, but maybe.

CKNJ

@ethicalpaul The Ventrite #11 is about 25% less venting capacity (at 1 oz. pressure) than a Gorton 5. By reducing the venting it was a game changer. I've thought about doing some experimentation with a Hoffman 40, but "if it ain't broke....".

jumper

Consider the inverted glass model. Air is in there at 0 psig.
Steam @ ~0 psig mixes with it so that partial pressure of water vapor is ~7.5 psia?
Steam condenses @ ~180° F ? Is that so bad?

If I had a single pipe heating system with those old fashion radiators I'd try sealing all vents and evacuating the system as far as practical. Close to 28" mercury. Probably easiest is to rent a powerful air compressor and use an air powered venturi vacuum generator.

The condensate hopefully returns thanks to gravity. No air required.

This is theoretical since I never tried it.

ethicalpaul

CKNJ said:

@ethicalpaul The Ventrite #11 is about 25% less venting capacity (at 1 oz. pressure) than a Gorton 5. By reducing the venting it was a game changer. I've thought about doing some experimentation with a Hoffman 40, but "if it ain't broke....".

Yes, there are smaller than the #5 (such as the #4 and the one you named) but the #5 is still a nice slow vent. I'm glad you had a good result regardless

CKNJ

@jumper Back in the mid 1920's, Hoffman made a No. 2 vacuum radiator vent. It was for single pipe systems, letting air out, but then when the radiator went into vacuum (like i describe above) it closed maintaining vacuum in the radiator. Do not know why they stopped making them, but cost was 3X that of a regular Hoffman No. 1.

PMJ

CKNJ said:

@jumper Back in the mid 1920's, Hoffman made a No. 2 vacuum radiator vent. It was for single pipe systems, letting air out, but then when the radiator went into vacuum (like i describe above) it closed maintaining vacuum in the radiator. Do not know why they stopped making them, but cost was 3X that of a regular Hoffman No. 1.

Hoffman called air "The Heat Thief". I attach his book on it here.

I've experienced the benefits of all he says in two pipe for years.

Steamhead

First thing, and simplest, is to increase the venting. These long, convoluted mains will work if vented properly.

SteamCoffee

Older thread, but it seems the vacuum argument is to try and NOT let air BACK in the system. Modulating burner?