vacuum vents

twopipe

I have a gas-fired two-pipe Webster system (100 yr old house).  The system has crossover traps from the steam mains to the returns, and there is one main vent on the return (the original Webster Vent Trap).  The vent trap has the small ball valve on top which is effectively a vaccuum vent, but there is no vaccuum pump nor any sign there ever was one in this system.  I've read all the comments that say I should replace that ball valve with a standard vent.  But I just don't understand any of the explanations on why I should do that.





This is my first winter in the house, and I noticed that someone else was thinking about the same thing because the previous owner left me with an unused new Hoffman #75 right next to the vaccuum vent- they can easily be interchanged.  Since I have both, I've tried running the system with and without the vaccuum vent.  When the system runs with the vaccuum vent, it hits a maximum pressure of about 1oz after the boiler fires for a very long time, and (not surprisingly) drops into vaccuum pretty quickly after the boiler turns off.  The system seems to hold vaccuum for a pretty long time- on cold days the system is under almost continuous vaccuum except for when the boiler is firing. 





When put on that Hoffman #75 (or I just take the vaccuum vent off and run with nothing) the system heats fine, but there is a lot more air hissing in and out of that vent.  The boiler pressure gets a little higher late in the cycle, but I've never seen more than 3oz. 



So is there any reason not to run this as a true vapor system (with the ball valve)?  The way I figure it, with the vaccuum vent there is less air in the steam mains for the boiler to push out every time it fires up, and the steam should remain uncondensed in the pipes for longer as they cool down (since the pressure is low when the boiler shuts down). 





I know that they designed this way for coal firing, but I don't see why the basic principle that low pressure is good would not also apply to gas or oil firing.

My question is, what exactly is the problem with running a gas fired steam boiler with a vaccuum vent?  Is it going to cause me problems down the line, or somehow cost more in energy?  Or should I do it?

Steamhead

The problem is

the relatively short firing cycle on oil or gas. The coal fire came up slowly, and all the air got out before the system went into vacuum. But now, it's possible to get vacuum before all the air is out. When this happens, the remaining air can expand and block steam circulation. This is obviously not good.



Use a Gorton #2 vent on that vent trap. It has much more capacity than a Hoffman #75.

twopipe

follow up

Thanks. But why is that a problem? If it goes to vacuum before the cycle is over, there will just be less air in the system and the steam will distribute better.



I can see why there could be a problem if there was vacuum in some parts of the system but not others. That's not possible, though because all pipes meet at the boiler- so they will all have vacuum.

Steamhead

Because

the remaining air can expand under vacuum and block steam circulation. No circulation, no heat.

twopipe

my 2 cents

The conventional wisdom on this board is as kindly described above- that is, to remove all vaccuum devices when converting a steam system to gas/oil.  But this advice should be questioned.  Consider:

a) My single data point says vacuum is just fine.  I have a gas-fired two pipe system running with a vacuum vent.  My heating system is warm, even, quiet, and (relatively) cheap to operate. 

b) It really doesn't make sense that the expansion of air will somehow expand cause problems (at least in my opinion).  If the sytem is half-full of steam and the boiler shuts down, then, yes, the remaining air will indeed expand if there is a vacuum present.  But if there is an atmosphere vent then air will just rush into the system at that point in the cycle, basically doing the same thing. 

c) Air in a steam system is bad, in the sense that you are paying to expel it every time the boiler cycles.  Making it easy to vent the air is one solution, but not having much air in the system is a better one.  Low pressure is also good because steam stays steam longer, and therefore you pull more BTU's out of your steam before it condenses somewhere other than the radiators and heads for the return. 

d) I've searched this board, and there is no mention of anyone who actually had a problem with air expanding and blocking steam circulation.  If anyone has actually seen this problem and fixed it by removing the vacuum device, it would be interesting to hear.

e) Lastly, its easy to check.  Put the vacuum vent back on and see what happens.  That's what I did, and I'm sticking with it.

Dave in QCA

I am very interested in how your experiments work out.

With the deepest respect for Steamhead and the other pros who hold to the belief that vacuum is bad, I accept that their wisdom may be correct and that their conclusions may be based on experience.

 

However, the only place in literature, both current and from the dead men, that I can find the recommendation to remove all vacuum devices when converting to gas or oil firing is from Hoffman Specialty in their reference and resource section.  While there may be a distinct reason that relates to problems caused by vacuum, I suspect it was more a matter of simplifying the system.  Gas was still cheap, and now that there was the ability to run a boiler full blast and off, as opposed to open damper coal blazing, and damped way down and smoldering, it probably seamed that the ability to finely regulate temperatures could be handled by that means alone.  Afterall, vacuum was in large part a means to provide better temperature control. 



 However, it seems to me that there are still great advantages of running in subatmospheric conditions. 

Number 1, Most systems today have way too much EDR as a result of insulation and storm windows being added.  If vapor at a lower temperature can be distributed in the system, it will more closely match the heat loss of the building, resulting in more even heating. 

Number 2.  A radiator heated with 190 F vapor will produce 78% of the heat it would at 215F, at 180F - 69%, at 170F - 61%, and at 160F - 53%.  This seems like a better way of modulating heat as opposed to the continuous battle of trying to get small portions of the radiators to even heat to 212F.

Number 3.  When vacuum vents are present on a tight system, when the boiler shuts off and steam condenses, it produces a vacuum.  Any residual air in the system will disperse evenly throughout the space.  It will not expand and produce more air, it is a partial vacuum, and that is how most of the vapor / vacuum systems operated.  At 10 in Hg vacuum, water boils at 192, at 20 in vacuum, 161 in, at 25 in, 133.



I understand the problems of partial expulsion of air on a short cycle - the air is still there on the next cycle, but isn't the system full of air on a non vacuum system?  It would seem that cycles where the system is only partially heated are still going to occur the same as an open system, or perhaps a little easier if there is any residual vacuum.  Any amount of vacuum will allow the water to boil at a lower temperature and also allow the steam to move throughout the mains easier - there is less air to move out of the way.



I read a dead mans account of a vapor system written around 1907.  He described a vacuum system being fully heated, perhaps coming up to a few oz of pressure.  When the damper was closed and the fire cut way back, the vapor continued to flow when it would not have in a non vacuum system.  As the boiler continued to cool, vapor continued flowing but at an ever decreasing temperature, with the boiler, vapor, piping, and radiators all running at the same temperature.  Vapor would continue to flow until the boiler got down to about 150F, at which point the vapor flow would cease.  Once the damper was opened and the fire raised, and the boiler was warmed, vapor would again begin to flow at the low 150F temp range.  The more/longer the boiler was fired, the warmer the boiler, vapor, pipes, and radiators would become.  If enough heat was added to bring everything up to 212, the vacuum would no longer register, and if more heat was added, it would rise to 215F at 1 PSI.



Sounds alot like a hot water sytem doesn't it?  However, the vapor/vacuum proponents at the time boasted that it was better than hot water because like hot water, the radiators could run at a moderate temperature, the system was silent, but the radiators were much quicker to heat than with water, the size of the radiators were smaller than with water because they could go up to full steam temperatures, and the cost to install the systems were less than hot water, because piping sizes were smaller. 



I have an old two-pipe dunham vapor / vacuum system.  All remnants of the vacuum operation have been removed, the gravity return using either just a Dunham air eliminator and perhaps a Dunham return trap have been removed and replaced with a condensate return pump, pressure was controlled by a 2 psi pressuretrol (which I have replaced with a vapor stat set of 8 oz -- cut in 4 oz.  But, in reality, with the t'stat set at constant 72 degree temperature, the boiler never reaches 8 oz unless the temp outside is 20 below 0. 



I am also very interested in seeing how my system runs in a vacuum or partial vacuum.  The way I look at it, if steam is both comfortable and one helluva lot of fun, then vapo-vacuum has to be twice as comfortable, and twice as much fun.  The vapor/vacuum system was in my opinion, pure genious.

nicholas bonham-carter

something out of nothing

i think the pro's who are anti-vacuum are more interested in speedy venting, than in maintaining the vacuum once the air is out. if your system is in good shape, as far as valve packing and joints, then the vacuum could persist quite a while. as it is, the vacuum vents currently available are pretty slow. maybe a check valve on the front of the vent antler would be a better arrangement. it would be important to have the persistant vacuum not be compromised in one part of the system, and not in another.

obviously, the difference between inside and outside system pressure would be much greater with a vacuum, than during steaming.

i look forward to more discussion on this subject!--nbc

twopipe

If you try this, some things to consider

If you do try run vacuum, here are a couple points I learned:

a) I made sure my thermostat was set for a "gravity return system" which will result in 1 or 2 cycles per hour.  That helps to make sure that the t-stat calls for a long enough firing cycle so that you don't get the system partially filled with steam.  Not sure this is specific to vapor, but it wasn't obvious on my thermostat on what it was set to or how to change it. 



b) As far as I can tell, the pressuretrol/vaporstat is mostly useless in this system.  If it fires for a while, I can get a couple oz (like 1-2) of pressure in the system, but it drops into vacuum really fast when the boiler shuts down.  Which means that the vaporstat will just cycle the boiler back on.  The vaporstat does provide a measure of safety I guess, but it doesn't do anything to save fuel. 



c) Most of the time, the system doesn't vent any air- since it is under vacuum from the last cycle.  So that vent hissing sound is gone except when firing the system up when it has been off for quite a while.

Jamie Hall

This is one of those issues

where it seems to me that everyone has a perfectly valid point, and everyone is more or less correct.  As nbc pointed out, the real issue here is speed of venting, not the vacuum by itself -- and perhaps only those of us who have actually had to cope with a coal burning boiler can really appreciate why, although several folks have touched on it.



Simply, it is quite impossible with a coal fire to go from cold (no fire) to full output quickly.  Trust me, I've tried.  For that matter, one can't even go from a reasonable bed of coals to full output quickly.  It takes time -- quite a bit of it.  Whereas with oil or gas, one goes from nothing to full bore in milliseconds.  The result is that a coal fired boiler will build steam very very slowly, and there is absolutely no point in having anything bigger than, say, a single Hoffman 75 on even a very large system.  This also means, incidentally, that a coal fired system won't heat all the radiators in a very large system at the same time -- the farther ones will take longer.  Can't help it.



Now with oil or gas, you can build to essentially full steaming capacity, even in a pretty good sized boiler, in a matter of minutes from a cold start.  Since it is desirable to have the various radiators all heat up at about the same time, it is also desirable to be able to vent the air as fast as the steam builds -- so the "modern" approach is to have a lot of big vents.  The result will be that all the radiators will be more or less evenly hot, or should be, by the time the thermostat is happy and the system shuts off.



Why wasn't this a problem with coal?  Simply because the tendency was to run longer cycles -- in effect, one continuous cycle (you can't turn a coal fire off quickly either -- I've tried that, too) so eventually all the radiators would be about the same amount of toasty, according to the heat requirements.  Which took some anticipation, by the way, by the fellow with the shovel.  With coal, the vacuum was a real asset, as it allowed the boiler to continue making steam, although at a lower temperature, as the fire died down -- among other things, minimizing the number of times the thing had to be stoked.



Could one run a vacuum system with an oil or gas burner?  Sure -- why not.  But there wouldn't be much advantage to it.  Is there a potential problem?  Yes -- and it has nothing to do with vacuum, but with parts availability.  As vacuum vents go, the 75 is a nice big vent.  As modern vents go, it's a pipsqueak.  One would need a pretty fair antler to carry enough 75s to get the venting capacity up to the point where it comes even close to the ability of a few Gorton #2s, for instance.  But one could do it, if one wanted to.  I personally doubt that there would be any significant benefit to it -- conceivably there might be a fractional gain in efficiency, but probably not measurable.

Charlie from wmass

If we had modulating burners

Vacuum would still be good. On most steam and all residential steam we have to the floor on or off for the firing. Vacuum took advantage of lowering the boiling point of the water. but also modulated the firing rate on the solid fuel by way of the damper. If you have a modern burner the vacuum causes issues of incomplete venting which causes unbalanced heating. Also vacuums are hard to maintain in real world 100 plus year old systems. pressure leaks can be found and addressed. Vacuum not so easy to find.

Charlie from wmass

To add one more point

Coal and wood never took away the heat just adjusted it. Modern fuel stops all new heat being added when it shuts down.

Dave in QCA

OK, your beginning to convince me....

Thank you Jamie and Charlie.  Your hands-on experience is quite compelling.  I am beginning to accept the advice of "Live Men". 

I spent some time looking for coal boilers, and checking on the price of coal.  I don't think there would be much savings, and even though I am retired, I don't know if I went to spend a lot of time shoveling coal!

twopipe

Vacuum doesn't really need big vents

Your point is correct- I have only a fairly small vent on the system.  But it isn't a problem, because the system rarely vents much air.

When it is fired up from a completely cold state (like its been off for a day and the vacuum has slowly subsided, then it does quite a bit of work to push the air out of the vent.  But that's a one-time deal.

When the system is running day to day, it holds vacuum for a pretty long time.  So there isn't much air to push out- in fact on a reasonably cold day the thing doesn't seem to hiss much at all. Each cycle just has to push out the amount of air that leaked in since the last cycle, which is pretty small. 

So the lack of big vacuum vents is not an issue I think. 

The question of whether vacuum has a cost benefit is hard to answer.  As pointed out, I'll get heat out of the system at lower temperatures, so the water in the boiler will keep producing steam down to 150 deg or so.  And the steam produced is going to really zip around the system since there is no air resistance. 

Seems to me it should be lower cost to run vacuum by some amount, and it doesnt seem to be hurting anything.

twopipe

leaks

Actually I did find some leaks when I went to vacuum.  I could hear them hissing pretty easily.   Had to tighten some fittings on the sight glass and on the radiator vents.  System holds tight now.

Mike Kusiak_2

Vacuum operation with oil firing

Vacuum operation and on-off firing were not always thought to be incompatible. Here is an article from the Wall library about retrofitting one pipe systems with vacuum vents for increased efficiency and even heating. From the 1933 Fuel Oil Journal:



http://www.heatinghelp.com/article/163/Older-Steam-Heating-Systems/1336/Vacuum-Valves-for-Winter-Profits

Dave in QCA

Is forced air better than steam heat?

Resoundingly NO!.    But yet, there are thousands of technician, engineers, and homeowners who will argue in favor of forced air.  The proponents of steam, and the installation of new steam heating systems began to dwindle long ago.  Why?  Not really sure.  Of course, it has something to do with the advent of AC, but steam had pretty much disappeared a decade before much AC was being installed on forced air heat.  It seems to me that perhaps it's just part of the changing times.... things come, things go.  

Perhaps vacuum heating wasn't as useful as it was when coal was still being burned, but in the post above is an example that someone thought it was very helpful, even on an ON/OFF oil fired boiler in the 1930s.

I was recently helping new owners of a commercial building analyze their heating and AC systems.  One section of the building is heated by a steam boiler with a 2 pipe system.  All traps are Dunham 1E, the end of the steam mains are vented by crossover traps to the return line.  The return line is vented by a Dunham air eliminator.  When I went to flush the LWCO, it sucked in air instead of draining.  I went searching around, removed the screw cap on the air eliminator, and found that someone had shoved a hex headed bolt, wrapped with Teflon tape, into the exhaust port.  I don't know how it even vented at all... in fact, from their description of the way it operated, I was not venting well.  I removed the bolt so that normal venting would occur, and the system started working great!  Now, I want to find a disc to put back in the check assembly, so that it will both vent well when needed, and maintain a vacuum too.  I am dying to see just how this system behaves as a vapor vacuum heating system.  I want to see what it's like to have a steam boiler continue to boil with the fire turned off, and circulate vapor to warm radiatators at 170 degrees F.

The current operation of steam heating systems goes as follows..... 

Hot Steam up at 212..... radiators HOT... then COLD.... HOT..COLD...HOT....COLD    Hmmmm.... sounds a little like forced air???

   OK... I'm just kidding.

Jamie Hall

Actually...

I'm right with you, Dave.  That is, I would be if I could figure out a way to build an antler which would fit in the space I have available which would mount enough Hoffman 76s to do a decent job of venting from a cold start...

Dave in QCA

Correction regarding Vapor Stat

In (b) above, you commented that the vaporstat was useless because it would cause the boiler to run when it went into vacuum. 

This is not correct.  The vapostat is a limiting device, it will not cause the boiler to operate by itself.  What it will do, is prevent the pressure in the boiler from going above the setting, regardless of the call for heat from the thermostat.  So, if the thermostat is calling for heat, and the pressure in the boiler rises to the set point of the vaporstat, the burner will shut off until the pressure falls to the cut-in point.  At that point, the burner will again start to fire.  When the Thermostat becomes statisfied, the burner will shut off.  The Vaporstat will not start the burner.

Thus, the vaporstat is a very critical device that prevents the pressure in the boiler from rising above 8 oz, or whatever the set point is.  If your system is very well matched to your building, and the boiler is closely matched to the radiation, the vaporstat may rarely or never have to do its thing and shut the burner off.  But, if someone came along, and decided to turn off the radiators in half the house, you would quickly find out just how important it is.