I had originally written this post on another thread on hoffman #2 vents. However, there seems to be quite a lot of confersation on vacuum systems in the last few days and I thought I'd repost this as it might have gotten lost.
Thoughts on the Subject of Vapor Vacuum Heating
Is Vapor Vacuum Only For Coal Firing?
I think part of the mystery of vapor / vacuum has to do with coal firing, how it worked and how it was regulated. Coal burning in a boiler or a furnace with reasonable good quality draft and check dampers did not burn uncontrolled like wood or coal in a fireplace. While some of us are familiar with how wood can be controlled in an airtight stove, coal is much more easy to regulate, especially after the initial gassing period when all that is left is a glowing pile of carbon. The draft can be shut down and the burning coal will nearly extinguish, producing little usable heat at all. It may appear as if the fire has gone out, but a small core of burning coal will remain in the center. A fire can be held like this for quite a period of time, 30 minutes to several hours. When the draft is thrown open, within a few minutes, the entire mound of coals will again glowing bright red with red - blue flames licking up out of it. A coal furnace, stove, or boiler can essentially be turned off and on by the movement of the draft.
Dunham actually shows diagrams of electrical control of coal-fired steam boilers where a room thermostat operated a damper motor, connected to the both the draft and check dampers by little chains. This would allow completely automatic control of the firing of the boiler and would produce steam cycles not unlike what we see today on gas or oil fired boilers.
On most other vapor systems, I see special controls that operate off of very low pressures, say 8 ounces or below. Some of these systems describe that steam is maintained in the mains at all time and the regulation of heat in the building is via the proportional radiator valves. Thus, if it were mild outside, the rad valves would be mostly closed, or perhaps all closed. This would cause the boiler firing to adjust down to a very low point, that held a few ounces of steam pressure in a static condition.
Even in these types of automatic damper control, there is the ability for manual adjustment that would turn down the draft and let pressures drop.
Steam vs Hot Water vs Vapor/Vacuum
In the time around 1890 - 1900, the options for quality heating were pretty much between steam and hot-water. Steam had the reputation of being noisy, hard to regulate, annoying vents, and in many cases, the heat was on too much of the time. When the steam went down, the radiators cooled quickly, resulting in the feeling that the heat was off, and a phenomenon known as "cold 70". Thus, the on/off character of steam was seen as undesirable. Hot water was mild and even. But the downfall of water was that it was slow. If you let the space cool down, it took a long time to get the system warmed up and even longer to get the space warmed up. The radiators were also out of necessity, quite large, and the initial cost of installation was high. And so, the battles between cheap hard to control steam and expensive comfortable hot water waged on.
Then, around 1905, someone realized that if you let a steam system fall into vacuum, you could boil water at a lower temperature, the steam would not be as hot and the same for the radiators. With the ability to modulate the temperature of steam, a vapor / vacuum system could mimic the operation of a hot water system, with its inherent ability to produce modulating radiator temperatures. Also, since many vapor systems operated with a separate return line, there was no need to have a hissing vent in your bedroom or living room. Since the temperature of steam could be reduced, the piping losses to unheated spaces were reduced as well. Most importantly, it seems that vapor/vacuum operation provided a much better ability to control the delivery of heat to the space and thereby prevent overheating and wasted fuel. The ability to improve distribution meant the the heated space was much more balanced, and this too prevented overheating of individual rooms and the waste of fuel. And, in addition, unlike slow hot water systems, vacuum/vapor could be up to temperature and steaming in a matter of minutes.
Many Vapor Vacuum systems and components were invented by numerous engineering companies and they flooded the market. It must have seemed like a modern miracle, that steam heat, which had the public misconception of high pressures and danger could actually operate under a vacuum and cooler than the temperature of boiling water! WOW! A Modern Miracle!
Now, 60 years after most coal boilers were taken out of service or converted to natural gas and oil, we find ourselves in a time where we wonder about the knowledge of the dead men. Dan Holohan has done an amazing job of learning what the dead men knew and of compiling about every piece of published information on the subject of steam heat that ever existed. He has put it all together into a packaged form that makes it easy to understand, and it's organized at our fingertips. Absolutely Amazing! I cannot begin to explain the importance of his work,not only in the present, but even more so in the future.
But, there is something that gnaws at my curiosity, and it will continue to do so until I am able to experiment on my own, make observations and tabulate findings. This is on the subject of Vapor Vacuum. I know that Dan does not make stuff up, and neither to the knowledgeable pros, who contribute so much to this site. But here is the reason that I am skeptical about the accuracy of the statement, Vapor/vacuum is for coal only". It seems to be an accepted truth. But, Dunham very clearly stated that their vapor vacuum systems were good for any kind of firing, whether coal, oil, or gas. A number of other vapor/vacuum equipment producers said the same thing. Were they wrong? Were they lying? I doubt it. Was the idea that vacuum operation would save money, just an elaborate marketing hoax? I don't think so, it sounds to logical.
So, where did "vacuum is for coal" come from? I know that Dan doesn't make stuff up because I have accidentally found a number of his citations and specific information in other old and hard to find publications. Some of them I found in the Heating Help library and others in Google Books or other places. The thing that is missing, or at least that I have been unable to find, is factual justification of the statement "vapor vacuum is only for coal." But, I have found one statement to this effect. It is not substantiated, and no explanation is given as to why. But, in the Hoffman Specialty vent selection guidelines it states,
"Determine if the vent is to be installed in a vacuum system. The Model 76 Main Vent is for vacuum service.
It should be used on systems with a vacuum pump or a vapor system with a coal or wood fired boiler.
Systems converted from coal or wood fired to oil or gas should use non-vacuum vents such as the Model 75."
Perhaps this is the source of the commonly accepted truism? I don't really know, but it's the only source I have been able to find.
So, if this is the source of this directive, why would they say it? Perhaps, they thought that with very precise thermostatic controls that were now available, combined with the use of the #75 main vent and #40 radiator vent, that they felt that very even delivery of steam throughout the system was now possible, even on a very short steam cycle in moderate weather where only one or two sections of a 15 section radiator would heat. Perhaps, because they were so confident in their vents combined with good thermostatic controls, that they thought temperature control would be perfectly acceptable. If that is what they were thinking, they were pretty much correct. And since fuel was so cheap back in those days when the conversions were taking place, the added economy of keeping the vapor vacuum system in good operation seemed unimportant, and things could be simplified by forgetting about vacuum altogether.
Am I right? I don't know. Maybe? Maybe not! But, over the next year or two, I'm going to give it a good try, and I'm going to get my 1909 Dunham system to operate in a vacuum and see how it works.
Assuming that my guesses as to why Hoffman made the statements that Vacuum doesn't run on gas or oil unless there is a vacuum pump are right. I have to remember that much of the savings produced by vapor/vacuum operation were because of good control of the heat and the elimination of waste. But, there are a few other factors that are left on the table.
When steam operates at 212, every time steam comes up, it has to raise the distribution piping to 212F. Even on insulated piping, the losses of pipes at 212 are going to higher than losses at 180F. The efficiency of the boiler, boiling water at 212 is going to be very slightly less than boiling water at 180F. And then, there is the whole venting thing. So much effort goes into getting the air out of the system as fast as possible to that the steam can travel around unimpeded. Then when the boiler shuts off, we want to relieve the vacuum as fast as possible and get that air back in there... and we do it as fast as possible. Every time the air comes in, we just have to take it out again. Every time that atmosphere comes in contact with water, it oxygenates it and causes corrosion. The CO2 in the air also forms a very week dilute amounts of carbonic acid, that causes corrosion in the wet piping. Why are we so eager for this to occur? Why not have vapor vacuum with gas and oil?
It seems to me, that we may wait along time for the boiler companies to give us what we want in the form of 3 pass gas fired boilers with all heating surfaces below the water line, and so many other improvements that would improve the economies of steam heat. These things should be done, and will be done, but when, is the major question. In the mean time, it appears to me that vapor/vacuum is worth another look. We know we're not going to find 45% savings like some of the old brochures stated. We have probably got most of that just by having better controls. But, we don't have all of the savings that vacuum operation can provide, and it seems to me like it would make good sense to go after it.
We've got some pretty darned smart pros on this site, and of course Dan ain't chopped liver either! These people are the leading minds in American steam heat today. Look at what has been accomplished in new advancements such as dropped headers, steam mini-tube, maximized venting, actual tabulated flow capacities of dozens of vents, main vents, and traps. etc.
I accept that my thinking on this subject may be all wrong, and there is not one penny to be gained. I also accept the possibility that more study and thinking on the subject is in order. And who better to do it than all of the people who contribute to this site and work on steam heat.
Dave in Quad Cities, America
Weil-McLain 680 with Riello 2-stage burner, December 2012. Firing rate=375MBH Low, 690MBH Hi.
System = Early Dunham 2-pipe Vacuo-Vapor (inlet and outlet both at bottom of radiators) Traps are Dunham #2 rebuilt w. Barnes-Jones Cage Units, Dunham-Bush 1E, Mepco 1E, and Armstrong TS-2. All valves haveTunstall orifices sized at 8 oz.
Current connected load EDR= 1,259 sq ft, Original system EDR = 2,100 sq ft Vaporstat, 13 oz cutout, 4 oz cutin - Temp. control Tekmar 279.http://grandviewdavenport.com