Vacuum vs Vented Steam Delivery Test

PMJ

I ran a test with and without vacuum.

When air is what fills the void created by collapsing steam every time the burner goes off, steam delivery to the radiators stops instantly. When air is not allowed to fill that void, steam already in the mains and boiler continues to flow to the radiators.

I ran my system with and without vacuum to illustrate the difference. Bear in mind that my system does a controlled fill - a timed amount from a satisfied remote temp sensor so the starting fill place was the same. The next burn is initiated when that same sensor opens again so it is an exact measure of how long it took the radiator to cool in the same conditions.

In vacuum the radiators cooled to the restart temperature after a burn in 18 minutes. Without vacuum they fell to the restart temperature in 14 minutes. The reason for the difference is that in vacuum conditions the lowest pressure in the system when the burner goes off remains the radiators and steam continues to flow from the mains into them. When air is allowed back in to radiators through vents either directly or through the dry return it stops steam flow to them instantly.

The point is that vacuum results in more of the heat produced in each burn actually getting delivered to the living space per minute of burn time. Considering 4 minutes at the rate all the radiators are condensing steam this is a significant amount of heat. Without the vacuum that same heat came out of the steam sitting in mains and boiler but went some place other than where you wanted it to go. Not a total loss for sure, but clearly not the desired result either.

Vacuum also has the advantage that at the start of every new burn the conditions are the best for new steam delivery. The system is sitting in its deepest vacuum state so there is no air to remove, and the temperature required to make steam and support its flow in the piping is lowered from what it would be at atmospheric conditions. The steam flowing during the burner off time is also favoring the radiators in the colder areas which evens out the heat.

Radiator pictures attached. I'm not an expert in FLIR but the radiators were painted different colors perhaps accounting for the difference in the peak values. Bottom line time difference was detemined by the temp sensor anyway.

Long Beach Ed

I did very similar experiments 20 years ago, reproducing the Paul system in a 300 sq ft. house. My savings were about 30% in fuel with nearly instantaneous steam distribution.

guzzinerd

So how does one switch from a vent to vacuum system?  Why do vents still exist if vacuum is so much better?  Is it a cost and maintenance issue?

ChrisJ

Long Beach Ed said:

I did very similar experiments 20 years ago, reproducing the Paul system in a 300 sq ft. house. My savings were about 30% in fuel with nearly instantaneous steam distribution.

Why is 30% always the number used?

I could see 1 to 3% but 30%?


So before you added vacuum your system was only delivering 50% efficiency and simply adding vacuum got it up to 80%?

ChrisJ

guzzinerd said:

So how does one switch from a vent to vacuum system?  Why do vents still exist if vacuum is so much better?  Is it a cost and maintenance issue?

This is a tough subject.

I say it's impossible on a single pipe system because you loose the only way you have to control distribution but pmj says it's not.

 I think pmj has a two pipe system.  

PMJ

ChrisJ said:

guzzinerd said:

So how does one switch from a vent to vacuum system?  Why do vents still exist if vacuum is so much better?  Is it a cost and maintenance issue?

@chrisj said: This is a tough subject.

I say it's impossible on a single pipe system because you loose the only way you have to control distribution but pmj says it's not.

 I think pmj has a two pipe system.  




Actually I merely quote George Hoffman who said it was possible and who manufactured and sold radiator vacuum vents to do it. He seems a credible enough source to me. The publications he produced on the subject make sense to me. I make no claims to know anything from personal experience with one pipe to prove anything one way or the other. I do know I'd be trying it if I had one pipe.

I do have a two pipe system. The conventional wisdom 30 years ago was that vacuum would be problematic and of no value without a coal fire. From what is now a lot of personal experience I found that wasn't true. I have found that the physics are the same banking a coal fire a little or in the case of intermittent fire a lot.

Long Beach Ed

I was replicating the Paul System, where a plastic or metal 1/4" line sucked the air out of the radiators using a pump. The 30% savings was in mild weather, which New York has lots of. Distribution was greatly accelerated, so 15 minutes distributing steam to radiators was reduced to five. I suppose you can call that a two-pipe system.

PMJ

Long Beach Ed said:

I was replicating the Paul System, where a plastic or metal 1/4" line sucked the air out of the radiators using a pump. The 30% savings was in mild weather, which New York has lots of. Distribution was greatly accelerated, so 15 minutes distributing steam to radiators was reduced to five. I suppose you can call that a two-pipe system.

If I had a one pipe system based on what I know about vacuum operation it would be worth a small fortune to me to snake those air lines to a central manifold in the boiler room. Pumped or not the flow control possibilities would be endless - vastly superior potentially to my two pipe system.

ChrisJ

PMJ said:

guzzinerd said:

So how does one switch from a vent to vacuum system?  Why do vents still exist if vacuum is so much better?  Is it a cost and maintenance issue?

@chrisj said: This is a tough subject.

I say it's impossible on a single pipe system because you loose the only way you have to control distribution but pmj says it's not.

 I think pmj has a two pipe system.  

Actually I merely quote George Hoffman who said it was possible and who manufactured and sold radiator vacuum vents to do it. He seems a credible enough source to me. The publications he produced on the subject make sense to me. I make no claims to know anything from personal experience with one pipe to prove anything one way or the other. I do know I'd be trying it if I had one pipe. I do have a two pipe system. The conventional wisdom 30 years ago was that vacuum would be problematic and of no value without a coal fire. From what is now a lot of personal experience I found that wasn't true. I have found that the physics are the same banking a coal fire a little or in the case of intermittent fire a lot.

So you feel all salesmen are a credible source?

Just because someone sold a gimmick a long time ago doesn't make it any less of a gimmick today.


Not to mention vacuum doesn't get you free heat from a burner that shuts off regardless.

If you manage to suck something extra out of the block and water you're just going to have to put it back next cycle.

Vacuum lowers the boiling point it doesn't magically create free energy.

Steamhead

@PMJ , where is the vacuum pump connected on your system? Did you use an existing central venting point?

PMJ

I'm looking for where I made a claim about free energy but didn't find it.

The subject is comparing burner run time to the time steam is actually being delivered to the radiators. Air works against that project plain and simple. The ratio of actual delivery time at the radiators to burner run time is better without air to remove. If that weren't true then it would never make sense to pay extra to run a pump to remove air in a Paul system or any other.

In Lost Art starting on page 104 Dan H. gives what shall I say is a more favorable view of George Hoffman and his knowledge of venting. It would appear that his accomplishments in the industry and with vents in particular did distinguish him just a little from "all salesmen". I'll be keeping George's stuff in my library on steam heat.

Chris_L

@PMJ, I am more interested in the boiler time on than the time it takes radiators to cool.
Do you have data on the actual firing time with and without using vacuum for the same thermostat set point and weather conditions?

Jamie Hall

There really are two fundamentally different approaches to using a vacuum in a steam system, and much of the confusion surrounding the concept arises from that.

One is the Paul system, and variations on it, in which a vacuum is applied to individual radiators by means of small pipes which lead to a vacuum "source" (it doesn't have to be a pump; a steam ejector will work, or a steam condenser). In many ways -- except for the vacuum lines -- these are one pipe steam systems, and, so far as steam distribution is concerned, need to be piped that way. The advantage to these systems is that it is very easy to control steam delivery to individual radiators. They also -- an advantage which may or may not be shared with the next type -- allow for more rapid (though still not instant) steam delivery once the boiler has started making steam -- which may be at a lower temperature than an all atmospheric system

The second type, which was often found on two pipe systems, uses the same distribution and piping as any two pipe system (including crossover traps) but, inntead of using regular vents which open on a vacuum forming, uses vacuum vents, such as the Hoffman 76, which, if the rest of the system is tight, will hold a vacuum (a more or less acceptable substitute can be assembled from very low cracking pressure check valves before conventional vents). These also allow the boiler to make steam at much lower temperatures, or may allow a boiler to continue making steam after the burner shuts off using the remaining heat in the block (with modern boilers, this is a rather dubious advantage). They also contribute to more rapid steam delivery, but not all that much. It is critical to remember when thinking about such systems, though, that the distribution is governed not by the absolute pressure of either the steam or the returns, but by the difference in pressure between the mains and the returns. Depending on how that difference is maintained, simply applying a vacuum to either the returns or the mains may result in balance problems.

One relatively little appreciated advantag%e of either approach is the opportunity to modulate the temperature at which steam is delivered to the radiation. Whether this is really advantageous, though, or just sort of a nice idea, is quite debatable. If the boiler burner is such that its output power can be modulated, this could be quite advantageous in that the system could maintain a constant burn and a constant temperature in the radiation. If the boiler burner must be modulated by turning it on and off -- as most steam boiler do -- the advantage is doubtful at best. In either case, it would be necessary to also modulate the vacuum in the system as a whole.. This can be done, of course -- but adds complexity.

I have to admit that claims of huge savings simply by introducing vacuum assist of either sort have to be viewed with some skepticism. A well tuned steam system, with a correctly sized boiler, should be capable of at least 75% overall efficiency from fuel input to overall system output, and the better ones should manage 80 to 85%, with the limitation being more from the fact that the boilers are, in general, non-condensing. Claims of much over 10% without other work on the system are, then, somewhat suspect.

PMJ

Chris_L said:

@PMJ, I am more interested in the boiler time on than the time it takes radiators to cool.
Do you have data on the actual firing time with and without using vacuum for the same thermostat set point and weather conditions?

@Chris_L , I have thought about running such a test, but running long enough each way in conditions sufficiently identical in Cleveland to draw conclusions is problematic, and convincing central command to go a long period without vacuum for a test is unlikely. I have never been one to claim a large efficiency difference like 30%. For me the main advantage to vacuum has always been in comfort - the system is shockingly quiet with no openings to the atmosphere in the living space. Eveness of the heat overall is related to the rate of change of the temperature of the radiators and as I explained above vacuum operation spreads this out a lot. It is a win win for me in comfort and knowing the operation is more efficient by some amount.

PMJ

@Jamie Hall , As you know I have never made claims of large energy savings. I know it is something, and the investment required in two pipe so small it is more of a why not for me. It is an equipment savings actually with no expensive vents to worry about.

In my two pipe system the return side of every radiator is connected to the same open pipe which then is all at the same pressure below atmospheric except at the very tail end of every burn. There is no mystery to it, and no way for any place to be dramatically different in pressure than any other place. As we have discussed before relative pressures in the system are basically the same as an open system with the burner on. It is just that the whole system pressure is shifted lower, and those pressure differentials within the system are maintained with the burner off, unlike the open vented system.

Jamie Hall

PMJ said:

@Jamie Hall , As you know I have never made claims of large energy savings. I know it is something, and the investment required in two pipe so small it is more of a why not for me. It is an equipment savings actually with no expensive vents to worry about.

In my two pipe system the return side of every radiator is connected to the same open pipe which then is all at the same pressure below atmospheric except at the very tail end of every burn. There is no mystery to it, and no way for any place to be dramatically different in pressure than any other place. As we have discussed before relative pressures in the system are basically the same as an open system with the burner on. It is just that the whole system pressure is shifted lower, and those pressure differentials within the system are maintained with the burner off, unlike the open vented system.

no argument with any of that.

PMJ

Steamhead said:

@PMJ , where is the vacuum pump connected on your system? Did you use an existing central venting point?

No pump. Just one 1/2" solenoid valve vent at the most remote point in the system in the garage. Same location as the temp switch I use to set fill. System gets to 5-6" Hg vacuum between cycles.

Last month call time 50% of total time, burn time 23%, and that vent was open 12% of the total time or 1/2 the time the burner is firing.

PMJ

PMJ said:

Steamhead said:

@PMJ , where is the vacuum pump connected on your system? Did you use an existing central venting point?

No pump. Just one 1/2" solenoid valve vent at the most remote point in the system in the garage. Same location as the temp switch I use to set fill. System gets to 5-6" Hg vacuum between cycles.

Last month call time 50% of total time, burn time 23%, and that vent was open 12% of the total time or 1/2 the time the burner is firing.

*by same location I mean in the garage but vent on dry return.

jumper

When boiler is off everything above water level line is air or water vapor? So applying suction to header evacuates to vapor pressure of water in shut off boiler. When burner goes back on then boiler has less work to do to push air (and vapor) through vents. That is an efficiency improvement.

In Toronto some designer/installer used city water powered eductor to generate vacuum that way. Maybe nowadays an air powered ejector is more acceptable? I can't say how efficient those installations were but installer certainly saved himself cost of vents and traps.