Vent placement in home 2-pipe steam heat system

mattmia2 · June 16

If you could actually make it leak free then some low pressure refrigerant would probably make more sense than water.

Jamie Hall · June 16

Actually in many ways water is an excellent refrigerant if you don't mind working at sub-atmospheric pressure. The latent heat of vapourization is remarkably high, so the mass of refrigerant required for a given power is low, and it's non-toxic, doesn't do global warming, etc. It does, however, have one problem. It freezes…

mattmia2 · June 17

https://forum.heatinghelp.com/discussion/comment/1858049#Comment_1858049

Also quite corrosive if there is oxygen available.

Jamie Hall · June 17

https://forum.heatinghelp.com/discussion/comment/1858052#Comment_1858052

true. In fact, there are remarkably few things that water won't attack, given an oxidizer such as oxygen and the right pH… It's about as close to a universal solvent as there is (although it can be slow).

pacoit · June 20

Pardon my absence.

Modulating boiler. That's what it's called? Yes, in my idea, the boiler would have adjustable gas flow, and then a vacuum pump (used to top-up the vacuum from time to time), and then some sensors and instrumentation, and a micro-controller programmed to modulate the base vacuum (i.e., set the boiling point as low as possible while still providing the space heating requirement). The concept is to minimize the differential in water temperature rise each cycle, and so reduce fuel. This goes back to previously discussed, e.g., 1/2 the temperature rise for 2x the time will require 1/2 the kinetic energy and therefore 1/2 the fuel. Perhaps it's sort of like emulating to some degree the continuous heat of old coal-fired systems. Or up to "always on system"—as you mentioned.

There is still the heat lost to the flue as a general inefficiency of today's boilers to resolve. The vacuum system (lower boiling point) would reduce flue heat loss. Seems also that boiler (burners) could be comparably smaller. Flue diameter could be reduced.

So, the amount of heat needed to heat a space is what it is. But the amount of fuel needed depends on the efficiency of the mechanism used to generate that heating. Still pondering.

mattmia2 · June 20

most of the energy is in the latent heat of vaporization. there is comparatively little energy in the specific heat of the water or the steam. vacuum won't change your fuel usage much.

the delta t affects the transfer rate from the flue gasses to the hx but the difference between 180 and 220 f degrees isn't going to be big. the bigger efficiency gains are made with stainless heat exchangers and low return water temps that can condense the latent heat of vaporization out of the products of combustion but even that is only another 10% or so

large commercial boilers have modulating gas trains and draft dampers. modulating residential size steam boilers aren't manufactured.

Jamie Hall · June 20

half the temperature rise for two times the time…

First off, you are not talking about kinetic energy here. You are talking about thermal energy. A very different matter.

The amount of heat energy — from the fuel — to raise a certain amount of water from one temperature to another is dependent entirely on the temperature difference and the mass of water. Further, the amount of heat energy — from fuel — to evaporate a certain mass of water is a constant and depends on the pressure at which vapourization occurs. In neither case does time have anything to do with it.

I encourage your pondering — but get the physics and thermodynamics straight!

pacoit · June 21

Thanks. As I understand it, kinetic energy applies to mechanics—-to which we are agreed, but also applies to temperature, for example, the kinetic energy in a gallon of water rises as the temperature rises.

I agree that the amount of heat needed to heat a space is what it is (the output). But the amount of energy/fuel needed (the input) depends on the efficiency of the mechanisms used to generate that heating. That is what I'm trying to investigate.

In the case of a small propeller spinning fast to move a large boat, a small column of water is accelerated to a speed much faster than the desired boat speed; there is the inefficiency of high propeller slip; and also of the high generated differential speed of the water column, impacting energy requirement by the square of the velocity. So, given a larger propeller driven by an efficient lower speed, high torque motor, that pushes a larger column of water at a speed equal to the desired boat speed (plus a little more), it would reduce slip and require much less energy to accelerate the water to a lower speed, and so be significantly more efficient. The kinetic energy input needed to create the same overall required thrust would be less.

So, perhaps I'm wrong (still studying), but I see an analogy with the operation of a typical (e.g., single family low pressure steam boiler). It burns fuel fast; much heat goes out the flue before it can be transferred to the water; it works to raise the water temperature beyond 212 deg in order to heat a space at 65-70 deg; it works to push air out of the piping each heating cycle. So, my theoretical notion is that generating steam at as little as possible above desired space/room temperature for longer (or continuous) heat cycle, using modulating boiler, etc., should be more efficient, less input for the desired output.

I brought up boats and propellers because mechanical things are fairly easy to visualize, make tangible. A volume of water with faster/slower moving particles as measured by temperature, not so much.

mattmia2 · June 21

Both in absorbing heat from the products of combustion in to the heat exchanger and from the heat emitter in to the air in the room, the rate of energy transfer is a linear function of the temperature difference between the gas and the surface.

In the case of the heat exchanger the products of combustion are something like 400 or 500 degrees f and the hx is around 200f.

In the case of the room, with the emitter around 210 f and the room air around 70 f the delta t is around 140 f degrees. If you change the delta t to 5 f degrees then you need a whole lot more surface area to transfer the same amount of heat, like making the floors, walls, and ceilings radiant panels.

Jamie Hall · June 21

It is true on a certain level that one can conceptualize heat energy as the kinetic energy of the molecules of the substance. But only up to a point. There is a lot more to heat energy that just the moving molecules — and the analogy disappears almost completely in the case of solid or liquid phases.

You are, frankly, delving very deeply into thermodynamics here, and I hope you have a very solid background in it.

Now your thought that minimizing temperature differences in a heat transfer system increases the efficiency is, again, broadly correct. But only broadly. The overall thermal efficiency of most combustion based heating systems, however, is governed not by the temperature of the heat transfer medium but by almost unavoidable losses: the temperature of the exhaust must be greater than the temperature of the intake air. With proper design of the overall system (not just the boiler), it is not al all difficult — as we see in condensing boilers — to achieve upwards of 95% of the higher heat of combustion of the fuel appearing in the output heat transfer medium. In steam heat transfer systems, it is simply not possible to get there, but in low temperature vacuum assisted systems it could, in principle, be done (I might note that in steam power work, it is possible to get very close to the higher heat of combustion out as heat energy, but not as useful work — thermodynamics again).

However, the system — not just the boiler — has to be designed and built to make this work and, while this may be feasible it may not be practical.

pacoit · June 22

https://forum.heatinghelp.com/discussion/comment/1858397#Comment_1858397

Thanks. I think we agree. Assuming an pre-existing radiating surface (less than optimum), it can be mitigated by up to always on, and regulating steam temp up (via vacuum control) on colder days as needed.

pacoit · June 22

@Jamie Hall , Thanks. I guess I'm trying to understand the sort of things you alluded to. I'll work on it.

Mean time, maybe I should steer this thread back to the immediately practical. I calculated volumes of mains pipe sections to determine vent sizing, positioning, and equal venting speed. I used

https://heatinghelp.com/assets/documents/Balancing-Steam-Systems-Using-a-Vent-Capacity-Chart-1.pdf

for the venting rates of vents. But should I base vent rates calc on 1oz of pressure or 3oz or more? My basic 1997 boiler pressuretrol (I think) cuts in at min (.5psi?) . Should I assume that supply system pressure is always at or above .5psi when system is on (boiling)? At what pressure should I calc venting rates?

Also, what time frame for venting the system should I strive for (single family home), 1min? 2min?

mattmia2 · June 22

1oz would be more conservative. there will be virtually no pressure if the venting is large enough. the progression of the steam will be limited by both the venting ad the time it takes to heat up the mass of the pipe. the steam will condense to water until it heats the pipe at the front of the steam to steam temps in infinitesimally small increments until it gets to the end of the pipe. you can't have too much venting but the time it takes to heat will be limited by the mass and size of the pipe once your venting is large enough.

Jamie Hall · June 22

The pressuretrol just tells the boiler when to shut off. As @mattmia2 said, most of the time the system pressure will be much less, if the venting is adequate. Most of us have found over the years that what happens is that when the boiler starts to steam the pressure will rise to an ounce or two per square inch gauge (maybe as much as 4 ounces) — and then just sit there until all the radiators are full and the vents are all closed. Then the pressure will start to rise again since more steam is being produced than there is capacity to condense it, and the pressuretrol will shut the boiler off until the rest of the system can catch up.

pacoit · June 22

Ah, ok. The pressuretrol cuts in when pressure falls below, e.g., .5psi; but it doesn't regulate system pressure (except to shut off the boiler if cut-out pressure is reached). The pressure in the system depends on the system design and may or may not ever reach .5psi.

And, since normally the pressure never reaches cut-out level, e.g., 2psi, the pressuretrol switch is always closed (on), which means the pressuretrol doesn't do anything (in normal operation)?

So, in my (single family) system, it is reasonable to expect, as suggested, 1-2 or 3oz pressure during heating while vents are still open.

pacoit · June 22

To aid venting (including mains), does it make sense to set fully open all 19 radiators Hoffman 1A, and then use radiator supply valve to control room temperature? (I'll still be adding mains vents.)

Steamhead · June 22

https://forum.heatinghelp.com/discussion/comment/1858441#Comment_1858441

Do your main vents first. Then see if any radiators are too fast or slow, and adjust accordingly.

mattmia2 · June 22

Since condensate drains through a separate return pipe you can balance the radiators by how far you open the supply valves.

Steamhead · June 22

True, but we can use the vents to establish a baseline, where all the rads heat evenly. Then, if you want to cut some back, you can do it with the shutoff valves.

pacoit · June 22

Yes, of course, I would start with fully open vents; then close a bit any fast heating outliers to get even heating. The thought is to set the Hoffman 1A as open as possible within the realm of even heating.

mattmia2 · June 22

https://forum.heatinghelp.com/discussion/comment/1858446#Comment_1858446

Balancing with the valves has the advantage that it is a constant rate. It will admit a relatively constant proportion of steam (depending on the pressure of the system at that time) no matter how long the cycle is.

If you balance it with the vents you are simply delaying how long it takes the radiator to heat fully. How much the slower radiators heat will vary by cycle length and as the cycles get longer imbalances in the radiator sizing will become more apparent. You can still do a decent job of balancing with vents and it may not matter if the system is reasonably well designed but balancing with the valves gives a more consistant result over a wider operating range.

Balancing with valves will have more variability in short cycles and balancing with vents will have more variability in long cycles.

Maybe start with sizing the vents based on radiator and runnout volume then tweak it with the valves.

Steamhead · June 22

Establishing a baseline is exactly what the Dead Men did when they installed Vapor systems. Each shutoff valve had a shutter or other limiter or orifice that limited the maximum steam input. This was matched to the size of the radiator. The exterior handle could be turned down if less heat was wanted, but if all the handles were wide open, the system heated evenly.

No reason we can't do that here.

pacoit · June 23

Thanks. I'd like to estimate the volume of the radiators to calc venting requirements. I could detach a radiator, and see how much water it takes to fill it. But is there a way to estimate rad volume from, say, EDR? I calculated my EDR at 700.

I tried using values from the link below. But I get value of 9.4 cu.ft or double that, either of which seems high?? It dwarfs the mains volume I calculated at ~4cu.ft.

https://www.gwgillplumbingandheating.com/webapp/p/237/radiator-capacity-chart#

mattmia2 · June 23

You can find the catalogs in the library here for some of the old radiators to get an idea of their volume to see if your estimates make sense. It will tell you how many gallons of water it holds, usually per section. US Boiler's radiator spec sheets will show the volume of their radiators too.

I think there are some rules of thumb of vent size per EDR of the emitter but others will know better here.

Remember that the steam can't move any faster than it can heat the radiator, it has to heat the mass of the cast iron to steam temps before it can proceed so that will be the limiting factor in the venting speed after a certain relatively small vent rate.

pacoit · June 24

Hey, my living room window seat radiator! 11 sections. 44sq.ft radiation: https://www.heatinghelp.com/assets/documents/nira-window-seat-rad-chart.pdf

Other radiators are all Pierce, Butler Co. unadorned two column, 20" tall jobs. But I haven't found any volume data on either. I'll look for other companies that have similar sized radiators and also volume data.

But does it ring true that the volume of radiators in a system is commonly 2x+ the volume of mains piping? If so, shouldn't typical radiator vents be much bigger?

mattmia2 · June 24

the fact that the emitters are designed to emit heat and they have more mass means that they will heat more slowly than the piping so faster venting won't help them heat faster beyond a certain point. the vent size can slow heating to balance spaces but it won't make it heat faster.

Steamhead · June 24

@pacoit , radiator air volume can be found here:

https://www.heatinghelp.com/systems-help-center/balancing-steam-systems-using-a-vent-capacity-chart/

You'll have to scroll down a ways, but they're there.

Jamie Hall · June 24

I think it has been said, but…

First, get the mains well vented. The objective is to get steam distributed to the radiation throughout the building as evenly as possible.

Second, the radiator vents for one pipe steam, or for a steam system where condensate, but not air, can is returned separately, are used to control how fast a given radiator will heat up and thus, to a considerable extent, how much heat per cycle it will put out.

Third, it is not necessary — and often not desirable in a space which has more radiation than needed for the space — for a radiator to heat fully across. The criterion is is the space comfortable, not is the radiator hot all the way across.

pacoit · June 24

@Steamhead , Thanks, I missed that last page when I first read the document.

So, I have (my house):

System works but gas bills are very high. Time for rads to BEGIN warming is ~10 min. Presume insufficient venting.

262,000 BTU input gas boiler (Carrier/Dunkirk)
3,400sf house, circa 1910s
Have unusual (I think) wet return system, near basement floor, with 1 return pipe per rad. Also 1 supply pipe per rad. So rad returns are isolated by water in wet return. All rad vents are in basement high on each rad return.
700 EDR rads (19 20" 2-column cast iron, 350 sections, 0.025 cu.ft/EDR)
~17.4 cu.ft of rad volume
~1.54 cu.ft of rad supply lines
~3.8 cu.ft of header/mains
~22.75 cu.ft of system volume to evac air from
3 mains and 2 Tees lead to 5 end points (1 ~50ft and 4 10-15ft). So should need 5 mains vents.

Current venting is 19 Hoffman 1A, 1 per rad. Combined provide 1.45-2.75 (pos 1-6) CFM @1oz. press.

So, if I want to evac all air in, say, 1 minute @ 1oz pressure, I would need 3.82 CFM venting for mains, and then an additional ~1 CFM vent per radiator. Do these numbers seem about right?

mattmia2 · June 24

You're not going to be able to do it in 1 minute because the steam needs to heat the pipe for the steam to progress in the pipe. It will happen faster when the mains are still warm from the last cycle, but it will take several minutes to tens of minutes for the steam to heat the main sufficiently all the way to the end.

Jamie Hall · June 24

Well… the math is OK. The physics isn't. There is simply no way that you need to — never mind can — evacuate the mains in 1 minute. Why not? Because the steam front in the mains simply won't move that fast, unless the pressure was very high. The reason is that the steam has to heat the main to steam temperature before it can move on. Depending on the insulation on the mains, figure 15 to 20 feet per minute isn't a bad ballpark.

You do need 5 main vents. Given the size and length of your mains, a Gorton #2 at the end of each main should be more than sufficient. In fact, probably overkill for the shorter mains.

As I said earlier, the radiator vents control how fast — and in some cases to what extent — the radiator in each space heats up. They need to be selected and adjusted so that the right amount of heat gets to each space. The Hoffman 1A is adjustable over a nice wide range — though a bit fiddly — so they can stay, starting on a low setting and opening them a bit as needed if there are spaces which need more heat. This is somewhat of a trial and error process.

Steamhead · June 24

Assuming the steam mains are 2" diameter ( @pacoit , is this correct?) you have the following air volumes:

50' main: 1.15 cubic foot- start with a Gorton #2

10-15' mains: average of 0.29 cubic feet- start with a Gorton #6 on each one. You can get these in either a horizontal or vertical pattern (but see below).

I would first install the #2 on the 50' main and see how it works by itself. I've seen cases where a comparatively short main will vent more quickly than expected without a main vent. In that case, venting the short mains would throw the system out of balance. You want the steam to reach the ends of all the mains at roughly the same time- this way, when the vent closes, each tee joint feeding a radiator line has equal access to the steam, and the system is balanced or nearly so. If venting the long main makes steam get to the end long before the short ones, then vent the short mains.

Jamie Hall · June 24

Go with @Steamhead 's recommendation… he's about the best we have.

pacoit · June 24

Thanks everyone. So, I figured evac in 1 min @1oz press would be considered ambitious. How about 3 min? And is assumption of 2oz press on average during evac period less reasonable or more reasonable than 1oz?

@Steamhead , Interesting the, start with Gorton #2 on long section and see what happens. Some details.

Large main (1) has 16' 3" section that tees to a 2" 10' and 2" 50' sections.
Large main (2) has 12' 3" section that tees to 2 2" 15' sections.
Small main (3) has a 12' 2" section. (connects to only 2 radiators)

Header is huge, 16" (x2) of 2.5" verticals to 4' of 4" horizontal. (Interestingly, boiler was raised 9" to mate with old boiler piping).

Jamie Hall · 12:42AM

The problem — once the venting has reached a reasonable level, which @Steamhead has suggested — is not "evacuating" the piping. It's persuading the steam to travel down the piping.

As I said before, with any pressure at all — 1 ounce is more than adequate — the apparent velocity of the steam —- defined as how long it take steam to get to a particular point on a main — is determined entirely by how fast the steam can warm the pipe it's travelling in. This is true even in high pressure power systems, though less obviously so.

It will travel no more rapidly in a pipe open the atmosphere at one end than in a pipe with a vent of any reasonable size.

Try and remember that saturated steam — what you are playing with — is at the temperature at which it boils and condenses. If it comes in contact with a surface which is below that temperature it will condense and move no farther. Once the pipe is up to temperature, the steam will pass with only enough condensing to keep the pipe warm.

Steamhead · 1:15AM

https://forum.heatinghelp.com/discussion/comment/1858642#Comment_1858642

Interesting. This may require some experimentation. Start with the #2 on the 50-footer, and see how the system responds, then add venting as needed. This will avoid buying lots of vents that you don't end up using. For perspective, if I were coming there to vent that system, I'd have a full stock of vents on the truck, so I could swap them in and out as needed. If any got installed and then swapped out I'd just use them elsewhere. But you don't have that luxury, so that why I'm saying start with just the #2.

On the 3" sections, the 12-footer has 0.636 cubic feet and the 16-footer 0.848.

mattmia2 · 1:37AM

of course you also have the option to use the radiator valves to balance it, if those aren't stuck or leaking, that is free.

pacoit · 2:44AM

@Steamhead , Thanks, It's all adding up nicely in my head.

@Jamie Hall , @mattmia2, The idea that steam can't progress until the pipe walls are heated bothered me a bit on finer points, but I get the meaning.

But just to clarify for me at the more detailed level, I see a lot of variables involved in the theory of steam progressing through a pipe. There is the rate of steam generation at the source; its temperature; the temperature differential in the piping; the resulting pressure differential including from the resistance in the pipe; pipe diameter; probably more. But it seems to me the steam front can move ahead of walls still not heated up fully, to some extent, if the rate of steam production is sufficient, especially in a larger pipe (more volume of heat to pipe surface area). But, yeah, to a limit.

One case where the limit would seem more generous I think would be in a vacuum system. Steam would shoot through the system much farther ahead of pipe still warming up, all dependent on the steam production rate to replace condensation, and all the other variables mentioned above.

How am I doing?

pacoit · 2:48AM

@mattmia2 , Agreed. But the radiator valves (edit: I mean vents) are working hard already, I think. So, I would try to increase and balance the main venting as much as possible so I can leave the rad vents as open as possible.

mattmia2 · 3:08AM

If more steam is produced it will progress faster, but the pipe being cooler than the boiling point of water is going to condense the steam to water until the energy absorbed by that condensation gets the pipe up to steam temp it won't push past it but if there is more steam coming to that point there is more energy there (and the steam is hotter and under more pressure). the steam is pulled along by the steam collapsing. when it collapses it contracts 1700 times so more steam is pulled in to fill that space. something collapsing the steam, usually a pool of trapped water is one of the 2 major causes of a part of a system that won't heat. the water kills the steam. in many cases the water eventually evaporates and that section heats later than the rest of the system.

The other major cause of a part of a system that won't heat is air that can't get out so the steam can get in.

The radiator valves can be used for adjustment, they can throttle how much steam is let in to the radiator(not the air vents). In a 1 pipe system the condensate has to drain out of the inlet so if you partially close the valve the condensate ends up getting trapped in the radiator. since you have a separate return for the condensate you can partially close the valve in rooms that overheat to reduce the amount of steam that emitter gets without trapping condensate.

Vent placement in home 2-pipe steam heat system

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