Vent placement in home 2-pipe steam heat system

pacoit

@mattmia2 , pardon me. You originally mentioned radiator "valves", and I understood and responded to radiator "vents". Yes, I agree with you 100%.

In fact, I currently balance the heating in the house by partially closing select radiators.

Steamhead

https://forum.heatinghelp.com/discussion/comment/1858653#Comment_1858653

You shouldn't have to do that. When all radiator shutoff valves are fully open, the house should heat evenly. That's our objective here. Then if you want to reduce or shut off the heat to a particular room, on your system you can do that with the radiator shutoff valve.

pacoit

@Steamhead , Agreed. I was just noting what I have been doing until now.

Something interesting. I just did a detailed assessment of radiators, EDRs, vents, supply piping, and how they are distributed along the 3 mains and their 5 end points. With this level of detail, the outsized long mains section is not so much the outsized culprit in the system as one might assume!:

Section		EDR	no. rads	supply pipe length	pipe diam.	pipe vol cu.ft	rad vol cu.ft	total vol cu.ft	vent @1oz CFM	evac time minutes
50ft SE L tee		289	8	66'	0.75"	0.202	7.23	7.43	1.16	6.40
10ft SE R tee		158	3	33'	0.75"	0.101	3.95	4.05	0.43	9.31
15ft N L tee		92	4	26'	0.75"	0.080	2.30	2.38	0.58	4.10
15ft N R tee		48	3	35'	0.75"	0.107	1.20	1.31	0.43	3.01
12ft W		87	2	15'	0.75"	0.046	2.18	2.22	0.29	7.66

Venting is based on no. of Hoffman 1A (no. rads) fully open (0.145 CFM). When proportionate header and mains piping volume and venting is added, note that evac times don't grow proportionately for each mains section. But not too big a deal…in this case.

+ mains time	+ header time	total evac time	total evac T / evac T
0.49	0.152	7.05	1.10
0.49	0.152	9.96	1.07
0.58	0.152	4.83	1.18
0.58	0.152	3.74	1.24
0.90	0.152	8.71	1.14

So, the obvious contender for bad guy ( the outsized 50' section with most rads—-and vents) ranks 3rd in air evac time.

Steamhead

Put the Gorton #2 on the 50-footer, and it'll help speed up the one that takes longest. According to the chart, both are fed from the same 3" main, and if the steam get to the end of that 3" faster, the other main gets steam faster too.

pacoit

So, I calculated the effect of overall air evac by adding a Gorton #2 at end of 50ft tee. The effect is quite small. As shown in the chart, the evac time from header, related main, and main tee (where vent is added) is reduced, but not in the related supply pipe and radiators—-as expected. And since the volume of related supply pipe and radiators is much greater than the related mains and header, evac at the radiators is only .7min less out of 7.99 min.

Before adding Gorton #2

Section	evac time minutes	+ main tee time	+ mains time	+ header time	total evac time	total evac T / evac T
50ft SE L tee	6.40	0.94	0.49	0.15	7.99	1.25
10ft SE R tee	9.31	0.50	0.49	0.15	10.46	1.12
15ft N L tee	4.10	0.56	0.58	0.15	5.40	1.32
15ft N R tee	3.01	0.75	0.58	0.15	4.49	1.49
12ft W	7.66	0.90	0.90	0.15	9.61	1.26

After adding Gorton #2 at 50ft SE L tee

Section	evac supl+rad minutes	+ main tee time	+ mains time	+ header time	total evac time	total evac T / evac T
50ft SE L tee	6.40	0.48	0.29	0.11	7.29	1.14
10ft SE R tee	9.31	0.50	0.29	0.11	10.22	1.10
15ft N L tee	4.10	0.56	0.58	0.11	5.36	1.31
15ft N R tee	3.01	0.75	0.58	0.11	4.45	1.48
12ft W	7.66	0.90	0.90	0.11	9.57	1.25

So, although the Gorton #2 is a help, I think that to make a meaningful difference, in my system, I should vent more at the returns. Unfortunately, I have isolated radiator returns (separate drain per rad to a common wet return). So I would need to buy 20 larger radiator vents.

mattmia2

Your logic is not correct because once the steam gets to a radiator it is going to mostly stop venting air, the steam will condense and pull more steam in from the boiler, but as the steam gets to each emitter along the main, the remaining radiators have to vent what is left in the main. put vents on the mains.

Jamie Hall

this^^^

Perhaps you are overlooking the fact that saturated steam is NOT a perfect gas. Far from it. You can't treat — or calculate — the system as though you are dealing with air and some other nice gas.

Steamhead

@pacoit , this is starting to look like "paralysis by analysis". You have to start somewhere, and we've given you a great starting point. Get a #2, put it on the long main, and go from there.

pacoit

https://forum.heatinghelp.com/discussion/comment/1858770#Comment_1858770

Funny! Nah, no paralysis (no rush); just analysis. When I do things myself it's because I'm interested in the topic, not just to get things done. Plus, I'm exploring the possibility of conversion to a vacuum system, as an option. So, I'm grateful for any further explanations on where my logic is wrong.

https://forum.heatinghelp.com/discussion/comment/1858757#Comment_1858757

You lost me here. Once the steam gets to the radiators, won't the mains will already be vented? Once the steam reaches the mains vent, it closes and no-longer contributes to venting; the radiator vents must finish the job.

Of course, the mains vents reduce the overall time for steam to reach the radiators, but only by reducing venting time through the mains.

In my system,
1. the volume of the 50ft tee section mains is ~20% of the total mains+supply+rad path;
2. the Gorton #2 will ~double the venting through that mains part, then closes when steam arrives;
3. So, the Gorton will ~1/2 the venting time of first ~20% of of the line; so the overall venting time of the path is reduced only ~10%.

So, I agree with adding a mains vent (or more). But since the supply+rad volume is much greater than mains volume, it seems that additional venting at the returns will also be needed to get venting times down significantly, say, 3-4 minutes.

https://forum.heatinghelp.com/discussion/comment/1858769#Comment_1858769

You lost me here. I can't figure what you are referring to. I mean, I'm simply talking about basic parameters of air volumes to vent and the venting available to do it.

Thanks.

mattmia2

the steam doesn't have a traffic cop to tell it to go along the main rather than to the runout to the radiator. once the steam gets to the first runout it will start taking that branch too. if the venting is all through the radiators the first radiators on the main will start heating a long time before the radiators at the end of the main. the steam starts dividing paths every time it gets to a runout so there is less steam heating the main to move further along the main.

If the main venting is a lot faster than the radiator venting the main will tend to vent first so all of the runouts will start getting steam to the radiators at close to the same time.

note that once there is steam in a radiator the collapsing volume of the steam in the radiator pulls more steam in so once it starts heating it keeps heating without the vent. this is why you want to make the main heat first.

pacoit

That's fine. I agree with that. It's important to clear the mains quickly such that the runouts get steam as close to the same time as possible. So, to be clear, I'm all for venting the mains.

What I was trying to point out originally was simply that even after generous mains venting, the venting of the runouts and radiators will still be slow on most legs, and so more venting at the radiators (returns) is needed as well to shorten those times, including different sizing for different rads as needed. How am I doing? Thanks.

Jamie Hall

Part way there at least. You are quite correct that one must consider — if one is going more into depth — the venting of runouts and risers, as well as the mains. Usually this is not really that much of a factor, as the runouts and risers are considerably smaller than the mains. However, in some installations they may have significant length or volume.

For this reason it is sometimes recommended — and sometimes even necessary — that longer runouts or tall risers be vented in much the same way as a main might be.

HOWEVER. This is not done with radiator vents (on one pipe systems, which yours is a variant on). This is done with additional vents on the inlet side, at the ends of the runouts or risers.

The reason for this is that the radiator vents on a one pipe system, or the inlet valves (or orifices) on a two pipe system and not there to vent air. Rather they are there to control the rate at which steam can enter the radiator and thus control the heat output of the radiator. The mechanism — or physics, if you look at it from that direction — is different on a one pipe system from that on a two pipe system, so they need to be analyses differently as well.

A bit of explanation. On a two pipe system — which is much simpler really — the radiator is vented through the trap, and that venting rate is (relatively speaking) extremely high (a standard radiator size thermostatic trap has about the same venting rate as a Gorton #2). Thus the inlet control serves to determine the rate at which steam can enter the radiator, and thus directly control the heat output of the radiator. In a really well set up two pipe system, it is entirely possible that at least some — if not all — of the radiators will be able to condense all of the steam admitted, and in fact the traps will never close!

On a one pipe system, there is no inlet control and thus, if the venting were unrestricted, the radiator would reach full power rather quickly. This is rarely desirable. Thus the vent size is reduced so that air venting is reduced. The effect is that steam is not able to reach all of the radiator essentially at once — the rest of the radiator still having air in it — and thus at the beginning of a cycle the output of the radiator is much less. The output will increase over time, as more of the air is vented and steam can reach more of the radiator. Thus the overall poser output of a radiator on a one pipe system is controlled by two factors: the venting rate and the length of the cycle (note that if the vent is closed during a cycle, such as might be by a thermostatically controlled vent, the radiator will continue heating, but at the power output it had when the vent closed).

Thus it can be seen that for a one pipe system the radiator vents are not — MUST not — be counted on to allow the air in the entire system to escape uncontrolled.

And in both types, venting to ensure that the piping is filled and steam reaches the radiators as close to uniformly as can be done must be placed before the inlets to the radiators.

A further note, not to confuse the issue — on many two pipe systems the mains venting is not to the atmosphere, but to the dry returns, which are in turn vented at the boiler. Don't worry about that for this discussion.

pacoit

Thank for the detailed response. I understand and agree. But, as a reminder (long thread), I have a 2-pipe system; and I have separate runouts from mains to each radiator. So, I don't have risers I can vent. But, I could add to the existing venting at each radiator return.

Now I get where you were coming from! And I hope now you get where I'm coming from.

So, for my 2-pipe system, how should one distribute the venting between the mains and the runouts+rads?

mattmia2

remember also that the vent rate of the radiator can only slow the speed at which it heats, it can't speed it up beyond a certain point. at some point it will be limited by the speed at which the steam can heat the mass of the radiator and the surrounding air to stem temps.

Jamie Hall

Well, you do have a two pipe system… and you don't. Eh? As I understand it, each radiator has a steam inlet — fair enough — and a condensate outlet. But, the condensate outlet does NOT go to a dry return, but drops all the way to a wet return line.

Thus the condensate outlet cannot vent the air, unlike a "normal" two pipe system, where the dry return is mostly dry and can vent the air.

Thus your system — so far as air venting is concerned — behaves like a one pipe system, Therefore the radiator vents control the output of the radiators — and can be placed on the radiator, or on the drips to the wet return above the water line, or… But NOT on the inlet line.

If you want to speed up how fast steam gets to the radiator, you need to vent the inlet line (whether its a riser or a runout doesn't matter). If you want the control the power output of the radiator, you need to do that with a vent on the radiator itself (the usual procedure) or on your drips to your wet return.

Two very different purposes for the vent — and two different locations.

pacoit

@Jamie Hall , Thanks. I agree with your explanations.

Regarding my system, the returns ARE mostly dry. General reminder: in my system, each radiator has its own dry drain/return to a common wet return near the basement floor. Currently, the radiator vents (Hoffman 1A) are attached to those drains, in the basement, up near the ceiling.

So, correct, I don't have a COMMON dry return, i.e., a convenient place to put a few large vents.

I do have individual runouts to each radiator (no "risers"). So, basically, I see each runout+radiator as a unit, to be vented at the radiator drain/return. Makes sense?

Jamie Hall

Terminology. Yes, your drips from the radiators to the common wet return pipe are "dry". Dry returns, however, technically, they are not — but that is an argument that has gone on for decades. Unfortunately. There are profound differences in how a system can be and needs to be controlled and vented depending on whether it has true dry returns or not.

In your case, as I noted, you have a system which will operate like a one pipe system, and your radiator vent locations are quite reasonable — and it is those vents which will control the power output — heat output — of the corresponding radiator.

Further, in your case, altering them to try to balance steam delivery time to the various radiators will upset the heat output balance to the various radiators and spaces. If you want to work on that steam delivery time, those additional vents must go on the feed pipe, NOT on the return line, dry or not. The question is, are you trying to balance or improve steam delivery timing, or are you trying to control heat output? Two completely different objectives, and two completely different vent placements and sizing.

pacoit

Thanks. I'd like primarily to improve steam delivery time overall, all the way to the radiators; then I suspect some balancing, as needed.

As to controlling "heat ouput", I'm not sure what you have in mind by that? I'd like to get steam to each radiator faster—-by which I mean shorten the period of venting air from the system.

JUGHNE

I recall reading above that they had to raise your boiler 9? inches to get the supply pipes lined up. This would raise the boiler water line at least that much and maybe more from the original water line.

Is it possible that your wet return may be above the current boiler water line?

Just more things to consider.

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1858901#Comment_1858901

Fair enough. Then you need to add vents to each radiator's SUPPLY line, as well as to all the mains and runouts.

What I mean by heat output is exactly that: how much power (BTUh) does each radiator put out, on the average, over time. That needs to match the heat loss from the space, or the room will be too hot or too cold. That has almost nothing to do with how fast the steam gets to the radiator, unless it is absurdly slow — slow enough that it is a significant fraction of the total cycle time of the overall system, in which case that room won't heat properly.

pacoit

@Jamie Hall, I'm a bit confused by various recent comments. It's probably due to some subtleties in terminology that I'm missing. To fix this, allow me to recap a bit my understanding. Thanks.

The transit time for steam to reach my radiators is slow. I'd like to speed it up, i.e., get steam to and into the radiators faster. For the current discourse, let's assume the only issue is insufficient venting. You know the system. So, as I understand it:

1. If there were no vents, steam would not reach the radiators;
2. If only the mains were vented, steam would not travel beyond them;
3. If only the supply ends were vented, steam would not travel beyond them;
4. If only the radiators were vented (at rad or drain), they would heat—-perhaps slowly.
5. If venting at all radiators were, e.g., tripled, they would heat faster—-and be as balanced or unbalanced as before;
6. Balance can be improved by venting the mains fast; or radiators can be set with different venting rates; or a mix of the two methods can be applied;
7. Having no risers and one radiator per runout, adding venting at existing radiator vents rather than at a new location on runout seems a simpler and superior option as it speeds both runout and radiator venting.

Are these fair statements? If so, then the venting is reasonably understood.

I recognize the above is only about venting the system. I created a spreadsheet to calculate how placing a vent in one section affects all the other sections. That's only a partial model. Adding a calculation of heat loss through each section of pipe and radiator will surely improve the measure of venting times and balance. But, one step at a time. How am I doing?

Jamie Hall

Not bad. I still think you are still overlooking the importance of the different function of the vents allowing steam to get to the radiators and the vents controlling the radiator output, however.

If the runouts to the various radiators are relatively short, then there will probably be very little to be gained in terms of speed of delivery to the radiators, and indeed only large systems need vents on these runouts (or in a tall building, on the tops of the risers). In that situation, the radiator/outlet vents perform the dual function of venting the runout (minor) and controlling the heat output and hence system balance (primary function).

Now you theorize that if you tripled the venting on the radiators they would heat faster and be as balanced or unbalanced as before.

If by "heat faster" you mean that steam would arrive at them faster that may — or may not — be the case. Beyond a certain point, however, increased venting — main or radiator — may not make any significant difference at all. There are two factors involved here.

The first is that when the boiler first starts to make steam at the beginning of a cycle the pressure at the boiler is extremely low and the flow velocity off the air is, thus, also low — and largely controlled by the pressure drop in the piping unless the venting is extremely restricted.

The second is actually more important: as I mentioned earlier, the steam is condensing in the main (or runout) as it tries to heat the main up. This is influenced most by the weight (physical weight) of the mains and the insulation around them. I honestly have never run through the math on this, but it is my experience that a maximum apparent velocity of around 20 feet per minute is to be expected for an insulated main or runout, and half that — or less — for an uninsulated one (I have observed an extreme case of a two inch, uninsulated main in an uninsulated crawl space which had an apparent velocity of around one foot per minute…).

It is interesting to note that during the warmup, the steam is moving in the warm portion of the pipe at a considerable velocity — but the air in the cooler portion of the pipe is still very very close to atmospheric pressure and hardly moving at all.

On the balance of the system. Increasing the venting proportionally on all radiators — tripling, for instance, as you suggest — may or may not affect the balance of the system, but as a generalization it will. This is because there are three major factors in determining system balance: the maximum power output of the various radiators, which is controlled entirely by their size (the "EDR") and does not vary; the rate at which the radiator reaches that capacity, the length of the boiler run cycles, and the interval between boiler run cycles. It is quite rare — it does happen — for most of the heating needs for all the radiators, or even the majority of them, to reach their full capacity in any one run cycle. This is true even for systems which have been carefully designed to match the available radiator output to the required space heating needs, and even then only under more extreme conditions. Further, it is also the case that for most normal heating needs the various radiators will need to provide varying amounts of their possible output; for example, a radiator which is three times as large as is needed for a space on a certain day will need to be limited to a third of its capacity, which a radiator which is only half again as large as the requirement would need to reach about two thirds of its capacity.

Now. What governs the capacity of any given radiator? Two factors: the vent size, which controls the rate at which air leaves the radiator and thus how fast steam can reach all parts of the radiator, and the boiler cycle length which, in combination with the venting rate, will determine whether a given radiator reaches full capacity — if indeed it ever does.

Now if you increase the venting rate on all the radiators on a previously balanced system, it is quite likely that some of the radiators which did not previously reach full capacity will now do so — and the system will be out of balance.

Sorry for the dissertation, but I have the impression that you are really interested in the actual physics and dynamics of your steam system — and it's not quite as simple as it might look at first glance.

pacoit

Yes,

pacoit

Pardon my absence; had technical problem, again, posting to this thread.

@Jamie Hall , thanks for the detailed explanation regarding my bullet point:

"5. If venting at all radiators were, e.g., tripled, they would heat faster—-and be as balanced or unbalanced as before;"

But, I must apologize for making the error of being overly concise; I was talking in simplified terms. I wholly agree that balance can change in certain cases.

pacoit

So, I'v built a spreadsheet to analyze how adding venting at one mains branch or radiator will affect venting at all the other branches and radiators. (Also working to add rate of steam condensation and flow and steam capacity analysis).

I was told that it's good practice to vent/balance the mains first, then balance radiator venting after. Sounds intuitive. Has this method been tested/proven/verified scientifically? Because I was surprised when I started playing with my spreadsheet model.

I first added vents at mains to speedup and balance mains venting; then I added to and balanced the radiators, which, in doing so, made the mains unbalanced. Hmm. Actually, it seemed quite logical.

Then I tried eliminating the mains vents completely and do all the venting and balancing at the radiators (returns). Here is what I found:

1. The mains were significantly more balanced; pleasant surprise.
2. The radiators were perfectly balanced; naturally.
3. The number of vents were reduced by about 1/2; pleasant surprise.
   1. Why? Mains vents work only up to mains; radiator vents work along whole system path?
4. Much lower cost of vents, and no labor to tap new vent locations.

One might say my spreadsheet is flawed. Perhaps. One might say that in the real world there are other factors to consider. Sure (e.g., different condensation rates in different size piping section). But this is simply a focused analysis of air volume and venting in a piping system. It's interesting. And I can't think of a theoretical reason for balancing the mains first.

mattmia2

does your spreadsheet account for the heating of the main, the runouts, and the emitters? that is mostly what governs the rate at which things heat. venting can only slow down the lower mass parts to balance them with the parts with more mass. you also have to account for how well the emitters are matched to the space they are heating in comparison to all the other spaces.

pacoit

Yes. I divide into header, mains, tee sections, runouts, and emitters, their paths, so I can analyze effect of vents in different places.

One possible reason why "balancing the mains first" seems to work well in experience is if one's experience is working mostly on larger systems where the mains and risers are the majority of the piping to the emitters. In my case, mains piping is only ~1/5 of total piping.

Jamie Hall

"simply a focused analysis of air volume and venting in a piping system"

Sounds simple enough, and if air or some other non-condensable, near ideal gas was what was involved I'm not all that surprised at your results.

Saturated steam is, unhappily, condensable and nowhere near an ideal gas. Even such apparently simple relationships for gasses, such as Bernoulli's principle, simply don't apply.

Look up enthalpy… among other things.

pacoit

Fair enough. What counts is what steam actually does in the system. Once I finish adding modeling steam flow through the system I'll report back.

pacoit

I was thinking about the possibility of adding orifices to my radiator valves and replacing the Hoffmans in the basement with a simple air check valve (no thermostatic function). The idea is that orifices will limit steam into the radiators such that it condenses before reaching the outlet; the check valve never sees steam; the check valve closes once the radiator fills as much as it's gonna that cycle; when the boiler turns off, the check valve remains closed, steam condenses and creates a vacuum; check valve impedes air from re-entering.

Depending on the airtightness of the piping and the boiler-off duration time (period of possible leaking), the next heat cycle will start with some degree of vacuum, which should help speedup warmup time.

I think suitable check valves may be found for roughly ~$10 each (x20 for me). And vents are no-longer needed. Any thoughts?

Jamie Hall

That type of system has been used — quite successfully. Two real considerations. First, the flow through the orifices is related to the pressure difference across them, so the steam pressure from the boiler needs to be controlled quite precisely. It doesn't really matter all that much what that steam pressure is — so long as the orifices are sized correctly, although most orifice systems operate on very low (a few ounces at most) pressure, since that pressure can be reached within a minute or so of steaming.

The other is that the check valves need to have an equally low cracking pressure. There is, also, still an advantage to having them on the mains as well.

You may be surprised at how difficult it is to maintain any significant level of vacuum…

pacoit

Check valve at mains would require combo thermostatic and check valve, correct? Are those available, reliable, pricey?

Yeah, I currently have no idea how airtight or leaky my system is. I'll have to figure out a convenient way to close it and connect a vacuum pump to test it. I could temporarily remove all 20 Hoffman 1A return vents and plug the holes. Or, I could close down all the vents (I doubt that would actually seal). Hmm… maybe I could simply put a rubber over each Hoffman?

mattmia2

i think people have done it with something like a rubber band

you can use a radiator trap and put a check valve on the outlet to make a vacuum vent. just orient it so the condensate runs away from it or you will need to drip it between the trap and the check valve. the issue most run in to here is finding a check valve with a low enough cracking pressure.

pacoit

Do you know of any product that would fulfill the cracking pressure requirement? I suppose it needs to be really sensitive, 1oz pressure?

I wonder what design type, mechanically, is suitable for this application, and how reliable and long-lasting it would be?

pacoit

In a vapor steam system like mine, what is the typical steam pressure at the different stages of the heating cycle (e.g., venting phase, radiator condensing phase, etc.)?

mattmia2

depends on how well the boiler is matched to the system. if the boiler matches the edr of the system including the piping it is very close to zero, more or less just whatever the resistance of the piping is.

if the boiler is significantly oversized then it will depend on the heat loss of the house, the output of the emitters compared to the current heat loss with the current weather conditions and the setting of the vaporstat or pressuretrol.

if the thermostat is satisified before the system completely fills with steam it may still stay near zero. if it operates for a while after the system is full before the thermostat is satisfied and the boiler is oversized it may ride the vaporstat or aquastat cutin and cutout settings.

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1859332#Comment_1859332

It varies with the system, but in most systems the operating pressure at the boiler will rise as soon as the boiler starts to steam, typically to one or two ounces per square inch gauge at the boiler, and will stay there so long as all the radiation is not completely filled. In a system where the boiler is closely matched to the radiation, it may never rise above that. Otherwise, such as in the biggest system I personally care for, where the boiler is very slightly oversized, after about 45 minutes to an hour of steaming (which means the house is close to or at design; -10F) the pressure will start to rise again as all the traps are closed. The vapourstat will cut the boiler at 6 ounces gauge.

The pressure varies throughput the system. There is no "venting" phase. The pressure at the outlet to the radiators (whether they are trapped or controlled by orifices) will be at zero gauge pressure — atmospheric — and the dry returns will be at atmospheric at all times.

It is well worth remembering that in earlier vapour systems, the dry returns — which were the only vents — were open to the atmosphere at the boiler. Later various devices were added near the boiler for various reasons related to system overpressure, but these made no difference under normal conditions. For all practical purposes, a vapour system dry return is always open to the atmosphere (this is true of most "normal" pressure two pip;e systems, as well).

To go to your check valves. I would suggest spring checks, and you are looking for a cracking pressure differential of 1 ounce per square inch or less.

pacoit

@mattmia2 and @Jamie Hall , thank you both. Your comments confirm my own thoughts.

Another question. I wonder if the flow of steam into a radiator is different—-in some important way—-when limited by an orifice plate versus a return thermostatic vent?

For example:

If an orifice plate, I imagine more restriction on ENTRY to radiator, more condensation before entry, and more condensate back down supply pipes.

If a thermostatic vent, I imagine free flow of steam into radiator, less steam condensed before radiator, and more condensate down return pipes.

Is it correct that the orifice plate creates this nuanced inefficiency—-even if a system designed for orifice plates may be more efficient overall?

mattmia2

It is more about where in the cycle it is more controlled and less controlled. Se where @Steamhead and I were debating about balancing with valves vs vents a couple pages ago.

Jamie Hall

On orifice vs. traps. No difference. Condensation occurs — in both cases — due to two factors: a very slight pressure drop (greater in the case of the orifice) and the resultant expansion of the steam and the encounter with a cool surface.

It is very important when dealing with saturated steam to recognize that expansion itself — whatever the reason — will cause condensation within the gas stream. This appears as microscopic entrained liquid droplets — can't see it right away! — but it does occur. This is one of the more annoying ways in which saturated steam does NOT behave like an ideal gas!