2 pipe steam boiler short cycling

Gary Segal

Good Evening Steamhead and all,
My first attempt to check main line venting agrees with your hunch that it undervented. Here is how I collected my data:
I fired the boiler with the vent on top of the boiler open. As soon as steam came out, I assumed steam had entered the main and started timing. With the Gorton #2 removed, it took a mere 5 minutes for the steam to reach the main. Later in the day, i repeated the test with the Gorton in place. That time it took more than 11 minutes. It was not a perfect apples to apples comparison as the boiler and the pipes were a little cooler when I did the test with the vent in place. I will repeat again to verify the measurements.
One question - Instead of using Gorton #2's, why not just use radiator traps? Based on the data in Gerry Gill and Steve Pajek's book, some appear to have the same flow rate and cost a lot less - I can get two Barnes and Jones 122's for the price of one Gorton #2.
As for the return venting, I stumbled upon the Illinois catalog that was in the Library. It had a picture of my return vent, with this comment:
"Illinois Vapair Trap and Air Check
An improved, simple, float operated Air Vent Trap, which permits free venting of air without the loss of condensate. This sensitive air vent valve instantly vents the system and prevents the return of air. This results in high vacuum, low temperature steam circulation. Air Checks may be connected in parallel for increased capacity.
Furnished in 3 sizes, from 4,000 sq. ft. to 12,000 sq. ft."

After reading this description and looking at the picture, I was struck by a three items:
1. The comment "prevents the return of air" I tried blowing into the vent on mine and air flowed freely. Now, instead of the 45-degree vent shown in the catalog, mine has a Hoffman 75 on it. My hunch is that valve is doing nothing but adding restriction to the system. Is that cap on the end of the 45 a check valve?
2. Since mine appears to let air in or out, I cannot pull a vacuum - was this intended to be a vacuum system?
3. Capacity. Mine is a No 1, which I assume is 4,000 ft2. My boiler is 608 ft2, so I guess I have a lot of extra capacity.

Gary Segal

short cycling on 2 pipe steam boiler

Hello all,
I am trying to solve a short cycling issue on a 2 pipe steam system in our home. The problems have been going on for a number of years. I'll start with the good news - the system is quiet (no water hammer) all radiators get hot.

Here is what happens:
1. When the boiler fires up for the first time, it takes about 45 minutes to reach the Vaprostat cutout of a little more than 1 PSI (It used to take less time, but we downfired the boiler a bit last year.
2. Once the Vaporstat cuts the boiler off, it only takes about 90 seconds before the pressure drops below the Vaporstat cut in setting (8 oz differential) and the boiler cycles back on.
3. The boiler will then run anout 2 1/2 minutes until the Vaporstat cuts it out again. This cycle repeats until the house reaches the setpoint on the thermostat. The off-on cycling speed is independent of outside temperature.
The house is 70 years old, but insulated and has mostly new windows.
I have done measurements where I have turned the boiler off when at the time the Vaporstat turns off the boiler so i could extend the off time. Here are the results:
Off time (min) Burn Time (min)
1.2 2.5
5.1 7.3
7.0 8.0
10.3 10.7

Gary Segal

2 pipe short cycling continued

Sorry - the first message posted before I was done!

So, when I tried to manually extend the off time, here is what happened:
Off time Burn time
1.2 min 2.5 min (regular operation0
5.1 min 7.3 min
7 min 8 min
10.3 min 10.7 min

So, it appears that increasing the cycle time gives me less burn time. I should note that there are no apparent steam leaks in the system and that all traps are new last year (part of the attempt to solve the problem)

I am attaching a boiler room schematic and a picture of the near boiler piping for clarity.

Thanks for any suggestions!

Gary

gerry gill

temporarily remove

the air vent from the return pipe and run the system..this will tell us if you have steam in the returns, cause you'll see it, and if you need more venting..then report back with what you find.

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Steamhead (in transit)

Hi, Gary

this makes me wish for the availability of 2-stage firing on residential steam and Vapor systems. I suspect you're still a bit overfired- I seem to remember you're running 1.25 GPH or so? Why don't we try a slightly smaller nozzle?

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Gary Segal

Steamhead, good to hear from you. I guess it is that time of the year where everybody remembers the issues with their heating systems!

Since the boiler is taking a good 40 - 45 minutes to reach cutout pressure, I am not sure I would want it to fire even less. Also, perhaps I do not understand something .... The firing rate controls how long it takes the boiler to make steam, but once it cuts out, it is the attached load (and/or leaks) that determines how long it will take to lose pressure and cycle back on - correct? Or is the desired mode of operation that you rarely reach cutoff pressure because the burner is firing so slowly?

From a making my wife happy perspective, I would rather fire faster .... we get heat more quickly.

Gary

Gary Segal

retunrs and the like

I played around with the return a bit last year, but not for the exact reason you mention. In the process of trying to understand the opreration of the system a little better (still have not nailed it after 15 years!), I popped the cover off a trap just before steam started to get to the convector. I discovered some back pressure (air) from the return line. Then I took the plug out of the return line (where I later added the second vent) to see if there was back pressure at that location. There was.

At that time, there was no steam in the return, but I installed the second Hoffman vent in the opening just because I had an extra one and there was some pressure in the return. There was one Hoffman vent on the "unknown black box" at the end of the return line. I say unknown, because I assume it is not an F&T, since it is on the end of the return line, not the end of the main line. It is marked "Illinois NO 0"

Gary

ttekushan_3

staged firing

I, like steamhead would like to see variable input residential steam boiler controls. In your situation such a unit would fire at the higher rate to get you steam faster, and if extremely cold weather conditions call for it. Once pressure begins to build, the firing rate decreases.

Larger steam systems utilize this feature quite commonly. These are always forced draft fired so that both air and fuel modulate so the proper fuel-air ratios are maintained.

Not available in a residential boiler near you. At least not recently. Worked on a 1929 boiler that worked this way but non-electrically (check out the B-LINE boiler info in the library -the ancients were very clever).

-Terry

gerry gill

hey Terry

joe szackas has the instructions for that old b-line model..it says to open more burner tubes as it gets colder..we didn't exactly make an advancement with the all or nothing firing today did we?

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Gary Segal

Variable input residential steam boiler controls sound pretty cool, but I agree, it does not look like something coming to a boiler near me any time soon.

One thought I had was to install an on delay timer in the circuit. The way is would work is that when the Vaporstat cycles off, the timer would start and prevent the boiler from starting back up for the preset time, say five or ten minutes. The house might heat a little more slowly, but the initial burn breaks the chill, so it is not a big deal.

Also, I should mention, we do run on a programmable thermostat - we basically do not fire the boiler when we are asleep or away from the house. But even when the temperature setpoint is not changed, the boiler cycles infrequently by the thermostat - short cycling is only via the Vaprostat.

I did notice that the thermostat has some non-adjustable presets that contribute .... it does not cycle off until the temperature overshoots setpoint by 2 degrees and it does not cycle on until 1 degree below setpoint, so there is a 3 degree swing minimum, normally. Once you throw in the thermal mass of the hot convectors and radiators, the overshoot of setpoint may total 3 -4 degrees.

Thanks again for all your input,

Gary

Steamhead (in transit)

Not a huge reduction, Gary

just one size smaller. It can be easily reversed if it doesn't work as we hope.

Remember, those little convector elements don't hold much steam to begin with, and shed lots of heat quickly. So the steam therein condenses quickly, causing a rapid pressure drop.

The "black box" is a float trap/air eliminator. Originally that was the only vent in the system, we have since added more.

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Gary Segal

Steamhead, I compiled some interesting data on oil usage over the past few years. I calculated consumption in terms of gallons per degree day. It is probably not a perfect measure since we keep the house pretty cold, but I could not think of a better measure.
In the 2001-02 heating season, we used .160 gallons per degree day, then .133 in 02-03,.145 in 03-04, .122 in 04-05 and .122 in 05-06. For the record, you vented the main on February 05, the Vaporstat was installed April 05, then we downfired the boiler in December 05. I finished replacing all the traps in February 06.


So, I am not sure whether we are reaching the point of diminishing return. The biggest efficiency loss seems to be the short cycling. I know once the boiler makes steam, the off cycle is independent of the nozzle, so I guess I am still hung up on reducing the cycling. I'm more inclined to fire a little stronger to make steam quicker and add some sort of time to delay restart after cycling off on Vaporstat pressure set point.

dana_3

2- pipe steam

is the pigtail on the pressuretrol blocked

Steamhead (in transit)

Gary, I use the same method

but I also try to break it down by month if possible if we're working on a system in stages. If I'm reading your figures properly, they are averages throughout a complete heating season. Have you had a chance to break them down by each month, or other appropriate interval such as when the tank was refilled? I bet you'd see a nice drop-off in 04-05, starting in February when we started working on the system.

The trouble with firing stronger is that it may decrease the "on" period to the point where the boiler would rarely reach steady state. Startup and shutdown are the least efficient periods of a boiler's running cycle, so we want to avoid generating too many of them. We're looking for a nice long burn that doesn't overheat the house.

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Techman

Pullup's-Pulldown's

Hi Gary! I have forced warm air, I also turn my heat off at night,on most cooler/colder mornings my FWA has a much longer run time , the 1st time ,them normal runs after that.So, does these two conflicting requirements have anything to do with your understanding of your steam system?My A.C. customers are ALL aware of the opposite effect on them and their comfort.Some Ac units can take hours to pulldown to proper humidity/temps from having been shut off. Now,Steamhead,I'm only thinking out loud here, about this !

Gary Segal

blocked line on pressuretrol

That is an interesting thought. I do not think it is, but it is easy enough to check. When I switched to a Vaporstat a couple years back, I kept the copper pigtail from the pressuretrol instead of using the black metal loop that came with the Vaporstat.

Gary Segal

Hi Techman,

My hangup is that the "normal" run is the first burn. After the first burn, which is currently 40 minutes (and basically independent of starting temperature), the on cycle is about 2 minutes and the off time is about 70 seconds. It does not matter how fast the first burn is, the subsequent pressure cycling is always the same - 2 minutes on and 70 seconds off.

Since the cycling is controlled by the Vaporstat as opposed to the thermostat, the short cycling continues without a long run should I bump up the thermostat a couple degrees after reaching initial temperature setpoint.

Thanks for the ideas! This system is a fun learning experience!

Gary Segal

gallons per degree day and steady state boiler op

Steamhead, Unfortunately, we only get about 3 tank refills a season, so the data is a bit noisy. I can track burner runtime on the t-stat, but it only stores 2 days of data, and I rarely remember to check it that frequently. The spreadsheet I attached has the data broken down by tank fill, but as you can see the data is noisy.

I guess the real question is "How long does it take a boiler to reach steady state?" My hangup is that the speed of the first burn has little influence on the number of additional short "after" burns required. It is not uncommon to have 10 - 20 of them on a heating cycle. I guess if we down-fired to the point that the thermostat tripped out before the Vaporstat tripped, we would have a winner.

On the other hand, I imagine there is an efficiency hit for making the steam so slowly. Right now, the Vaporstat is set just to the point that the most distant convector gets hot all the way across, so a slower burn would mean a greater delay heating that room, right?

I look forward to more conversation!

Gary

jay_33

short cycle

Gary sounds to me you need 2 stage firing. There are pros on this site that have perfected fine tuning thru stage firing. It is the solution we all seek for the many oversized systems that shortcycle. This is real outside the box practices, but sound none the less.I wish they would clime in. Good Luck

Gary Segal

Re: short cycle and 2 stage firing

Jay, 2 stage firing does sound intriguing. It would probably solve the issue I have. I do see a few other ways around it. I've noticed that each burn of the boiler (from a cold start) is good for a 3-4 degree rise in the house temperature. I also know the range of burn time as currently fired is 30 minutes to get steam to all radiators and convectors and 40 minutes until the Vaporstat kicks out at 1 psi. Technically, I guess I can drop the pressure further, but then it really short cycles.

So, if I can find a thermostat that lets me program in short increments, I can "trick" the system into being off for 10-20 minutes between burns. It will not work, of course, if we come home early and decide we need heat at a non-programmed time. On the other hand, if I could locate a thermostat that treated the boiler like an AC compressor, with a delayed restart, perhaps I could control the off cycle with it.

The path I was taking last year was to install a delay relay in series with the Vaporstat, so that each time the Vaporstat cycled off, there would be a set delay before firing back up. In essence, I would obsolete the cut in side of the Vaporstat. I selected the relay, but the weather got warm before I bought it.

All these ideas go under the assumption, that I do not need to make more steam 70 seconds after cycling off. Given that the house continues to increase in temperature for a good 30-60 minutes after the boiler shuts off, that assumption carries minimal risk to comfort (read that my wife will not notice a problem!). From the data I have collected, a 10 minute off delay would create another 10 - 12 minute burn.

I do thing graduated downfiring could be more efficient, but it does sound technically complex in terms of draft management and air/oil mixture management. Of course, we could borrow from the automotive industy and add an O2 sensor in the flue along with a process control computer and control the air inlet accordingly.

Gary

thfurnitureguy_5

Gary, why does this sound like a venting problem? It sounds like you have added venting, but 30 min for steam to reach the radiators and only 10 to close the vents and build pressure??? Is there any way that steam could be backing up the returns and leaving the mains with air trapped? or could you still be under vented?

Gary Segal

Undervented?

I do not think we are under vented. I have pulled one of the return vents numerous times when the boiler was in operation with the standard 1 PSI cutout and never found steam in the return line.

Originally (you can look at the photo in the original post 11/26/06), there was a thermostatic trap used to vent the main to the return line, although it appeared to have been installed reversed (outlet was toward steam main). Steamhead worked on the system, found the issue and replaced the trap with a Gorton #2.

At that time, it took 29 minutes to cut off on pressure. Later in the season, we downfired the boiler a little to slow the cut out time to about 40 minutes. To be clear, it takes 30 minutes until the last convector gets warm. The first fets warm in 15-18, minutes, I believe.

The steam main runs a circle around the basement (about 78 feet from the header at the boiler to the return vent - 3" main - all insulated). Smaller diameter lines come off the main leading to convetors and raidiators on the 2 floors above. Each is fed individually, but there are no other vents on the supply side piping.

The condensate return is piped in the exact same method as the main, with the lines running parallel around the basement. The return line has 2 Hoffman 75 vents at the boiler end. Under normal operation, there is no steam in the return line, however, I did bump the Vaprostat up to 2 PSI cutout yeaterday to blow out the LWC and auto refill. I popped off one of the Hoffman vents in the return and there was definitely steam in there at that pressure. Once I returned to 1 PSI cutout, the steam in the return went away. FYI, all the radiator traps were replaced last year.

I can only assume that since I have 46" of "B" dimension, 2 PSI was enough to back up the return. Interestingly, I discovered that there is a check valve (in somewhat working order)between the Hartford loop and the wet return. When I say "in somewhat working order", I mean it holds back water from the Hartford loop down to a flow of a drop a second or so when the boiler was off. There was a drain plug on the end of the return that enabled me to verify this fact.

Finally, the return of the steam main to the boiler does not really match any of the pictures in "The Lost Art ..". There is no F&T trap at the end of the main. Instead, after the vent, the line drops to about 2" diameter and goes directly to the base of the Hartford loop. Also, since the boiler had been replaced, I assume, the return side of the loop is (" higher off the floor than the boiler inlet side.

Thanks,

Gary

Steamhead (in transit)

If there's still that much difference

between when the first convector gets steam and the last one does, you're still undervented.

One of the things we have now that we didn't have back then, is better information on the throughput of different vents. The Gorton #2 is still the champ, but with Gerry Gill and Steve Pajek setting up lab equipment to test them, we have a much better handle on what this vent will do. At Vapor pressures (1-3 ounces) it has less throughput than we thought.

The next step will be to add two more Gorton #2 vents to your steam main. This will direct the steam toward the end of the main first, before it goes to any convectors. We stock the #2- give us a call.

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Gary Segal

underventing

I can not remember whether we did it or I just read it in "The Lost Art ...", but I was thinking that we compared the time to get to the end of the main with no vent and with the #2 installed.

I will give it a try this weekend. I think I will pull the # 2 this and fire from cold to see how fast steam gets to the end of the main, then repeat the test Sunday with the vent installed.

Given that there is no other venting other than on the main, isn't it possible that the time difference is just due to the difference in the lenghts of the runs? The last 2 convectors to get warm are at the end of the main and on the second floor, which kind of makes sens to me.

Gary

Steamhead (in transit)

The basic principle

is that the steam should go to the end of the main first, before it goes into any of the runouts to the radiation. You make this happen with proper main venting.

Compounding your situation is the fact that you only have one steam main and the entire boiler's output goes into it. With only the one vent, you might not have enough venting capacity to get rid of the air as fast as the boiler produces steam. So some of the steam would spill into the first few runouts.

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Steamhead (in transit)

The problem with using traps that way

is they won't close against water, if the main should somehow flood. The Gorton #2 will close against water, as any good main vent should.

The Vacuum feature worked great on coal but doesn't do nearly as well on oil or gas. The problem is that you can get vacuum long before all the air is out of the system. This causes the trapped air to expand, blocking the steam.

Add a couple #2 vents to the steam main and watch how well it works. We stock them.

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Christian Egli_2

Reading the cards on this two pipe thermostatic traps at the rad

It takes 18 minutes to steam in the first rad, then at 30 minutes you have steam everywhere and at 40 minutes you’re done cramming steam, after that the boiler modulates(!) on 120 sec, off 70 sec (120 of a 190 sec cycle is 63% fire)

When the system is hot, of course, the start up time is shortened. And of course, it makes no difference in the 63% modulation when everything is steady.

Here is what I think.

The first 18 minutes (even though cold) was too long to only get to the first radiator. Then, the next 12 minutes to reach the last radiator seemed even more out of proportion. I'm glad your latest timing showed great improvement, that gets the first 30 minutes out.

Next the 10 minutes it takes to build up a certain pressure, plus the fact that your boiler cycles at 63% when rolling indicates you do not have too small a boiler, quite the opposite. Too small a boiler would never develop any visible pressure and it would not either heat all of your radiators (either all partially, or some of the last ones plainly forgotten).

You have two pipe steam, and beyond partial filling of radiators and possible cold spots, there isn’t much more trouble with undersized boilers here. How are your radiators currently heating? All fully hot top to bottom? If so, then you really haven’t got an undersized fire.

Whether the boiler is over or under sized, and within the partial filling limitations of under firing, we should still expect the same quick venting (this is two pipe). The cloud of steam forming around the boiler still moves with the same speed of light rate, it just doesn’t go as far. We won’t vent as much air, but we’ll still vent it quick.

Here is what I’d do.

To fix the venting problem, I’d definitively scrap the air vents past the big black box, the air eliminator. Steam must be stopped from escaping, and it is stopped at the radiator traps. Only these radiator traps along with the main crossover trap are crucial to good system performance. Stopping hypothetical steam further down these traps into the return makes no sense. The air eliminator never ever was meant to stop steam - in fact stopping steam at the breathing hole on top of the air eliminator will only pressurize your returns and proceed with the active destruction of your radiator traps.

Closing air vents on two pipe B dimension atmospheric returns with traps at the radiators only sets the stage for a demolition derby once steam wiggles itself in there.

Furthermore, beyond removing the current thermostatic air vents, I’d remove the air eliminator. Air eliminators only shut on rising B water column. You have an accurate vaporstat to prevent a flooding rise in B height (at what, perhaps 2 PSI max), thus no need anymore for the air eliminator - it’s not like you’re still burning difficult to control raging wood and coal fires.

The air eliminator away, you’ll have that much more of a breathing hole to vent your whole system air. Insert in its place a big pipe, 1/2, 3/4, which you leave open and which you carry over to where you can see it and where it can best barf into a floor drain. (or bring a drain to it)

Then watch what comes out of it. If steady steam pours out, go fix your radiator traps and cross over traps - imperatively - immediately - (You did say your traps were newer, but you also said you observe pressurized steam in your returns, thus some radiator traps must have already gone bad again in the demolition derby)

Next danger

If ever you flood your system with water from a faulty feeder valve, then at least this flood will not rise into the living space and onto your carpets (around valve stems for instance...) Instead the dangerous flood will come pouring out the breathing pipe we just pointed to the drain - no catastrophe - just fix the feeder valve

Under normal operation, this hole should simply exhale puffs of air without pressure. There might also be some whiffs of steam on long cycle runs.

For the main venting, I would go back to crossover traps, this is two pipe steam. There isn’t anything wrong with what thermostatic air vents do, but thermostatic crossover traps vent more and faster. This is because a crossover traps purges itself into an atmospheric return - this allows it to do a sloppy job of closing on steam, it does not have to stop the very first trace of steam like a thermostatic air vent does, we don’t want spitting vent, while we don’t care about spitting traps, if all the messy dribble lands in a return, it all ok. Since a trap can operate more closely to the cliff, it can vent more and faster and better. It’s ok to add air vents along with the cross over traps on the mains, but I would not switch one for the other.

A further reason why your system might seem to be dropping in pressure unexpectedly quick.

Right now, with the air vents in place, once these shut, they don’t reopen until things get significantly cooler - air or no air. You now might have a system in which there still is a squishy balloon of tramp air. This balloon will play with the pressuretrol on your boiler. Pressure up, the balloon shrinks a bit and retreats into a corner. Fire out, it immediately bounces back, meanwhile, it limits the steam capacity of the system. This may or may not be the case in your setup. Just food for thought. Your pressurized returns could also be causing this yoyo effect.

Later on, for limiting the on off firing, perhaps playing with the differential gap on the vaporstat might suit you. Perhaps also investigating alternate end of main vaporstat placement might suit you.

But first, unbutton the breathing hole, and next, fix the radiator traps and the crossover traps that may have gone bad.

Gary Segal

Thanks for taking the time to

look into all my information and making sense out of it! Let me see if I understand what you are suggesting, then I'll answer your questions.
1. Switch back to using crossover trap(s) from the end of the main to the dry return (currently, there is a Gorton #2 at the end instead of a crossover trap). Use as many as are required to get close to the time it takes to get steam to the end of the main with the port open at the end. To be clear, when you say crossover trap, I'm assuming you are referring to the original setup where a radiator trap was used to cross over and vent from the end of the main to the return line.
2. Eliminate the air eliminator and replace with an open vent, which ideally ends at a drain. Now, I pulled the Hoffman 75 off the end of the air eliminator and it vents pretty well through its 1/2" diameter opening. Since the inlet and return plumbing are at least 1" in diameter, the return will be capable of venting even more with the eliminator gone.
3. Replace any failed radiator traps.

In response to your questions:
1. At the moment, the Vaporstat is set to cut out at about 1 psi. I have a 0 - 4 PSI Vaporstat so the resolution down at that point is not that good. (I was too chicken at the time to buy a 0 - 16 oz unit).
2. The only time I saw steam in the return line was when I cranked the Vaporstat up to a 2 psi cutout to get a good blowdown. Under normal operation, there is just air in the return (along with condensate). When steam first gets all the way to the end of the radiators and convectors, the return line stays cold for a good while, so I assume the traps are OK.
3. Yes, all the radiators and convectors get hot the whole way across.
4. I agree that the boiler is not too small. I tried estimating the connected load, despite not having any EDR data for my unusual convectors. Independent of my method of estimation (comparing them to various similar devices), I came up with a connected load that was about half of the boiler's net rated capacity after applying the 1.33 pick up factor (all original convectors are intact and pipes are insulated).

Thanks again,

Gary

Steamhead (in transit)

If you revert to crossover traps

you will need several of them, and will need to do a lot of piping to connect them into the dry return in such a way as to allow their full throughput to enter the dry return and be vented.

The additional air coming into the dry return would in turn slow the venting of the convectors- especially when one of the crossover traps leaks, which will pressurize the dry return and further slow or stop the steam flow into the convectors.

That's why I installed that Gorton vent in the first place, and suggest simply adding more Gortons to further speed the steam to the far convectors.

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