What *exactly* is short cycling?

realolman

I'm more ignorant than you... what about me?

Vermonter

Any \"official\" timeframe?

Is there an official definition for short cycling? Less than 5 minutes firing per cycle? Less than 3?

Brad White

Let's Vote on This!

Personally, it really does depend. To me, with regular on-off equipment, any run time less than 2 minutes is a waste- it is just getting warmed up and combustion products stabilized. I would be happy with five minutes in mild weather.

Weather- now that is a big factor- What I would consider short-cycling especially with an on-off boiler depends on how cold it is out. It is somewhat relative, meaning I will settle for shorter on-time in warmer weather in exchange for a good long burn in cold weather.

ModCons change all of this. Same timing (2 minute minimum) for any firing but darn it, I expect it to be long and steady any time the outside temperature has the boiler running above its minimum turndown.

scott w.

Can you give us unlearned folks the definition of turndown?


Much appreciated

Brad White

The ratio

of low fire to high fire in a modulating or staged boiler.

For example a boiler with 80,000 BTUH input that can modulate down to 20,000 BTUH input would be said to have a 4:1 turndown.

A two-stage boiler at 80,000 BTUH and 40,000 BTUH would be said to have a 2:1 turndown, but the distinction is in staging (two steps) versus modulation (almost infinite change from high to low).

"Turndown" can also apply to the lending industry. It is not a pleasant term, I am told.

chuckNJ

steam cycle period

Brad, for at least some of us end users who have gotten hooked on trying to optimize our residential steam systems this question arises in the context on "modulating" under vaporstat control at the end of a long run (like recovery from setback). We can raise the cutout preassure (and energy consumed) to lengthen the period, and/or we can cut back the fire a bit (but at the expense of available pickup for response in the shoulder seasons). How to decide? A couple details perhaps you can offer fact and/or opinion on with regard to your suggested 2 min minimum: first, is this combustion stabilization different for different fuel and/or burner (oil vs gas and atmospheric v power burner)? Secondly, if the "off" time is relativly brief (say less than 60 sec), does that effect the answer? thanks for any thoughts.

Brad White_9

Interesting things to consider, Chuck

As far as steam control goes, I will leave that to others with more direct experience; observing, tuning, tracking... I have not really thought much about it in those terms but you piqued my interest so I will find out more for myself at least.

My least thoughts are, if the generation of steam is proportional to fuel input, one could modulate fire (if commercially available on smaller boilers) to take a rolling boil back to a simmer. I suppose if you are controlling to vapor pressures, a simmer would create very low pressures and take the bottoms out of what would normally be "dips" in a heating cycle. Again, this is the first time I am thinking about this.

(My steam experience surrounds commercial/industrial/institutional steam boilers such as for hospitals and universities where maintaining a fixed and constant pressure is the goal. Modulation is key to this kind of control versus one-pipe steam with expected cycles. So, full disclosure here.)

For the combustion stabilization point, when a burner first ignites, we have all occasionally smelled the waft of unburned gas for a second or so (in atmospheric burners). That tells you there is no combustion taking place at that point and the quality of flue gasses has to leave something to be desired.

Once ignition is established (against cold HEX surfaces especially), spikes of CO, CO2, NOX, water vapor, all the good stuff, are vying for stability (relative to measurements you will get from a steady-state combustion stream). Take your combustion tester and see what you get.

The "2 minute" time I got from a Viessmann course some years ago which opened my eyes to several things (probably obvious to many of you in the field). Higher mass boilers would take longer naturally but the 2 minute spike period seemed to be a minimum.

My point on that is, your heat transfer rate does not reach peak efficiency until the passages are warmed, the products are stabilized (signifying as efficient combustion as can be) and the thermal momentum is established.

(By thermal momentum, I mean that if you were to remove the heat source by shutting off the boiler, you would still add heat to the medium being heated for a period afterwards as opposed to the heated medium being possibly warmer than the HEX at that time. An analogy to this would be kinetic energy in a car accellerating from a dead stop.)

Imagine getting all of that work done, or not quite reaching the point of optimum thermal efficiency then shutting off the burner- what a waste of the energy needed to accellerate.

I see the "atmospheric versus power burner" comparison like this: Atmospheric would tend to have a longer required stabilization time to my thinking. When the burner is off, the HEX is open to ambient so cools more than a sealed power burner. Maybe that has a longer warm-up period. I do not know because I have not measured it or heard one way or the other. But I can suspect based on logic. Does that make sense?

As for short off times, I am less concerned from an efficiency standpoint so long as I have good, long "on" times. I will have to think about this more, what such a scenario means in terms of load at a given time. If a long burn time is followed by a short off time it would tell me that there is a low mass emitter side and a higher mass boiler side maybe? Mmmm. My head is beginning to hurt. Let me think about that some more. I really do not know.

Thanks for prodding me awake, Chuck!

Ken_40

Interesting thread

I didn't see the system "form" stated anywhere. Since steam, radiant, F/H/A and HWBB would all have hugely divergent responses/answers, I am puzzled as to which form we are responding to?

Then too, the fuel used makes some difference as well. The inclusion of D/H/W and whether coiled or indirect - also has a huge impact on an answer.

So, before we all continue to look like savants, what do we have as the form and intended use of the cycling equipment?

Only after we know the basic parameters can we respond properly, or did I miss somtehing yet again?

Brad White_9

A lot of variables, Ken

But you are absolutely right, a basis of operation should be stipulated first. I took it as a "heating demand" question and that should have been stated.

A run to make DHW with an indirect seems like an acceptable departure from modulating and condensing and with a conventional cast iron boiler might be a welcome long run just the same.

The other part of the approach I was taking for one, is to ferret out the common elements regardless of fuel used. All have different stabilization times I am sure, but the point remains that it is detrimental to efficiency if one aborts a firing before achieving even that minimum stability. (One would hope for a long run afterwards too, indicating properly sized equipment.)

I am glad you gave your perspective- it should have been stated up-front.

Thanks!

Brad

ALH_4

how about this?

How about this definition of short cycling for a hot water system? Has this been used before?

The determination of whether cycling is normal or if it is "short cycling" is based on the ratio of the minimum boiler input to the minimum anticipated design load.

For example if you have a bathroom zone with a 5000 btu/h design load and a mod/con with a minimum boiler output of 20,000 btu/h, you have a 4:1 cycling ratio.

It's more complicated than that, but different boiler designs might tolerate different "cycling ratios". What I'm trying to get at is that the reason is more important than the actual times.

What do you think? Does this idea have any merit?

Vermonter

Single pipe steam...

I was asking about short cycling for a single pipe steam system equiped with a W-M EG55, vaporstat, natural gas, electronic pilot, and automatic damper.

I'm tuning the vaporstat and want to make sure that I'm not short cycling; especially since it seems like the cycles are so much shorter now that I removed the pressuretrol.

Lots of interesting things already in this thread.

chuckNJ

specifics

I thought it would be interesting to try and bring out the relevant considerations for the specific case I raised that are independent of a specific configuraion - of course the correct application of those considerations depends on all the system specifics...In my case it is a 125KBTU input atmospheric gas steamer, no DHW. Currently cycles by vaporstat on preassure at the end of a long run (when either the house or the iron has to be picked up from cool or presumably on design day, if I ever see that), about 210 seconds on to 45 sec off (approx 4 min period). Currently cycling between about 14 oz and 4 oz. Since by the gas meter it is running 5-6% underfired, I could be tempted to have the fire increased a bit to improve responsiveness (pickup) in the shoulder seasons when it has to start from cool frequently. By load the system could certainly support 5% additional input. (295 sq ft CI radiation) On the other hand, increasing the fire would shorten the end of run cycle period (and reach it a bit sooner as well). Enough specifics? thoughts on what is "optimum" guys?

I also point out that since my question relates specifically to the situation where the system is already at tempurature and has been steaming for a while, the HEX and combustion area are alerady close to stable conditions. And I also point out that this preassure cycling IS a form of fire modulation (digital, or pulse, modulation. Hey - its digital!). Even if a low enough capacity hi-low burner does exist, it certainly would require a significantly more complicated (expensive) gas train and one would still cycling a valve so the wear/tear factor (at least for gas atmospheric) would be the same, no?

chuckNJ

change

Yes, we definitely burn less fuel not bringing the preasure, and temperature, up with vaporstat v preassuretrol. In my case, 15-20 min less burn time before I enter preassure control mode. Your original question is, I beleive, the same as mine.

Uni R_3

Both a ratio of good time over total time against duty cycles

Defining short cycling to me would imply a ratio of the useful running time over the total running time of the cycle. To me that would mean a number like 80 or 90%.

So if an oil boiler is on a cycle and it is 25 seconds just for ignition, 2 minutes warming up, 5 seconds shutting down, it could be factored like this. Give credit for half efficiency while warming up and it has 1½ minutes waste in a cycle. If you want an 85% ratio then it needs to fire for 10 minutes (10-1.5)/10 = 85%.

This needs to be viewed in the context of the duty life cycle of the equipment. An electric boiler might not be short-cycling according to the above even if it cycled every 5 seconds. However, if this meant that is was shortening the life of the boiler then obviously it would need to be matched to a cycle length appropriate for longetivity.

Uni R_3

2 Parameters

I really think you need to satisfy 2 parameters:

A minimum required ratio of fully efficient cycle time of the total cycle time.

and

Not be jeapardizing the lifespan of components

Examples:

If a boiler wastes 1½ minutes, prepurging, warming up etc then it is 85% fully efficient doing 10 minute firing cyles. If 10 minutes does not affect the lifespan of components it isn't shortcycling.

An electric boiler might be well over 90% efficient doing 2 second pulses but cutting it's life by 3/4. MTBF analyses should be used to say that perhaps it needs to do 2 minute minimum cycles or the elements might burn out faster.

At times one might need to strike a balance between these two parameters. At that point it should be fully costed, including a fully costed risk analysis.

Ken_40

OK!

Mr. Vermonter (now my home state as well(:-o) has given the parameters.

Set the vaporstat to the absolute lowest setting it will reliably (every time - without fail) "reset" to "make," and set the "break" pressure to ~10"w.c. lower than dimension 'A'

If a true vapor system, and merely using a vaporstat in no way implies it is a true vapor system, the maximum should be whatever the basic designs called for as per TLAOSH. If unknown, I'd use 1 p.s.i. as the maxiumum.

If the boiler is sized properly, the 'stat will give you most of the control, not the vaporstat. Unless of course, the house is a thermal seive?

Vermonter

Now we're talking!

So it sounds like it's going to be different for every system, but that being said how do I determine my minimum safe cycle time for the beast in the basement?

Maybe I should covert back to coal...

Mitch_4

less than

> Is there an official definition for short

> cycling? Less than 5 minutes firing per cycle?

> Less than 3?



126.321657 seconds is the ballpark I use.

;-}

Actually I use anything less than 5 minutes as too short a cycle as the unit never reaches steady state. I like to see at least 10

jim_110

Vermonter "short cycling" in trade lingo usually means a more ominous symptom associated with a boiler's LWCO shutting it off due to water levels dropping during firing. On this site consumers more often then not use the term to explain the cycles your system is experiencing. All things being equal, if a system is vented and piped properly but is grossly oversized for the amount of EDR, it will make more steam then it has the ability to condense. This will eventually cause the system to build pressure during the initial cycle and shut down on cut-out. In this example the advantages of a vaporstat over a pressuretrol are somewhat muted since both will shut the system depending on cut-out.The length of subsequent "cycles" will be dictated by your high setting(cutout), no more then 1.5 lbs ,and your systems ability to hold pressure until cut-in(1 to 6 ozs) is achieved.Your concern about "SAFE" will be placated if you keep within the above range. The number of cycles that occur until the stat is satisfied are mainly a function of the tightness of your home. This cycle malady can be reduced an d in cases eliminated through a careful derateing of the burner (PROS ONLY HERE).Oddly if you called your boiler maker and was able to speak to a Tech ,he would likely say that your cycling is normal and the boiler was doing what it was designed to do,make heat to satisfy the stat.As for efficiencies , if you notice at cut-in your system still contains pressure and upon firing pressure is stabilized very quickly.This suggests that that the performance efficiencies loss are minimal and that the greater concern should be the eroding effect of excessive cycling on controls.But the fact remains, if your system is grossly oversized your using more fuel then necessary .Good Luck

.

Jamie Hall

Congratulations!

This has been a superb discussion of an uncommonly murky subject....

Dan, are you listening?

hvacfreak

most here say it's normal

( edit : What exactly is " short cycling ? " )It's what my electronic lwco " gaurd dog " does to the burner.

I do understand control logic and safeties ...and this thing should be be the defendant in a major class - action law suit. For those who don't know , this device " interupts " a clean burner start and fire after like 2 minutes ( on a NORMAL CALL FOR HEAT )...at this point it does " whatever " ( proof of water ) ....starts again with a warm combustion chamber.

On the next fire ( after delay ) , my efficiency is way down ( I bypassed it and watched it for many cycles ( call for heat and pressure cut-out ) with the lwco jumped out to compare ). A wonderful thing that this device has cost me countless gallons of fuel , and purchase a quality soot vac.

I understand that the origional post asked about " system short cycle " ....but understand...if the burner is " short cycled " for whatever reason , the end result is the same. I need to replace this device with a mechanical lwco , bottom line. And I can't understand why the " Guard Dog " can't prove water while keeping the burner " on " . Some will say " foaming "... **** ever...this thing must be marketed by Exxon or something.

Vermonter

I'm glad my ignorance has contributed!

I've enjoyed this one too.

Vermonter

How can you be so sure?

Perhaps you underestimate my ignorance!

chuckNJ

Ken

I'm curious what the underlying basis is for your suggestion to set the cut out at 10"w.c. less than the "A" dimension? I'm probably missing the obvious...

Also, would it be correct to say that setting the cut in point should also be constrained to not allow the boiler to drop into vacuum (allowing for ignition delay items such as vent dampers that need to open or prepurge cycles)?

thanks!

Ken_40

Dimension 'A'

is presumed to be the point at which water will naturally back out of the boiler and block the main and main vent.

If you allow ~ 10" for, friction loss, and potential minor surging, you have a small yet critical margin of safety before spitting water from the main vent and return pipe "stacking."

Which ever method is used to determine pressure max. (dimension 'A' less 10") (or 1-1/2" psi), the lower number would become the upper limit/setting to "break" the burner.

realolman

wouldn't that underestimation

prove me even more ignorant, still?

Never thought I'd become embroiled in this discussion.

Just shows you how ignorant I am.

Vermonter

I understand the Dimension A provision, but why would I want to set the vaporstat to cut-in at the minimum possible by the vaporstat? Wouldn't it be better to cut in at the minimum needed to supply steam to the farthest radiator?

Vermonter

Well done [nt]

realolman

All right !

in yer face!!

Doh!

Ken_40

Because...

With time (and there would be a LONG time between first firing - and the point at which the system would get enough pressure to cycle off on limit, then drop back to another firing time) the system will virtually eliminate pick-up losses, and be at near thermal stasis. At this point, the pressure at the boiler is almost the pressure in the entire system and the furthest rad is already near completely heated - thereby making so-called "friction losses" almost nil.

Also remember, the lower the pressue the faster the steam.

Ken_40

Time out!

Guard Dogs are fine.

Clean the boiler water with TSP. Remove the probe and clean it with a sift brush and strong solution of detergent (TSP is fine gere too).

Betcha lunch at a good restaurant the problem dissapears!

Vermonter

So.....

Once everything is nice and hot, once the air vents are nice and hot, then pressure drop from the boiler to the farthest radiator becomes negligible?

chuckNJ

MTBF

or really what it would be good to know is MCBF (mean cycles before failure) on burners and other mechanical components...I certainly would like to be able to do a fully costed risk analysis...but these component life expectancy specs don't seem to be published - I guess you probably need to be a boiler manufacturer under non-disclosure to get them...

chuckNJ

equalizer

I thought I understood from THLAOSH chapter on Hartford loops that a properly sized and installed equalizer effectively removed boiler preassure from Dimension "A" analysis.

It seems to me from this discussion that once you have 2+ minute long burn time on each cycle, the cut-out setting becomes a balance between incrementally increasing fuel usage pushing to higher preasure with longer "on" periods vs incremental decrease in component life expectancy due to an increased number of shorter cycles at a lower cut-out setting.