Pressuretrol cut-in/cut-out cycle time

BrianStMarie

I recently had my pressuretrol and PSI gauge replaced on my system. However, I'm seeing some weird behavior with the new setup.



The pressure gauge is 0-15 psi and is mounted on its own pigtail. The pressuretrol is a Honeywell P404A, mounted on another pigtail, and I have it set to 0.5 cut-in with a diff of 1.



My system is fully hot and all the vents are closed after about 1 hour of constant running. At this point, the pressure starts to build and then it cycles between the cut-in and cut-off settings of the pressuretrol. However, the run time is about 3:30 to get to cut-out and 1:30 to get back down to cut-in. Is this an accurate cycle time for a steam system under pressure? It seems very short.



Also, I never see the pressure on the gauge rise much at all. I know the gauge is working, because if I set the pressuretrol settings higher, the gauge will start to creep up. So either the pressuretrol is running at much lower than 0.5-1.5 or the gauge accuracy is absolutely terrible at under 6 psi. Best I can think of is that because they are on different pigtails, they are experiencing different pressures.



Any thoughts? Should I be concerned about the short cycle time while under pressure? What's a normal cycle time between high and low pressure settings?

nicholas bonham-carter

short-cycling?

in a perfect system, with massive main [not rad] venting, the steaming capacity of the boiler would so perfectly match the condensing capacity of the radiators; that no pressure would ever build up, even though the radiators were full of steam.

in your case, the main venting may be unable to let the air gently escape, and so the gas company is pushing the air out for you. unfortunately, the 15 psi gauge is as useless as the 30 psi for steam system diagnosis. a 0-3 psi gauge will tell you when the gas company can stop charging extra for air-pumping. another problem may be an over-sized boiler whose capacity needs more radiators than you have.

i would always put a gauge on the same pigtail as the pressuretrol, so you can see what the pressuretrol sees. if the pressuretrol is an older mercury switch model, make sure the axis of the pigtail is perpendicular to the face, so as the metal heats up, and the curl unfolds slightly, the mercury switch will not be tipped into a false actuation.--nbc

BrianStMarie

Vents are ok

The main vent and radiator vents are working fine. Once the system is hot and full of steam, they all close, as they should. This is the point when the pressure starts to build. Since I have an oil system and a non-modulating burner, the system just cycles on and off to keep the pressure in the correct range. It just seems that the cycle time is short and I'm concerned it should not be so short. I don't have the experience to know what a typical cycle time is for a small residential steam system.

Dave in QCA

normal

You had indicated that it took an hour of firing before your boiler started to build steam and that this happened only after the system was entirely heated.  So, this is not a venting problem. 

As to the cycling, once the boiler shuts off on high pressure, the pressure will fall very quickly and will start to build pressure again as soon as firing begins again.  What you describe is common in a boiler that has been firing for a prolonged period of time.  I am assuming that this occurs when you're coming out of a set back.  Most boilers do not operate for that long of a period unless the weather is extremely cold.

crash2009

3 TO 1 RATIO

Mine short cycled a few times this winter.  The timing on this one is on for a minute, off for three, on for a minute, off for three.  Don't know if this is normal, wouldn't be surprised if it was.  It is re-assuring to know that the pressure controller is working.

Luv'nsteam

Cool chart

Crash.  Where did you get the instrument to write it and what is it called?  How did you wire it in and is it permanent to your boiler?



Thanks,

Mike

Jamie Hall

Cycle time once pressure starts to build

Yours sounds pretty typical.  The time from cutout to cutin -- the off time -- will usually be quite short, as the pressure drops very fast as the steam condenses.  The actual time will be controlled either by condensing rate -- or by the minimum reset time of your burner, which may be longer (often is).  The on time is simply the time it takes for pressure to build again.  The ratio of the two is related to just how much more steam your boiler can make than your system can condense once it is fully heated up.



I've always enjoyed Crash's charts.  Way too much free time... !

Dave in QCA

Crash's Charts

Jamie, yes, I too enjoy, appreciate, and envy Crash's fantastic charts.  But I think you may be correct..... way too much time on his hands!  He obviously needs a much bigger house!

crash2009

There is not much to it Mike

  JPF and Mike K turned me on to it awhile back.  Manufactured by a company named Lascar Electronics.  The model I have is EL-USB-TC. Mine included a thermocouple which I inserted into a crack in the mortar, just touching the flue.  As I understand it, others have inserted the probe directly in the flue (possibly using the analyzer hole).  The logger has a USB interface to allow it to be plugged into your computer.  Sampling rates can be adjusted from 1 per second to 1 per hour.  Software is included.  The data can be imported into EXCEL for further manipulation if desired.   The model I ordered was less than a hundred.   http://www.lascarelectronics.com/data-logger/  Everybody should have at least one.  Sure is a good timesaver, teacher, and watchdog.

Luv'nsteam

Useful AND cool

Diagnostic tool.  This little gadget would also be quite helpful in understanding odd or intermittent sealed-system problems with fridges, freezers, a/c's, etc.  Thanks for sharing this Crash.  I will be getting one soon!



Thank you,

Mike

crash2009

Here I am

with more time on my hands.  Jamie, so are you saying that you can tell how much a boiler is oversized by the cut-out cut-in ratio?  It never occured to me before but now that you mention it, if on for 1, and off for 3 is a cycle having 4 parts 3+1=4, and if those 4 parts are compared to some real EDR ratings such as my place for example.  The connected EDR is 377, the boiler is oversized by 123, and the boiler is rated for 500.  377+123=500  (3+1= 500) It's amazing that you can see that just by looking at some timings on a chart.   

You and Dave are correct about me having a little extra time.  As you both know I was too green to burn a couple years ago.  Still a little wet behind the ears.  Every fall I look forward to moving my office into the boiler room.  It's like a little mini-steam-school in here.  These charts really do help me to understand what is going on inside those pipes. 

Now watch this!  Jamie is gonna say I havn't learned sh#@ and I got it all backwards.     

Mike Kusiak_2

Lascar and cycling

This place has the best price and free shipping



http://www.testequipmentdepot.com/lascar/dataloggers/elusbtc.htm



Crash you are precisely correct about the relationship between pressure cycling on-off time and oversize factor. The duty cycle, or the percentage of on time divided by the total cycle time is inversely related to the oversize factor. If the boiler generates twice as much steam as the rads can condense, then it needs to burn only half the time to completely fill the system. So a 50% duty cycle ( say 2 min on, 2 min off )  corresponds to a boiler generating double the steam that the system can accept.



So oversize factor = 1/ [Ton/(Ton+Toff)]

crash2009

I seem to remember

 reading a thread about this a while back.  What is the name of this theory?  Lewton? Loutom?

Mike Kusiak_2

"Lutorm's law"

Yes, this relationship was first noticed a few years ago by a guy working on his system, who happened to be a nuclear physicist.



http://www.heatinghelp.com/forum-thread/128792/More-boiler-cycling-thoughts



Over the years I have found that it accurately confirms the sizing of the boiler, provided the venting is adequate to allow all the air to leave the piping and rads before pressure cycling occurs.



It accurately measures the steam condensing capability of both the piping and rads. The typical "pickup and piping factor" includes the piping loss. So if the piping loss is considered as being included, the remaining oversize for "pickup" might be about 15% of the EDR. In this case the "ideal" cycling on pressure would be 1/1.15 or 87%. This would correspond to a ratio of about  7 min on, 1 min off.

crash2009

I think I get it now

Back to BRSM:  You have declared that your boiler is cycling at 1:30/3:30

If I am getting this right.  Welcome to the oversized club.  Mine is 33% oversized.  Your 1:30/3:30 cycling is normal for a boiler which is 30% oversized.  Man these guys are smart.  They just took one look at your numbers and figured it out in their head. 

The 0-3psi gauge is pretty accurate.  Everybody is always talking in ounces around here so I went and got a 0-20 ounce gauge.  Others are using gauges even lower.  The lower the gauge the more accurate a reading you get.  If you are going to look for some new gauges, look into a snubber to protect your investment Model 722BG 1-888-SNUBBER.  While you are at it, you might as well look into a vaporstat.  The vaporstat will help to keep your pressures even lower than a pressuretrol.  Less pressure equals a lower fuel bill.  

crash2009

I just re-read

Lutorms Law.  Maybe I am not getting it 100%.  In your example there of on for 3, and off for 1, you calculate 33% oversized, somehow I calculated only 25%.  I think I figured % of oversize=1/4 of 4, instead of % of oversize=1/3 of 3

Might have to read it a couple more times.

Mike Kusiak_2

Reciprocal of duty cycle

Crash, in your case duty cycle is (3 on,1 off ) or 3/3+1 = 3/4 or 75% on.



Oversize is 1 divided by duty cycle, or 1 divided by 3/4 = 4/3



4/3 = 1.33 or 33% oversize compared to condensing ability of system.

crash2009

Condensing ability

Ithink I see where I was going wrong.  I was using the rating of the boiler (500) and subtracting the existing radiation (377) to arrive at (123) which is about 25% of 500.

This new (to me) method of calculating oversize you don't even have to consider the connected EDR, boiler rating, or the EDR missing.  The timing is the whole equation.  This feels weird to divide something I am unable to put my hands on.



I have read that condensing ability can be improved with venting, Mr Gill has mentioned it several times, and you have also said "provided the venting is adequate", NBC suggested a venting check 20 minutes into this thread. 

How does venting or "lack of venting" play into the condensing ability of a system?

BrianStMarie

Points to a bigger issue

"Lutorm's Law" works out to about 43% oversized for me. This makes some sense, because I only have about 140 sq ft of EDR in my condo and my boiler (Burnham V83) is rated for 283 sq ft of steam. Living in a small place seems to always guarantee you'll have an oversized system.



What's got me worried now is that when under pressure, the pick-up of the system should no longer be a factor, since all the pipes and radiators are already hot. This means that my boiler should be dumping the fully rated MBH into the pipes, which in my case is 91,000 btu/hr, less the time it's off per hour due to pressure conditions. So in my case, the duty cycle is 70% on, times the 91 MBH, which gives me an output of 63,700 BTU/hr while under pressure. But based on my EDR, I'm only actually getting 33,600 BTU into my condo through my radiators. So now the big question is, where the heck are the other 30,000 BTUs/hr going? That's a depressingly high number to see lost through radiance of the boiler piping, especially considering I just finished insulating the system. Probably a couple thousand BTUs/hr can be accounted for by the difference in efficiency of my real-world system versus Burnham's hypothetical claims, but even taking that into account leaves me with a large number of missing BTUs/hr.

Mike Kusiak_2

Downfired?

Is your boiler actually putting out 91K BTU? Perhaps the previous owner had the burner downfired with a smaller nozzle.



 Without knowing the actual nozzle size and the operating oil pressure it would be hard to tell what the firing rate would be. Also a combustion test might give you some clue as to efficiency and output.



Are you sure the EDR of the radiation is accurate? Is there much uninsulated piping in the walls, etc?

crash2009

Burnham V83

Isn't that boiler oil fired?  If it is, why not have it downfired?



Thats scary, we must have been typing at the same time.

Mike Kusiak_2

Venting

If all the air is not driven from the piping and radiators, then the system may not be completely filled with steam when pressure cycling begins. If that happens the measurements will not be accurate, as any air left in the system will reduce its effective EDR.



To be valid the duty cycle must be measured when the system is completely filled with steam and the rads heated all the way across with the vents closed by contact with the steam. Then you are at the point of steady state condensing, at a rate determined by the total amount of radiator EDR and the piping loss.

crash2009

Missing 30,000 BTU's

Can you lose 30,000 up the chimney?  Are you also missing some water?  Is it possible the boiler has a hole in it?

BrianStMarie

Could be downfired

Entirely possible it's downfired, but I have no idea how I'd go about checking.



Crash, not sure what you mean about missing water. Haven't had to add any water to the system yet this season. Everything seems very tight.



As far as the chimney loss, it's my understanding that the MBH takes the combustion efficiency and stack losses into account already.

crash2009

Maybe the boiler is shot

I was just looking back on some of your previous posts.  You have collected a lot of reasons for replacement of the boiler.  Maybe you could get a pro over to confirm your suspicions.

eats oil at an impressive rate, boiler uses 800 gallons a year,

the hot water also sucks, as a tankless coil combined with the 10 minute on/off of the standard CG450 LWCO is a lousy combination, 

9 years is pretty old for a V8,

Then there is some little things, such as the sight glass.  If its nine years old, the water should not be crystal clear.

The pressuretrol stopped working recently.  Whats next?

different pigtails, they are experiencing different pressures

Now recently you have calculated that there is 30,000 btu's missing

What I am suggesting here is that maybe there is a rotted/rusted hole somewhere inside the boiler that you can't see.  This is a time when I hope I am wrong.  Maybe this is where your 30,000 btu's are getting out.

Pick a cold day for this one.  When it is running, go outside and have a look at the smoke stack.  Can you see any thick white smoke?

Pick a warm day for this next one.  Shut it off at about 6 am, let it cool until about noon, then flood the boiler to check for leaks.



Search the Wall for "rotted out V8" and you will understand why I suspect what I do.

Mark N

Not losing water

Crash he claims he hasn't had to add any water this year, so I doubt any has gone up the chimney. I would guess his supposed missing BTUs are going to generate pressure and not heat. He claims the system is fully heated and the vents are closed so why hasn't the tstat been satisfied. What surprises me is that it takes 1 hour of constant firing before the system starts to build pressure. If his boiler is truly as oversized as he claims 100%, output of boiler 283sqft EDR vs 140 sqft of rads. I would think that the short cycling would start long before that. The boiler is putting twice as much steam into system as can be condensed, pressure would have to rise. Being that it takes an hour for this to happen and only after the vents are closed I would think the output of this boiler is well matched to the amount of radiation.

BrianStMarie

Only from set back

I'm only cycling on pressure after the system is recovering from a temperature set back. For instance, the situation that caused my initial post was after we had been gone the whole weekend and I had set the heat down to 50 while we were gone. When we got back, the system had to recover from 52 to 68. In total, that recovery took a bit less than 3 hours, but the second half was pretty much constantly cycling on pressure.



My typical nightly set back is only 6-8 degrees and the system can recover from that in about an hour, so it rarely reaches pressure under those circumstances.



Mark, I agree that it seems like the boiler is relatively well matched to the heating load, so I'm also confused as to where the extra power of my boiler model is going. The oil burner is rated at 0.75 gph, but if was downfired to 0.5 gph, suddenly all the numbers make a lot more sense. I just don't know how to verify if my system is downfired. I'll ask about it at my next burner servicing, which will be in March.



As for my past posts, most of those issues have been resolved. There were a lot of misconfigurations on this system when I purchased the place, but I have been steadily fixing them. For instance, the previous pressuretrol was completely non-functional, the vents were clogged, the pipes were uninsulated, and the LWCO was cycling the system ever 10 minutes. It took forever to get heat and ate oil like mad. All that is pretty much solved at this point. I've replaced the pressuretrol and gauge, "fixed" the CG450 to disable the intermittent test, and insulated all the pipes. Our oil usage so far this winter has been a bit below 200 gallons, significantly less than the previous owners. From a cold start, it takes about 8 minutes to make steam and 45 minutes for the rads to be hot all the way across 9 sections. Under ordinary circumstances, we have heat at the radiators within about 10 minutes of the system kicking on. This all seems pretty normal to me and doesn't seem to indicate I have an oversized system. While we are on the second floor, that's only about 12 feet of risers on 5 radiators, so that doesn't seem like enough piping to be losing significant amounts of heat.



As for leaks, I don't think there are any. I ran the water level up near the top of the sight glass for the entire fall to help with getting hot water through the coil and I never saw the water level drop. When I drained the system down to the midpoint in early December, the water came out nice and clean.



All in all, I think this is a pretty good system, especially for its age. I just think the service company is a bunch of knuckle heads and never had the system set up correctly from the day of install. I won't be keeping them after this year; one reading through "The Lost Art" has made it pretty clear I know more than any of the guys they've sent out to meet with me. As an example, the guy who replaced my pressuretrol last month left it with a plugged pigtail and set to cut-in 4, cut-out 9!



At this point, I just need to figure out the firing rate of the burner and that will hopefully answer a lot of questions.

Mike Kusiak_2

Checking the firing rate

When the burner tech comes for his next visit, ask him to check the size of the nozzle in GPM and the burner oil pressure. The nozzle size is marked right on it, and the oil pressure can be measured with a gauge. Together these two factors determine the actual firing rate.



Also have a combustion analysis performed as it can also give some idea of the actual firing rate and efficiency as compared to the published specs of boiler.

Jean-David Beyer

I just don't know how to verify if my system is downfired.

If you are lucky, you may not have to wait. When I had oil heat, the tech replaced the nozzle at each service, and often left the old nozzle on the floor inside the boiler (not in the combustion chamber, just inside the outer jacket). Mine used to have a number on them that told me the nozzle size and maybe the angle (I do not remember anymore). Mine was usually 0.5 gallons per hour, and they changed the angle from time-to-time in what I now suspect was a problem they did not understand.

crash2009

I am glad

 that you have eliminated the possibility of a hole in the boiler.  I even hate to suggest it.  When you brought up the missing 30,000, all I could think of was up the chimney.

BrianStMarie

Thanks!

Thanks, Mike. I'll make sure to do that. Combustion efficiency was 85% last March, according to the service card. Measured at 84% in December when I had an energy audit. Lower than Burnham's claims, but only by a couple percent.