Q: about cycles, what is short cycling and what not

MrDvorak

My boiler has a 3PSI gauge and a new shiny Vaporstat installed. I had had it running for long minutes until it began cycling earlier today and here is what the cycle looked like:



T-zero: cold start, pressure hovers around .1 PSI after 5 minutes, after approx 25 minutes the vaporstat trips the burner at .5 PSI

T-one: approx. 50s later, vaportstat cuts in when as the pressure drops to ~0.25 PSI

T-two: it takes about 15s for the burner to get going, the pressure drops to .1-.05 PSI in the mean time and the pressure begins to rise immediately

T-thre: 55s later, the vaportstat cuts out at .5 PSI again and the cycle repeats



The whole cycle was about 2 minutes long, the heating took about 70s and the cooling 50s, the pressure oscillated between ~.05 to .5 PSI. I set the vaporstat for the pressure not to go above .5 and below .00 PSI. I assume that if I wanted longer cycles, I would either have to go with a higher cut-out pressure, or a smaller boiler, or bigger heat losses in the radiators (more radiators, larger house, colder weather, etc.).



Does this look right to you? How much oil gets wasted during repeating ignitions every two minutes? I have a pro coming to inspect the system before the Winter hits hard, but I would like to know your opinions, too.



Thank you,

-MrD

nicholas bonham-carter

short-cycling

next time you are observing the boiler, make a note of the pressure in ounces as the boiler first begins to boil. this is the back-pressure of [main] venting, and that should be as low as possible. my system vents at no more than 2 ounces, so i am pretty well vented.

it is possible that your short-cycling is just the pushing the air out of the mains, and the steam chest of the boiler itself. naturally, you can't do anything about the size of the boiler, unless you have a modulating burner, but you can make the venting capacious. i suppose you could take the vents off and fire the boiler, to see if the short-cycling behaviour changes, but be careful of steam--it's hot, and wet enough to remove wall paper!--nbc

nicholas bonham-carter

short-cycling

next time you are observing the boiler, make a note of the pressure in ounces as the boiler first begins to boil. this is the back-pressure of [main] venting, and that should be as low as possible. my system vents at no more than 2 ounces, so i am pretty well vented.

it is possible that your short-cycling is just the pushing the air out of the mains, and the steam chest of the boiler itself. naturally, you can't do anything about the size of the boiler, unless you have a modulating burner, but you can make the venting capacious. i suppose you could take the vents off and fire the boiler, to see if the short-cycling behaviour changes, but be careful of steam--it's hot, and wet enough to remove wall paper!--nbc

BobC

Short cycling example

This boiler is much too large for the load but it runs well so it is what it is. I only used 300 gallons of oil last year so I'll keep it till something happens to it. The unfortunate result is it makes a lot more steam than I can use, maybe i can sell some to my neighbor?



I'm assuming the four minute dwell at 0.2psi is the radiators venting?



This is a log of my first boiler run since it was cleaned in August.

    46F outside - 1st boiler run of season

       Tstat set for 65 room at 63



00:00:00   Tstat on

00:06:10   Steam at boiler outlet

00:09:35   0.05psi   Steam at main airvent

00:10:30   0.2psi

00:14:10   0.2psi

00:16:29   0.4psi

00:16:50   0.6psi

00:17:10   0.75psi    Vstat off

00:17:59   0.3psi      Vstat on

00:19:12   0.75psi        off

00:20:05   0.3psi          on

00:21:02                       off

00:21:59                       on

00:22:57                       off

00:23:59                       on

00:24:50                       off

00:25:55                       on

00:26:53                       off

00:27:59                       on

00:28:50                       off

00:30:00                       on

00:30:54     Tstat off



00:23:52 Total run time



Bob

MrDvorak

Good stuff

BobC: Thank you very much. Your cycling is similar to mine. How do I tell the exact point at which there is fsteam at boiler outlet?



Nicholas: I was quite confident that my system was well vented, what makes you think otherwise? The pressure is not measurable for the first five-ish minutes (I need to start measuring times from scratch tomorrow). Later it hovers around 0.1 PSI for looong minutes until the system gets saturated, then pressure shoots up quickly and begins cycling. During initial venting, the pressure never raises above 0.1 PSI; and during cycling the pressure falls/raises quite rapidly - another sign that venting is not an issue. Btw if I let the boiler cool, the pressure drops to -0.5 PSI before the Gorton #2s let in air. They open instantly at this pressure and the air suction is audible.

BobC

to hot to touch

My boilers steam outlet comes out the top of the boiler, I left about 1" of bare pipe uninsulated that I can feel. You will know when it has steam in it, to hot to touch for more than a couple of seconds.



Bob

Dave in QCA

Short Cycling is.....

Short Cycling is a condition where the boiler shuts off and on well before steam has filled the system.  This can be caused by:

Improper anticipator setting on the thermostat.

Improper venting on the system causing pressure to build once the boiler starts to steam, but well before the system is filled with steam.

Other faulty conditions where devices are causing the boiler to cycle off and on even though the system is not fully filled with steam.



When the system is fully steamed and pressure starts to rise, it is expected that the boiler will cycle off.  The frequency with which this is happening on your system would suggest that your boiler my be oversized for your system.  It might be possible to derate the burner.  Also, I would expect that if your thermostat was left at a constant setting, this probably does not occur.  I am guessing that it occurs only after coming out of a setback condition.

MrDvorak

Makes sense

Thank you Gentlemen for your help. I have an independent mechanic with good references coming to clean and adjust the burner tomorrow. I will ask him if it would be possible to tune it down a bit. Your assumption about the cold start is correct Dave, I have a plain on/off switch installed temporarily during my maintenance exercise and it was switched on for a longer time than normal as I was observing the behavior of the system with a new Vaporstat.

David Nadle

"late" short cycling

I think your (our) situation is not uncommon but is not really textbook short-cycling. The key is the 25 minutes till fhe first cutoff. My system is almost exactly sized for the attached radation and it acts the same way. Whenever the vaporstat cuts out before the thermostat the system oscillates 1 minute on, 1 minute off. But usually the thermostat cuts out first. I don't know what could be done to make the system hold pressure longer anyway.

MrDvorak

Big boiler

The burner mechanic told me today that my boiler is a commercial application six segments beast (IIRC), clearly oversized. He set the burner to use 1.2GPH instead of the default 1.5GPH. I am not convinced this is going to help much, it will probably just take longer for the steam to show up. Thank you for your help gentlemen!

Dave in QCA

Significant Decrease

It would have been a good idea to do a steam survey first.  That would involve examining all of your radiators, measuring them, and determining the total EDR or sq ft surface area of the radiators.  However, with the speed with which you were building pressure after everything was heated up, I think you will be OK.



You may notice that it takes a bit longer for the steam to make it to the radiators, but not all that much.  You will notice that it will take longer to bring all of the radiators up to a fully steamed condition.  That is when you pressure begins to build, and it will also be a slower process.  If you get to the point that the vaporstat shuts the burner off, the pressure will drop just as fast as it did before, but when the burner cycles back on, the pressure will not build as fast.



Your radiators have the ability to accept and condense a certain amount of steam.  If your boiler is sized just a bit larger than the radiator rating it will take a very very long time for the boiler to build enough pressure to cause the burner to cycle off.  In your case, the boiler was WAY oversized and you were making much more steam than your radiators could handle.  You have made adjustements to bring the boiler closer inline with the capacity of the radiators.



Let us know how it works.

BobC

Should save fuel

my boiler was rated at 1.65 GPH and originally installed with a 1.5gph nozzle. Over the last 5 years I've winnowed that down to 1.1gphand that has saved fuel but I still have short cycling.



Bob

MrDvorak

How do you like them cycles?

With the boiler set to 1.2GPH, the cycle is a little longer at 2 min 10sec.



This is what the heating cycles look like in my home. I have a temp. logger with a probe taped to the exhaust pipe where it enters the chimney. I can quickly calculate how much oil I spent per day from this data, and it makes an interesting correlation with degree days data available at i.e. http://www.degreedays.net