Short cycling and what sends off signal

[Deleted User]

I'm dealing with some vent issues after a lot of help from people on this forum. Right now I have Gorton #2's on my mains although one #2 is slower than the other and I already returned one because it didn't vent at all.



But in playing with this and reading the forum, another question came up. There is a lot of talk about short cycling, but I've never seen it defined. My boiler is currently running for about 5 minutes and then starting again after about 10 minutes. I'm guessing that is short cycling?



My other question is, how can you tell whether it's the thermostat or the pressure that sends the signal to stop? Is there a way to find out with a multimeter or something?



I don't know what my pressures are. The pressuretrol is set for 4 and 2, but I suspect the pressures are much lower than that. I ordered a 0-3 gauge, the 0-30 gauge never budges.

Jamie Hall

What you are describing

isn't classic short cycling -- nor is it end of run pressure cycling.  What it sounds very much like to me is a thermostat problem.  Thermostats aren't quite as plain vanilla as one might think.  Some have an anticipator in them.  This is a device which actually heats the thermostat itself while the boiler is running, and causes the thermostat to think it is warmer than it is and turn off sooner than it should; properly adjusted, it can pretty well prevent overshooting.  Some have a "swing" adjustment, which sets the cut in and cut out temperatures, if you will -- a thermostat set to swing 1 degree will cut in at a degree less than the setpoint, and cutout at a degree over.



However, many thermostats have "cycles per hour" setting.  This has to be set correctly for the type of heating system you have, and your description sounds exactly like a thermostat set up for hot air heat: 6 cycles per hour.  Steam should be set up for 1 cycle per hour (atthough a couple of folks have said that 2 also works).  The instructions that came with the thermostat should tell you how to check this setting, and set it correctly.



Try that, and see if it helps.



As to checking whether it's the thermostat or pressure, there are nifty ways of doing it with a multimeter.  In fact, that or a test light or jumpers is about the only way to be absolutely sure.  However, if it is pressure the off time will almost always be very short -- a minute or two at the most, and often just long enough for the burner to do a post purge, if that!



Now as to definitions of short cycling -- there are several flavours.  The most common occurs towards the end of a longer run -- say coming back from a setback -- and is caused by the radiation not being able to condense the steam quite as fast as the boiler makes it.  It's pretty well harmless, and doesn't mean anything is wrong.  On the other hand, if you get the boiler cycling fairly quickly on pressure early in a cycle -- say five to ten minutes in -- then it is more likely that the problem isn't slight over capacity in the boiler, but inadequate venting not letting the air out as fast as the boiler can steam.

Steve Nichols

Ptrol too high

I'm sure the pros will weigh in on this, but let me start things off (disclaimer: just a homeowner that loves steam).   The pressuretrol should keep the boiler in between about 0.5 and 1.5psi.  If you have a perfectly matched boiler to radiation system, you can operate on ounces of pressure, but let's assume you are working on the .5 to 1.5 psi range.

If you have enough venting on the mains (assuming you calculated the volume of air in the mains and have sized the vents to exhaust that air in a few minutes), then other issues that pop to mind are an oversized boiler.    Can you time how long it takes steam to get from boiler risers to vents?  How long is it--a few minutes or longer?

Did the installer calculate the connnected radiation load (measured in Square feet of steam EDR) by measuring all your radiators?  Boilers will have a rating indicating the Square feet of steam they produce.  That needs to be close to the same value as the EDR calc. for the radiators.  If the boiler produces say 358 Sq Ft (Burnham 5 section) and all your radiators add up to 250 sq. feet, you have my system.  The boiler is producing more steam than the rads can condense and that causes pressure to increase. 

Moving to the radiator vents--if your radiator vents are not working properly, then the air will have difficulty getting out and the pressure will rise. Also, the heat will be uneven in the rads.

Basically, if Tstat calls for heat boiler runs until the max pressure is reached (controlled by Ptrol). That will cut power to boiler and let Pressure go down until you hit the "cut in" pressure.  IF the Tstat is still calling for heat, the boiler fires up and the cycle repeats.  You can see on the Tstat when the boiler is runnning as it should give some signal or message like "Heat" or something like that.

Do you set the thermostat to a lower temp at night?  The difference between day and night temperature is called a "setback".  Steam doesn't do well with large setbacks.  My system can' t recover more than 2 degrees without a few short cycles.  Typical on cycle for me is about 6 mins and 20 mins off (during the cold weather).

Once you get that 0-3psi gauge, you'll really see what's going on. That will tell us a lot.  I don't know how much other information you have posted, but the near boiler piping is where I'd start looking and go from there.  Please post pictures as they help us see what's going on.

[Deleted User]

old school thermostat

Thanks Jamie and Steve. It's just an old mechanical thermostat. I'm not saying it is working as it should, but it certainly isn't programmed with cut-in and -out temperatures or cycles per hour.



Based on Jamie suggesting pressure-off would only last a few minutes at most, and mine is lasting ten minutes, does that mean the thermostat is doing it? I can't imagine, with the boiler running only 5 minutes, the thermostat is already registering a temperature increase, but who knows. Should I replace the thermostat?  Any suggestions on a replacement?  It sounds like if I had an electronic thermostat, I could easily tell whether the thermostat or something else (pressure) is turning off the boiler?

Inadequate venting is another possibility you both brought up (though that would suggest the pressure is shortening the cycle).  Would that be mains or radiator venting, or both?  It's possibly inadequate.  I'm sure I can figure out how to calculate the volume of the mains, but how do I determine what kind of vents are needed to clear that out in say, two minutes?  I assume that would depend on the pressure too, which I don't know (should have a lo-pressure gauge by sometime next week).

I had to smile when you asked about what the installer did Steve.  I'm quite sure this radiator was put in when the house was built in 1918.  So this is a classic deadmen installation!  I posted photos in another thread http://www.heatinghelp.com/forum-thread/149215/New-main-vents-but-mains-seem-unbalanced  Eventually I will try to do all that radiator calculating myself, but need to learn more first.

Thanks for the great info.  I'll leave you guys alone till I get the gauge on. 

ChicagoCooperator

Two-pipe short cycling...

Would short-cycling be caused similarly in a two-pipe system as well? I'd love a good definition myself. 

[Deleted User]

Sometimes my ignorance overwhelms me

How do I take this stuff apart to add the new gauge? Do I have to unwire the ptrol so I can spin it off? I plan to clean the pigtail while I'm at it

Jamie Hall

Unions are useful gadgets...

and it would be so nice to have them where you want them.



Yes, you have to disconnect the pressuretrol to take it off.  Just be sure to note carefully what connects to what, and put it back together just as it was.  And don't drop it.



While you are at repiping to add the new gauge -- probably the easiest thing to do is to replace the pressuretrol's elbow with a T, and extend the horizontal with a nipple and an elbow to the new gauge -- I'd suggest adding a union under the pressuretrol...  it will mean another nipple, but that's trivial.



Good idea to clean the pigtail -- and the opening to the boiler -- while you're at it.