Boiler Short Cycling (single pipe stem system)

JMervin

I have a 15 unit apartment building with an old American Standard boiler that has started short cycling lately (confirmed after tenant complaints). I have admittedly not been very diligent blowing the boiler down weekly during the past few heating seasons mainly because it has always worked well (no excuse). I have read the Holohan books, and I have been working my way through the possibilities. The short cycling appears to be caused by the Honewell Pressuretrol because the boiler pressure is getting up to 2.5 PSI if I am reading the gauge right and the burner shuts off. It will re-start after the pressure drops. I have ruled out the thermostat because it measures the temp in the hallway and is on a timer for set hours of the day.

Other Symptoms:
1. After blow down the site glass is very slow to refill. I recall it refilling faster in years past.
2. I had contractor replace a small section of cold water return in copper (he chose copper?). I have since fired that individual, but I have not changed out the approximately 1.5 feet of copper which is just above the Hartford loop.

Equipment:
2. American Standard boiler 420,000 btu (it appears to have had two of the burners removed and capped prior to my ownership)
3. ITT McDonell & Miller boiler control model#N-47-2 (automatic water feeder and cutoff combination)
4. Honeywell Presuretrol set to Diff = 1psi or kg/cm2 = .8 and Main 3 psi or kg/cm2 = .25 (if I read it right)

So far I have:
1. Checked the main air vent located on the roof by removing it holding it vertical and blowing through it. It seemed to be clear and it gets very hot when the steam is on (i surmised there is no trapped air). The main vent is 2 years old or so.
2. I replaced 4 of the radiator vents. I will replace the rest just to be sure they are good.
3. I removed the Honeywell Pressuretrol to clean the pigtail which seemed to be clear enough with just a little crusty material; However, there was no water in it. I reinstalled the presuretrol and leveled it using the swinging guide arrow mounted inside.

What I plan to do:
2. Change the automatic water feeder / cut off (at least 15 years old) and confirm water supply lines are not restricted by corrosion.
3. Change the remaining radiator vents not already done.
4. Perform a thorough boiler cleaning to include the Hartford Loop.
5. Change out the copper mentioned above.

Am I on the right track here? So far it still short cycles.

nicholas bonham-carter

First thing is to find out what is causing the short-cycling. It could be as a result of:
1.overpressure, and bad venting, now plugged up.
2.disappearing water (lWCO)
3.thermostat problems.
If you have a low pressure gauge, (and if you want to save money you should have one), then you can see if the main vents are doing their job., by the back-pressure of venting, as the boiler starts up.
Has there been any addition of boiler snake oil into the system?
Have any changes been made to the system lately? This includes thermostats.
Post some pictures of the boiler piping, so we can see, and advise.--NBC

Steamhead

You say the main vent is on the roof?

JMervin

Yes the main vent is on the roof above the apartment furthest from the boiler. It is 7 stories above the basement where the boiler is located.

Abracadabra

JMervin said:

I have admittedly not been very diligent blowing the boiler down weekly during the past few heating seasons mainly because it has always worked well (no excuse).

I'd service the float chamber and clean it out.

JMervin said:

1. After blow down the site glass is very slow to refill. I recall it refilling faster in years past.

Clean/replace the strainer on the feeder. Also verify old iron pipe supplying water to boiler thru feeder is not corroded restricting flow.

JMervin said:

Checked the main air vent located on the roof by removing it holding it vertical and blowing through it. It seemed to be clear and it gets very hot when the steam is on (i surmised there is no trapped air). The main vent is 2 years old or so.

I have to guess that if you have only 1 main vent on a 15 unit building you need more main venting. If the main vent is on the roof, without some more pics of the whole piping arrangement, I am surmising that this is some kind of a vent for the main riser. (7 story building? 2 units per floor plus one in the basement = 15 units?)

JMervin said:

I replaced 4 of the radiator vents. I will replace the rest just to be sure they are good.

What did you replace them with?

JMervin said:

Change the automatic water feeder / cut off (at least 15 years old) and confirm water supply lines are not restricted by corrosion.

I wouldn't say it's necessary to replace the feeder/lwco. MM provides parts to make that thing last for years with good maintanance.

When pressuretrol hits first cutoff, which and how many radiators are hot? How long to reach first cutoff from warm start? Has main vent closed?

JMervin

Abracadabra,
In the order of your comments:
1. Darn I was not aware the float chamber could just be cleaned and serviced. I ordered a new one anyhow. There were 3 other old ones sitting in the boiler room, so I assumed it was a consumable item.
2. Is the strainer inside the feeder / lwco? Will that be fixed by replacement? Again I did not know it could be serviced. I will check inlet piping during change out.
3. There are 5 floors of apartments 3 apartments per floor. The other 2 are commercial on the ground and mezzanine levels not heated by steam. They a small units with just one radiator per. I can only find the one vent on the roof with a Hoffman #75 vent.
4. Radiator vents are Hoffman #40 type. They look like a silver bullet with hole in the top.
5. It appears when the Pressuretrol shuts off the burners we have steam at main vent and it has closed due to being very hot. I assume then that the air has been pushed out. Most of the tenants indicate they have heat but it is intermittent. The radiators will get hot then cool back down slowly. It takes approx. 15 to 20 minutes for the start up to Pressuretrol shut down the first time.

When I swap out the feeder / loco that should address most of the items you mention. I don't understand why the feeder might be causing excess steam pressure and subsequent Pressuretrol shut down? hmm. It needs to be serviced in any case.

Abracadabra

JMervin said:

Is the strainer inside the feeder / lwco? Will that be fixed by replacement? Again I did not know it could be serviced. I will check inlet piping during change out.

Yes it's part of the valve assembly where the in/out feedwater supply pipes connect. Locate 4 bolts underneath. You'd need to replace the entire valve assembly if it doesn't have the hex shape at the in/outlet. And yes.

JMervin said:

radiators will get hot then cool back down slowly. It takes approx. 15 to 20 minutes for the start up to Pressuretrol shut down the first time.

Sounds about right. You might just have an oversized boiler. Do all radiators get hot across evenly? You might benefit from a hi/lo burner setup. How many cycles until thermostat is satisfied on a very cold day?

JMervin said:

I don't understand why the feeder might be causing excess steam pressure and subsequent Pressuretrol shut down? hmm. It needs to be serviced in any case.

It won't cause excessive steam pressure. It will cause a shutdown of the boiler if lwco kicks in and will kick boiler back on when level rises again. Either due to condensate returning or feeder eventually filling boiler. How much does water level drop when boiler cuts out first time? How do you know it's the pressuretrol cutting out?

Jamie Hall

I might add that excessive pressure -- anything over 1 1/2 psi -- may cause water to back out of the boiler and trip the LWCO.

JMervin

So nothing has changed with regard to boiler size since my ownership. I have had several years where the steam has worked well and without issues. It may be possible that there is too much fire off the burners? Is that adjustable? I know it is the Pressuretrol shutting off the burners because I can watch and hear the murcury switch tip right when the burners turn off for what appears to be around 2.5 psi. I am virtually positive that excessive pressure is the issue. Could a dirty boiler cause too much pressure, or could excessive flame off the burners be caused by a change in gas pressure? When the Pressuretrol mercury tips it is measuring pressure in excess of the setting? Is that correct?

The site glass when cold starts about 3/4 inch from the top of the tube and lowers to about 1/3 to 1/2 full when operating at full temp.

Thanks for all the ideas. I appreciate the help.

Fred

Is that mercury Presuretrol perfectly level and is it on a pigtail? It must be level and the Pigtail must have the loop at 90 degrees to the face of the Pressuretrol as that loop expands from heat and will move side to side when the boiler is running. That will throw the merury switch off. Has anything happened that would change the gas pressure or the way the burner fires?
Has any piping been done on the system that might have put oils in the boiler water/ Does the water in the ssight glass bounce a lot when the boiler is running (more than 1/2 to 3/4 inch)? If so it may need to be skimmed.
The water level seems high. It should be no more than 3/4 of the way up the sight glass. Not 3/4 inch from the top.

Captain Who

JMervin said:

I know it is the Pressuretrol shutting off the burners because I can watch and hear the murcury switch tip right when the burners turn off for what appears to be around 2.5 psi. I am virtually positive that excessive pressure is the issue. Could a dirty boiler cause too much pressure, or could excessive flame off the burners be caused by a change in gas pressure?

Oily water will cause wet steam to be more likely to make it up into the mains and into the radiators. That will be less effective at heating the rooms in your house and will result in the boiler having to fire longer to satisfy the thermostat. Having to fire longer will tend to make it more likely to hit the cut-out pressure on the pressuretrol and/or cycle on pressure more times.