New Gas Steam boiler short cycles

Gaz

Hi,
I just had a new williamson gsa 238 gas steam boiler installed.  The sight glass is showing some surging but not enough to set the lwco.  I'm wondering why the system is short cycling every 3 minutes to the tee.  3 minutes on and 1 minute off.  Pressure setting is 0.5 cut in and cut out diff is 2 = 2.5psi.  Any ideas?

Jamie Hall

How was the new boiler sized? That cycle timing -- assuming that is once things have more or less warmed up -- is telling me that the boiler is about a third again as large as it should be to match your radiation.

Gaz

Last one was a 250,000 btu weil mclain. 40 yrs old.  Not sure how they sized it..  Its a 4 unit 3200 sq ft building with 18 radiators throughout.  From initial cold startup it does this 3:1 cycling.

Mad Dog_2

Have you asked your installer why?  Mad Dog 🐕 

Gaz

They're saying it sized right.
The last boiler worked fine and it was 250k btu.
This ones 238k btu.

pedmec

what are you using to initiate demand

Jamie Hall

If they didn't measure up the radiation -- radiator by radiator -- they didn't size it right. Was the old one putting out it's full rated power? Or had it been downfired -- many can be. Not sure about your Williamson, but at least one boiler I work with is rated at 623 mbtuh gross but is currently fired at 450 mbtuh gross, just for example.

Gaz

I'm using the 10 yr old digital honeywell thermostat. Plumber came by again and I know he opened it up to look at it.
Old one was working great.  It was a a little bigger. 

Gaz

I Think Since plumber opened thermostat  it's been finnicky. 

Mad Dog_2

The problem with NOT doing your own measurements of the connected load of the building is that you're relying on men that came before you..Did they measure? Did they know what they were doing? Its a big question mark. Mad Dog 🐕 

Gaz

I see

Gaz

So exactly what happens during operations of an oversized boiler?

bburd

An oversized boiler cycles on and off during a call for heat because the pressuretrol limits the steam pressure to a safe value. This slightly reduces operating efficiency and increases wear on electromechanical parts.

A correctly sized boiler will run continuously during a call for heat if all radiators are turned on and the system is otherwise operating properly, and will not build enough pressure for the pressuretrol to turn off the burner. This is seldom achieved in the real world, in part because of the generous "piping and pickup factor" included in the industry standard sizing calculation. Most boilers in the field are oversized to some degree.

On the other hand, a boiler sized too small for the connected radiation load is likely to lead to uneven heating because there isn't enough steam to fill all of the radiators, and it is not likely to distribute evenly throughout the system.

pedmec

Have you removed the thermostat out of the equation by bypassing it. You need to rule this out. Process of elimination.

EBEBRATT-Ed

Did they skim the boiler after installing? Even though the sight glass may not be surging much it could be surging at the low water cutoff probe.


You need to get an electrical meter and find out which control is shutting the boiler down

1 Low water Cutoff
2 Pressure Control
3. Thermostat

You won't get anywhere until that is done. Just guess work.

Gaz

Thank you all.  Good to see people sharing thoughts.  I'll do all checks this week and fill you in.

BobC

Was that boiler skimmed after it was installed? Dirty water does nothing to improve a boilers operation, a steam boiler needs clean oil free water to work right. A new installation introduces oil and dirt tyo a system and that has to be removed by skimming it off the surface of the water

Did the installer install a skim, port?

Bob

Gaz

He did not.  I will do it in a few days.  
Can this trip the lwco without making the feeder give water?  No extra water has been fed.
The boiler shuts off exactly every 3 minutes and turns back on after 1 minute. 

Mad Dog_2

Don't short the LWCO!!!  Mad Dog 🐕 

EdTheHeaterMan

Gaz said:

He did not.  I will do it in a few days.  
Can this trip the lwco without making the feeder give water?  No extra water has been fed.
The boiler shuts off exactly every 3 minutes and turns back on after 1 minute. 

Gaz said:

Last one was a 250,000 btu weil mclain. 40 yrs old.  Not sure how they sized it..  Its a 4 unit 3200 sq ft building with 18 radiators throughout.  From initial cold startup it does this 3:1 cycling.

This is a clue. If the water was cold and the burner starts and runs for only 3 minutes even with no pressure on the boiler yet, then I don't believe it is going off by the pressure control. If it was cycling by pressure control, the very first cycle would take longer to get the cold water up to a pressure high enough to trip the limit . Like maybe burn for 6 of 10 minutes, go off by pressure, then the pressure would take a minute to drop, then go on every 3 minutes and continue until the call for heat is satisfied.

Since you say the 3 minute on 1 minute off starts right away, then I would look elsewhere for the cycling problem. Unless of course, the very first cycle IS longer and you have not observed that longer cycle

EdTheHeaterMan

There are 7 things that can cause the gas valve to close then restart the way you are describing. All 7 have to do with losing the 24 volt power to the gas valve. The 8 circle (blue) is a one time fail and done so that will not be the problem.

The two most likely that will happen from initial start up without having the initial delay due to colder water would be Spill Switch (in the green circle, unless that is a manual reset) and the flame sensor, which might be caused by the pilot flame is not making the flame rod connection after 3 minutes of operation. If either of these two are the problem, you can diagnose this with a meter and placing the leads on the ignition control at the 24 volt and 24 volt ground terminals.

I don't believe the roll out switch is the problem since that is probably a one time fail and done, like a fuse.
The next likely is the thermostat having some sort of cycle or if there is an auxiliary limit that may not be shown on the diagram, but the installer put on for some reason. Can you post some pictures of the job and the individual controls that are connected? Try following the wiring diagram I posted and find each of the components by following the wire from one control to the next.

nde

As mentioned above jump out the thermostat to rule it out as it sounds like a thermostat issue. Should be set to steam cycle.

JUGHNE

Do you have a Cycleguard LWCO?

Jamie Hall

Now this is important. If the boiler is cycling on a fixed schedule right from the very beginning -- before the boiler has started making steam (feel the pipes) your problem is not the boiler or its controls, unless something really weird is going on. Even a Cyclegard has more patience than every three minutes. However, it could very easily be the thermostat. Most modern digital thermostats are shipped set up to run for forced air heat, which does run short cycles. They can be programmed, however, to run for steam. It will be in the instruction manual for your thermostat. So check that to see what it really is set for.

ChrisJ

@Gaz can you share some pictures of the boiler and all of the piping around it for us?

Also any controls that are related to the system.

BobC

I missed the 3 minute cycling part and I agree the thermostat could be your problem. If the thermostat has been recently changed it probably came from the factory set for hot air, you need to set for steam or gravity hot water so it can run for 30 to 60 minutes if it has to.

I'd still skim that boiler to get the water clean and consider using something to get your boiler PH up to 9 or 10. Ive used steam master in my boiler since it was new (10 years now) and my boiler water has stayed clear for 10 years now.

Mad Dog_2

Pictures please..Mad Dog 🐕 

Gaz

So upon cold start the  boiler fires nonstop for 30 minutes.  I watched the psi gauge and noticed it go to the 2.5 and shuts off which is the cut out setting.  Then afterwards the boilerbwill cycle 3 min on and 1 minute on till the desired thermostat is satisfied.
I checked the thermostatat by by jumping it and it made no difference.   To me it seems I have an oversized boiler with old headers that where never piped right.
Suggestions?

EdTheHeaterMan

There you have it. The boiler short cycles after the initial call for heat gets the boiler up to pressure.

That sounds like the pressure control
to verify that you will need to check the electrical contacts at the pressure control (the middle red circle at the top of the diagram) with a multi-meter. It may be that the vent damper closes after the limit opens and before the limit closes when the pressure drops. Then the cycle time of the vent damper is added to the off time. adding up to the 4 minutes total

Does the vent damper close then reopen on every off cycle?

Mad Dog_2

Pictures 📷 will really help.  Did anyone size the Connected load, first?  Mad Dog 🐕 

EBEBRATT-Ed

We need to see the boiler and the piping connected to it.

Look in the boiler manual it will show a drawing of how the piping is supposed to be and the various pipe sizes.

If they did not follow the MFG minimum piping requirements for this boiler it will not work. It does not matter if the old boile did work with that piping.

nde

It does sound oversized if 40 minute burn kicks in pressure control. Not the end of the world though. That is a long burn run, are you using big swings? try a set temperature on the thermostat as typically at set temp most steam cycles are 20_30 minutes. If running long burns at set temp then you need to vent mains and rads faster. Properly vented mains should have steam hitting all rads at about 10 minute mark so long as they too are properly vented.

Dan_NJ

My Cycle Guard seemed to help mitigate my oversized boiler problem. Short cycling didn't really come up except maybe on the coldest days and then only a few cycles at the very end of the call for heat. Also had no setback helped so no unusually long runs in the morning for example.

Gaz

Gaz

Boiler from cold start runs a straight 30 min non stop.  Then does 3:1 on:off till thermostat is satisfied.
One of my 2 mains returns is missing a vent as well. My thermostat is in the hallway a little far from the radiator.
Its a 4 unit 3200 sq ft building with 20 radiators throughout. 

KC_Jones

Not that it's all the problem, but that boiler is piped wrong and that's not helping your situation for sure.

So they probably didn't size it correctly (must measure all radiators in building to do it right), and piped it incorrectly. If the boiler was sized properly, and the venting is proper, you basically shouldn't be capable of getting to 2.5 psi in the system. So either oversized boiler, inadequate venting, or both, oh and mix in the boiler piping to add insult to injury.

I hope you haven't made final payment, seriously.

Is it really that hard for a professional to read a manual and follow a picture?

Gaz

Yes
The vent damper does close and reopen after every cycle

nde

If I had to guess the old boiler steam outlet was probably positioned a "corner" away from now with a swing joint to the header, then feed those odd main supplies. Now it looks like the steam/water carryover is slamming straight up into that first main vs carryover to the equalizer. It there banging and/or one main heats up way faster? If so that main is getting too much steam and locks out on pressure while the other is probably not getting enough but keeps calling.

EBEBRATT-Ed

Pictures are deceiving sometimes but the boiler piping looks like 2" and should be 3" as @KC_Jones mentioned.

EBEBRATT-Ed

Either way it's not going to work like that right....ever.

Gaz

The old boiler looked like this one (was a Weil Mclain but a little higher btu). The pipe orientation is identical.