How to correct short cycling on a residential steam boiler

Nycjs99 · February 16

Steam boiler short cycling, causes

Nycjs99 · February 11

What problems can a defective preasurertrol cause on a steam residential boiler cause

KC_Jones · February 11

The question is a bit vague, but I’ll give it a shot.

It depends how it’s defective. Is it defective that it’s not stopping the boiler at the safety cutoff pressure? If so it’s a potential safety concern in that the boiler can run out of control. In reality the thermostat would probably cut it off before any real problems occurred, but it should still be fixed.

Is it defective in that it cuts off, but not at the pressure it’s set it’s set to? For me, not really a huge deal as they are known to be wildly off and not much to be done about it, a new one could have the same issue.

The only other failure mode I can think of is it won’t let the boiler run, but I’m doubting that’s what you have because that one would be obvious.

bigbillynyc · February 15

Change the pigtail and retest. Could be dirty

Nycjs99 · February 16

My Weil mc claim GE-40 Pi steam boiler is short cycling when the thermostat is calling for heat.
the preasuretrol is set at .05 psi with a #1 differential . The boiler short cycles at least 5 or 6 times before the thermostat is satisfied.

Nycjs99 · February 16

The boiler is natural gas

Jamie Hall · February 16

There are so many causes... the most likely one is that the boiler is simply oversized -- possibly by quite a lot -- for the system. Nothing to be done about that. If the cycling begins before all the radiators are warm, it's also likely that the system does not have adequate main venting -- if it has any at all.

Nycjs99 · February 16

With the thermostat calling for heat, the boiler seam to cut off at around 4 lbs , then the pressures goes down to a little above zero and the boiler starts up again, goes up to 4 lbs and shuts down on pressure, the the pressure goes down at just above 0 lbs and the boiler stars up again, it does that about 5 or 6 times before the thermostat is satisfied and shuts down the boiler

KC_Jones · February 16

If it's short cycling after the mains are hot and the radiators are heating, the boiler is most likely too big and short of downsizing it, nothing you can really do except live with it.

Oversizing iss one of the most common problems in this industry.

Nycjs99 · February 16

The pressuretrol is set ot .05 pai with a #1 set on the differential wheel, it seems to cut out and back in at the approximate psi, but it seem to short cycles . It cycles like 5 or 6 times before the thermostat is satisfied. My question is this normal.
the pressure could be off to it is an older boiler though.

Ok and thank you for you help !
charlie

ethicalpaul · February 16

It's normal for an oversized boiler, which is indeed very common.

The house needs heat, so the boiler wants to run. But the pressure control says "you can't run for awhile". Repeat until call for heat satisfied.

But having said that, you should either fix or replace your pressure control because 4psi is WAY more pressure than you want. You should consider a vaporstat which can be set to a much lower cutout pressure.

Jamie Hall · February 16

duplicate thread, I think

Nycjs99 · February 16

Thank you both for your reply, it is appreciated.
best regards
Charlie S

Nycjs99 · February 16

Thank You Paul,
I was thinking of adding a vaporstat, what is the ideal setting? I’m thinking .05 psi cut in and 1.5 psi on the cut out. What are you thoughts?
Best Regards,
Charlie

ethicalpaul · February 16

Personally I would go much lower than that. I think the vaporstat lets you go to the ounces

Keep in mind, and lowering of the cutout will increase your cycling. If you have a gas atmospheric it's not a big deal. If you have oil it might be a bigger deal, I don't know much about oil.

You can also set up a timer circuit that makes the boiler rest longer after it starts building pressure, that's what I have. It's good, but it does increase the number of atmospheric "inhales" into the system when the steam collapses.

Make sure your main venting is good, you want to put off the pressure as long as you can.

EBEBRATT-Ed · February 16

@Nycjs99

If the boiler is oversized some on this forum have been experimenting with various timers to limit the short cycling with some success

The idea is when the rads start to heat the timer shuts the burner down while the rads are giving off heat but the thermostat has not, yet been satisfied hoping the stored heat in the rads will satisfy the thermostat

When the timer remakes contact and the stat is still calling the boiler fires again. The timer lengthens the time between cycles and cuts down the short cycling

ethicalpaul · February 16

Yes, it works great. We don't really "hope" that the stored heat in the rads will satisfy the thermostat, but if it happens to, then the call for heat ends.

If it doesn't, then the boiler starts back up to provide more BTU into the house.

EdTheHeaterMan · February 16

If we could know more about your boiler , there may be more we can suggest to reduce the number of cycles per call for heat. The make and model number of the boiler will help in that regard.

Oil or Gas?

guzzinerd · February 16

ethicalpaul said:

Personally I would go much lower than that. I think the vaporstat lets you go to the ounces Keep in mind, and lowering of the cutout will increase your cycling. If you have a gas atmospheric it's not a big deal. If you have oil it might be a bigger deal, I don't know much about oil. You can also set up a timer circuit that makes the boiler rest longer after it starts building pressure, that's what I have. It's good, but it does increase the number of atmospheric "inhales" into the system when the steam collapses. Make sure your main venting is good, you want to put off the pressure as long as you can.

How low would you go? I replaced the trol with a vaporstat at the start of this season and it's set at 1.5 and 0.5 psi.

Right now it calls for heat once or twice an hour and shuts down with the t-stat (set @ 69° prime time and 67°overnight) between 0 and 1 psi.

ethicalpaul · February 16

He did say it was gas in the other similar thread

EdTheHeaterMan · February 16

If you could use this chart to determine how much Radiation EDR you have by looking uo what your radiator looks like and counting the number of sections. there is a little math involved but if we know the EDR of your system, and the size of the boiler, that will answer a lot of questions.

Sometimes it is difficult for someone to select what type of radiator they have from that chart above. This chart is intended for the professional that has encountered all kinds of radiators form many homes, so at a glance they can select the proper match. I have noticed some DIY folks on this site select the wrong radiator from this chart. To give you a guide to selecting the radiator from the chart that most represents your radiator, hear are some photos to help.

To be sure you are counting the EDR correctly, we can help if you post pictures of your radiators.

Erin Holohan Haskell · February 16

Jamie Hall said:
duplicate thread, I think

I've merged these duplicate threads into one. Thanks!

109A_5 · February 17

Hello @Nycjs99,
If there is nothing else wrong with your system like a plugged up pigtail, etc. And your are just dealing with a boiler size to EDR mismatch (oversized boiler). Just add an inexpensive timer in series with the pressuretrol or vaporstat. Excessive cycling whether is it at 5 PSI or 5 Ounces is excessive and can be eliminated or significantly reduced.

https://www.supplyhouse.com/ICM-Controls-ICM203FB-ICM203F-Delay-on-Break-Timer-6-Wire-Leads-03-10-Minute-Knob-Adjust-Delay

How to correct short cycling on a residential steam boiler

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