I have a gas powered steam radiator heating system with 8 radiators in the entire house. I replaced my old heating boiler with Crown Bsi138enpzzpsu 138K BTU which is way too powerful for my house.

My boiler short cycles for these 2 reasons.

  1. The boiler gets too low on water if I start with water level slightly more than half on the sight glass tube. Boiler runs fine for a while until it gets low on water - I think boiler is powerful enough to push water up into the pipe. After low water cutoff is hit, it shuts down for a few seconds until water level reaches back up. Also, some time this doesn't happen fast enough and boiler gets over filled because the auto feeder kicks in before all the water gets back into the boiler. Eventually, boiler gets overfilled after a few days of operation.
  2. I noticed that once my boiler gets overfilled with water, it kind of masks the issue with low water cut-off being hit. However, now it short cycles because of pressuretrol being hit after certain pressure is reached in system.

Here are my questions (Info on any of these would help me):

  1. I do not like the option 2 - i.e. using overfilled boiler - but I am not sure if there is any risk associated with using overfilled boiler. Could it cause boiler to explode?
  2. I have good air vent (Hoffman 1A, 1/8" Adjustable Angle Steam Radiator Air Valve) installed with fastest vent setting on the radiator in the room with the thermostat. I think this vent opens up after a certain pressure builds in the system. Would it help if I install similar vent in other radiators as well to reduce the pressure in system?
  3. What is my option to solve the issue with overly powerful boiler. enter image description hereenter image description hereenter image description hereenter image description hereenter image description hereenter image description hereenter image description hereenter image description here
  • Can you specify the exact model of boiler? Also, what is a "presser troll"?
    – FreeMan
    Jan 18, 2021 at 13:38
  • It is Bsi138enpzzpsu. Thanks for bringing Typo to my attention, I meant PressureTrol. Jan 18, 2021 at 19:32
  • Is it supposed to lose water? I thought systems like that were closed systems like your car's radiator, and shouldn't lose water at all. Frequent top-up is simply a leak. Jan 18, 2021 at 20:56
  • Thanks for the response. It is not losing water. I think the boiler is powerful enough to send water up the pipe and all the water eventually get back into boiler but it takes a while. And in the mean time, autofeeder also add more water. Jan 18, 2021 at 22:12

1 Answer 1


I have to ask, "who sized the replacement boiler". An oversized boiler will have numerous operating problems and you are witnessing a few of those problems.

The input of the boiler or as you say "powerful" has little to do with why the boiler water line falls when the boiler fires. The "near boiler piping", the system piping , the water quality and to a lesser degree the size of the boiler are causing the problems. Was this boiler installed by a licensed boiler installer? Did he flush out the boiler to get rid of the oils and debris that is in the boiler when installed? Is it piped as per the MFG installation booklet? What is the operating pressure on this system? If you have vents on each radiator then you should have a 1 pipe steam system. What controls are installed on the boiler? Has the gas input been set and checked?

Take a few pictures of the boiler and it's piping. Show the size of the piping leaving the boiler and how it is piped. Include pictures of a couple radiators. Oh yes, do not overfill the boiler and I will be interested to see the pictures of the system.

Adding to my answer; if you search for "Arco cast iron radiators (1959)" you should come up with a pamphlet on the size of Arco rads that you can utilize for the heat requirements for your radiators. Even if your rads are not American radiator company rads (arco) you can still utilize this chart since the output will only be a little more or less for another companies rads.

To check the boilers input, it can be done if the boiler will fire long enough to read your gas meter for 1 minute. With the boiler firing it should use 2.3 cu ft of gas per minute. Your boilers input is 138,000 BTU/hour and natural gas has about 1000 btu/cu ft. So, 1000 btu/cu ft X 2.3 cu ft X 60 minutes = 138,000 BTU/hour. If the input is too high then you need to get a boiler guy or an HVAC person to adjust the gas input.

Take a picture or 2 of a radiator and post it. Also, the boiler may be oversized but I believe that the boiler needs to be cleaned and skimmed by a professional boiler person. By the way, in what city are you located?

  • Thank you for your response. Unfortunately, I hired a bad contractor for basement renovation and plumber who replaced the boiler was also hired on his recommendation. I had to hire someone else to redo all the near boiler piping and install the drop header to deal with water shooting up the pipe. I don't think anyone actually sized my boiler. However, the 2nd plumber who installed the drop header flushed the system afterwards. I am not sure what it means to set and check the gas input, but I am sharing pictures here. Any more info or insights into solution would be helpful. Thanks Jan 18, 2021 at 19:28

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