Overheating and burping on Central Boiler

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New Member
Nov 17, 2022
Boise, Idaho
I posted regarding this issue a few months ago. I inherited a Central Boiler when I purchased my home outside of Boise, Idaho last year. We didn't use it last winter as we were at the house sporadically, but we now live here full-time and began using the Central Boiler in November. My problem is the system is overheating.

The model is CL5648SB. The metal plate indicates it was manufactured in June 2001. It appears to have a huge firebox (it indicates the heat transfer area is 114) and a water capacity of 385 (I assume this is gallons). The solenoid opens the damper when the water temp hits somewhere in the 150s. That's about all I know about the machine. The former owner provided an owner's manual and showed me two valves to turn on for water flow when the machine was operating. I don't know how to set the water temperature on this model or if I even can.

I have been experiencing overheating; not always, but typically at night. I realize this isn't a pressurized system, but the amount of steam can be a bit alarming at times and occurs in temperatures below 185, often in the 170s. I could tell there wasn't a good seal on the door, so last week I replaced the rope seal. Additionally, I cleaned out the firebox and had the chimney swept. I had high hopes that my problems would be solved, but no. It also just started doing some smoke burps out of the damper door. The smoke temporarily stopped coming out of the chimney and "burped" out of the damper.

I've added water through the opening at the top of the roof. It appears to be at the right level per the tube by the door. Could air be creating the overheating situation? I've never bled the system from lack of knowledge how to do it. I am not very knowledgeable about plumbing issues in general.

Right now I'm running the thermostat in the house at a higher temperature to take some heat off the system. That definitely helps to some degree. But, we like having the temperature at 62 when sleeping and we've recently been running it at 70 to 72 because of the overheating. This is resulting in us being uncomfortable sleeping, but is also leading to increased electricity and wood usage.

Any thoughts or suggestions? I'm going to try to contact the closest dealer (in Montana). We aren't that remote, but service technicians are often difficult to find in our area. We have a solid journeyman plumber, but he is taking some time off to help his wife recuperate from a surgery. I'm looking for another plumber, but don't currently have one.

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You shouldn't be seeing steam at 185. Mine has hit 190 before and I don't see any steam. Water doesn't boil until 212F at 0 PSI. Is the steam coming from the fill pipe ?

Not a CB expert, just read comments about them mostly on FB. These seem to be common causes.
  • Circulator is too small, causing hot spots - very common
  • Pull from the bottom, not the top. This should help with hot spots and better stratification
  • Maybe try turning the circulator up for betting mixing. Same issue as above....
  • Seal on the damper plate on the door is letting air in - very common
  • Make sure the circulator is on the boiler not in the building. It's a circulator not a pump, so it needs water pressure from the water in the boiler pushing on it. This is a hotly debated topic.
If the boiler sat for a long period of time without water in it, that's bad. Without any water in it, the boiler was rusting from the inside and the outside. If you are losing water on a daily basis then the boiler has a leak. Need to find the leak and patch it, or junk the boiler and start from scratch. Those boilers are maybe 30% efficient on a good day so they are going to chew through a ton of wood.
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Might help to put in less wood towards the night time. Especially as the days n nights get warmer.

This may result In starting a fire in am but that may help with the over heating.

And is your temp gauge fairly new? 170 or even 185 shouldn’t boil out. The boiler itself is an open system. I have a GARN which is an open system and that won’t boil out until it’s over 200. It will roll some steam but not boil out.

Learning the amount of wood(per loading) needed is a bit of a guess/experience job. So I’d cut back at night. Sort out your smaller splits for a fire starter in the am. Should be coals enough
I've attached some photos of "The Beast." This is an old one, built in 2001. I have no idea how to set the temperature or whether you even can. I spoke with the closest CB dealer--he's in Montana and I'm in Idaho. He suggested trying a new pump (I ordered one from him) and possibly a Heat Trac to replace current LED. As we explored that idea further, it doesn't appear there is a replacement or after market part for this unit's LED.

As for the non-sealed damper door, if no air at all can get through when the damper is closed, wouldn't the fire suffocate?

As for reducing the amount of wood overnight, I have no problem using less wood. I'm amazed how expensive it is in the Boise area. But, it does still do it during the day occasionally.

I'm looking forward to getting the new pump and seeing if that may be the cause.

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I would start researching it's replacement...
You probably don't have a lot of time left in that water heater,especialy as you don't have 20 years of history with it.
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