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Author Topic: Overheating/boil over prolems  (Read 5395 times)


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Overheating/boil over prolems
« on: November 13, 2019, 09:24:07 AM »


I inherited this boiler 3 years ago when I purchased my home.  I believe it is a home made unit (guessing 15ish yrs old), as there are no manufacturer indications anywhere and none of the local dealers recognize it as anything they have seen before. It worked well the first year and then started having boil over problems.  I am looking for some advice to trouble shoot this.

Here is what I have done so far:

1.  Last year the furnace would blow cold air even with the pump running.  The boiler would run up to max temp on the temp controller (176 deg F) and shut off the blower to the fire.  Usually the temp on the fluid (190-200 plus) would keep rising until steam would shoot out the top.  Once the fire died it would cool down and I would try again.  Because of life circumstances and being a novice with wood boilers I struggled through the winter trying to figure things out.  My house also has propane, so I just used that and let the boiler sit idle.  In early spring I realized that I had not checked the boiler fluid level and discovered that it had fallen below the outlet port.  This meant that I had run the pump dry, potentially all winter.

2.  Prior to the fluid level discovery, I had removed the pump, drained the inside portion of the system and blew air through the heat exchanger; thinking that some sort of blockage had occurred.  Everything blew out without any plug coming out, so I assumed it was fine.  I fixed the fluid level, re-installed the pump, and got it running.  The pump seemed to function as expected, as evidenced by the fact that the plumbing on the down stream side of the heat exchanger would get hot when I had a fire going and the pump on.  Since it all seemed to be working, I decided to leave well enough alone and let it sit all summer.

3.  This summer I decided it would be a good idea to insulate the boiler enclosure (reflective compressed polystyrene) as well as put new insulation (reflective bubble wrap kind)on the fluid reservoir itself.  Previously, there was just reflective bubble wrap on the reservoir.  I was just hoping to increase my wood burning efficiency.

4.  Now with cold weather I have attempted to use the boiler three times.  The insulation seems to be working (??maybe too well??), as the fire rapidly heats the reservoir fluid and shuts the blower off.  Twice it has maintained temps above 160 F for more than 12-18 hrs even with the pump running.  However, it wasn't cold enough either time for the house furnace to kick on and pull heat from the system. 

5.  Two days ago, it got cold enough for a couple of days to run the system and heat the house as intended/expected.  I stoked a big fire, turned everything on, and monitored the situation.  Outside, the situation repeated itself: fire heats fluid up rapidly to 190-200 F and blower shuts off.  Inside the pump seems to be running and is getting hot fluid up to the heat exchanger.  The house furnace is blowing warm air (although not as hot as I remember it getting in year 1). Here is where things get confusing:  The heat exchanger coils are warm but not that hot and the downstream plumbing is not hot, almost downright cold.  Outside, the boiler is blasting steam doing its best impression of Old Faithful for more than an hour.   I shut everything down and am waiting for more Cryo-Tech 100 to be delivered so I can refill the system.

I'd be happy to provide more info/pictures if that would help.  I also have questions about the firebox side of things but want to at least get it functioning properly first.

In what order should I approach things?  Is there something wrong with my pump?  Do I try to test it or just get a new one?  Should I pull out the heat exchanger and take a look? If yes, how should I test it?  Did I mess up a delicate balance by adding insulation and should I consider removing it? 

I am trying to avoid a multi-hundred/thousand dollar or more bill from my plumbing, heating,& air guy (who is willing to take a look but admits to having no experience with these systems).  Or should I bite the bullet and have him deal with it?

Thanks in advance


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Re: Overheating/boil over prolems
« Reply #1 on: November 14, 2019, 04:34:38 PM »

It sounds like an air lock before the pump. Need to make sure no air between the pump and boiler or no water will move. If it has been run that way for a long time then pump is probably bad too.

Where are you located?


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Re: Overheating/boil over prolems
« Reply #2 on: November 15, 2019, 06:20:09 AM »

Id guess you have an air bubble in the lines somewhere.  Every system is different on how to bleed it so it'd be hard for us to help out.  Try to use some common sense and hopefully you can find it.  I installed a valve with an open pipe leading to my floor drain just for bleeding air.  Maybe the previous owner did something like that and you're not seeing it yet...?

As to the boiling over, my CB unit doesn't use a fan, it uses a damper to cut off the incoming air once it gets up to temp.  And once it reaches the temp and the damper shuts off, the heat barely rises as the flames pretty much stop immediately.  I'm not sure about those that use a fan and I could be wrong here, but It sounds like it's still sucking in air once it reaches temp and letting the fire get hotter. 

A question for the group... Do stoves that control a fan also shut off a damper, or do they just rely on the stopped fan to control the air to the fire?
Hudson, WI

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Re: Overheating/boil over prolems
« Reply #3 on: November 16, 2019, 03:26:51 AM »

Every outdoor boiler should have some kind of flap or damper that closes tightly when it's up to temp, fan or no fan. There might multiple issues with this particular unit. Air leakage, too little or no flow (air lock or damaged pump), I've also seen internal piping in the water jacket on a homemade boiler cause issues if it isn't designed to properly mix.
My guess a HVAC guy might not be able to diagnose unless unless he's really experienced with troubleshooting, you don't have homemade HVAC equipment.
Adding insulation shouldn't cause issues.
Look for air leaks (smoke from the stack should almost stop when up to temp), purge piping with house pressure (replumb if needed to do so), make sure pump is pushing from the stove, not pulling somewhere in the loop.
Post pictures if possible.
HeatMaster dealer serving southwest VA.