Burnham Aquastat Prblm - Runaway Condition


  #1  
Old 05-14-20, 01:05 PM
C
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Burnham Aquastat Prblm - Runaway Condition

Howdy all:

I'm new to boiler maintenance, but I have a good grasp of how the system works and how to properly maintain it. Recently, my circ pump failed, and while I was diagnosing this I noticed the boiler overheating and the pressure relief valve leaking or totally dumping (old pump was a near silent Grundfos - I never heard it run in 25 years). After some investigation I also discovered the aquastat (Honeywell L4080D 1036) appeared off by anywhere between 20-50 degrees from the boiler temp gauge causing a runaway condition. Temp / pressure gauge seems fine - PRV was dumping as expected when temp reached ~230 and pressure climbed ~30 d/t steam formation.

I purchased a compatible replacement recommended by Honeywell, but new Astat (Honeywell L6006A 1145) is behaving the same. Switches work, verified by manually lowering Astat temp to ~130-140F once the boiler temp exceeds high limit (180F), and burner shuts off as expected and allows circulator to continue. Pipes are hot, distribution system heat feels normal, but high limit does not function as it should to stop the burner. On re-heat after reducing Astat temp to ~130-140F, system heats to ~180 and shuts off burner (Astat set to 135F).

I believe this is a well contact or probe issue. Probe fits in Iwell fine, and in fact was too loose prior to adding heat conduct paste - now I have problems getting it in and out of the well (which, obviously I'm not trying to do often as I want best contact). I've done everything I can think of to ensure good contact with the immersion well (pulled the well and cleaned it, put it back, used thermo conductive paste, even HELD IT IN AGAINST THE WELL WHILE IT HEATED). The high limit does not function at all, and boiler is in runaway condition until I shut it down. I'm about lost as to why this is happening. I know HW Astats are not scientific accurate, but I definitely don't think this is correct operation.

Any insight or advice on what might be the problem here? I really don't want to drop another $150 for another new Astat. The Iwell seems fine, but it's cheap to replace so I could try getting a new one before calling in reinforcements. Good news is the weather in NW Ohio looks like it's good behave like summer from now on, but I'd like to get this all figured out before putting the boiler to bed for the season.

While system was drained to replace circ pump / pull Iwell (just to clean - did not replace), I went ahead and replaced / repaired several other components. Don't worry - I added several isolation valves so (hopefully) I won't have to drain the entire system again. Work completed while down:
1. Replaced Grundfos pump w/Taco pump & added iso flanges
2. Added iso valves at air scoop for vent and ET
3. Repaired solders on air scoop loop (left side)
4. Replaced backflow preventer due to slow leak / age
5. replaced copper w/PEX for cold water feed from pressure inlet to halfway down to system tap (didn't want PEX to get too hot)

SYSTEM:
Boiler: Burnham Series 2 Model B (204) - installed 1995 / no major issues until now
Astat: Honeywell L6006A 1145 (replaced old L4080D 1036 2)

Pics should be attached - let me know if you need any more pics / info. Thanks in advance for your help!
 
Attached Images
File Type: pdf
OldAstat_1.pdf (926.1 KB, 22 views)
File Type: pdf
OldAstat2.pdf (1.06 MB, 20 views)
File Type: pdf
Iwell1.pdf (1.43 MB, 17 views)
File Type: pdf
NewAstat1.pdf (1.86 MB, 23 views)
File Type: pdf
AirScoopLoop1.pdf (940.7 KB, 23 views)
  #2  
Old 05-14-20, 05:51 PM
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Are you absolutely positive that you have the a/stat wired correctly? If I recall you want to be wired to R-B, open on temperature rise. Follow the wiring inside the cover. One way is open on rise, the other is close on rise. Just can not remember if open is R-B or R-W.
 
  #3  
Old 05-14-20, 09:01 PM
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Welcome to the forums.

I'm not the boiler pro.

It would be pretty hard to mess up the wiring as there should only be two screw terminals on that stat which would be the R and B terminals. The W is a 1/4" push on terminal.

R-B OPENS, R-W CLOSES ON TEMPERATURE RISE.

It's definitely a problem with the aquastat reading the actual water temperature.

pulled the well and cleaned it
I would have said the well was corroded where it contacted the water but obviously that isn't the problem.
You said the boiler comes up to 180 when the stat is set to 135. That has to be some type of well issue. Possibly one of the other boiler gurus can shed some light on the problem.
 

Last edited by PJmax; 05-14-20 at 09:20 PM.
  #4  
Old 05-15-20, 11:24 AM
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C,
As was mentioned it is an aquastat problem if the boiler doesn't shut off. The aquastat you have has 3 terminals and a white differential ajustment wheel. You want to use this as a high limit control so you need to use the R & B terminals which will shut the boiler down on the high limit setting and come back on when it drops below the high limit minus whatever you set your differential at. Say you set your high limit at 180, If you set your differential at 10 your boiler will come back on at 170.

As far as your pressure problem, that usually doesn't have anything to do with temp. You mentioned your boiler reaching 230. At 12 psi it takes 242 deg to make steam in a boiler and the higher the water pressure the higher the temp it takes. To change water to steam at atmospheric pressure, like in an open kettle is 212 deg. A boiler is a closed vessel so your 212 doesn't apply.

This is just something that I believe but never had it verified by a manufacturer like Honeywell. If you notice all aquastats high limit settings is 240 deg. I believe it's because the industry standard boiler pressure is 12 psi which would need 242 deg to make steam. Your boiler when working properly cannot be set any higher than 240. That is just my thinking.

Another issue is your pressure problem. It looks like your expansion tank is White which is usually a Therm X tank for potable water and not for heating systems. Do you have a 3/4" thread on that tank. If so that tank is for a hot water tank. The extrol you need has a 1/2" threaded fitting. Another thing is if that tank is a ST-5 it only has a 2 gal capacity instead of the 4.4 gal you need with a #30 extrol tank. That 2 gal tank is too small for your system and would cause preesure problems.

Just a thought, hope this helps a little.
 
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Old 05-15-20, 05:28 PM
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Good catch on the expansion tank, Spott. It most definitely is a thermal tank and not a hydronic tank! Note the 3/4 x 1/2 reducer !
 
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Old 05-15-20, 07:59 PM
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Thanks for responding spott.

Question.... you addressed how he should be setting the aquastat but when he did that the boiler temperature went way over the setting. Set at 135......... meaning when the aquastat measures 135 ..... the boiler should be shutting off at 135 but it's running until it reaches 180.

This seems to be a problem that needs to be addressed further.
 
  #7  
Old 05-16-20, 02:42 PM
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Pete,
I agree it does need to be looked at further. The way I'm looking at this is there are 2 aquastats, 1 old and 1 new having the same results so I'm guessing it's not the aquastat but the way they are sensing the water temp which could start with the sensing bulb if he hadn't tried a new one so that would seem to eliminate that. The next thing is the well but he cleaned the well of debris but I've never seen one look black like that and wonder what caused it. If it is some kind of coating that got on there stopping the bulb from properly sensing the water.

I believe the origional aquastat could be used with or without a well. It should have a 3/4" thread that can go right into the boiler for more accurate sensing. Before buying a new well I would try installing the old aquastat withour a well to see what happens if it has that 3/4" thread. The only other thing is to make sure your aquastat is sitting in water and your boiler doesn't have an air lock. I mention this because you mentioned steam which cannot happen at the temp you had. By looking at your gauge your pressure looks low also. Try raising it to twenty and see what happens,

Pic of the old aquastat removed would be helpful.

Just my thoughts but it does seem to go beyond the aquastat.
 

Last edited by spott; 05-16-20 at 03:54 PM.
  #8  
Old 05-17-20, 11:48 AM
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Verify the circulator is pumping the proper direction. If the flow through the boiler is backwards it will cause the boiler to way overshoot setpoint temperature.
The flow should be into the lower taping and out of the upper taping on the left side of the boiler.
 
 

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