Failed Aquastat - Narrowly escaped a runaway boiler.

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Old 03-21-18, 07:32 AM
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Failed Aquastat - Narrowly escaped a runaway boiler.

So just how common are failed aquastats? Especially ones that have the temperature bulb fail, leaving the circuit closed?

About 4 years ago I installed a Honeywell L6081A in my old peerless boiler with tankless coil, wen I rebuilt the firebox, added a 4 zone controller, and eventually upgraded to a new AFG-II burner. Recently, our domestic hot water had been a bit hotter than usual, but we really didnt think too much of it until we noticed the pressure relief valve had blown. Looking at the gauge, I saw the boiler was running over 215* while the aquastat hi limit was set for 190. I had to lower the aquastat hi limit to 140 to cut off at 190 actual. My first thought was that the bulb was not making good contact in the well, so I pulled the aquastat, cleaned it, recoated it with thermal grease, and reinserted. Even set at hi limit 140, I watched the boiler climb to 200+. Dropping the hi limit as low as I could, I never heard the click. I pulled and reinstalled a few more times, trying different bulb positions and depths, and making sure the bulb lead was not touching the aquastat case, but no matter what, the DHW lo limit was never satisfied, and the high limit never broke open.

As best I can tell, there is a mechanical failure in the aquastat, in the bulb or the actuator (for lack of a proper term). How common is this? I'm very thankful that this as caught early, and a disaster was avoided.

A quick trip to lowes a half hour before they closed, and I was able to get a 4006a a'stat to temporarily take over high limit duties. We wont have a call to fire the burner for DHW, but at least we will stay warm as the fourth nor'easter in a row dumps 12 to 18 inches on us today.

I have ordered a replacement L6081A, but now I'm wondering if I should be concerned this one may fail the same way? I dont know of a place to add a second well to install a backup/safety aquastat. I'm seriously considering rigging something up with an arduino and a thermistor to control a relay, or at the very least, alert me of an overtemp condition.
 

Last edited by GuitsBoy; 03-21-18 at 09:47 AM.
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Old 03-21-18, 08:30 AM
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Guits-

I’m not a expert for sure. But are you saying you installed the 6081a and then had the problems, or you had the problems and then installed a new 6018a and still had the problems? So the next 6081a would be your third. It sounds like it all started when you installed a 6081a.

But it seems to me it’s not just a high limit problem. I think if everything else was working properly the thermostat should have been satisfied and the burner stopped long before hitting the really high boiler temperatures. Maybe I’m wrong. (Also I think sometimes you can’t trust the temp gauge)

Since the l6081a also controls the circulator maybe the circulator wasn’t running also. I think that would explain why the thermostat would continually call for heat. I would check to see if the circulator is running when you have this problem.

I’m sure the experts will be along soon.
 
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Old 03-21-18, 08:55 AM
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My apologies, I see some areas of my post were a bit ambiguous.

The failed 6081a is about 4 years old. It replaced a simple high limit fixed diff aquastat that kept the boiler at full temp year round for DHW duties. We did a lot of work to the boiler to keep it running while we wait for the natural gas line to make it the extra 2 houses down the block. We swapped to the 6081a, rebuilt the firebox, added a 4 zone controller, smart thermostats, new beckett AFG-II, new exp tank and PR valve, new fill regulator, new temp/pressure gauge, air purge, and re-routed all the copper-in-slab runs with pex, etc.

We installed a pellet stove on the main floor of the house, which kept the main zone from running frequently. One room, the den, was always very cold, where the heat from the pellet stove does not reach, so we put this on its own zone. This new zone is very short, and thus the boiler is overfired when only the den zone is heating and main zone is satisfied by the pellet stove. This is when the boiler was most likely to bang up against the hi limit.

We are not using the 6081a to drive the circulators directly. Those are driven by the taco 4 zone controller. The low limit and diff side was hooked to the thermostat terminals of zone 4 (priority) to fire the burner for DHW. In essence, it acted like a low voltage thermostat for hot water. The high limit side of the aquastat broke the TT terminals on the AFG burner, as opposed to killing the hot 120v, that way the burner cool down cycle could finish up. Hope that makes sense.

Regardless of our particular setup, the aquastat most definitely failed. Even without anything hooked up, with the bulb in a hot well, you should hear the contacts open and close as you move the high and low limit knobs past the actual water temp. This would no longer happen. It was as if I had cut the bulb off, and the aquastat would not recognize any heat.

Since the aquastat did not sense any heat, it was continually calling for heat for DHW. The burner kicked on, but DHW would never be satisfied, and the high limit would never be reached. Eventually the PR valve would have spilled its guts had I let it run longer.

Thanks for chiming in, its always good to have a fresh set of eyes.
 
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Old 03-21-18, 09:32 AM
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Old 03-21-18, 09:37 AM
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Interesting. I had no idea there was such a thing as a strap-on aquastat. Seems like I could easily run this in series as an extra failsave, set just a few degrees higher than the primary aquastat. Where are these typically strapped to? The main 2" output pipe at the top of the boiler?

I can probably just re-purpose the L4006A I'm running temporarily, if I come up with a good way to mount it.

Thanks for the suggestion.
 
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Old 03-21-18, 11:17 AM
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Guits –

I see what you are saying and it looks like the aquastat has a problem.

But it seems to me there is something funny in your setup. That 6081a opens contacts (presumably for circulator) below the low limit, and closes them above the low limit to allow the circulator to run.

But you are using those contacts as T-T signal for Zone 4 on the Taco. So it seems to me above the low limit there would be a continuous call for DHW (i.e., T-T is always ON) but below the low limit there would be NO call for DHW (i.e., T-T is always OFF). That seems backwards.

It seems like what would happen is that you would be continuously cycling around the High Limit for DHW, which I guess would work but maybe not efficient?

But I guess that is off on a tangent and is not your current problem – and might be a total misunderstanding on my part – lol.

(just had a thought – if anything I said above were true ( lol-big if) could an extraordinary exercising of the High Limit cause this unique problem? I guess that’s a real stretch.)
 
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Old 03-21-18, 12:49 PM
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Just curious. Why were you using the 6081 instead of a triple action aquastat which is common wit boilers with a tankless coil. What purpose was the 6081 serving. What was there originally before you installed the 6081.


http://www.supplyhouse.com/Honeywell...ronic-Aquastat
 
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Old 03-21-18, 02:13 PM
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zoesdad -

Youre quite the eagle-eyed observer. However the 6081a has both low limit AND circulator functions, depending on the terminals used.

https://customer.honeywell.com/resou...0s/60-2105.pdf

Action on Temperature Rise to Setpoint: Breaks R-B / Makes R-W.

This was the reason I chose this particular aquastat. When we first bought the house and changed all the thermostats from line level to low volt, we wanted to change out the aquastat to idle at a lower temp for domestic hot water, rather than simply bounce between the high and low limit as was the case with the original single aquastat.

Spott -

I dont recall the exact model aquastat that was originally installed when we bought the house, however it was a very similar design to the 4081 and 6081 series, but only had a high limit and a diff. This meant that it kept the boiler between the setpoints year round. The circulators operated without regard to boiler temp or DHW production.

The 6081a appeared to be very similar to the existing equipment (for fitment), was inexpensive, and had the functionality I needed to let the boiler idle at a lower temp for DHW, yet fire up hotter on a call for heat.

Now looking at your link, I'm second guessing ordering up another 6081a. But then again, I'm not sure what functionality I would actually gain or be able to make use of. However after this recent failure, I am beginning to think that I'd trust an eletronic thermistor more than the mechanical bulb. I would have to assume the digital aquastat would fail open at least.

Regardless, its all a temporary measure until we can finally convert to natural gas and kick this 40 year old boiler to the curb, so to speak.
 
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Old 03-21-18, 02:57 PM
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Murphy's Law Believer

Also as protection for boiler over temperature use a separate $23 PID control. Am Murphy's Law believer. PID is protection in case out-door-reset / aquastat or something else malfunctions.

One PID with 3 control channels can be used for Aquastat set to 180F, Alarm #1 boiler over temp set at 190F and Alarm #2 low temp circulator cut out set at 135F. I use a separate ODR /Aquastat for burner on off

Each function can be set with different temperature and delta T. PID displays both set and actual water temperature and alarm channels status.

https://www.ebay.com/itm/TA4-RNR-Dua...wAAOSwEW9aXtF7

Note: on ebay make sure to get 120 VAC model with F/C Fahrenheit scale and two alarm channels
 

Last edited by doughess; 03-21-18 at 03:15 PM.
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Old 03-21-18, 04:12 PM
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Guits-

Gotchaa!!!Thanks for taking the time for my education. I see that there are obviously more wiring options than I thought of. Looking at the diagrams I can see it now.

Good luck!!

(btw- that Patriot-Supply.com is a good place for parts. They seem to have really good prices and they deliver very fast. Just my opinion.)
 
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Old 03-21-18, 06:14 PM
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doughness -

Is the PID simply an alarm to alert you if something exceeds specs? Or do you actually make use of the relay for control or shutoff? Interesting and inexpensive, though I might question its reliability if used as an actual control.

zoesdad -

Thanks for looking over things and asking questions. Its a relief if my setup checks out by you. But I'm surely in no position to educate you on anything.

Patriot Supply and Supply House are both my go-to places to order online. Theyre literally only minutes from my house too. The only down side is sales tax, but most orders arrive the next morning for me.

EVERYONE -

Thank you all for your help and comments. I'm still curious as to what caused this to fail in the first place, but you guys have certainly given me plenty of options for the future.
 
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Old 03-22-18, 08:37 AM
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Guits-

I agree that unfortunately your original question still goes unanswered. Here is a thread that might give you a little information – but I don’t think it would answer your original question. It talks about the L7224U Aquastat. That’s the one discussed in the thread (the one Vermont has and I have) and those have a replaceable plug-in sensor bulb. I think in the thread Vermont clearly sees that the problem was his bulb. But I think in your case you have not seen any obvious bulb damage.

It looks like the 6081A doesn’t have a replaceable sensor bulb which I’m sure you know, so I know a replaceable bulb would not be your answer.

(IMHO one thing to notice is what Grady, who is a pro, says about thermal paste.)

https://www.doityourself.com/forum/b...0001464-a.html
 
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Old 03-22-18, 11:14 AM
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GuitsBoy: Is the PID simply an alarm to alert you if something exceeds specs? Or do you actually make use of the relay for control or shutoff? Interesting and inexpensive, though I might question its reliability if used as an actual control.

PID''s provide contact close/openings based on user settings. What you do with it is your option. Reliability is very high. Far better than bimetal or bulb type controls.

PID's are used by the millions worldwide in industrial control applications. USA brands go for $200. Chinese ones go for $10 to $20. Have both, see no difference.

See PID definition: https://en.wikipedia.org/wiki/PID_controller

For the new comer PID's have a learning cuve. Recommend running one on the bench before putting to use in system. Can always use it a digital thermometer. I use one for stack temperatue with 9” staineless probe (eBay - $10) to check efficiency.
 
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Old 03-23-18, 01:21 PM
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doughness,

Thanks for the info. Seems pretty similar to the temperature controllers homebrewers use for to regulate fermentation temperature. I mean, I guess thats pretty much all an aquastat is anyway. Could probably just do away with the aquastat completely and just use a these things, but with so much at risk, I'm not quite sure Im ready to trust it yet. Theres something comforting about knowing that I'm using a purpose built component for the task.

Since I already have the two aquastat's (replacement 6081a and the temporary 4006a) Ill probably just use what I have to add a failsafe. But it might be worth exploring using a PID or TC unit for an audible alarm if it falls outside normal range.

https://www.amazon.com/Inkbird-All-P...dp/B00OXPE8U6/
 
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Old 03-23-18, 03:16 PM
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GuitsBoy: There is something comforting about knowing that I'm using a purpose built component for the task.

We are dealing with a whole heating system. Focusing on components, especially old generation types can loose sight of benefits newer technologies offer.

Aquastat is just a $10 name for temperature controlled switch used on hydronic systems. https://en.wikipedia.org/wiki/Aquastat

Outdoor-Boiler-Reset- is an aquastat with 2nd outside temperature sensor and computer control. Many DIY's think incorrectly of it only as aquastat, oblivious to the benefits and features.

ODR's optimize water temperatue to what is need based on various setup data and outside temperatues. Without going into details they save money in ways that is impossible with your old component units.

For $156 ODR's provide quick pay back in fuels savings and make for a more comfortible home. Read the Installation Instructions to get a better understanding: https://www.supplyhouse.com/Tekmar-2...iler-4150000-p

I have a Tekmar 256 and found when delta-T is set to “automatic differential” it caused water temp to go to 190F. The “AD” setting was designed for those paranoid about short-cycling, less than 4 minutes burner cycles. AD setting actually decreases efficiency.

My Tekmar 256 now use a fixed delta-T and 170F max temp setting to allow for overshoot. As over-protection set PID set at 190F. Same PID disables circulator below 137F.

Most PID have far more features than your Amazon link: https://www.amazon.com/Inkbird-All-P...dp/B00OXPE8U6/ which seems to be limited to simple heating an cooling applications.
There are far more features PID https://www.ebay.com/itm/TA4-RNR-Dua...wAAOSwEW9aXtF7
 

Last edited by doughess; 03-23-18 at 03:46 PM.
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