I have a Weil-McLain oil boiler with a Honeywell L8124A aquastat. I was having trouble controlling the boiler heat (heating to 220-235). I called a boiler technician and he came to look at the system and said there is definitely something wrong with the aquastat. He quoted me $335 to install a new aquastat and suggested that I find a used one to get me through the winter (since I will be removing the oil burner next fall and install a heat pump). I purchased a used aquastat of the same model and the same issues occur.

The aquastat bulb fits snuggly in the well and has never had heat transfer paste during the previous 16 years. A different technician that I talked to suggested using paste and it may take care of my problem. I tried and it did not. I adjusted the aquastat last night so that it works pretty well. With the upper limit set at 120 +/- and the lower limit set at 100 +/-, the furnace temp stays between about 170 and 200 degrees (dif. setting set at 10 degrees).

Does anyone have any idea what might be going on? The aquastat seems to think that the furnace is hotter than it really is, so I don’t think it is a heat transfer issue in the well.

Thanks in advance for any suggestions.

Kevin

 

Just a thought but If 2 aquastats have the same exact problem, drain the boiler and remove and inspect the aquastat well. The probe on the well may be coated with crud which acts an insulator and this may be the reason that your aquastat isn't reading the proper boiler temp.

 

That's bizarre!

Are you absolutely CERTAIN, beyond a shadow of a doubt that the temperature gauge is accurate?

As OldBoiler says: "ASSUME NOTHING, SUSPECT EVERYTHING"

 

I have used a temperature gun to verify the boiler temperature.

Lightsout: If there was that much crud on the well, wouldn't I have the same problem with the temperature gauge?

 

One thing to keep in mind about infra-red guns is that you won't get an accurate reading on copper pipe. Cast iron is pretty durn close though... if you do want to measure copper pipe, you have to paint a spot flat black... magic marker won't work... paint will. Even so, copper will read cooler... it can't read hotter! Just an FYI ...

Isn't the temp gauge in it's own well?

Never underestimate the power of coincidence either! The odds are slim, but do exist, that both aquastats are defective!

mantra: Assume nothing, Suspect everything.

 

Awesome, this explains alot...

OP, maybe your aquastats are in Celsius. Seems like the observed temps are closer to the limits in C than in F. And yes... I have no idea what I am talking about or if they even make aquastats with delineations in Celcius, just an observation...

 

For more info on the _whys_ of IR guns reading differently on different materials, Google up "emissivity" and check it out... the $expensive$ guns have an adjustable setting for same.

 

Kevin, you said:

That doesn't make sense to me.

If the aquastat thought the boiler were HOTTER than it is, wouldn't it be shutting the burner down when the water was actually COOLER than the setpoint?

If you have to set the aquastat to a LOWER temp to get the boiler to shut down at a reasonable high limit, doesn't that mean that the aquastat thinks the boiler is COOLER than it really is? And thus keeps right on firing until it thinks it is hot enough?

This COULD be a problem with heat transfer... but I'm somewhat skeptical of the crud buildup on the well... but stranger things have happened.

 

Swap the gauge. Check all wiring under the primary control back to the aquastat. Very odd situation.

Good luck.

 

NJ Trooper - you are correct, I had a brain malfunction. With the aquastat upper limit set at 120 and it heats to 190 before it shuts off, the aquastat detects that the water is actually 120 degrees when it shuts off - COOLER. So, with use of heat transfer paste with the proper size well, it sounds to me that it could be an issue with the well itself.

 

Maybe the bulb is not bottomed out in the well. Use a permanent marker and put a mark where the well is on the capillary tube. Pull the bulb out. Run a small wire into the well and mark the wire at the opening of the well. Same as you did on the capillary. Pull the wire and hold it next to the bulb/capillary, and the mark should be the same place on both. If not the control is reacting to what it is sensing. The bulb must be bottomed out in the well for proper sensing.
See link
http://www.comfort-calc.net/Service/Aquastats.html