View Full Version : Boiler Temp 240F - Bad Aquastat?

02-10-2011, 04:12 PM
Walked into my sisters home tonight and noticed a burning smell. The CI radiators were way hotter than usual and one of them had a slight hiss at the vavle stem.

The boiler's temp was at 240F, pressure about 22 PSI - so the relief valve wasnt blowing off. Is this most likely a failed aquastat relay? The aquastat is a Honeywell L8148 model, so there is a HI setting only, and a fixed differential @15deg. The HI setting dial is set to 180F (it hasn't been messed with since it was installed 2 years ago).

The entire plumbing system is only 2 years old (except the old radiators) - new Dunkirk boiler, Honeywell Aquastat, flue damper valve,taco zone valves, and grundfos circ pump.

02-10-2011, 04:27 PM
The relay is a possibility, but not the only thing in the circuit. this should be addressed IMMEDIATELY. Water that hot will instantly flash into steam if it gets released. The thing saving that from happening is the 22# of pressure, like in a car's radiator.

02-10-2011, 05:51 PM
I turned all thermostats off immediately - they can rely on the gas fireplace and spaceheaters until I fix this.

Jim - what else in the circuit could cause the boiler to not shut off?

02-11-2011, 07:05 AM
Not familiar with the details. They should have the manuals which will show a block diagram, or if not, find them on-line, or there's usually one glued to the inside of the cover. You need to follow the interlock path...it could be a stuck valve, a control board, the aquastat, a shorted line, and possibly something else. Having spent a lot of time with military block diagrams, I find the ones used by the HVAC people as pretty criptic, but if you study it and figure out their symbology conventions, they eventually make sense. The aquastat could be electronic or mechanical. If it is something like a thermistor, it might simply be that it got knocked or pulled out of its well and is not actually sensing the water temp, but the nearby air temp. If you are lucky, all you have to do is push it back into place, or if mechanical, latch it back down. A multimeter and some knowledge would go a long way to isolating this fairly quickly. A hot water boiler is not designed to run at 240-degrees, and the life of any attached components are at risk, so there may now be multiple failures. the manual typically gives some troubleshooting steps, and a call to the manufacturer may also help in isolating the problem.

At 2-years old, it may still be under warranty, and may be the best path to take.

02-11-2011, 12:15 PM
thanks. replaced the whole aquastat with an identical unit and solved the problem. perhaps the old one could have been repaired - but it was easier to just switch the whole thing out.

02-12-2011, 11:08 AM
The Honeywell L8148 is an absolute money-waster and it is not a reliable control. In 30 years in the field I have every reason to state with accuracy that the L8148 is partially responsible for Global Warming. Seriously, I do not know why they came up with that design, as it only allows for 1 zone of heating with it's built-in circulator and burner switching. However, just because you replaced the aquastat and everything works well now, this does not mean that the 'old' aquastat was defective - it could mean ttat when you installed the new control you actually pushed the temperature sensing bulb all the way into the immersion well and it sensed temperature correctly - I have seen this minor problem dozens of times over the years as a service technician. Other clues would point to this and confirm it such as, when your expansion tank fails prematurely as a result of the exceedingly high water temperature it was exposed to (for how long?). Simply replacing parts is not a definitive troubleshooting outcome, so you could experience other problems.

As far as a more fuel-efficient control application: Replace the 8148 with a Honeywell L6006 1145 SPDT Aquastat and switch the burner only as a "high limit" control and run your circulator off a circulator relay with end switch, which connects to the "T T" on the burner primary control. 1 conductor of the end switch/TT wire will go through the 6006 common and "opens on temperature rise" terminals to the primary control. This control configuration will allow you to do a few things that the 8148 can't - namely, you can have a temperature modulation control installed to save quite a bit of fuel, the 6006 rarely fails, and the 6006 has a bottom limit of 100 degrees, whereas the 8148 bottom limit is 180. Further, the 6006 is a less expensive control than the 8148.

Good Luck!