Boiler Aquastat Temperature Response FAQs

Aquastat temperature response Q&A

Aquastat Response to Boiler Temperature FAQs

Tip: while the aquastat sensor probe itself may fail, requiring control or probe replacement, be sure to also read

AQUASTAT SENSOR PROBE HEAT CONDUCTING COMPOUND - required on some, not all, aquastat temperature sensing probes.

Also see BOILER PRESSURE & TEMPERATURE SETTING, CONTROL FAQs

My boiler fires up immediately on a call for heat

My oil fired boiler fires immediately when the thermostat calls for heat. Aqua stat is set 180 high, 140 low, 10 dif.

No matter what boiler temp. Is at. Circulator is OJ.

I just seem to remember circulator running until boiler temperature Got to LO setting, then the burner would fire. On 2017-04-10 by Joe Fortuna

by (mod) - explanation of normal boiler burner turn-on point

Joe

If the boiler starts immediately on a call for heat that's being driven by the actual boiler temperature, not simply the aquastat settings.

What you describe is correct in general, that is the thermostat (on U.S. typical setups) turns on the circulator and the boiler temperature is what turns on the burner.

But if on a call for heat the boiler temperature is already low the boiler may fire immediately.

Boiler actual temperature is not at the aquastsat set temperature - why?

My aquastat is set to turn off at 180. According to my new temp/pressure gauge the water temp is only 150. Any advise On 2018-01-31 by Ed

Reply by (mod) -

Ed

That could be perfectly normal.

With the room thermostat set all the way up so that the call for heat will be continuous, check the boiler temperature on the boiler gauge when the boiler's burner has just turned off. The temperature gauge should read close to 180F.

(These are not precise lab-grade instruments).

If the gauge is way off then I suspect the gauge itself is defective.

What are Normal Boiler Temperatures When the Aquastat has Turned OFF the System?

Our aquastat shuts off burner at 190 degrees but old and new temp gauge continues to rise to 215 degrees. is this normal? - Warren G.

Reply: It is normal for boiler temperatures to sometimes rise a bit after the burner has shut down

Warren: it is normal for temperatures inside a heating boiler to continue to rise a bit after the Aquastat has turned off the oil burner at the HI or when the thermostat has stopped calling for heat.

This temperature rise may occur because:

• The latent heat in the heating boiler combustion chamber and in the steel or cast iron body of the boiler itself continues to heat the water inside the boiler even after the oil or gas burner has stopped firing

and

• The circulator pump(s) have simultaneously stopped sending hot boiler water out into the heating zones.

As long as boiler temperature does not rise high enough to cause the pressure/temperature relief valve to spill you should be fine.

Aquastat tripping off on "safety" when an heat zone is satisfied at the set thermostat temp

We have a 14 year old system that has performed very well over that time period. There are 3 zones served by a Raypack boiler.

Main floor served by air handler with forced air hot water heat. Basement and garage served by in-floor heat.

Following failure of main circulation pump in 2013 and one system circulation pump recently elected to replace 2 remaining pumps.

Garage floor system was partially drained in that process. Tech has bled air from system.

We have been operating for 2 weeks with the Acquastat tripping when any heat zone is satisfied at set thermostat temp.

Also boiler will only run if 2 zones called. Static water pressure is 12psi and operating pressure is approx. 30. Any thoughts? On 2014-01-27 by Jim Hamilton

Reply by (mod) -

Jim,

RE: "Aquastat tripping when an heat zone is satisfied at the set thermostat temp"

I don't quite understasnd. An aquastat "trip" to me means going off on safety reset.

That condition is not caused by the thermostat but usually by a flame or burner operating problem - such as a bad flame.

I think you mean that at the end of a "heat-on" cycle the aquastat is tripping off on "safety" and the reset button pops up.

Many service techs would start such a service call by doing two things

1. Replace the cad cell

2. Observe the flame at shutp-down.

A "sloppy" shutdown flame could mean that the burner's fuel unit isn't closing off sharply and unburned oil is peeing into the combustion chamber as well as making a smoky flame that trips the cad cell relay.

In that case

Watch out: you risk a dangerous puffback explosion if the problem isn't corrected.

Watch out: do not keep pushing that reset button, also because of the risk of a puffback.

Also, the fact that you say the boiler only runs if two zones are calling for heat might mean that your thermostats or zone valves or circulator controls are not properly wired - something to ask your service tech.

Keep in mind that it's easy to get confused about when the boiler should actually run. The thermostat calls for heat. The circulator sends hot water to the heating zone. Only when the boiler temperature then falls to or below the boiler's aquastat CUT-IN temperature will the boiler itself or its burner actually "run".

- Daniel Friedman