Aquastat Probe Failure FAQs

Aquastat setting vs temperature sensor probe failure

Boiler Temperature Sensor Probe Failure 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

And see AQUASTAT TEMPERATURE RESPONSE FAQs

How can I test to see if the aquastat control has a bad sensor?

Is there a way to test control sensor problem - (Feb 16, 2014) Anonymous

I have 24 volts at each dial when burner is firing is there any other test I can do with a meter to see if this a control sensor problem (Feb 16, 2014) henry s

Reply:

Anonymous: look at the boiler pressure and temperature on the boiler gauge. If you see that the boiler is not turning off at or below its HI limit setting then the system is unsafe and needs repair.

See details at AQUASTAT SENSOR PROBE HEAT CONDUCTING COMPOUND

Interesting question, Henry, I don't know but a wiring diagram for your control might answer it.

If you are talking about an aquastat on a heating boiler, typically there is a boiler temperature sensor that is inserted into a well that itself projects into the boiler water;

The manufacturer recommends that the copper tubing connecting the sensor to the aquastat be bent enough to assure good pressure or contact between the sensor and the well sides, and they also may recommend use of a heat conducting paste to assure good contact; If contact is poor (or if the sensor has failed) the control may fail to respond properly to the actual boiler temperature.

So my thinking starts by observing how the boiler actually responds compared to the gauge temperature.

Unfortunately to check the sensor mount and contact you'd have to remove the whole control - a lot of trouble so not what people would try first.

That's why I stressed the importance of looking at the HI LO and DIFF settings first

What temp should a steam heating system be set at

At what temp should a steam heating system be set at had a pipe replaced an now it seems like the water takes longer to get hot thinking the repairman turned the temp down an now seems like the water is taking longer to shower (Dec 23, 2015) Dexter

Reply:

Dexter:

Steam is made by boiling water - so at sea level that's 212 degF or 100C. It's not adjustable.

What is adjustable is the room thermostat or individual radiator thermostats, or steam system pressure - none of those have to do with the time for hot water to arrive at a shower: that's hot water, not steam, and is a different system.

If your hot water is made by a tankless coil mounted in the water reservoir of your steam boiler the coil may be clogged.

Or if your hot water, made by a tankless coil, has its temperature controlled by a mixing valve or anti-scald valve (an important safety device) that valve may not be adjusted properly.

See those details at MIXING / ANTI-SCALD VALVES

Boiler shuts off at Aquastat HI but doesn't come back on

My boiler is shutting off when the water reaches the aquastat hi setting but its not coming back on until after the water temp drops significantly and the pump is circulating cold water.

I have a L8148 A. The temp is set at 200 as it came and the low temp is not adjustable. On 2018-01-27 by Lucas

by (mod) - Check the aquastat sensor probe or replace the whole unit

Lucas

Possibly the aquastat's actual sensor is not in good contact with the side of the steel well into which it's inserted into the boiler - or the installer omitted the heat-conducting grease recommended by the manufacturer.

Otherwise I suspect a defective aquastat sensor probe.

That will require replacing the whole aquastat as few technicians will try to just replace the probe.

Burner won't turn on until temp falls way below the LO setting on my Aquastat

My aquastat is set at 195 high, 175 low and 25 differential.

The low setting seems to work as designed, however when my thermostat calls for heat, the burner doesn't turn on until the temp falls to around 120. My understanding is that it should start the burner immediately. Don't know what the problem is. Any help would be appreciated. Thanks. On 2016-12-28 2 by Bruce

by (mod) - problem with the temperature sensor of the aquastat control

Bruce,

I suspect a problem with the temperature sensor of the aquastat control - perhaps it's not in good contact in the well into which its inserted, or the thermal grease was omitted.

Boiler not turning on at call for heat?

I have just recently installed a new Monitron Slant Fin electric boiler to my 20 year old system. I have 5 zones...two downstairs in floor heat and 3 upstairs in baseboard heat. I have the L7248L Aquastat and I have added the I also installed the Honeywell C7089 sensor and Outdoor Reset Module to this system.

My boiler settings are 180 High and 160 Low with 10 differential. The outdoor upper temperature setting is 40 degrees. In northern Minnesota when the temperature gets below 20 degrees the system works very well. have 2 questions:

1) When the the temp is ~35 degrees for a long period of time, the boiler stays between 180 and 160 degrees even if the thermostats are not calling for heat.

This results in hot water circulating upstairs by simple gravity and it gets too warm.

Shouldn't the boiler shut down with no calls for heat?

2) If the outside temp is 42 and cloudy for several days, the ORM shuts down the boiler and the house gets cold because the boiler does not kick in even if the thermostats ARE calling for heat. Shouldn't the boiler kick in if the thermostats are calling? On 2017-04-26 by Steve -

Reply by (mod) - burner turns on in response to water temperature, not thermostat call for heat

Steve

Typically aquastats come factory set assuming that there is a tankless coil on a boiler; the boiler will keep some heat in the boiler year-round (in case the tankless coil needs it) even if there is no call for heat.

If heating water is actually circulating out of the boiler even when your thermostats are not calling for heat, that too will cause the boiler temperature to drop and the burner to run to re-heat the system.

That's an error: I suspect a zone valve that's not closing or a system check valve that's not closing when it should - permitting hot water to circulate by convection or as you put it, by gravity.

More detailed diagnosis of this problem is at HEAT WON'T TURN OFF - - https://inspectapedia.com/heat/Heat_Stays_On.php

The ORM, as I understand them, changes the operating temperature range of the boiler - it should not be disabling it; rather its intent is to give longer boiler-on cycles on a call for heat when the outdoor temperature is not too low. It does that by changing the effective HI setting.

I addressed this question where you (I think it was you) asked it previously: if the room temperature is below the set temperature on the room thermostat then hot heating water should circulate out of the boiler through the room's radiating devices.

Unless your home is in Canada where installers set up the system differently, it is the temperature of the water in the boiler itself that turns the burner on and off.

Review the ORM settings with the manufacturer of your module - or its instructions. It may not be working properly.

Details are at OUTDOOR RESET MODULE AQUASTAT ADJUSTER

Raised the aquastat but one zone still gets no heat

My boiler aquastat is raised to 160 Hi and 140 LO because part of my second floor is not getting heat but I have not seen any improvement. While the front half of the house radiators are hot the back part especially the second floor there is no heat.

What could be the problem with the hot water circulation?
Thank you for a response On 2018-10-18 by Randy

by (mod) - aquastat treats all zones the same; the problem is elsewhere, probably a bad circulator, relay, or airbound zone

if one of your zones is getting no heat at all then the problem is not the settings on the aquastat on the primary controller. That's simply controls the temperature operating range of the boiler.

More likely you have a zone Valve or circulator pump that is not working, or you might have an air bound heating Zone.

If you can confirm that the zone is calling for Heat and the circulator is running

read the repair procedure at

AIR-BOUND HEATING SYSTEMS

Honeywell R8182d aquastat ignores call for heat and boiler gets cold - bad control?

I have a Honeywell R8182d aquastat. On a random basis it seems to ignore a call for heat and consequently the boiler goes cold.

Is this likely due to a bad control or could other things be causing the failure to fire? Like maybe dirty electrodes or plugged nozzle? Although why would this be an intermittent problem. Any ideas appreciated. (Oct 25, 2015) Anonymous

Reply:

If the thermostat calls for heat on a hot water heating system the boiler may not run (in most U.S. installations) until the boiler temperature drops below the cut-in temperature controlled by the system's aquastat or primary controller.

But if the boiler never turns on at temperature drop, and doesn't even try to run then there is a control or wiring problem - provided it's not off on re-set.

Check for an open wire in the thermostat circuit and check that the boiler will run if the therostat terminals are temporarily connected by jumper at the aquastat. Check also for a bad primary control or control relay.