If the water temperature is running well above the setting on the aquastat there are a few reasons for this. We are not concerned about 5º to 10º above or below the setting. There will usually be a discrepancy between the gauge and the control. The only way to accurately check the temperature would be to use a calibrated temperature sensor in the well of the control. Let's list the common problems with relationship of aquastat settings and higher temperature readings.
First let's remember what happens to the steaming temperatures as we increase the water temperature. We know water boils at 212º at sea level under zero pressure. A heating system should always operate at a minimum of 12 psi. If you increase water pressure you raise the steaming point of water. At a minimum of 12 psi (minimum a heating system should operate) the boiling point of water has increased to about 236º. Of course, as you raise water temperature you raise the pressure slightly due to water expansion. The steaming point will increase slightly.
1. A difference between the control and gauge due to different locations in the appliance
2. The control may not be located in a good location. These will normally be aftermarket applications.
3. Mismatch of well and sensing bulb. A newer control added to an older well that may have utilized a larger bulb.
4. The newer electronic aquastat sensors are smaller and do not need to fill the well completely.
5. A mineral buildup on the exterior of the well. If this is a hot water boiler system this is also a problem for the boiler. Adding too much fresh water is adding oxygen, minerals and chemicals too the boiler water all of which will shorten boiler life expectancy.
6. A bad control
7. The sensing bulb or electronic sensor is not bottomed out in the well. They must be all the way into the well.
How to determine if the bulb is bottomed out.
Before removing the control and pulling out the sensing bulb, make a mark on the capillary tube at the face of the well.
Get a small wire or anything less than 1/2 the size of the well opening. Slide the wire into the well all the way to the back. Place a mark on the wire at the face of the well. Remove the wire from the well and lay it beside the sensing bulb. The lines should be at the same place. If the mark on the wire is higher up on the capillary the bulb is not all the way in the well. The well needs to be removed and replaced. The diameter may be off slightly or a small dent in the well.
You probably will not have the well adapter. If not mark the same place up against the well. We are just looking for a reference point. The important part of this test is both the wire and the capillary tube be marked at the same reference point.
