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Aquastat high limit for indirect water heater?

Jersey2 Member Posts: 166
I have this Resideo aquastat on my tank. There is only one dial to change the temperature, the low limit. So what is the high limit? Is it adjustable? Also, this type has the sensor in a well without water in it correct? My attention has been drawn to my aquastat because when I ran bath water, after a while the water got cold and the aquastat did not call to heat up more water. I had it set at 125 and I had to bump it up to 130 to get it to turn on. And then it ran for about 45 minutes for the 30 gallon tank. For showers the water is always hot, but showers use a lot less water than baths.

I'm not a plumber or hvac man and my thoughts in comments are purely for conversation.


    HVACNUT Member Posts: 5,737
    The thermostat wire is blocking the most important letter.
    Is it an L4006A? Should be. 

    It is a "break on temperature rise" high limit. If there's no white dial on the right side of the aquastat, then it has a fixed differential.

    45 minutes to recover a 30 gallon tank?
    What make and model tank?
    What circulator for the tank?
    What's the BTU ratings on the boiler?
    Is the indirect on a priority zone?

    Also make sure the capillary tube on the aquastat is fully seated in the well. It's a tight, snug fit, but it must be fully inserted. 

    Once all is rectified, it might be a good idea to install a thermostatic mixing valve. Then you could keep the aquastat set to 140° or more and temper back down to 120° to the fixtures. The higher tank temperature will help prevent or kill bacteria in the tank and it will act as more capacity because it won't be drawing only from the tank.
  • MikeGordon
    MikeGordon Member Posts: 10
    Hello Jersey2;

    That appears to be a Bradford White indirect water heater (SW2 model). If so, it would come from the factory with a Resideo/Honeywell L4006A2211 aquastat. That looks like just what you have. This control has an adjustable high limit (the graduated dial you can see in your pictures) and an adjustable differential accessible under the aquastat cover. See this link to the indirect water heater manual, Figure 5, page 23.


    Just as HVACNUT said, this control is the high limit. The setpoint is adjustable up to 160 degrees F, and the differential is adjustable from 5 degrees to 30 degrees. Factory settings are 120 F high limit and 5 degree differential.

    Once the domestic hot water temperature inside the tank reaches the setpoint shown on the dial, the electrical contacts inside the control open up and that then interrupts the call for heat/circulator operation to the boiler from the indirect water heater. Once the domestic water temperature then drops to below the setpoint minus the differential, the control contacts close and there is another call for heat to the boiler/circulator.

    Per the above linked manual, Table 5, Page 9, your 30 gallon water heater has about 5.5 minutes of hot water availability. So, if your boiler and piping are starting from a realtively cold start, and there is a high domestic hot water flow demand, tub filling for example, you can indeed run out of hot water before the boiler/piping have a chance to come up to temperature. I'm thinking this is what you experienced.

    Hope this helps

    EBEBRATT-EdAlan (California Radiant) Forbes
    EBEBRATT-Ed Member Posts: 15,302
    How old is the indirect/ You could have a little sludge around the aquastat well. Also make sure the capillary is pushed all the way in as @HVACNUT mentioned
    Alan (California Radiant) Forbes
  • Jersey2
    Jersey2 Member Posts: 166
    MikeGordon, that is exactly what I have. A Bradford White indirect model SW230L. Your explanation and links helped a lot. I did use up the hot water filling the tub, but the aquastat acted like the water was still hot. It didn't click on until I increased the temperature on it, which was about 5 minutes after the water got cold. My boiler is 180K btus, no priority but the two zones to heat my house were not calling at the time.

    Edebratt it is a little over 1 year old. It is possible that there is some sludge around the aquastat well, I do have well water.

    Havacnut I believe it is inserted fully, it was installed by a professional a little over a year ago.
    I'm not a plumber or hvac man and my thoughts in comments are purely for conversation.