1. Welcome Hearth.com Guests and Visitors - Please enjoy our forums!
    Hearth.com GOLD Sponsors who help bring the site content to you:
    Hearthstone Soapstone and Cast-Iron stoves( Wood, Gas or Pellet Stoves and Inserts)

Question about Honeywell Aquastat L8124 on oil Boiler- Issue when wood boiler is running

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by NewBoiler, Nov 2, 2010.

Thread Status:
Not open for further replies.
  1. NewBoiler

    NewBoiler New Member

    Joined:
    Feb 23, 2010
    Messages:
    45
    Loc:
    Canada
    I have a Slant/Fin Oil Boiler with a Honeywell L8124L Aquastat which heats 4 zones (cast iron rads) and an indirect hot water tank. I just installed a new Kerr Highlander wood boiler in parallel which is working great.

    My problem is with the existing circulator that is installed in the return to the oil boiler. Its basically the main circulator for all the zones. It is connected into the Honeywell L8124L aquastat on the oil boiler. The aquastat's settings are 170 high, 150 low and 15 differential. When the wood boiler is running, I have a relay wired in to the oil burner power to shut it off (controlled by an aquastat on the wood boiler).

    What I am finding is that the main circulator is not running when there is a call for heat from one of the zones, and the oil boiler is up to temp (when wood boiler is running). The circulator runs sometimes, so I know its working, but not all the time. If I adjust the differential on the L8124 aquastat either up or down, it will turn on.

    I just do not get when the circulator is supposed to run off this aquastat. I believe there is a difference depending on whether the temp in the boiler is rising or falling The manual for the aquastat is shown below. Using the settings above, can you tell me when (at what temp) the circulator should be running (on a call for heat)?

    Attached Files:

    Helpful Sponsor Ads!





  2. ewdudley

    ewdudley Minister of Fire

    Joined:
    Nov 17, 2009
    Messages:
    1,783
    Loc:
    Cayuga County NY
    Terminal C1 should be hot when terminals T-T conduct and the aquastat sees temperature rising above 155 degF. The low-limit-make temperature is low-limit-setpoint minus 10 degF plus low-limit-differential = 150 - 10 + 15 = 155 degF.

    Terminal C1 will go not-hot when terminals T-T cease to conduct or the aquastat sees temperature falling below 140 degF. The low-limit-break temperature is low-limit-setpoint minus 10 degF = 150 - 10 = 140 degF.

    Once the low-limit aquastat circulator C1 circuit breaks on falling temperature, it will not make again until temperature rises above the low-limit-make temperature.

    Likewise, once the low-limit aquastat makes, it won't break until the temperature falls to the low-limit break temperature.

    (The high-limit aquastat cannot participate directly in determining whether the circulator runs, except in the sense that when the oil boiler is active the circulator can't run until the burner heats the water enough, which can't happen if the high-limit is set too low relative to the low-limit.)

    --ewd
  3. NewBoiler

    NewBoiler New Member

    Joined:
    Feb 23, 2010
    Messages:
    45
    Loc:
    Canada
    EW, your a genius, Thank you.
  4. ewdudley

    ewdudley Minister of Fire

    Joined:
    Nov 17, 2009
    Messages:
    1,783
    Loc:
    Cayuga County NY
    I'm in a similar situation with an old house with and over-sized cast iron radiators. When a radiator zone calls for heat, especially this time of year, it's only once in a while and at least the bottom of the radiator has gotten quite cool. When the zone valve opens up the radiator dumps a shload of cool water into the boiler and it shuts down the circ on low-limit even though the gauge indicates a temperature above the low-limit-circulator-break temperature.

    It's no big deal in and of itself, but the real problem is that now the boiler is below condensing temperature and if the oil burner is the source of heat it's a bad situation. What I did was to add a recirculation loop with its own pump on the oil boiler, which runs whenever there's a call for heat. This way the return water from the radiator gets mixed into the entire boiler contents rapidly so that the low-limit aquastat can shut down the load circ before the boiler gets cold.

    But in your situation if you're not going to be running the oil boiler much, if any, then you probably don't care if the load circ shuts down intermittently while the wood boiler catches up, you'll probably get plenty of heat soon enough.

    [And BTW, Ronald Reagan is probably the last genius we will have seen for many years to come.]

    --ewd
Thread Status:
Not open for further replies.

Share This Page