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Thread: Weil-McLain possessed after midnight

  1. #1
    DIY Junior Member PatrickC's Avatar
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    Default Weil-McLain possessed after midnight

    Hello All, I am new to boilers, first house with one, I have:
    1. Weil-McLain boiler
    2. L8124A aquistat (set to 160-180 20 diff)
    3. Taco 007-F5 circulator
    4. CT87A Honeywell thermostat
    5. Vent-Rite VR30 expansion tank
    6. Cast Iron radiators


    All pictured in this link https://www.dropbox.com/sh/r8fvtowz6s6fhlk/EfNJytBgBi

    The problem I am having is this, I crank my thermostat to 70, the system will call for heat and it will get within 5 degrees of 70, during this process it comes on and off frequently until it reaches the 68, I assume the on off is normal until it reaches the desired temperature?. Personally I thought it would run continuously until it reaches my temp.

    Now it seems that every time after 12 or 1, I am guessing the coincidence is when the temperature drops in the 20's outside, the broiler runs continuously, but provides little to no heat, this morning when it happened around 3 am I checked the radiators and they were cold to the touch.

    Where would I start looking to fix the problem, could it be a faulty thermostat? any help would be appreciated.
    Last edited by PatrickC; 12-19-2013 at 07:00 AM.

  2. #2
    DIY Junior Member jac04's Avatar
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    Can you tell us what the boiler temperature is doing during the cycles, and also during the times when the t-stat is calling for heat but the radiators are cool?

    The on/off cycles are most likely the boiler hitting the high temp cutoff (which shuts the burner off), then dropping back down to the low limit (which fires the burner). The output of the radiators is not enough to satisfy the t-stat before the boiler hits the cutoff temperature.
    Last edited by jac04; 12-19-2013 at 10:00 AM.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    Depending on how your system is setup, it may be constantly trying to maintain the boiler temp between the min/max, even with no call for heat into the house. Actually getting the heat into the house requires the thermostat to open up any valves (if they exist as in zone valves) and turning on the circulator pump. Without the pump, you might get a small amount of heat via convection, but probably not much, if any (there may also be check valves, making that hard to impossible).

    So, can't help more without more info. Also keep in mind that if you have air trapped in the lines, even if everything is trying to work, the pump often cannot move the water without help from it falling back down after getting over the top at the high points - IOW, most circulator pumps are sized without enough head to pump water over the top...if the pipes are full, it's just friction, with little effect of gravity but when not full, it must push the water up without some helping to push back down on the back side.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    DIY Junior Member PatrickC's Avatar
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    Thanks Jac04 and Jadnashua, after going home and trying to document what was happening to report here, I saw that the low on the aquastat was set to 200 much higher than the high was set to at 180.

    After setting it to 160 the unit worked well throughout the night. Hot water wasnt all that this morning though I figure I might try +10 on the low.

    On another note I need an affordable wifi/smart thermostat Jadnashua what do you think about the Nest?

  5. #5
    In the trades Dana's Avatar
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    If it's a gas-fired boiler you can usually set the low limit to about 135-140F which would help the net efficiency considerably by reducing the standby loss and jacket losses to the boiler room by a large fraction.

    If 20F is the max differential, seting it that low would put the max temp at 150-155F, which or may not be high enough to fully heat the place, but it probably is. Almost all systems that old were oversized when installed, and the house had a much higher heat loss then, with crummier windows, less insulation, etc. It's pretty common to be able drop the max temp back to 140F and never get cold, even without adding more radiation.

    But a cast iron gas-fired boiler shouldn't be operated continuously with the water coming back from the radiators and entering the boiler at temps much lower than 130F or it can have issues with corrosive exhaust condensation inside the boiler. It's common to have near-boiler plumbing that mixes some of the boiler output with the returning water to protect it from return water that is too cool, and often the mix is adjustable with a ball-valve. When lowering the operating temp of a boiler, it's important to monitor the entering water temperature, and keep it above 130F (140F if oil-fired.)

    If your domestic hot water is served by an embedded coil in the boiler you'll probably have to keep the low-temp at 160F or higher to get reasonable mid-winter performance, if that's what you meant by "Hot water wasnt all that this morning..." If that is your situation, an indirect fired tank operated as a separate zone would allow you to run the boiler at lower temp/higher-efficiency and deliver even BETTER domestic hot water performance. But that's a whole other kettle o' fish.

    Sounds like you found the source of your short-cycling issue though.

  6. #6
    In the Trades Tom Sawyer's Avatar
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    Quote Originally Posted by PatrickC View Post
    Thanks Jac04 and Jadnashua, after going home and trying to document what was happening to report here, I saw that the low on the aquastat was set to 200 much higher than the high was set to at 180.

    After setting it to 160 the unit worked well throughout the night. Hot water wasnt all that this morning though I figure I might try +10 on the low.


    On another note I need an affordable wifi/smart thermostat Jadnashua what do you think about the Nest?
    Ok so setting the low above the high was the case of your problems but, that you can't get decent hot water means that your tankless coil is most likely limed up.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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