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Thread: Help ASAP: Short cycling

  1. #1
    DIY Junior Member grumpy's Avatar
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    Default Help ASAP: Short cycling

    Hi, First post thanks in advance for any suggestions!

    I have a Rudd Multi-Tube boiler ser 480-01974 model CB576C for 6 small apartments and 2 commercial units. Guessing it is 1974 or earlier model.(maybe 60's, building is 100 yrs plus)

    Pump--->Flow Switch--->Boiler--->Air Tank/auto fill--->Pressure/Temp--->Level Control---Honeywell Control.

    max input btu 576000
    pump in return 2" Armstrong 1/6th HP Pump Motor P/N 817025-005
    Boiler piping 2" to 3" to 1.5"
    Limit control set at 175 degrees
    Pressure is 16PSI

    The problem is that the boiler is on for <5 min and temp gauge says 140 degrees when it stops
    Temp Gauge reaches 120 Degrees after 12 mins and the cycle starts again.
    Pump only operates when there is a call for heat.
    Outside temp today is 3 degrees Celsius = 37.4 degrees Fahrenheit

    Basement has no rads but it sure is a sweat shop, all units very warm too. The thermostat is on the second floor hallway.

  2. #2
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by grumpy View Post
    max input btu 576000 = 9600 BTU/min
    Boiler piping 2" to 3" to 1.5" = 16 gallons and 130# of water per 100' of piping
    The problem is that the boiler is on for <5 min and temp gauge says 140 degrees when it stops
    Temp Gauge reaches 120 Degrees after 12 mins and the cycle starts again.
    What's your fuel usage during this 5 minute "on" period?
    Last edited by Thatguy; 10-24-2010 at 02:40 PM.

  3. #3
    DIY Junior Member grumpy's Avatar
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    Quote Originally Posted by Thatguy View Post
    What's your fuel usage during this 5 minute "on" period?
    Don't really understand the question, but the boiler does have a 1" gas line feeding it. My concerns are two fold cost of course and comfort.

  4. #4
    Master Plumber Redwood's Avatar
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    Sounds like its not circulating right.

  5. #5
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by grumpy View Post
    Don't really understand the question, but the boiler does have a 1" gas line feeding it. My concerns are two fold cost of course and comfort.
    It seems your furnace has become more efficient than it used to be, or your units take way less heating, both of which seem impossible. Fuel usage may give us a clue.
    Do you use NG or oil? Each has a different "heat of combustion".

    What were the "on" and "off" times before the short-cycling started? Being on for 5 out of 5 + 12 minutes means you're using 5/17 of 576000 BTU/hr, about 170,000 BTU/hr. 60/17 is 3.5 cycles per hour and not all that high.

    To check if the furnace turns off because the high limit switch is opening, rather than t'stat contacts opening, you'll probably need a multimeter.
    Having a flow meter in the line may also give a clue.

    Can you post a schematic?
    Last edited by Thatguy; 10-25-2010 at 09:09 AM.

  6. #6
    DIY Junior Member grumpy's Avatar
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    Thanks for the replies

    Redwood that is part of my concern, who knows what has been done over the years, surely not me as I have only owned this building since May. (Been a commercial tenant for 25 years)

    Thatguy it is NG, history is unknown. If you think that those times are not bad then I will worry less as this is my first boiler. But why is the limit switch set to 175 while the thermostat is working between 125 and 140?
    Schematic, would have to break out the crayons I'm not even sure if this is primary secondary or parallel. Will check next time there, but I do know there is several feeds going upstairs in many different locations and the other boiler another Rudd but 210000 BTU in another part of the building is plumbed parrallel.

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