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Short Cycle

Condoman Member Posts: 91
A few weeks back I posted in the DHW thread.  Since then I completed the installation of the Crown MS-40.  I have noticed a few times and I mean no more than three, while working at my desk, one room away from the boiler, I can hear the burner cycle on for about 5 seconds then off and nothing happens after that until the next call. Today I went to the ZVC and none of the zones were active.

Any clue what is going on?

The attachment describes what I have.



  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    I think it's a classic short cycle

    Without staring at it when it happened, my guess is that right as the aquastat called for the boiler to fire for one of the zones, the call for heat ended . Like I said, kinda hard to tell without looking at the entire system/controls

    There was an error rendering this rich post.

  • Condoman
    Condoman Member Posts: 91
    ASC Setting

    Thanks for replying.

    I did more research and noted the ASC value is off on the L7224A.  I may try setting it to 2 minutes to see if that helps.  As I recall the boiler had not been running, then the short cycle occurred, and maybe 10 minutes later normal call for heat.
  • Zman
    Zman Member Posts: 7,573

    I would look for a faulty t-stat. Maybe one with the anticipator incorrectly set.

    I think you are going to have to catch it in the act.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Condoman
    Condoman Member Posts: 91
    Gathring data

    I just set up a camera to catch what is going on by taking a picture every 10 seconds.  I'll report back when I have some data.

    The tstat is a Honeywell RTC 8100.  Did not see any setting other than system type which I verified a set for gas/oil.


  • Condoman
    Condoman Member Posts: 91
    Research Results

    I have a time lapse camera I

    bought last summer to capture the addition we had put on.  I set this camera to take a picture every 10

    seconds between 6:00AM and 10:00PM.  The

    picture captured shows the Taco ZVC with two active zones; #1 heat and #4

    priority DHW.  Also in the picture is the

    digital boiler temp on the Honeywell L7224A and a cheap analog 110VAC clock

    wired across the burner 110.  The clock

    lights up and the second hand moves to verify the burner is on.It takes some time to review

    all the thousands of frames I upload each morning and afternoon.  Today I caught the short cycle I had heard

    that prompted this research.

    During a heat call the temp

    went over HL 160 and the burner went off. 

    The heat call was not satisfied yet so, it kept dragging the BT down till

    it went below 150.  The burner fired

    again and the call was satisfied.  The

    burner was only on 8 seconds.  Apparently

    once the call is over and the BT is above LLD 130, it is off, no matter when it

    came on.

    I think I have my

    answer.  Thanks for the replies.
  • icesailor
    icesailor Member Posts: 7,265
    Short Cycles:

    Taco 572 zone valves are notorious for doing that if you can't set the heat anticipators on the thermostat to higher than .9 amps or higher. Its the wax motor cylinder going up and down and tripping the end switch.

    If you are using a Taco ZV controller, watch for the yellow light coming on at unusual times and the red light coming on for a very short time, if at all. The Yellow light shows the call from the thermostat, (#1 and #2) the red one shows that the valve is open and the end switch (#2 and #3) has closed. Some electronic thermostats really need an isolation relay between the controller and the thermostat.
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