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Short Cycle on Hot Water Zone off Steam Boiler

18651865 Posts: 3Member
New gas fired steam boiler installed with a hot water zone off it. Steam heat seems to be working fine, but short cycling is occurring. Boiler fires for just over 1-minute, then shuts down for just over 3-minutes, then fires again and shuts off along similar intervals. Honeywell Aquastat L4006A set at 180 degrees with a differential set to what looks like 5 degrees. During the short cycling, I do not hear the Taco 007 circulator pump run, but the thermometer (Moeller Bimet) attached to the hot water loop deviates between 160 and 200 degrees. Watching that thermometer I observe the following:

@ 160 degrees the boiler fires up
@ 180 degrees the boiler turns off and the thermometer continues to rise to 202 degrees then begins to fall back.
When the temperature falls back to 160 degrees, the boiler fires back up and the cycle starts over.

During all of this short cycling, the radiators are generally cool, so it seems the short cycle is only occurring when the system is not calling for steam.

For reference, I see the following make/model and settings on the new equipment:
US Boiler Independence I6
Honeywell Pressuretrol (set to a Diff .5 kPa / 1PSI) and Main 10 kPa / 1 PSI)
Honeywell Aquastat L4006A
McDonnell & Miller Lower Water Cutoff for Steam Boilers PSE-802-24
Honeywell RA832A relay
VXT VXT-24 Programable Water Feed (showing 05 in the digital display)
Taco 007 Circulator Pump (on output circulating away from boiler)
Honeywell RTH2003B Programmable Thermostat (Hot Water Zone)
Honeywell Thermostat (Steam Zone)

The hot water zone / loop does not have a heat exchanger or compression tank.

Well, hopefully I have supplied enough information to start the conversation.
I'm a homeowner who's trying to study and learn their system (and its problems), so you may need to break it down in laymans terms for me. Will need to be able to relay the info correctly to the heating co to hopefully resolve the problem.
Was hoping not to have any issues right off the bat.


  • Jamie HallJamie Hall Posts: 10,302Member
    I suppose the first and obvious question is... is the hot water zone calling for heat when this is happening, and if it is, why isn't the circulator running? Am I missing something?

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • GBartGBart Posts: 753Member
    Over sized? over fired?

    call a pro
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,641Member
    Possibly when they installed the new boiler air got in the hot water loop which must be 100% full of water for the hw zone to work. I suspect the aquastat f& Thermostat for the hot water zone are calling for heat. The boiler heats up but the water is not moving so the boiler shuts off on high temp.

    You need to find out deffinitely if the circulator is running or not.

    seems like a circulator or air issue to me
  • 18651865 Posts: 3Member
    Thank you all for the replies.

    I don’t know for certain if the thermostat for the HW loop is calling for heat during the short cycle.

    This evening during what seemed to be a normal steam producing cycle, the hidevectors within the loop were running, so I decided to see if the circulator pump was running. Based on sound, from the cartridge area of the pump and slight sound of rushing fluid, I believe the pump was on. The thermometer on the loop read close to 208 degrees.

    When I was experiencing the short cycle, I was seeing the fluctuating temperature between 160 and 202 for the HW loop, but no circulator running from what I could tell. Will the Aquastat self regulate the temp of the boiler water without a call for heat?

    My old boiler had a tankless coil for domestic HW, thus I’d thought the Aquastat regulated the boiler temp to keep a minimum for the coil based on its setting. The only difference is now I don’t have the coil, but a separate gas fired HW heater which is independent of the boiler. I still had the steam and hw loop off the old boiler.
  • Jamie HallJamie Hall Posts: 10,302Member
    That's exactly what the aquastat is supposed to do -- run the boiler to maintain the water temperature it's set for. Since there is relatively little water in a steam boiler, if the hydroloop isn't running it doesn't take long for it to heat up.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • 18651865 Posts: 3Member
    edited December 2018
    I noticed that the water in the sight glass is not quite 1/2 way up the tube when the boiler is not running. Looking through the boiler manual, it shows 28.6"H as the normal water level (from the floor) which by measuring, coincides with the top of a piece of tape wrapping the sight glass. Currently, the water level in the sight glass measures approximately 26.25" which means I'm under by about 2.35”.

    First, am I supposed to measure water level with the boiler running or off? With it running, the water level fluctuates some.

    Second, should I add water to bring the sight tube to coincide with the normal water level of 28.6" or could there be a reason why the installers left the water level lower?

    Lastly, could the low water level be part of the culprit for the short cycle? I'm not sure what an additional 2.35” of water would equal in volume, but if I added water, there would be more water to heat up. Thus, maybe a longer cycle to reach the Aquastat’s HI set point of 180 degrees. The lowest permissible water level tag on the side of the boiler is measures 22"H, so I'm at least above that by 4.25".
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