Heating boiler short cycling off and on troubleshooting and repair.

Page top illustration: if the boilers temperature sensing probe is not properly installed, a result can be improper temperature sensing and boiler short cycling.

This article series explains how to recognize & diagnose problems with residential heating boilers, including loss of heat, heating boiler noises, leaks, odors, or smoke, and high heating costs.

Heating Boiler Short Cycling Causes & Cures

Here we list the causes and cures for hot water heating boilers or steam boilers that are cycling on and off too frequently or at unexpected intervals.

For forced warm air heating sytems that are short cycling see FURNACE FAN CYCLES DURING HEAT 

If you don't know whether your heat is provided by a furnace (hot air) or boiler (hot water)

see HEATING SYSTEM TYPES

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What Causes a Heating Boiler to Cycle On and Off too Often?

Boiler keeps cycling on and off

Our InspectApedia reader Norm asked:

We have a Slant Fin boiler that keeps kicking on and off.

We have replaced the control panel. had the pumps checked.

We don't know what else to do. do you have any suggestions? - On 2019-10-25 by Norm

Causes & Cures for Heating Boiler Short Cycling On and Off

Watch the boiler temperature; is it turning off because it's reaching the cutoff temperature (set on the primary control aquastat) ?

If so then yeah maybe a circulator pump isn't pumping or the system is air bound as you suggest.

But the problem can certainly be elsewhere.

You don't give the fuel (oil or gas) but often the problem with a boiler whose burner is short-cycling on and off is one of those that we list just below.

• Air bound heating zones?
  
  If a hot water heating zone is air-bound or partly air-bound, on a call for heat the boiler may quickly reach the aquastat HI LIMIT while heat has not yet satisfied the room thermostat, leading to boiler short cycling.
  
  Repair procedures are at AIR-BOUND HEATING SYSTEMS
• Aquastat settings: if you set the DIFF to the maximum of 25-30 F that will reduce boiler short cycling.
  
  Details are at AQUASTAT HI LO DIFF SETTINGS  and at BEST DIFF SETTING  and at AQUASTAT TROUBLESHOOTING

• Aquastat temperature sensor: An aquastat that is not working properly because it's not sensing boiler temperature consistently
  
  See AQUASTAT SENSOR PROBE HEAT CONDUCTING COMPOUND
  

• Bad primary control or flame sensor on oil fired boilers discussed at
  
  RESET SWITCH, CAD CELL RELAY  and at
  
  STACK RELAY SWITCH
• Boiler oversized: if the boiler is over-sized for the building's needs, a result is likely to be short on-cycles or more-frequent on-off cycles and less efficient, more costly boiler operation.

• Combustion air deficiency
  
  Watch out: this is an unsafe condition
  
  See COMBUSTION AIR REQUIREMENTS
  
• Chimney that's blocked
  
  Watch out: this is an unsafe condition
  
  Inspection tips that can help spot a chimney or flue at risk of being blocked are
  
  at CHIMNEY INSPECTION INDOORS
• Circulator Pump defects, circulator runs intermittently
  
  See CIRCULATOR PUMP RUNS INTERMITTENTLY 
• Electric Boiler Short Cycling - improper control or delay settings
• Gas burner short cycling and other furner and flame problems are explained
  
  at GAS BURNER FLAME & NOISE DEFECTS
  
• Oil burner short cycling:
  
  are at OIL BURNER SHORT CYCLING 
  
  also see
  
  Oil Burner Adjustment or Nozzle - boiler over-fired: Poorly-adjusted burner or improper oil burner nozzle that is firing at too high a rate (too high a GPH nozzle)
  
  See OIL BURNER NOZZLE GUIDE
• Steam boiler excessive water level or condensate return delay problems can cause boiler overfilling and lead to steam boiler sort cycling.
  
  Repair details are at STEAM BOILER FLOODING / SURGING REPAIR
• Thermostat Heat Anticipator Settings: while it would be unusual for a room thermostat to itself cause boiler short cycling, you can fine tune the boiler on-off cycle if your thermostat has a heat anticipator.
  
  Details are at HEAT ANTICIPATOR OPERATION and at HEAT ANTICIPATOR vs SHORT CYCLING HEATER FAQ
• Thermostat wires that are loose or shorted can also cause irregular burner operation.
  
  See THERMOSTAT DIAGNOSTIC GUIDE
• Zone Valve not Opening: on a call for heat the boiler may quickly reach the aquastat HI LIMIT while heat has not yet satisfied the room thermostat, leading to boiler short cycling.
  
  See ZONE VALVES, HEATING 
  

If your concern is for a heating furnace (forced warm air) short cycling, see FURNACE FAN CYCLES DURING HEAT

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Research on Heating Boiler Short Cycling Causes & Cures

• Argo AI-B ELECTRIC BOILERS for FORCED HOT WATER, IO MANUAL [PDF] (2004) Argo Technology, Inc., Berlin CT 06037, USA - Division of ECR International, PO Box 4729, Utica NY USA 13504-4729 Website: www.ecrinternational.com
  
  Excerpt:
  
  Adjustable time delays:
  
  The parallel wired time delays allow optimumcontrol of the heating elements being energized. Certain electric boiler applications including elevated outside temperatures, small heating zones, radiant heat installations, and flow rates usually require maximum time energizing heating elements.
  
  By increasing delay time in above cited applications, short cycling and excessive wear and tear on boiler components may be minimized, resulting in greater operating efficiencies and boiler reliability. - p. 8.
• ATH Cast Iron Condensing Boilers Models KN-2 and KN-4 INSTALLATION & OPERATION MANUAL [PDF] KN-2/4-CM2-0714 42-9452 - Advanced Thermal Hydronics Co., (Mestek Co.) In United States: 260 North Elm St. • Westfield, Ma 01085 • (413) 564-5515 • Fax (413) 568-9613 In Canada: 7555 Tranmere Drive • Mississauga, Ont. L5s 1l4 • (905) 670-5888 • Fax (905) 670-5782
  
  Include an adjustable "limit band" explained below, allowing the firing rate to be adjusted to avoid boiler short cycling.
  

• Smith Boiler Co., SMITH 8 SERIES STEAM or HOT WATER BOILER-BURNER UNIT Installation& Operating Instructions [PDF] (2013), Smith Cast Iron Boilers, Westcast, Inc., 260 North Elm Street, Westfield, MA 01085 USA (413) 562-9631, FAX: (413) 562-3799 Website: www.smithboiler.com
  
  Smith Cast Iron Boilers, 7555 Tranmere Drive, Mississauga Ontario L5S 1L4 Canada, Tel: 905 672-2883
  
  Excerpt: The most commonly available reverse acting aquastat is a Honeywell L4006B. The aquastat should be mounted in an immersion well directly installed in the boiler.
  
  The use of heat conductive grease, Honeywell part #972545, in the immersion well is strongly recommended for fast and accurate temperature response. Set this adjustable aquastat to make at no less than 130°F.
  
  While this method can cause the circulator to cycle more frequently, setting the aquastat’s differential to the maximum 25-30°F will minimize short cycling.
• McDonnell & Miller's HEATING BOILER BASIC OPERATION [PDF] McDonnell & Miller, retrieved 2019/11/06 original source: controlcomponents.com.au/Open-File/557/Boiler%20control%20basics.pdf
  
  Excerpt: If the automatic water feeder adds water to the boiler (to maintain a safe minimum operating level), and then the condensate pump returns its water to the boiler, the boiler will most likely wind up with too much water. This excess water limits the boiler’s steam making space.
  
  Without enough room to break free of the water, the steam can carry water up into the system piping. That leads to higher fuel bills, uneven heating, water hammer, scale formation in the boiler and burner short cycling.
  
• LAARS Endurance EBP EDP EDN BOILER IO MANUAL [PDF] Document 1187D (2009) Laars Heating Systems Co. (A Subsidiary of Bradford White), 20 Industrial Way, Rochester, NH 03867 • 603.335.6300 • Fax 603.335.3355 1355 Kuehner Drive, Simi Valley, CA 93063 • 800.900.9276 • Fax 800.559.1583 (Sales, Service) 1869 Sismet Road, Mississauga, Ontario, Canada L4W 1W8 • 905.238.0100 • Fax 905.238.0105
  
  Excerpts:
  
  EDP/EDN boilers installed in radiant (in floor) systems and other low mass boilers should be provided with a buffer/blender tank to assure a controlled supply temperature, and to prevent short cycling. - P. 16
  
  11.2 Short Cycling Possible causes
  
  a. Reduced system flow - EDP
  
  b. Thermostatic union (EDP) or anti-condensing valve not opening (EBP).
  
  c. T-T wires cross connected (jumped) – improper installation - EBP or EDP/EDN
  
  d. Leakage of flue products into the combustion air stream – Direct Vent Installations only, see Section 9.2.3, Resolving Lockouts. - P. 36

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Continue reading at BOILER OPERATING PROBLEMS - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see BOILER OPERATING PROBLEM FAQs questions & answers about our list of boiler operating problems that help troubleshoot hot water or steam heat

Or see these

Recommended Articles

• AIR-BOUND HEATING SYSTEMS
• BOILER CONTROLS & SWITCHES
• BOILER INSPECTION GUIDE
• BOILER NOISE SMOKE ODORS
• BOILER OPERATING PROBLEMS
• BOILER OPERATING STEPS
• CIRCULATOR PUMPS & RELAYS
• DIAGNOSE & FIX HEATING PROBLEMS-BOILER - home
• GAS BURNER FLAME & NOISE DEFECTS
• GAS FIRED HEATING BOILER PROBLEMS
• HEAT ANTICIPATOR OPERATION
• HEAT WON'T TURN OFF
• HEAT WON'T TURN ON
• NO HEAT - BOILER
• NOISE, HEATING SYSTEMS
• OIL BURNER INSPECTION
• PRESSURE & TEMPERATURE SETTINGS, CONTROLS
• RESET SWITCH, CAD CELL RELAY
• SHORT CYCLING AC COMPRESSOR 
• SHORT CYCLING FURNACE FAN DURING HEAT
• SHORT CYCLING HEATING BOILER
• STEAM HEATING SYSTEMS & CONTROLS - home
• THERMOSTATS, HEATING / COOLING
• WATER HAMMER NOISE DIAGNOSE & CURE

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