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Inspecting HVAC Systems Topics include heating, venting, and air conditioning inspections.

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  #1  
Old 1/18/06, 7:42 AM
twheeler
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Default Short Cycling

I would like to throw this question out there.
What causes short cycling in a gas forced air furnace?
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  #2  
Old 1/18/06, 8:27 AM
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Bruce A. King Bruce A. King is offline
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Default Re: Short Cycling

besides a bad safety switch or component it can be that too many vents are closed off in the house or improper ducting.



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  #3  
Old 1/18/06, 9:18 AM
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Paul Hinsperger, CMI Paul Hinsperger, CMI is offline
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Default Re: Short Cycling

Most common would be;
- over sized furnace
- restricted air flow (ie, dirty filter or ducting problems as Bruce mentioned)
- faulty limit switch
- faulty thermostat
- theromostat improperly installed or located



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Paul Hinsperger, CMI
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  #4  
Old 1/18/06, 1:52 PM
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JLilly JLilly is offline
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Default Re: Short Cycling

One more thing, a dirty secondary heat exchanger normally located just above the discharge of the blower (if a 90% furnace) On most furnaces it's a fin-tube device just like the A/C evaporator coil.
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Old 1/18/06, 3:12 PM
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David P. Valley David P. Valley is offline
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Default Re: Short Cycling

Short cycling on the high limit switch is caused by overheating. Normally caused by a lack of air flow. Clogged filters, faulty fan, restricted duct work. It can also be a cracked heat exchanger, both can be very serious and should be dealt with immediately.

The anticipator setting on the thermostat. On a mercury thermostat, there is an adjustment 0.2 to 1.4 which sets the cycling rate of the furnace. Lower numbers for shorter cycles, higher for longer cycles. Older standard furnaces are usually 0.2 to 0.4. Mid-efficiency and high-efficiency are longer 0.8 to 1.2 on mid-efficiency furnaces it is very important for the burner to run a few minutes before turning off. Short cycles will result in flue gas condensation and will prematurely rust out the furnace, vent connector and chimney.
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Old 1/18/06, 3:15 PM
Ted Allen Ted Allen is offline
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Thumbs up Re: Short Cycling

Nice post David
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Old 1/18/06, 4:01 PM
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Default Re: Short Cycling

Could be a blocked exhaust vent, safety shut-down on high efficiency.

Raymond Wand
Alton, ON
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Old 1/18/06, 10:13 PM
Dean Call Dean Call is offline
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Default Re: Short Cycling

Quote:
Originally Posted by rwand1
Could be a blocked exhaust vent, safety shut-down on high efficiency.

Raymond Wand
Alton, ON
Not normally because those safety switches must be reset manually so it would only cycle off once and not come back on.

Most common cause I've found is heat anticipater set for too short of cycles, like David said.
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Old 1/19/06, 7:30 AM
rwand1 rwand1 is offline
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Default Re: Short Cycling

I have not seen an anticipator setting on electronic thermostats. As to the safety shut-down for block flue there is a vacuum switch, where can the reset switch be found?

Thanks
Raymond Wand
Alton, ON
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Old 1/19/06, 7:57 AM
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Default Re: Short Cycling

Quote:
Originally Posted by rwand1
Could be a blocked exhaust vent, safety shut-down on high efficiency.

Raymond Wand
Alton, ON
Happened to me, the rain cap collapsed above the roof on one house and the insulation on the equipment panel came loose and lodged against the blower....both cause short cycling.
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Old 1/19/06, 8:02 AM
rwand1 rwand1 is offline
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Default Re: Short Cycling

I have seen toy cars, balls pushed into ABS/PVC vents. One instance the furnace tech took the furnace apart (High Eff) to find a toy car ontop of the heat exchanger. No one knew how the toy got ontop of the heat exchanger. The toy created a hot spot and cracked the heat exchanger. The furnace was under warranty. The puchaser had to pay for a new heat exchange but no labour charge.

Raymond Wand
Alton, ON
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Old 1/19/06, 9:30 AM
twheeler
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Default Re: Short Cycling

The Most Common cause of short cycling, is "failure of the thermostst".
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Old 1/19/06, 10:09 AM
rwand1 rwand1 is offline
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Default Re: Short Cycling

Tyrone

Mercury thermostats or electronic? I have electronic thermostat at home here, and I can't find any anticipator settings.

Cheers,
Raymond Wand
Alton, ON
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  #14  
Old 1/19/06, 1:49 PM
briddle briddle is offline
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Default Re: Short Cycling

I installed a new thermostat last week and it did have an adjustable anticipator. It was a 2 wire heat only thermostat.
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  #15  
Old 1/19/06, 3:20 PM
jrupert jrupert is offline
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Default Re: Short Cycling

The actual meaning of short cycle in the trade means that the furnace is turning on then off in a very short time frame from what might be normal due to current weather conditions. The determination is it normal or abnormal, on very cold night a short cycle could actually be normal as the house is loosing heat on a very accelerated level.

Abnormal cycling can be cause by lack of air flow from dirty evaporator coil, dirty air filter, dirty blower wheel and I have even seen the blower running backwards, too many supply’s closed, returns blocked off. Any or all or the above are very common, less common would be an oversized unit.

Short cycle may also be caused by a component failure, depending on the age of the unit modern units are loaded with things that can fail, 20 year old units no so many things. Example would be inducer motor proving switch defective, inducer motor not spinning to speed all the time, circuit board failure etc.

The thermostat heat anticipator too can cause this, they are best set using a heat anticipator meter, no guideline is correct since the length of the thermostat wire, the volt amps of the control transformer etc determine this. The stamped number on some furnaces is usually incorrect. But set incorrectly the furnace will short cycle. Most better digital stats will allow for anticipator adjustments, many cheaper ones will not and are pre set, how these are set is normally in the installation guide.

The purpose of the heat anticipator is to turn the heating unit off just prior to the house reaching the desired temperature, allowing the furnace to cool down with out over heating the house beyond the set point. Gas , oil and electric furnaces use them, heat pumps do not.



John Rupert
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Solitaire Heating & Cooling
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