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Boiler short cycling

rlrefalo's picture

Hey guys, I've got a problem with my boiler. It's an oil fired boiler connected to a boilermate HWH. The problem is it will fire for actually 5 seconds and then shut off. This short cycling must be a waste of oil. I can't think of any reason it would run for that short a time. Anybody have any insight ?

Edit to say it will run properly and keep up to temperature no problem , just this short cycling.


Edited 6/23/2008 7:06 pm ET by rlrefalo

(post #115142, reply #1 of 11)

Could be the flame sensor dirty or not "seeing" the flame.

(post #115142, reply #2 of 11)

Good thought. Would that be the case even if most times it fires normally and keeps up to temperature?  Would it be possible that there is no heat transfer compound in the aquastat well ? would that cause erratic firing ?


(post #115142, reply #3 of 11)

What brand of boiler?

How is the Boiler Mate piped and wired?

Does it do it when space heating as well?

Do you have hard water, could be the coil is coated with minerals.


(post #115142, reply #4 of 11)

Mike, it's a Crown boiler, it's wired so as not to be a cold start boiler, ie, it maintains the temperature set on the aquastat. I'm not sure precisely how the boilermate is wired. I do know I have no direct control of when the boiler fires.

  My thinking is that firing for five or ten seconds is not enough to satisfy any call for temperature rise, the firing chamber would barely be getting hot. The boiler is connected to two fan coils for space heating and it does short cycle during the heating season also.

   Anyone know if omitting the heat conducting compound in the aquastat well could cause this anomaly ?


(post #115142, reply #5 of 11)

Haven't seen this, could be a bad boiler control. What are the settings? What's the model # on the aquastat?

(post #115142, reply #8 of 11)

Mike, the aquastat is  a Honeywell L8148A.  It's set at 155 but the temp gauge on the boiler reads about 185. I checked the temp on the outlet with an IR thermometer and the boiler gauge is pretty close.


(post #115142, reply #6 of 11)


Just a quick thought,  waterflow issues will also cause your symptoms.  IOW, are all of the flow controls properly set?  Is the pump pumping water?  Is it pumping the right direction?  The boiler could simply be keeping itself warm and the pump isn't getting the hot water out into the system quick enough, allowing the boiler to short cycle.

Also, if the aquastat is in series with the room stat it could also short cycle.


(post #115142, reply #7 of 11)

if it starts sometimes and not others thats a clue, flame sensor should be ok if it starts at all, problem may be hydraulic.

(post #115142, reply #9 of 11)

Chris, everything seems to work well with the exception of this short cycling issue. It will actually run for about 10 seconds and then shut off. There is no way that that amount of heat is satisfying the call for temperature increase.  So I think something must be wacky.


(post #115142, reply #10 of 11)

Rich, if the aquastat (operator)  is set at 155, and the boiler is at 185 then you have to now figure out what is controlling the boiler,  remove the cover from the aquastat, take your multimeter,  put one leads on the wires that are on the aquastst.  Hold them there and watch as the boiler fires and shuts off.  If you have a reading when the boiler is off and nothing or zero when it fires then you have a bad aquastat. 

If it is constantly zero then you need to look at the wiring diagram closely and check your water flow carefully.  Check all of your valves to make sure they are opened fully, clean your strainer, check for air locks, check your flow control.  Depending on piping arrangement,  it can be possible to heat your domestic hot water, especially in the summer when demand is less, without a pump.  Homes were heated for years by hot water boilers without pumps for decades.  Check your pump,  is it pumping?  It will feel hot whether it is pumping or not, remember, it is full of 185 degree water. 

Think about what has changed,  did the boiler work all winter properly?  What has happened since then to change that?

To answer your question about the heat paste in the well,  yes you need it but it isn't going to cause the problem you are experiencing.


(post #115142, reply #11 of 11)

Chris thanks for the info, I'll check that tonite.