Boiler cycling on and off quickly

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Old 11-04-14, 06:44 PM
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Boiler cycling on and off quickly

Hi folks - I'm a new owner of a residential home with a Slant/Fin S90HXPD boiler with an L4081B Aquastat. The boiler is in the attached garage. When evening comes and the thermostat calls for heat, the boiler fires up and warms to 140 (which is the setting of the LO dial) and the pump starts. It runs for 5-7 seconds and then stops. During this time, the water temp drops about 10-15 degrees, which likely trips the Aquastat to stop. 1 minute later, the whole process kicks in again. Each time, the pump circulating time extends a little longer. All the while, the boiler is still firing.
Is this normal? Can I set it up so that it circulates constantly until the thermostat says it warm enough? It seems to be wasting a lot of gas.
Thanks in advance for your help!
 
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Old 11-04-14, 07:16 PM
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Is this normal?
With that type of system, yes it is.

Can I set it up so that it circulates constantly until the thermostat says it warm enough?
Maybe...

It seems to be wasting a lot of gas.
Probably not. What it probably IS doing is preventing excessive flue gas condensation which would ultimately corrode the boiler from the inside out, destroy the flue pipe, and the chimney.

Your boiler is not also making domestic hot water, correct? You've got a separate stand-alone water heater?

If so, you certainly could turn the LOW setting down to say 120. This would allow the pump to run a little longer each time.

What is the HIGH setting at?
 
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Old 11-04-14, 11:52 PM
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Thank you for the reply. There is a completely separate hot water heater so this boiler is only heating the rads for heat. Hi is set at 180. The highest temp it ever gets is around 150.
 
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Old 11-05-14, 05:14 AM
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Trooper I cant believe any heating would be designed to cycle for 5-7 seconds, it cant be normal for a properly operating system.
 
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Old 11-05-14, 05:53 AM
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Saves, it's the CIRCULATOR that is connected to a reverse acting aquastat that is cycling.

The LOW setting on the circulator control is set a little high...

But yes, with a reverse aquastat system it IS completely normal that the pump may start and stop several times at the beginning of a heat cycle.

This is yet another form of BOILER PROTECTION that does not require a bypass. It's done to allow the boiler to heat up quickly and if the return water is enough flow and cool enough to cool the boiler the pump is shut off to allow the boiler to get hot again without the continuous flow of cool return water.


It's NOT the burners that are cycling!
 
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Old 11-05-14, 03:22 PM
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Perhaps I should have explained a little further. It will cycle on and off like this for an hour or more. After 15-20 minutes or so, the amount of time the circulator stays on will increase but only by a couple seconds each time. All the while, the boiler is still firing and the temp is going up and down between 150 and 135'ish.
 
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Old 11-05-14, 04:09 PM
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There is a completely separate hot water heater so this boiler is only heating the rads for heat. Hi is set at 180. The highest temp it ever gets is around 150.
So cast iron rads and seperate HWH??

I think trrop will help you elimiate the lo function on the aquastat..

Another issue after that is the 150F boiler temp.. May need some type of bypass installed...
 
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Old 11-05-14, 04:42 PM
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It will cycle on and off like this for an hour or more
Yeah, that ain't right...

I think Mike hit the nail on the head, cast iron radiators, large pipes, large water volume. Is that it?

Can you take some pics of the system and post them here?
Maybe we'll see something.
 
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Old 11-08-14, 07:45 PM
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Sorry for the delay in responding - Internet died and needed new modem.
Yes, 1940's big pipes and old rads with newer boiler. Pics attached.


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Last edited by NJT; 11-08-14 at 08:10 PM.
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Old 11-08-14, 08:32 PM
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Might wanna read through this thread, very similar issue...

http://www.doityourself.com/forum/bo...eed-issue.html
 
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