Boiler short cycling

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Joe Henderson

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Hi my boiler keeps short cycling .. I think it might be because my thermostat looses its temperature very fast.. if I have it set to say 68 And it’s 67 or 68 my boiler will turn on for a minute then turn off then the will go back on in about 3 min when the thermostat goes down by one ,, can I fix this issue by programming my thermsast
To only go on at certain times.. I have been running a space heater at night for a few hours to keep it from kicking on so much , thank you
 

jadnashua

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A common reason for short cycling is having a boiler that is way oversized for the structure. A second reason is if the thermostat is located where it can be influenced too quickly for a local change, i.e., it's near a heat source and is sensing that versus the general room temperature.

On a boiler, there's usually a min/max. The boiler usually heats up to its maximum, then turns off, while waiting for things to cool off, and if it gets that far, turns on again. On many, this is an adjustable range. If that's too small, it will quickly hit the high limit and shut off. If you have the system zoned, and only one zone turns on, if the amount of radiation is too small, it can't dissipate the heat from an oversized boiler, so the temperature rises quickly, and when it reaches its high limit, it shuts off. Ideally, the boiler would be sized so that it can just produce enough heat to match the heat lost to the outside, and thus maintain the desired setting. THat rarely happens since the outside isn't a constant level, and solar and wind effects will also affect how much heat is gained or lost, in addition to how well the home is air sealed and insulated.

If the actual sensing devices are not working properly, that can cause the boiler to shut off. Does the boiler actually fire properly, and if so, how long does it stay on? What temperature does it get to?

So, without some more info, it's really hard to help. Need more info on what radiation is available, the actual boiler in use, where in NY you live (zip code is enough) and maybe a picture of your heating system.
 

Joe Henderson

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A common reason for short cycling is having a boiler that is way oversized for the structure. A second reason is if the thermostat is located where it can be influenced too quickly for a local change, i.e., it's near a heat source and is sensing that versus the general room temperature.
On a boiler, there's usually a min/max. The boiler usually heats up to its maximum, then turns off, while waiting for things to cool off, and if it gets that far, turns on again. On many, this is an adjustable range. If that's too small, it will quickly hit the high limit and shut off. If you have the system zoned, and only one zone turns on, if the amount of radiation is too small, it can't dissipate the heat from an oversized boiler, so the temperature rises quickly, and when it reaches its high limit, it shuts off. Ideally, the boiler would be sized so that it can just produce enough heat to match the heat lost to the outside, and thus maintain the desired setting. THat rarely happens since the outside isn't a constant level, and solar and wind effects will also affect how much heat is gained or lost, in addition to how well the home is air sealed and insulated.

If the actual sensing devices are not working properly, that can cause the boiler to shut off. Does the boiler actually fire properly, and if so, how long does it stay on? What temperature does it get to?

So, without some more info, it's really hard to help. Need more info on what radiation is available, the actual boiler in use, where in NY you live (zip code is enough) and maybe a picture of your heating system.


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The boiler fired properly I just had a plumber here replacing a part on it because it started to steam all over the place so I’m going to assume he checked it ..it fires on about every 3 min for about a minute.. I’ve been putting a space better on near the thermostat to keep the temp up so it doesn’t come on
 

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jadnashua

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See if you can find the theory of operation for your boiler. It is often in the owner's manual, but sometimes also more detailed in the installation manual. Often, those can be found online, if you don't have them. The timing sounds more like the system is failing to detect the flame, going into shutdown and purge, then trying again. You'd also want to try to see what the temperatures in the boiler are. It could be a defective or maladjusted aquastat.
 

Dana

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Short cycling is far more closely related to being oversized for the radiation than the heat load of the structure or the hysteresis in the thermostat. But on steam boilers are normally (and rightly) sized for the radiation, not the building load.

On systems like yours if the thermostat is installed too close to a radiator or drafty window it could create a short-cycling situation. There is simply no way that the average room temperature is dropping by a full degree in three minutes. Sometimes moving the thermostat to a different location (best bet) works, or putting a vented box around the thermostat to interrupt cool drafts &/or reduce direct-radiation gain off the too-close radiator.

Running 1-minute burns 15 times per hour is pretty extreme short cycling, taking quite a toll on efficiency and on the ignition components.
 

Joe Henderson

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Can I just set my thermostat so it turns on and off when I want it to ? it only turns on and off like that when it’s Cold this is a house not a buliding
 

jadnashua

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You have to determine why the boiler is only running for that short time. If it's because it thinks it has reached its upper limit or the thermostat says it has, it will shut off. If it's a sensor issue and not actually getting through a full cycle, it could be a flame sensor, the upper limit switch, or other sensor issues. If the system circulator is not moving the water, it won't take long for the burner to overheat the limited amount of water in the heat exchanger since it's not flowing.

So, without knowing more about what's actually happening, can't really help you. First thing I'd do is check to see if there are any status or error lamps on (may or may not exist on your boiler).

If you have an electronic thermostat that has replaceable batteries, change them.

Then, see what the supply and return temperatures are to determine if they're rising or not, and if so, how high. Does the burner actually light? If so, how long does it stay on?

A common sequence is something like this:
- call for heat triggers it all off
- (depends on the boiler) - purge the exhaust and burner of stray gasses
- turn on the ignitor (could be a hot wire or sparker)
- turn on the gas
- sense if the flame actually lit
- if so, continue until the call for heat is over or max temp is achieved
- if not, turn off the gas, blow out the fire chamber of flammable gasses, then, usually
- start a timer before it tries again. Many of them will stop after a certain number of attempts to fire, but older ones will just continue that try to light, fail, rest, try again ad infinitum until you fix it.

Sorry, if this is beyond you, it's time to call in a pro, or someone who has more diagnostic skills.
 

Dana

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Can I just set my thermostat so it turns on and off when I want it to ? it only turns on and off like that when it’s Cold this is a house not a buliding

The thermostat doesn't control the burner directly- it enables the boiler's controls.

To clarify jadnashua's explanation, a steam boiler's controls are operated to an upper pressure limit, not an upper temperature limit. Asal a steam system does not have a "...system circulator..." pump moving water through the system. The heat from the burner is entirely moved by the steam pressure, and the heat of vaporization that goes into creating that steam gets recovered at the radiator when steam condenses back to liquid.

The pressure control is what turns off the burner, and what re-enables the burner once the pressure falls enough. For a house it doesn't take much to reliably get steam to the furthest radiator on the uppermost floor- between 0.2-0.5 psi (1/5th to half a pound per square inch) is the most it will ever need, and not more. The temperature of the boiler at 0.5 psi will be about 215F, and only about degree colder when the pressure is 0.2 psi. Do not set the pressure control higher than 0.5 psi- it won't fix the short cycling, and can create other problems for the system.

As long as the thermostat is still asking for heat from the boiler it will cycle the pressure within some small pressure band. If the pressure control has too tight a control band it may short cycle, but it can also short cycle if the burner output isn't correctly matched to the amount of radiator. Setting it up so the burner fires continuously during a call for heat from the thermostat would create dangerous pressures and temperatures.
 

Ron Beck

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I am not sure the boiler can even build enough steam in 1 - 3 minutes to fill the piping. If you have a programmable thermostat verify the thermostat is set for a steam boiler. Check your cycle rate if there is one. If the thermostat is an older one with an adjustable anticipator increase it a bit.
 
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