Oil burner short cycling

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Old 01-03-19, 04:08 PM
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Oil burner short cycling

Crown Boiler, Riello burner, 4 zones, TACO 404 ZVC, one TACO circulator, Honeywell zone valves, external DHW.. L7224U aquastat with Lo limit set to OFF.
I thought that my problem was the 24 year old aquastat, Honeywell L8148A. So I replaced it with a new Honeywell L7224U electronic version. Nice thing is I can see a readout of the boiler temperature. The old analog temp gauge is much too unresponsive and the scale is to broad.
Everything seems to be working as it should. Thermostats, zone valves, circulator, burner, etc.
But here's what happens to cause very short burner cycles every so often:
A zone calls for heat, the burner fires, the zone valve opens, the circulator is energizes, until the hi limit is reached and then the burner shuts down. If the zone is not satisfied the circulator continues to run, the boiler temp drops. Sometimes it drops below the hi limit differential and restarts the burner (so far so good). But sometimes, within seconds, the ZVC indicates the zone is satisfied and shuts off the burner, zone valve and circulator. This short cycling (2-10 seconds) happens often enough that it seems the ZVC is holding onto the thermostat's signal after the zone is satisfied. I know that sounds ridiculous.
I'm wondering if adjusting the Hi limit differntial either more or less (default is 10º) would tend to minimize the problem. Any other thoughts would be appreciated.
 
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  #2  
Old 01-04-19, 10:19 AM
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Why not raise the high limit to 190 (or maybe even 200) just for testing purposes and see what happens? If you don’t bang up against the high limit as often maybe you will see a difference in burner cycle time, and at least that might throw some light on the problem.
 
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Old 01-04-19, 03:38 PM
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Thanks. I did try increasing the differential to 15º and that had no effect. Set it back to 8º but haven't had time to see any result.
Maybe I should go to 20-25º differential which would allow the zone more time to get satisfied before the boiler temp goes down to 155-160º. In that case, maybe raising the Hi limit would also help the zone get satisfied faster. I do note that the actual boiler temp continues to rise 7-8º after the burner shuts down upon reaching the Hi limit.
 
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Old 01-04-19, 03:44 PM
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Are you suggesting raising the high limit and increasing the differential? There is no low limit set to maintain a minimum boiler temperature. It only fires on demand, the so-called cold start mode.
 
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Old 01-05-19, 07:24 AM
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What I meant was that if you raise the high limit you might typically satisfy the thermostat before you hit the high limit. In other words, if I understand what you are saying, you typically hit the high limit and the burner shuts OFF, then the burner turns ON again after the water temp drops below the [High Limit – Differential] – and sometimes, after the burner turns back ON after dropping through the differential, it only runs for seconds and then turns OFF, because (presumably) the thermostat is now satisfied.

It seems to me that certainly could occur sometimes, but you wouldn’t think very often unless the differential is way too small or not working properly. I think that’s what you are thinking.

So maybe if you raise the high limit to 190 for a test period, you will still have the same short cycling problem, but now just a higher temperature. That I guess would tell you something. But on the other hand …maybe, the water temp will never hit 190 and the burner will continue to run, and the circulator will continue to run, until the t’stat is satisfied. No short cycling. That would also tell you something.

I’m no expert but I don’t think the boiler should be running up against the high limit most of the time. I think in a properly functioning system for a given geographical area, with right sized emitters etc, you shouldn’t be hitting the high limit that often. Maybe only in extremely cold weather.

(I also have the L7224U, but I use the Low Limit also))
 
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Old 01-05-19, 09:53 AM
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This article may help you understand your control and what a differential does.

https://www.pmmag.com/articles/89949...rential-anyway
 
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Old 01-05-19, 02:28 PM
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Thanks, I see what you're saying now and agree there is some logic to boosting the Hi limit. This I will try. And perhaps later will also increase the differential. I see others who suggest that boiler temps are better set at 190 or 200º though I haven't heard their logic.
 
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Old 01-07-19, 09:07 AM
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Set the Hi limit to 190º. Watched an entire cycle while a thermostat was calling. Boiler temp. started at 172º, hit 190º, overran to 194º after the burner shut off, dropped back down to 175º (190º less 15º diff), burner started, got to 176º and shut off.
Now I will increase the diff to 20º and see what happens.
The condition of short cycling is variable and probably depends on how cold it is outside (the load). I'm sure it also relates to the volume of water in the boiler vs. the baseboard units in the zone which is most used.
Perhaps there is some kind of control which would override the aquastat to set a minimum run time as long as it doesn't exceed the Hi Limit? Doesn't seem to be built into the L7224U.
 
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Old 01-07-19, 10:42 AM
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I’m no expert but this is starting to sound like something else is wrong here. Maybe not, but it sure seems funny.

If the burner came back ON after it dropped below 175, but then shut OFF again when the water temp was raised and hit 176, that seems like an awful coincidence (to me anyway). As the water temp dropped from 190 down through 175 while being circulated, the thermostat was not yet satisfied, otherwise the burner would have not have started again at 175.

But then, the burner runs just long enough to bring the water up one degree to 176 and now the thermostat is satisfied. That doesn’t seem probable. Seems like that should only happen once in a while. How long did the burner run to bring the temp back up to 176?

Perhaps there is some kind of control which would override the aquastat to set a minimum run time as long as it doesn't exceed the Hi Limit? Doesn't seem to be built into the L7224U.
I think you are right. That doesn’t exist. At least I don’t remember seeing it.
 
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Old 01-07-19, 12:46 PM
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S,
It sounds like you expect your boiler to run steady without any cycling when you call for heat. Lets use some good old fashion logic here.

You have 4 zones which means your boiler was sized for the total heat loss of the house. So when only 1 zone calls you are only using 25% of the heat the boiler can deliver. Naturally the boiler is going to make heat faster than it can be delivered because for 1 zone the boiler is oversized and thus causing cycling.

It's inevitable, your boiler is 4 times as large as it needs to be when only 1 zone calls and so forth as the other zones call.

In my opinion you never really needed another aquastat but that being said the upgrade was good and can be put to good use. An 8deg. diff. I think is too low and surprised Honeywell even makes one when even they recommend a minimum of 10deg. ordinarily.

With this new control you have flexibility to adjust your settings. A couple things to keep in mind.

Raising your boiler temp in itself is not going to stop short cycling. It will give you more heat output from the emitters but the boiler will lose heat and cycle just as rapidly. Your emitters heat output are rated with 180deg water going through them. If you deliver more, they will put out more, the same with less. Running at 190 or even 200deg. I think is smart. You will get the rated heat your emitters were designed to put out which is good when multiple zones are calling at once.

What will help stop your short cycling is longer times between runs which is only done with your differential setting which is why way back in your other thread I recommended this particular control.

The higher the diff. setting the longer it will take for your boiler to cycle but the price you pay for that is you are going to be running lower temps before the boiler restarts and those lower temps will give you less heat from your emitters and keep the stat calling longer. What you don't want to do is run the temps so low that it gets uncomfortable between the time the stat calls and when it's satisfied.

I would try at least a 20deg. diff. in your case and go from there but remember you have 4 zones which could be calling and overlapping at any time so I believe a little short cycling may be inevitable but this may minimize it.
 
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Old 01-10-19, 07:34 AM
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I think Spott has hit the spot (arghh......). Intuitively, I boosted the differential to 20º. I thought it helped a little, but still get some short cycles. Of the 4 zones, two are upstairs bedrooms which we keep cool, plus with normal upward migration of heat, they hardly ever call. The fourth zone is DTW which again is low usage with only two people in the house. So the zone for the downstairs living area of the house accounts for probably 80+% of the calls.
Given the boiler is oversize for the one zone which is always calling, I might try 25º diff which would result in a low of 165º. When really cold outside, the burner will be running frequently and retaining a reasonable amount of heat. When only cool outside, it won't matter so much if it takes a little longer to get up to heat.
Thanks all for your input.
 
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