rapid/short boiler cycle question

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Old 11-10-08, 03:22 PM
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rapid/short boiler cycle question

oil fired cast iron hot water boiler, 9 years old, running baseboard heat, 4 zones, 4 round honeywell thermostats. the system seems to work pretty well, but occasionally there is a short cycle, lasting from about 20 seconds to a minute. the boiler jsut seems to fire up and then stop. then silence, and in a few minutes it will fire up normally. i see no pattern, but wonder what possible causes there may be for this. any thoughts appreciated.
 
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Old 11-10-08, 03:55 PM
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This may be normal. If a thermostat is calling for heat the boiler may hit high limit and shut down. After it cools down a bit as the circulator is still running, if the thermostat is still calling it may fire back on again. The next time it could fire back up about the time the thermostat is satisfied. Everything shuts down. Than 20 seconds later another thermostat calls and it starts all over again.
 
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Old 11-11-08, 10:14 AM
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your answer makes perfect sense, if the thermostat were calling for heat. however, the short cycle i am describing does not involve the circulating pump, even thought the water in the boiler is hot but below the upper limit and above the lower limit. in other words, the pump is not running, but the boiler fires up for about 20-60 secs and then total silence, including the pump. i cannot understand why it is firing on its own without a call for pump circulation, since the pump is turned on first and then the boiler only if the water temp drops to the lower limit?? bizarre.
 
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Old 11-11-08, 01:06 PM
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short cycle

There may be a difference between the temp reading on your gauge and what the aquastat is sensing. Steve
 
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Old 11-11-08, 05:28 PM
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You said:

...and above the lower limit...
Which implies that you have a 'triple aquastat' on your system.

Are there three setting dials in the aquastat ? Know the model [of the aquastat] ?

What are the temp and diff dials set at ?

This would also imply that your boiler also makes domestic hot water ?
 
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Old 11-12-08, 08:56 AM
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trooper
i have to wait until this weekend to check the model number etc of the aquastat, so i can give more info next monday. the boiler used to heat a hot water tank. however, i replaced that with an lp tank and used that available zone to heat a family room addition. it was a 3 house zone system with a 4th zone used for a hot water tank. then, the house was enlarged. now it is a 4 zone house system. all zones are fed by 3/4 inch copper pipes; all zones have the same zone valves; all zones use round basic honeywell t'stats; no kickback; no digital; all zone valves are wired exactly the same way.
the house is about 4000 sq feet and is somewhat spread out over two floors. the master bedroom suite has 1 zone [about 650 sq feet]; the rest of upstairs [about 1000 sq ft has a zone]; the lr and dr and foyer [about 1000 sq ft] have a zone; the famly room and kitchen and breakfast room [about 1400 sq ft] have a zone.
by memory the aquastat has two round dials[an upper and lower cutoff], and a small knurled knob that programs in the about +/-10 degree swing around the upper cutoff.
by memory, the lower cutoff is set at about 120, the upper about 180 and the knurled knob +/- 10.
so i figure that in the winter the boiler water will never go below 120. when heat is called from any of the 4 t'stats, the pump kicks in. if, at that time, the boiler water goes below
170, the boiler fires, independent of the pump, to keep the boiler within the range of 170 to 190. i hope this makes sense and that it is a correct setup. obviously i am not an HVAC guy.
 
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Old 11-12-08, 03:33 PM
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Good description!

Yes, it does sound as though it's a 'triple' aquastat.

What is the make/model of the boiler ?

I believe your understanding of the control action may not be quite accurate though... the way that thing works is:

There is a High Limit (HL) and a Low Limit (LL) and an adjustable Differential (DIFF).

The HL is generally a FIXED DIFF ... built in, non-adjustable.

The DIFF acts on the LL, not the HL.

The action of the DIFF is kinda hard to explain/understand ... there is usually a FIXED portion and an adjustable portion. The FIXED portion is a NEGATIVE DIFF and usually 10 ... meaning that if the LL SETPOINT is 120 the FIXED portion of the DIFF is SUBTRACTED from the SETPOINT and the bottom of the range is then 110. The ADJUSTABLE part of the DIFF is ADDED to the bottom number, in this case 110. So if you set the adjustable DIFF to 10, the TOP of the range will be 120.

If you increase the setting on the DIFF dial, you are pushing the top of the range UP.

Example:

Your setpoint is 120. There is a fixed 10 negative diff built in.

120 - 10 = 110 bottom of range.

You set the DIFF dial to 20

110 + 20 = 130 top of range.

In this example, if the boiler cools to 110, the burner will fire until the boiler reaches 130, with no circulator running.

The circulator will not run unless the LL is exceeded. The burner will fire all by itself to maintain the boiler at the LL setting during periods of non-heating.

What you might be seeing is the burner firing up on a LL heat call ... satisfying that call and shutting down ... then, a short time later, a heat call comes in and fires up the burner AND the circulator. ?

With all that said ... my next question would be:

If you originally had an indirect water heater ... that unit did not require a 'warm start' boiler.

When you removed the indirect water heater, there was still no requirement for a warm start boiler.

Why not ditch that aquastat and convert the boiler to cold start ?
 

Last edited by NJT; 11-12-08 at 07:19 PM. Reason: corrected per post below... I wuz bassackwards... happens a lot!
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Old 11-12-08, 05:48 PM
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The control works a little diferent that that.
The differential down from the LL and HL are both 10f.
The adustable is added to the LL-10f
The settings are as follows
120LL
180HL
Diff 20F
The burner will fire as the the water cools to 120f- 10f or 110f
The burner continues to run until the temp rises up to 130f
120fLL - 10f (built in) plus 20f the adjustable diff
On a call for heat the burner will raise the water temp to the HL setting and shut off. When the water temp cools down 10f the burner will fire back up providing there is still a call for heat.
When the call for heat end the water will cool down to the LL setting - 10f before the burner fires back up. It will than raise the water temp the amount of the adjustable differential again.
Your numbers are right but the drop ion the LL is 10f and the rise is the adustable diff.
 
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Old 11-12-08, 07:14 PM
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Yup ... I got it backards agin... I'm gonna edit my post to correct the error ... thanks !
 
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Old 11-13-08, 07:32 AM
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you guys are fantaastic for taking the time to answer my question. i will know more about the aquastat model after this weekend, but i am starting to suspect that i have the wrong aquastat in the system, now that i have removed the indirect water heater.
thanks again.
 
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Old 11-17-08, 11:10 AM
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ok, i'm more confused than ever. my wife and i both heard the oil burner fire up for about one second and then stop; this happened a couple of times this weekend. the pump was not running, either before or after, so there is no call for heat.
also, i have the aquastat settings at 180 over 150 [the diff at 10], and at the time the boiler just spontaneously fired up, the temp gauge said the boiler temp was aobut 175. this is driving me crazy. no call for heat; boiler is hot; boiler short cycles, apparently on its own.
also, although the upper limit is set at 180, the temp gauge recorded a temp rising to 215, on more than one occasion, before the flame shut off.
finally, wierdest of all, my round honeywell thermostat on one zone has a set temp of 61 degrees. the ambient temp part of the t'stat said 64 degrees and the unit kept calling for heat about every 15 minutes. i could hear the boiler fire up for about one minute and i could hear the water circulating through the slant fin baseboard, and the slant fin was hot. so it seems that even theough the room is way too hot for the t'stat, it is calling for heat. the anticipator, like all of the other 3 t'stats is set at 0.2 amps. the other 3 t'stats seem to work ok. today i bought a new $45 round honeywell t'stat [both heat and air], but before i install it, does anyone have any ideas??? i can return it unopened.
my boiler is a burnham 3wnh and the aquastat is a honeywell R8182 [no letter after the numbers]. i'm losing sleep over this one!! help me; save me from the poltergeists. puhleeese.
 
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Old 11-17-08, 11:57 AM
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I review your original post and now have a question. You mentioned a circulator. Do you have zone valves and one circulator. If so the anticipator is not set right. If you have zone valve what brand and model do you have?
 
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Old 11-18-08, 08:05 AM
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good questions. since i am only home on weekends, i will have to answer this next monday. however, i have 4 of the same zone valves and one circulator. i will get the additional info and report. also, i suppose i will have to get a milli ammeter that measures in the 200 milli amp range [o.2 amps]. as i recall, radio shack sells a decent digital model for a reasonable price, although any advice would be appreciated. thanks for the reply.
 
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Old 11-18-08, 08:42 PM
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You shouldn't need to measure the current, the proper anticipator settings should be stated somewhere in the zone valve literature ...

I don't think the anticipator would have anything to do with the wierd boiler firings though ?
 
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Old 11-19-08, 08:45 AM
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i don't either. i will look up the zone valve literature and see about the anticipator settings this weekend. i am totally stumped.!.
 
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Old 11-19-08, 08:21 PM
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Way too high of an anticipator setting CAN cause short cycling, just as too low a setting can cause temp overshoot ...

but this isn't a 'short cycle' thing ... it's a ghost ...
 
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