Need help determining if boiler short cycling

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PBPB

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I have an older house that I purchased last year with a boiler that was installed in 2017. The boiler runs when the thermostat calls for heat, but once the high limit gets hit it seems to cycle on / off quicker than I'd expect, running for only around 4 minutes per heating cycle until the boiler's hi limit hits. I am looking for some help on determining if this is expected or not....

Boiler Model: Smith 8HE
Controller: Hydrostat Hydrolevel 3250 Plus

To test the timing I set the high limit high @205. Timing (mm:ss) of the "short cycle"
  • 00:00 - T = 205F - Hydrostat hits "hi alarm", boiler turns off
  • 02:05 - T = 195F - "hi alarm" clears, boiler fires
  • 06:37 - T = 205 - "hi alarm", boiler turns off
does this seem normal, if not - are there some usual suspects I could check out on the system? the on/off cycling goes on for ~hour to get the house from 65-68F, and we . have been able to get over 70F if we wanted to warm the house on a cold day, just takes a while.
 

Fitter30

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That is some hot water! Most systems are design for 180* water at 0* unless the radiators or fin tube is short of capacity. Heres a link to a manual.
https://hydrolevel.com/fuel-smart-hydrostat-3200-plus3250-plus/
Under setting 9 is the differential setting for control 10-20-30* start at 20*. High temp setting 9 on dial in the control set that at 190*. The control has a sensor at the end of a wire and should be in a well. Check to see if its all the way in.
Model of thermostat and type of heating devices radiators or fin tube. The boiler should have a high temp limit and a operating control mounted on its front. Limit should still be active but operating is inactive by hydrostat.
 

Dana

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Which size/model 8HE?

How much of what type of radiation do you have (broken down by zone, if multi-zoned)?

Any high-mass boiler needs to have minimum burn times in excess of 5 minutes (1o minutes is better) to approach anything near it's steady-state combustion efficiency. If it's more than 1.7x oversized for it's design load it's not going to hit it's AFUE either.

Setting the differential to it's maximum setting will increase the minimum burn times, utilizing the thermal mass in the system to the extent possible.

Odds are pretty good that you don't need anything like 180F water to heat the place (most systems are WAY oversized, to the point that it cuts in to actual comfort), but the lower the water temp, the shorter the burn cycles become: Since the emittance rate from the radiation drops with cooler water, the rate of excess heat going in to the system is higher so the water temperature goes up faster, making the cycles shorter.
 

PBPB

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Thanks for the replies guys,

The sensor is +/- 1 degree F from the analog thermometer installed, so I think the probe is located correctly.

Even though it's listed in the literature for the 3250 (and there is a label indicating that's the model installed) - this particular model has a sticker that says the "Differentials are controlled by the HydroStat and not adjustable". I even tried to reset it and there is not a setting "9" (goes from 8 to default reset). Pretty bummed was hoping this would be a good quick fix.

Model is the smallest 8HE-3, radiation is baseboard (mostly SlantFin). Basement level - 1 baseboard, first floor - 10 base boards, second floor - 5 base boards.
 

Dana

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A typical Slantfin baseboard can be as short as 2 feet, or as long as 6 feet per section, and the sections can be joined to make much longer total lengths. It's the total running length per zone that matters, not the number of sections it's broken up into.

The three plate 8HE puts out 104,000 BTU/hr if the burner fitted with the 0.85 gph nozzle, 126,000 BTU/hr if fitted with the 1.05 gph nozzle. (Clearly the smaller is going to short cycle less.)

With the 0.85 gph nozzle and a boiler output temp of 180F, with 160F on the return (=average water temp of 170F ) it takes about 200' of Fineline 30 or similar to emit the full output of the boiler. If the house is zoned floor by floor each zone would need to have a substantial fraction of that 200' to limit short-cycling.

There is only about 100 lbs of water thermal mass in the boiler, along with another 55lbs of "water equivalent" thermal mass of cast iron. The thermal mass of the plumbing & baseboard water & metal is pretty small, about 10lbs per 50' if 3/4" copper, 10lbs per 100' if half-inch. If boiler + zone is only adding up to 175 lbs water-equivalent and the zone is only emitting 50,000 BTU/hr there's at least 54,000 BTU/hr , or (/60=) 900 BTU/minute of extra heat going into the system (assuming the 0.85 gph), and the temperature slew rate is about 900/175lbs= 5 F per minute, so even with a 20F differential you'd be looking at minimum burn times in the 4 minute range.
 

Fitter30

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Went on supply house.com looked up slantfin and not knowing what model you have or pump about 580- 600 btu's per foot @180*. Measure all fin tube see where your at in btu's.
 

Dana

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Went on supply house.com looked up slantfin and not knowing what model you have or pump about 580- 600 btu's per foot @180*. Measure all fin tube see where your at in btu's.

That's the performance at a 180F AVERAGE water temp, not ENTERING water temp. If the boiler's high limit is set to 180F assume the AWT is going to be more like 170F (or even slightly cooler- it depends). Don't assume 180F AWT performance out of the fin-tube unless the boiler's high temp is set to at least 195F.
 
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