Boiler Short Cycling - Multi zones

[Kengie]

Hi everyone,

I have a strange issue in which I don't understand how it works. So I have a Williamson Oil-based boiler Series 2 (W_OWB_OWT_Brochure.pdf (williamson-thermoflo.com)) that has a Hydrostat and I'm using Lux Smart Kono for 3 zones in a 1700 sq. feet two story house. The water based heating element in all zones are using 3/4" baseboard of varies sizes.

My issue is that my short cycling can be as short as 3 minutes in between to around 30 minutes. The only variable that causes this from my finding is that if I turn on all 3 zones to call for heat. It will be short to 3 minutes cycling. On the Hydrostat that is attached to the boiler, I notice that the high temp cut off is 190 and with the Lux Kono thermostat calling for heat. The boiler will fire back up when the temp shows 170 on the Hydrostat. On the thermostat, I have already set the temperature differential to be 2.25 degrees. I have seen that the temperature on the Hydrostat LCD drop from 181 to 170 in a matter of minutes.

Now if thermostats are off, I can see the temp on the Hydrostat reach as low as 150 and the boiler wouldn't fire up. I'm wondering how does the zone thermostats connect/link/communicate with the thermostat on the Hydrostat.

So my question is timing of the short cycling of 3 minutes normal when all 3 zones are calling for heating? Will this short my boiler life expectancy (Installed 2019).

As a side note, in one of the zone in the larger room. The temperature would never reach the set temperature (e.g. set to 70 but the highest it would reach would be 68 degrees). Does the baseboard need more than 1 feet of clearance?

Thanks in advance.

 

[Fitter30]

There are two different controls for boiler water mounted on it. One is a high limit set at 190*-200* and would have a manual reset. The other is a operating control that would have two settings one is shut off temp the other is the differential. If set at 180* off, differential 25* the on 155*. Three zones, three stats and three pumps if this what you have there could be a control that when a zone calls for heat turns it's pump on and turns the boiler on. Water temp is controlled by operating control unless theres outdoor reset. If your boiler is cast iron minimum water temp is 140*. Model of the lux kono stat.

 

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[Kengie]

Thank you for your reply.

Yes I have three zones each with it's own Taco Value Head controlled by Lux Kono Smart Thermostat (https://www.luxproducts.com/kono-4/ The model name is actually Kono Smart Theromstat). The water boiler is an cast iron Williamson OWB/OWT-3 S2.

What I think I don't understand is why the duty cycle is so short as the water boiler would fire up again when the water temp in the boiler (according to the Hydrostat LCD) is 170 to fire up again.

Previously the 3 zones was using Honeywell 'dumb' thermostat by the previous owner. I just replaced them with the Lux Kono Smart Thermostate if this information will help with explaining the short duty cycling of the water boiler.

Thanks in advance.

 

[Fitter30]

Fin tube 170* water 4.25" x 4 .25"fins 3 /4" 1052 btu's per foot. #2 oil 139600 btu's per gallon. Don't know what size nozzle you have or how many feet of fin tube by zone and total. If your system holds 30 gallons of water 170* has 30,000 btu's that why the temp drops if all zones are calling. Kono stats did you set system setup 06 to boiler? Three wires connected to Taco valves then you're using a set of contacts in the valve to either start a pump or boiler.

 

[Kengie]

Thank you fitter for looking and replying to this.

For Kono stat, yes I have set the setup 06 to be boiler. The wiring from all three of the Kono stat are wired directly to the Taco value head to call for heat.

As for the size of the nozzle, I don't know how to find that on the Carlin Ignitor model 41000. As for the boiler, according to the brochure it has an output of 114,000 BTU (https://www.williamson-thermoflo.com/sites/default/files/field-file/T_OWB_OWT_Brochure.pdf). Not too sure it that info helps.

Zone 1: Approx. 26 feet of aluminum heat fins
Zone 2: Approx. 19 feet of aluminum heat fins
Zone 3: Approx. 12.5 feet of aluminum heat fins

 

[Fitter30]

Napkin math. Total fin tube btu's 60490, 200 ' 3/4" copper tubing 12800 btu's ( 64 a ft.) 73290 btu's total. Boiler 114ooo 80% efficient 91200 btu's.
Fin tude design 20* temp difference between supply and return.

 

[Kengie]

After looking at my set up, the three Lux Kono stat are connected to 3 Taco valve heads and it's connected to a single boiler Honeywell control board. So when all three zones are calling for heat. I think the three lux Kono is telling the boiler control board to fire up the boiler thus the short cycling as the Hydrostat internal temperature hits the high limit. Sorry about all the question or about my bad explanation about this as I am not familiar with these stuff at all.

Based on your calculations, I guess I can resume that my short cycling is normal and fine.

Thank you fitter for your time to take your time and for your calculations of the btu outputs.

 

[Fitter30]

Next time burner is serviced find out what nozzle is in it there stamp with pm and degree of flame. If at the coldest days of winter the boiler is still short cycling nozzle, smaller nozzle might be able to be used. All depends on the fire box how the flame spreads in it.

 

[Dana]

After looking at my set up, the three Lux Kono stat are connected to 3 Taco valve heads and it's connected to a single boiler Honeywell control board. So when all three zones are calling for heat. I think the three lux Kono is telling the boiler control board to fire up the boiler thus the short cycling as the Hydrostat internal temperature hits the high limit. Sorry about all the question or about my bad explanation about this as I am not familiar with these stuff at all.

Based on your calculations, I guess I can resume that my short cycling is normal and fine.

Thank you fitter for your time to take your time and for your calculations of the btu outputs.

Based on the napkin math the short cycling is expected, but that's different from "normal" or "fine". The Hydrastat is a heat purging economizer control that is trying to suppress firing on zone calls as long as possible, to make use the thermal mass of the system to lengthen the burns. But any burn shorter than 5 minutes is pretty lossy with a cast iron oil boiler, and sub-three minutes is getting on to an efficiency disaster, and delivering dozens of burns per day puts wear & tear on the boiler and it's ignition equipment.

With burns that short the average operating efficiency of the boiler will usually be ~10% or more lower than the nameplate's steady-state efficiency or AFUE. For the "as-used AFUE" to reliably hit the labeled AFUE number the minimum burn times would be on the order of 10 minutes each, and the seasonal average duty cycle on-time would be about 20%. So with the system as-is your ~85% efficiency boiler is more likely to be delivering only ~75% AFUE.

The boiler is just plain too big (more than 2x) for your total radiation, and LUDICROUSLY oversized for individual zone loads. Even the very smallest in that series boiler with the smaller 0.70 gph nozzle your OWB/OWT-3 would be putting out 98,000 BTU/hr. With a total of 58' of fin tube that's a whopping 1690 BTU/hr per foot of baseboard, which can't emit more than about 600 BTU/hr even when running 190-200F water out the boiler. Even with all three zones calling for heat the remain "extra" 1000+ BTU/hr will cause the system temp to rise to the high limit and turn off the burner fairly quickly. With the 1.00 gph nozzle (133,000 BTU/hr out) it's worse, of course, but even throttling back to the 0.70 gph burner isn't going to get the burn times anywhere near 10 minutes, and not even 5 minutes when serving single zones.

The solutions aren't necessarily simple or cheap. An insulated buffer tank to add thermal mass to the system might get you there, but it would need to be properly designed-in (not "design by web-forum") to have guaranteed results. Adding more radiation to each zone sufficient to emit around half of the boiler's output on any zone could also work, but that would be a huge amount of fin-tube baseboard.

Right now you've set the thermostat differentials to 2F, which is a pretty big swing, which is bad for comfort, and does NOTHING for efficiency given the excessive boiler output/radiation ratio. In this case the thermostat isn't capable of short-cycling the boiler. With a tighter differential on the thermostat the pump will cycle on/off more often, but the Hydrastat will maintain the boiler's burn characteristics to roughly where they are currently. On the smaller zones with a tight differential on the thermostats it's possible that the thermostat will sometimes be satisfied without firing the boiler at all (TBD).

Try downloading the programming instructions for the Hydrastat model that is installed on this boiler. There may be further tweaks to be made there. The improvements in burn times are likely to be marginal at best, but anything is better than what it's suffering through right now.

If you want to install air conditioning you may be able to use a right-sized heat pump instead of a short cycling oil boiler to heat the place. In most markets the operating cost of a decent right-sized heat pump would be comparable to or cheaper than a right-sized oil burner, and the comfort levels higher. Since we're going into the heating season it's possible to get a firm estimate on the heat load using the boiler as a measuring instrument, logging fuel use against heating degree-day data, as outlined in this bit o' bloggery.

 

[Kengie]

Thanks Dana for your time and your reply.

Some background info about the previously installation were using radiators of the tubular type of 22 inches in height and varies widths (31 inches, 36 inches. 48 inches, and 41 inches). Not sure if that was the reason the previous installer was choosing the Williamson OWB/OWT-3 S2. I just swapped out the radiators for baseboard heating element. I was also able to find the nozzle info of 0.75 - 60B DEL, Pump pressure: 160 according to the label on the Carlin Part Number. Also I think my actual BTU is 98,000 instead of the 114,000 BTU as mentioned before and the unit was recently installed in 2019. The first floor was using radiators while the second floor was all using baseboards. The total sq feet of the house is 1700 sq ft.

I find that one zone is very strange, the baseboard pipes are pushing out heat but the sun room would never reach the temp of 72 degrees (outside is 32 degrees). The angled ceiling is about 9 feet at it's tallest. So not sure if that is the reason.

So I was able to find the manual for the HydroStat 3150 and I have modified the high differential setting to be 30 degrees. The cycles I know is listed below:

Outisde Temp: 32 degrees.

Two Zones
On Cycle: 3 mins (150 degress - 184 degress)

Off Cycle: 14 mins (183 degrees - 150 degress)
On Cycle: 5 mins (150 degrees - 184 degrees)

Off Cycle: 24 mins (184 degrees - 150 degrees)
On Cycle: 5 mins (185 degrees -> 150 degrees)

Off Cycle: 18 mins (184 degrees - 150 degrees)
On Cycle: 5 mins (185 degrees -> 150 degrees)

Off Cycle: 22 mins (184 degrees - 150 degrees)
On Cycle: mins (185 degrees -> 150 degrees)

Three zones
Off Cycle: 9 mins
On Cycle: 8 mins

Off Cycle: 13.5 mins
On Cycle: 5 mins

Thank you for your informative post and I found that I have learnt something today and your blog post was an excellent post too. I do agree that I wouldn't be able to fix anything from a web base forum but it is more to understanding how things work just for knowledge.

Happy holidays to you and your family, Dana. Thank you so much.

 

[Dana]

Sounds like it will work-mostly with the 30F differential at play, even if some of the single zone calls short cycle it a bit. A 5 minute burn isn't an efficiency disaster- it will deliver about 1-2% less than the steady state AFUE.

The 3 minute burns (or shorter) still deliver pretty crappy efficiency, but by improving the average burn length you're probably looking at a 15% or more savings over what it delivers when short-cycling on all calls. If this has the 1.00gph nozzle and it gets swapped out for the 0.70 gph nozzle at the next tune-up those 3 minute burns become 5 minute burns- definitely worth doing eventually, but not worth calling somebody out for the swap until it needs annual maintenance.

 

[Kengie]

Thank you for your insight and looking into my issue. I will definitely take you up your advice with my boiler service company.

Many thanks.