oil boiler cycling -- too frequent?

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Would like a perspective on cycling of an oil boiler.
So I replaced a spent Burnham RSA110 steel boiler with a Peerless WBV cast iron boiler. Everything else remained the same -- still three zones, configured essentially the same way, and the same size / model Taco circulator.
I was expecting, due to the increased water capacity and thermal mass of the Peerless (new) boiler, plus a smaller nozzle, that there would be less frequent cycling. However, the new boiler, when all three zones are cranked open, cycles approximately like this:
- 2 minutes on - 4 minutes off.
That strikes me as excessive, and as more frequent than the previous RSA boiler which is counter-intuitive to me for the reasons above.
Does this rate of cycling seem excessive and/or inefficient for a new cast-iron boiler ?
A couple more data points:
The Aquastat is different, a newer digital model, but the settings are confirmed the same: 180F high limit, with a (fixed) 10F differential. I made sure the new digital temp sensor bulb was well seated in the well. Judging both by the digital readout on the 'stat and the temp guage, the new Aquastat seems to be doing the job "OK".
The old boiler had a nozzle that flowed 1.00 GPH. I put in the smaller of the nozzles available on the new boiler -- it flows 0.85 GPH. Now I know that the cast-iron model will be a tad more efficient, but I would not expect shorter on-cycle times with the lower flow rate!
I had the burner adjusted by the oil company guy -- seems to have the right flame -- but he has no idea about the cycle times.
I'm pretty sure the system is substantially evacuated of air, and the zones seem to be flowing well. And in the correct direction of flow :).
The old boiler was about 150 lbs of steel and 8 gallons of water. The new one is about 400 lbs of iron and 17 gallons of water.
Everything tells me the cycle should be longer -- but it's not. What else could I be missing.
Tman.
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wrote:

It appears you have missed quite a few things. Yes. The cycling you are describing is way too much. Did you do a "load" on what you have in the home? Im not concerned with the size of the home. You need to calculate the amount of radiation you have. Then and only then can you decide on which size boiler to use. Now, after that is done and sized properly, you need to find an oil tech that knows how to use and interpret digital combustion efficiency equipment. Looking at a flame on a new oil boiler and guessing at what size nozzle to use is like throwing darts at a dart board blind-folded. You will quickly destroy a new piece of equipment without setting it up properly. Bubba
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Bubba wrote:

SO I can do a load and have done them before... but what I don't get. The original boiler was sized professionally to the load, and certainly did not have any cycling issues. Now this boiler is very close to the same BTU rate, if anything (intentionally) a little lower. I would expect the lower output to result in a longer "on-time"... but something else is not right. T
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The load may be the same, but the efficiency is greater. If you went from putting 50% of the heat up the stack to only 15% up the stack, the same sized burner will cycle more often. There is a difference between input BTU and output BTU. I went from 100,000 Btu to 85,000 Btu due to better efficiency.
In either case, you must have a very large house to need a boiler of that size. Mine has 4 gallons of water and gets up to temperature in 90 seconds.
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Ed Pawlowski wrote:

I've thought of this, but it doesn't stand up. The old boiler was not a terribly inefficient unit. Specifically, the IBR rating of the old boiler was 111,000 BTUH and the new boiler is 90,000 BTUH -- this is as per the installed nozzle. So it went down a bit. My understanding is IBR ratings are not the heat input (fuel rate), but rather the heat output after adjustment for various inefficiencies such as stack heat loss, etc.

The WBV-03 installed is actually one of the smaller (not the smallest) residential boilers that Peerless makes.
I live in a development that was built by one builer. All the houses are similar... about 2400 sq ft, 2 stories, some with finished / heated basement, some without. Everyone I know has either a RSA110 (my original) boiler, or this Peerless unit, if they opted to pay a couple extra bucks.
So, I'm just not seeing that there is a terrible mismatch in boiler size to house, and the new boiler is quite similar to the old, just a little smaller in terms of heating capacity.
T
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I would not want a 2 min on cycle, you never get rated efficiency and its hard on controls cycling so much, does temp remain the same through cycles. I have about 2 degree spread from on to off and adjusted it in the thermostat to the maximum I feel comfortable with. I am about 30 min on 1 hr off at 32f. Was a real load calculation done that you saw in writing. Your thermostat should have a swing- differential setting, or anticipator, or get one that does have it. I saw a cheap Honywell that can be also be set to 30 or 1 hr off periods to allow temp to fall. Go for the longest fall back that you can feel comfortable with to get a longer cycle. I thought the newer units trend was less water in the boiler for highest efficiency.
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I guess I made this a little unclear. In this situation: The boiler unit itself is not cycling. The oil burner is cycling.
E.g. house is at 60deg. Thermostats (all three zones) are set at 70F and remain _on_. Circulator and zone valves remain open, return water lines to boiler on all three zones are pretty hot.
In this situation, the unit stabilizes at "2 min on 4 min off", approximately, for the oil burner unit. I would expect a longer cycle.
**Is that too frequent of a cycle?
OK now that I made that clear (it wasn't; sorry). A few more factoids.
High limit setpoint for the burner is 180. I observe the digital readout swing between 180 and 170 as the burner turns on and off -- as expected given the 10deg differential. There is a little bit of overshoot on the high side, e.g. the burner is off at 180 but the temp swings up to say 185 before sliding down to 170. The boiler temp guage (tridicator) indicates a reading that is roughly commensurate with the aquastat readout.
If this stimulates any ideas:
Water circulating through the radiator loops looses about 10-15 deg on the trip. E.g. 180 deg out of boiler, 167 into boiler, typically.
Oh yeah, this has a tankless coil on it. I haven't hooked that up (yet) -- I plan to in the next day or so. So the tankless coil is vented to the air.
T
Tman wrote:

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Tman wrote:

If you're loosing 15 degrees worth of heat from the boiler every 6 minutes, it's going somewhere. Makes me think you have an issue with a missing check valve and continuous convection circulation through the loop. With the circulator pump off, there should be no flow through the loop and the boiler temp should take a lot longer than 6 min to loose 15 degrees.
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Pete C. wrote:

I'm getting quite a complex for not being able to communicate, lol. Sorry buddy. I know where that 15 deg of heat is going every 6 minutes -- into the rooms via the radiator! Why do I think this? Cause the circulator is on and the zone valves are open -- as it should be during the call for heat.
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wrote:

As I said before, if your burner is cycling off after only two minutes of run time and the water is at its set point (180) and you have all 3 zones open and the pump/s running (like on a call for heat) then your boiler is WAY the fuquer oversized. Bubba
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Try setting the differential to 15 or 20 degrees. IMO, 10 is very tight, but even at 160 you'll be getting good heat transfer. Some new systems will reduce the temperature of the boiler in warmer weather for just that reason. They use an outside sensor to determine the water temperature to circulate. I've not timed my burner, but I know it will go more than 10 deg. differential. It will shut down the circulator if it drops below 120.
As for the tankless coil, I'd skip it. They are very inefficient when no heat is needed. They put out a lot of hot water and can keep up with your shower forever, but the chamber is poorly insulated and you lose a lot of heat to the house. OK when you need heat, wasted when you don't. You add an indirect fired tank and zone. How many times in the summer do you hear the burner kick on just to keep that water hot? Too many. Think about that 400 pounds of cast iron being kept hot "just in case" 24 hours a day. Move the hot water into a 40 gallon well insulated tank instead.
Bubba says your boiler is oversized. I don't know you climate, but I tend to agree with him. My house is 2000 sq. feet and the output is 15,0000 Btu less than yours. During the coldest weather we had in 27 years, it easily maintained temperature, an 80 degree rise.
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I am having trouble finding the issue yet because the information is incomplete. The thermostat is calling for heat so the circulator stays on but the burner cycles by the aquastat, right. He says house is 60, he wants it 70 and to warm it the boiler cycles 170-180 and back. That to me is normal the house is taking all that the radiators can give to warm it back up. The question is what happens maintaining 70 without setback. And how long at 170-180f water does it take to go from 60-70 and at what temp outside. Increase the differential if you want to cycle less, but what happened on the coldest day this year, how much did it run. I set mine down from 180 to 165 this year and will go lower as it warms up out, I heat and get more even heat. I have read you save by setting aquastat lower and some new controls track outside temp to vary water temp high limit, but I have no idea if it really saves money.
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ransley wrote:

The issue / concern is that the oil boiler cycles 2 min on, 4 min off, which seems excessively frequent to me.

On these Honeywell Aquastats in general, and mine in particular, the only adjustable differential is on the low limit, not the high limit -- it is fixed at the manufacturer (somehow) at 10F.

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wrote:

Dont need to know his climate unless you are actually sizing it. The burner runs 2 mins !! then shuts off because it reaches limit temp on the water. Either that not water is not circulating or it is. The op stated it is because all zones are open, pump is running and heat is being sent to the radiators. ITS WAY OVERSIZED! Id be concerned that the home is very poorly insulated too. It losses those 10 or 15 degrees of water and then comes right back on in 4 mins!? Are the radiators sitting outside? :-) Bubba
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A question here, could radiators be undersized, Baseboard needing 180f.? And What is outside temp, 65f like it was Saturday or -15f below with 30 mph winds like it was 2 weeks ago. Still no ext temp or rise info, its a trick question with no answer, Mrs Poster.
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On Thu, 12 Feb 2009 18:32:11 -0800 (PST), ransley

Certainly undersized radiators, which in turn, still means the boiler is oversized for the amount of radiators. Depends how well the system has or has not heated in the past. Bubba
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Bubba wrote:

Well, system has heated quite well in the past, with the same radiators and a slightly larger (in terms of BTU output) boiler. That boiler did not cycle the burner anywheres near as frequent.
I believe (not so sure though) that the heating effectiveness of this unit is inferior, from the standpoint of how quickly the house comes up to temperature. Not so sure, cause it's all the variables e.g. wind and temp are different and I have to go by memory.
My current thinking is that their is something in the current setup that is preventing effective circulation through the zones. It has the same circulator setup, a 1/25 TACO on three zone valves....
T
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I never timed mine,but this morning it did run for 5 minutes and I've not heard it go back on yet and it has been 10 minutes or so. It is so quiet I don't usually year it anyway.
If the water is 180 degrees and the same as the old unit, the house should be heating up at the same rate. The speed of the temperature rise is a function of the radiation area and as long as the water temperature is the same, the heat rate will be also. If the temperature is lower or the heat rate is faster (as in more radiation) the burner would stay on longer to keep up.
If you can get an IR thermometer, take a reading of the temperature of each radiator in the loops and see how much variation there is from first to last. It is possible that in the swap, some scale in the pipe got loose and there is a blockage of some sort restricting flow.
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Are pumps new this year and same output, did you re-blead all radiators of air and only water comes out, were any main valves shut off during install. Air can be hard to remove reducing radiator output. Is it baseboard radiator.
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But is it heating it at the lowest temp of the year, or is his info total BS...
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