Mechanics of Short Cycling

Property Manager

Why does short-cycling lead to higher fuel bills for a steam system?

I'm just curious on the physics of this.

For example, if my boiler is too big for the system, then the boiler reaches the required pressure very quickly and then cuts off. Why would this lead to higher fuel bills? How would this be better than a boiler that burns for longer but at a lower burner rate? I hope this doesn't sound like a stupid question. Just new at this.

Jamie Hall

Not a stupid question

there aren't any -- just stupid answers. Short cycling resulting from too big a boiler, or an over-fired boiler, wastes fuel because of the losses bringing the boiler -- and all that water -- up to temperature. Much better to get it warmed up, and keep it warmed up.

Note that this is very different from the cycling on pressure which is almost inevitable at the end of a cycle; there the fire isn't off long enough to let things cool down at all.

That help?

Property Manager

Definitely helps.

I'm still confused but I'm starting to get it.

In my current system, the heat cycle starts by first, starting with a long purging period of several minutes. Second, the boiler fires for a couple of minutes as it reaches 2 PSI. Then the system cuts off. In exactly 1 minutes, the sytem reaches 1 PSI and then the gas purge period starts again. This cycle repeats 2-3 times. (Entire heating period is about 20-30 minutes)

So, you are saying that with an oversized boiler, the boiler gets to a PSI of 2 too fast. When the boiler shuts off by the pressuretrol, the water in the system cools down quickly during that 5 minutes. And, once the PSI reaches 1 psi, the boiler has to heat all that cooled water back up.

On the other hand, with a correct sized boiler, the boiler keeps working longer as the pressure slowly creeps longer from 0 PSI to 2 PSI. A lot more heat is able to be given off during this period and the actual heating cycle could be less?

Sorry for the long post. Just wanting to know the physics of this.

Mark Eatherton

Your case is the worse case scenario

Each time the boiler goes thru pre and post purge, you are blowing paid for BTU's right out the vent stack.

The "ideal" situation is where the boilers output matches demand, and that actually only comes close to occurring when the house it is heating is at or near "design" conditions. (EXTREMELY cold outside)

Hopefully, one of these days, someone is going to come out with a fully modulating steam burner boiler that will have the ability to match demand with capacity like we are able to do with the high efficiency modulating/condensing boilers we are using on hot water heating systems today. They turn on when it gets cold outside, and stay on until it warms up, and typically reduce energy consumption by around 30%.

I think the technology exists, I think someone (Burnham) needs to step outside the typical BANG BANG burner box and make it happen.

For an automobile analogy, think about how inefficient your automobile would be, if you fired up the engine, immediately put the pedal to the metal, slipped the clutch until you got to your target speed of say 50 miles per hour, then shut off the engines fuel system, but kept air flowing through the engine (clutch still engaged, cooling the engine off) and coasted to 45 miles an hour, where the whole process starts all over again, except you have to push the clutch in to restart the engine, and again take the engine to full RPM.s.

Now, if you had the ability to allow the engine to idle, and put the pedal to the metal as needed, your auto would be much more fuel efficient.

Contrary to popular belief, bigger is not better. Driving tacks with sledge hammers gets the job done in a hurry, but wastes a lot of energy in the process...

ME

Property Manager

Great answer. I totally understand now.

Thanks a lot.

Mark Eatherton

My pleasure...

ME

The Steam Whisperer (Formerly Boilerpro)

These boilers do exist.......

especially in larger sizes and with greater and greater modulation ranges. In larger boilers they typically can have 3 to 1 modulation and with some burners about 10 to 1. I had a church whose boiler was operating just like Property Manager's. It had modulation available on the burner but nobody ever tuned it in to work. With some tuning, the boiler now runs down on low fire quite quickly and stays there for long cycles. I haven't got documentaion on the fuel savings, but it was significant.

I had another that wasn't running quite that bad, but did have way too much excess air and needed some better system venting and controls. With a small investment, the fuel usage dropped 40% and there were no more complaints about uneven heating. Payback on the improvements..approx 60 days. This structure now heats with the same fuel usage as the similiar one next door that has all 90% efficiency funrnaces...with one caveat...the steam heated structure is 50% bigger!


Boilerpro

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Mark Eatherton

What range capability?

Minimum, maximum etc?

It obviously has to be controlled based on pressure/transducer. whose controls?

ME

Wayco Wayne_2

Driving tacks with hammers

I like that.:) I think I'll steal it and use it. I once told a lady using her oversized boiler was like driving a 500 hp drag racer down a city street and hitting every stop light. WW

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The Boiler Dr.

Analogies

Mark and Wayne
I use those same analogies all the time... right size with ODR is like running your boiler on cruise control
Customers usually understand it immediately - in the boiler's case you don't even have to hold the steering wheel!

McKern_2

2 questions.

1. Boilerpro: Is this the same as lo-hi-lo function on Weil McLain boilers? Our boiler appears to have this capacity, but I don't know that it's hooked up. Will this work with a pressuretrol or does it require a vaporstat to be effective.

2. I understand why 1 longer cycle is preferable to 2 or 3 (or more) shorter cycles.

But, I'm missing something here, i.e., property manager says "if my boiler is too big for the system, then the boiler reaches the required pressure very quickly and then cuts off." And Jamie Hall says "...that this is very different from the cycling on pressure which is almost inevitable at the end of a cycle..."

Isn't PM's boiler cycling on pressure? How is this different from cycling on pressure at the end of a cycle?

Gene_5

Kewanee

we had a large Kewanee once that was all messed up and short cycling like a mother, after we pulled 25 bags of soot out and set it up we still had very bad short cycling

it turned out this 60gph boiler was designed with an addition in mind, a big one, when we figured out the buildings overhead heaters and fancoils we found we needed something like 20gph

this was the sub base in Groton CT and the Navy had no plans to finish the project or change the boiler, luckily were able to modulate the burner down using stack temp as a guide to keep it from condensing, if I remember correctly we got it to run 5 minutes, which was a far cry from 40 seconds and got it to stop sooting

short cycling on a steam unit is concerning because once you get a head of steam up the burner should be off for a bit, maybe a vapor stat would help, I would also wonder if you have any zone valves or strange set up or surging?

nicholas bonham-carter

short-cycling

i think most of the back pressure, causing excessive short-cycling in steam systems, is due to ineffective main venting. if you have enough venting to let the air out, then the steam will be in the radiation quickly condensing and reducing the system pressure. on the other hand, if venting is so small, that the steam pressure is trying to squeeeeze the air out through constipated little openings, and therefore short-cycles more as it is just on the edge of the cut-out pressure.

not sure whether i was as clear on my theory as i like to be-maybe i will think of an automobile simile later--nbc