?re short cylcing gas boilers

[Deleted User]

chip into an RM7800 Control. Read 1900 hours and 18,000 cycles. Look at the building and then at the boiler. Yep. Someone used the old 1 Horsepower per apartment sizing chart.

Don't care whose ignition module, ignitor assembly, gas power burner, or oil burner, you use. You are doing a disservice to the customer. Where do you think an excessive number of btus went on each of those short cycles? If you answer up the stack; you know better than to ask the question. And. That does not address the excessive wear on components or the price for larger than necessary equipment.

Sizing multi-family domestic water heating sometimes forces our hand. The 7:00 to 8:30 AM draw, 5 days a week, must be accomodated. But, there is no excuse to grossly oversize heating boilers. We have too many control options and boiler configurations to choose from. The old Engineers SFX X 4 is dead. And in some places, it will cost you big bucks if you use it. Don't mean to preach. But, I am tired of having to tell customers why their fuel bills are so high. And, why their equipment may, or may not, last past the depreciation period.

In answer to a question. Yes. Constant circulation is still the Cadillac, or depending on your preference, the Mercedes, of hydronic heating.

Bill Clinton

? short cycling gas boilers

question rattling around the recesses of my subconscious for some while now: Why is short cycling a problem? I mean, "everybody knows" short cycling is a problem! How do they know this? Bear in mind I'm not talking about 10 second cycles: How about 90 second to three minute cycles? How bad is that?

I can see three potential down sides here:
1. ignition equipment operates more often, so maybe it will wear out sooner. Don't know, but Honeywell spark ignition seems pretty robust. Almost never see a failure. My guess is that short-cycling won't kill it.

2. A drop of efficiency occurs: Usually, the vent damper stays open when the burner is turned off on operating limit, allowing more draft up flue on off cycle. I suspect that current boiler designs taking dilution air from a point almost as low as the burner (no mushroom on top) minimize this effect.

3. Some incomplete combustion happens during burner light up and shut-down. High CO; maybe some soot. I don't see this as a problem on gas-fired iron boilers. Maybe oil is worse--personally know zilcho when it comes to oil.

What does your light does your experience shed on the question?

Anyway, as Arlo Guthrie might have said (but didn't quite):

I don't want a pickle; just wanna let my boiler cycle.
and I don't wanta die; just wanna let my boiler cy.............
..................................cle.

Dennis - Reliable Services

That's why

I like to size close, take that big converted coal boiler out and install a suit case size boiler. Bid em high and still under bid the competion.

hot_rod

The biggest issue

I see in short cycling, is the inability of the flue to warm up above condensating temperature. Doesn't take long for wet flue corrosion to take it's toll. I have seen the inner aluminum liner in a B-vent corrode away in less than a years time due to short cycling. I imagine a long cold masonary flue would really present the problem.

Next the particles of the corroding flue sprinkle down and start plugging the HX, next the flame rolls out the front of the burner tray and licks at the wiring and gas valve. By now someone has taken the roll out switch out of the circuit not knowing why it keeps failing. You know where it goes from here.

hot rod

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Duncan_2

hot rod is absolutely right.

The vent corrosion issue presents real danger. And with galvanized, the problem is a LOT worse. It's more reactive to vent gasses, and corrodes a LOT faster. I've seen systems where the galvanized is rotted through while the aluminum B-vent liner has just started to develop little white crystal blossoms of corrosion where the metal is starting to pinhole.

More of a problem with the tiny passageways of copper fin tube boilers, but I've also seen cast iron flame baffles and flame passageways compromised by rust flake buildup. CFT boilers with a draft box offset from the burners are a safer design, IMO. If the vent does drop debris, it drops out of the draft box onto the floor where it's a visible indicator, rather than onto the top of the HX where it creates problems.

It's a real concern, because it's typical practice (from what I've seen around here) to use galvanized for vent material once distances to combustible materials are OK. Not only that, but many contractors use 28 gauge galv pipe for vent, thin enough to rust through in a year. A no-no, but most inspectors don't catch it. Even though that's the hottest part of the vent, short cycling will corrode it. Seen it with my own eyes.

I wouldn't necessarily underestimate the on/off cycling of components as an issue. Most parts -relays, zone valves, and I would assume gas valves- are designed with a certain number of lifetime cycles. For example, I've seen relays normally designed for 100,000 cycles and special purpose relays of the same configuration especially designed for 1,000,000 cycles. I have read Honeywell zone valve literature that mentioned 100,000 cycles. Compare short cycling and equipment life to car use. Stop and go city driving is harder on a car than highway miles. The car was designed for stop and go, and stop and go driving may not "kill" the car, but which car is in better shape ten years down the road, the city car or the highway car?

As far as "how do I know this" about short cycling being a bad thing... In the mid '80s, I was involved in boiler efficiency studies and fuel use with a non-profit company I worked for, and we studied hundreds of boilers. Part of this involved combustion efficiency. It typically took at least a couple minutes for boiler stacks to reach steady state combustion efficiency, to stabilize. We set a benchmark of five minutes burn before measuring steady state combustion. I don't remember the exact reasons why (sorry), but ten minutes was accepted as a good minimum on-time. Possibly because that would mean over 50% of the burn time was at steady state? I don't honestly remember how that figure was arrived at.

Short-cycling was one of the issues addressed by de-rating. System efficiencies were also improved by lowering operating temperatures when possible. Results were analyzed using hard numbers and statistical analysis.

Unfortunately, you rarely see any zoned system reach the ten minute mark. And with over-sized boilers, exaggerated short cycling is actually BUILT IN by (lack of) design. In the real (imperfect) world, I suppose the ideal cycle would absolutely be when the vent temperature exceeds the dewpoint of vent gasses, and this would vary with each installation. Five minutes is a pretty good starting point.

Incomplete combustion is a real issue around here where propane is the fuel of necessity. Carbon builds up to the point where heat exchangers become completely blocked. I can't say this is a result of incomplete startup combustion, though. It's usually bad burner setup or a vent problem. I haven't seen carboning with natural gas either.

Like you, I look at commonly accepted facts and practices with a skeptical eye. But sometimes, seeing is believing, and most commonly accepted practices are at some point based on fact. After paying attention to a few hundred boiler installs in varying conditions, you start to get a feel for what's going on as you become aware of causes and effects. Once you see a few glaring examples of the effects of exaggerated short cycling, you start noticing these effects on systems with less exaggerated short-cycling.

hot_rod

Oh it gets worse

if left un-checked. Both of these were still firing when I arrived!! I have replaced dozens of these low temperature application boilers. All had no, or in-adequate return temperature protection.

Remember, most boilers start below their dewpoint at some point. Even cast iron will develop little sweat droplets on the hx at cold start up. This is why it is critical the boiler come up above it's dewpoint before the fire goes out!
Ideally on every cycle!


A recent column in Fuel Oil News suggested the average oil burner starts about 10,000 times each year. That pens out to 27 times per day. (I guessing this would be a tankless coil boiler!)

If that is accurate the 100,000 cycle life would be used up in 10 years or less. Think about a boiler with a 3 minute burner on/ off short cycle! Even if I could be convinced this if fin, to me it's like fingernails on a chalk board. That relentness on off on off drives me nuts

It would be pretty hard to convince me that HSI ignitors, vent dampers, made in Mexico relays (grin), etc do in fact have a 100,000 cycle life! I've got piles of "parts" that show the opposite to be true

Depends if you're selling or repairing the stuff, I suppose


Burn 'em long and burn 'em hot, if you got 'em is my motto. Preferably with water inside them, see below.


hot rod

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Duncan_2

Fingernails on a black board.

Do they still have black boards? [G]

Hope you didn't misunderstand my take on short cycling, hot rod. You know, I just re-read my post above, and that one sentence kinda sounds like I believe a five minute cycle is ideal. That sure isn't what I meant! I meant that was a STARTING POINT, a minimum - that most boilers reach steady state combustion by that mark.

I'm not trying to convince anyone a three minute cycle is fine, hope I'm clear on that. I'd like to see more like ten minutes.

But I think that's more rare than a three minute cycle, from what I've seen - unfortunately.

Really kinda makes an argument for constant circ, doesn't it?