Boiler Size Reason?

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Old 02-19-14, 11:19 PM
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Boiler Size Reason?

Just to make sure I have it right...

Whether your boiler is correctly sized or oversized, it supplies water at the same temp. The reason not to have an oversized boiler is because you'll be using the larger firing rate (whether it be oil or gas) to maintain the same temp as you could with a smaller, correctly sized boiler, which is costing you money in fuel. Right?

Is there another reason?

I've seen some talk about this subject recently and just wanted to make sure I knew the facts.

Thanks.
 
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Old 02-20-14, 01:26 AM
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An oversized boiler will "short cycle" which means the burner will come on for a very short period of time, could be less than a minute or a few minutes, go off for a short time cycle and then repeat as long as the room thermostat is calling for heat. This is hard on the equipment and also does not allow the flue gases to reach ultimate temperatures. With the flue gases running cold there will be more fireside corrosion in the boiler which will ultimately reduce its service life, maybe a significant reduction in service life.

In addition there is the additional cost of the oversized boiler and the increased fuel costs from inefficient operation.
 
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Old 02-20-14, 05:24 AM
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What causes the oversized boiler to short cycle? With the same setup (number of circulators, etc.) as the correct boiler, won't it run for the same amount of time?
 
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Old 02-20-14, 05:39 AM
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Bigger boiler will heat the water faster. This is the short cycling. short cycling will use more fuel plus as stated hard on the equipment.
 
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Old 02-20-14, 06:07 AM
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So if a boiler runs for the whole time that the thermostat is asking for heat, can we say that the boiler is not that oversized?
 
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Old 02-20-14, 06:07 AM
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Think of an oversized boiler as a new (small) chevy Nova, with a older chevy nova (ss) big block in it.
It will get up to speed real fast, but if your on and off the throttle a lot your going to burn a ton of fuel and it will be hard to regulate your speed.
Since a boiler (old cast iron) typically is either on or off, and cannot modulate as you do with the gas pedal in your car, it's either full power of nothing.
Once the water temperature is up to the operating limit, the boiler will start to cycle on and off. Its the on / off cycles that cost fuel, and rein equiptment.
Boilers are rated in % efficency when they are firing.
When a boiler isn't firing, is losing heat. It essentially becomes a big rad with a huge chimney on it.
All this short cycling is doing is heating up the core, then putting some heat into the water, then using the chimney to vent and cool core. Even with a vent damper it will still lose heat, either up the chimney or into the room...
We all know about nice warm boiler rooms, thats misplaced money in my books.
 
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Old 02-20-14, 06:11 AM
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if a boiler runs for the whole time that the thermostat is asking for heat, can we say that the boiler is not that oversized?
I'd say that's a pretty good indicator.

Remember though that with the exception of modulating boilers, ALL boilers are oversized about 99% of the heating season anyway.
 
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Old 02-20-14, 06:49 AM
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[ The impact of boiler size vs. heating load on dynamic efficiency ]
Dynamic efficiency provides a much more accurate indication of energy efficiency as it accounts for the varying nature of the heating load. Specifically, dynamic efficiency is the ratio of useful output to total input at a specific load condition. Dynamic efficiency includes the effects of stand-by losses, and the variation of steady state thermal efficiency vs. firing rate. With exception of on-site seasonal efficiency metering, dynamic efficiency provides the single most accurate method of determining the energy efficiency of a system.
. . .
The dynamic efficiency advantage of modulation is graphical[y] shown below in Graph 4 which compares the dynamic efficiency of modulation to each of the other three firing configurations.
As shown, the primary energy efficiency advantage of modulation occurs at the low fire rates associated with light heating loads. It is appropriate to point out that a properly sized boiler system rarely operates at its design load. Typically a boiler system operates at less than 40% firing rate more than 60% of the time. Furthermore, if the boiler is even moderately oversized, the percentage of time the boiler spends at low fire will increase exponentially. . . Advantages of Modulation Raypak
. . .Boiler “short cycling” occurs when an oversized boiler quickly satisfies process or
space heating demands, and then shuts down until heat is again required. . .
Minimize Boiler Short Cycling Losses U.S. DOE Steam Tip Sheet #16 [PDF] Dec. 2000
Minimize Boiler Short Cycling Losses U.S. DOE Steam Tip Sheet #16 [PDF] eere.energy.gov Jan. 2012
". . . There are two problems with short cycling, one mechanical, one economic.

The mechanical problem comes from the effects of rapid cycling on boiler components. The burner material, for instance, rapidly heats and cools, and sometimes cannot run long enough to dry out. This can create stress and corrosion failures. Gas valves can see decades of use in a few short months. There also tend to be nuisance shutdowns and unexplained flame failures with flame programmer fault codes that have no easily identifiable cause. If you want to make a thirty year boiler fail in five years, and drive you nuts in the interim, short-cycle it.
. . .
The Load Profile
The facts are these: design heating loads almost never occur, and boilers spend nearly all of their operating hours only partially loaded. . . A boiler of a given input must do its work on a minimum volume of water.
. . .
Whatever the minimum incremental energy input of a boiler, one thing must happen if the boiler is not to short-cycle: the heat must be carried away from the boiler fast enough to keep the operating limit from tripping off. . . .
Short Cycle Prevention For Double-Digit Savings (Part I)[pdf] Patterson-Kelley
[ If you don't measure the load you don't know the load. Measure First! ]
Abstract: This is part one of a series of shameless “White Papers” on boiler plant design. . .

. . . In part 1 of this series we wish to make the case for a better understanding of a building's load for comfort heating hot water boilers. We intend to show that current methodology is inadequate for determining the load for an existing structure that requires a retrofit of the boiler room. We will show how current methods can lead to increased installed costs. Further, we will show that there is no reason at all to
make a mistake in boiler plant sizing for plant retrofits. Last, we will present TPC's solution to this largely unknown problem. . . Boiler Plant Sizing for Maximum Efficiency at Minimum First Costs: Measure First! [PDF] Thermodynamic Process Control (LLC) March 2012
[ Page 12 Figure 20. "System 3. A multiple-boiler system piped as primary/secondary" : Each boiler 25% of design load ]
". . . we’d like you to consider an independent study which the National Bureau of Standards conducted in 1988. The NBS compared these systems and came up with some interesting results . . ." The Laars Heating Systems Approach to Primary/Secondary and Multiple-Boiler Systems [PDF]
[ Oh, you want more? ]
-- Preface --
Because of escalating fuel prices, it has become far more important than ever before for the heating industry
. . . to provide building owners with the greatest possible amount of useful heat from the fuel burned. The purpose of this of this publication is to make the consulting engineer, the installing contractor and the owner more aware of the factors that affect the performance and operating costs of heating systems using low-pressure steam or hot-water boilers . . .
Energy Management with Commercial Hydronic Heating Systems [PDF] - Weil-McLain
 
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Old 02-20-14, 07:38 AM
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When a boiler is oversized it (the boiler) maintains temperature for the duration of the heat call. BUT, the burner only has to run in short spurts to maintain that temperature. If the boiler is correctly sized the burner will run for fewer but longer cycles. The previous posts explain the benefits of that scenario.

How is the correct size determined? Only either through a heat loss survey or through fuel use of the existing boiler.
 
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Old 02-20-14, 08:58 AM
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I don't want to get too off-topic, but doesn't the fact that the circulator is on make the boiler turn on, and won't it run until the temp hits its high limit?

If not, how does the boiler know to turn on?
 
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Old 02-20-14, 09:16 AM
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Yes, that's correct. By having an oversized boiler it is making heat faster than the system is using it. This is what causes the sort cycling. Why use a burner that uses 1.5 GPM when .75 will do the same job. Just as a car gets better milage on the highway than in town. That's because of the stopping & starting.

You wouldn't fill a 10 qt. pot to cook 1 ear of corn. It's all the same thing.Boilers, burners and pumps are all made to run, not to cycle on & off. That's what shortens the life of the equipment and raises the operating cost of the system.
 
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Old 02-20-14, 09:24 AM
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Got it... thanks for the info!
 
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Old 02-20-14, 03:45 PM
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Question

Maybe I'm off my rocker, but is it possible to change the nozzle to reduce the GPH rate that the boiler is firing at to slow the heating rate of the water, resulting in longer burn cycles? Or would this cause other problems or issues...
 
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Old 02-20-14, 03:47 PM
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I like to use the analogy of docking a boat. Imagine if the motor only has an on off switch. Would you want 5hp or 500hp if there wasn't much current? Slow and steady wins the race.
 
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Old 02-20-14, 03:55 PM
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is it possible to change the nozzle to reduce the GPH rate that the boiler is firing
It is, but to a limited degree. Maybe 10% could be 'gotten away with'. If your firing rate is twice what you need, it would be foolish to put half the nozzle size in. Lots of reasons why...
 
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Old 02-20-14, 04:05 PM
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My System 2000 EK-1 uses a .75 nozzle, could I go to a .65 without issues?

Input (gal/hr), Nozzle @ pump/PSI, Output (BTU/hr)
0.68, .60@130, 83,000
0.74, .65@130, 90,000
0.85, .75@130, 104,000
1.00, .85@136, 121,000

I am guessing heating time or recovery time for the indirect domestic hot water would take a bit longer if I changed to a .65 nozzle.

Any other issues NJ?
 
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Old 02-20-14, 04:57 PM
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I don't know the System 2000, but as 'finely tuned' as they are I would not recommend it... could be wrong though.
 
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Old 02-20-14, 05:02 PM
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ThomasDB,

A: EK-1 has the lowest standby losses in the industry (0.15% vs. 2.5-3.0% Cast Iron Sectional.) You will not save any money by going with a smaller burner-jet. No return on investment

Q: Is the burner short cycling, < 4 minutes ON time?
 
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Old 02-20-14, 05:14 PM
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Generally the thermostat calls the boiler in, and the boiler will control the circulator
 
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Old 02-20-14, 06:11 PM
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HeatWorm,
No issues with short cycling. System usually runs each time longer than 4 minutes.
Everything works just fine, although this thread got me thinking that using a smaller jet could reduce wear and tear, and possibly save another gallon of oil each year
 
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Old 02-21-14, 06:29 AM
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Thomas, the .75 nozzle is the smallest recommended if the system is chimney vented due to stack temperature issues with the lower firing rate. If the system is power vented you can use .65.

Even if the firing rate is more than needed to heat the home, the post-purge feature negates any standby loss encountered in conventional systems.
 
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Old 02-21-14, 08:13 AM
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I know someone who is going from oil to propane in the spring (I wouldn't do this... can't imagine he'll be saving money with propane). But I asked him if he did a heat loss, he said no. I asked how he chose his boiler size, he said "it's plenty big". I also spoke to someone else who recently had a gas boiler replaced, and same thing, no heat loss done, they just made sure the boiler was 'big enough'.
 
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Old 02-21-14, 08:28 AM
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That's pretty sad Slade... oh well, it's their money I guess!
 
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Old 02-21-14, 03:52 PM
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Thanks HeatPro. I will stay with the .75 nozzle. I do have a chimney vented system. Why fix what ain't broke ( is that one of NJ's sayings?)
 
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Old 02-22-14, 08:02 AM
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Boiler Sizing
Boiler sizing should always be done by a heat loss, blower door test, K factor or some other type of boiler sizing program. The idea with proper size boilers is to keep the boiler running longer without short cycling. Short cycling could be a very short run time or a very short off time. Once you have a heat loss number the amount of BTUs required for the home, you next have to choose a boiler. Choose the boiler from the DOE output rated for the boiler of your choice. You will more than likely end up with a The lost size between the two boiler sizes. I will normally use the 50% idea. This means that if the heat loss is halfway between two Boiler sizes I will drop down to the smaller size and of course if I am in the upper half between the border sizes I will go to the larger size. If I have more than one thermostat in the house I will normally always drop to the smaller size due to the heat loss having a buffer Broden and also extra zones allowing you to drop the boiler size.
Flow
Cycling can also be a problem due to flow rates. A boiler always has a minimum and maximum flow rate and we need to be within that flow right all the time. A cast-iron boiler would have a maximum flow rate of a 20° delta T at a minimum floor rate of a 40° Delta T. I talked to many contractors that use either balls all valves or all circulators and this could be an improper application in some situations. Circulator sizing in a large water volume systems and your near boiler piping could also be a problem. As more more people decide to choose properly sized equipment we have to pay more attention to zone valves, circulators and even the possibility of a verbal speed circulator. For instance, I am installing a US Boiler series 202X for a friend. I have chosen to use zone valves with a verbal speed circulator. This boiler has a DOE output of 42,000 BTUs. The system has three zones. If I used 3 pumps there's a possibility of having 12 gallons a minute flow through this boiler. Since most applications have three-quarter inch pipe and the circulators used could be a Taco 007 or a Grundfos 15-58 3 speed this would cause excessive flow and the boiler. The excessive flow would reduce the temperature rise in the boiler to a level that is excessive for the boiler. Above I have mentioned the Delta T required through the boiler, to achieve this you divide the DOT output of the boiler by 10,000. That would be the maximum flow through the cast-iron boiler. The minimum flow would be one half of that. For instance, the US boiler series 202X has a 42,000 BTU DOE output. If we if we divide 42,000 x 10,000 the maximum flow rate in the boiler would be 4.2 gallons minutes. Half of that would be 2.1 gallons. By using three set speed circulators this could give us again a maximum 12 gallon flow that would mean that each gallon with it last week you sent for exceeding the maximum heart rate. I have chosen to use zone valves and a variable speed pump which happens to be a Grundfos Alpha pump. The pump will Vary it's Speed according to the flow required in the system. As the zone valves close the pump will slow down and vice versa. This will help keep the flow closer to the requirement of the boiler. This was made easier by using this boiler as the Alpha pump is a choice offered without an up charge. Since the boiler control uses pre-purge I could take advantage of that and does also offer post purge but I cannot use that due to using zone valves.
If I chose to use separate circulators on this job I would have had to pipe it primary secondary to keep the flow of the boiler in the proper range. When I do this I would normally put shut off valves on the boiler piping and reduce the floor to the border to keep my 20 to 40° delta T. The flow rate is different in the system due to the three circulators and they will not affect the boiler flow rate because of the closely spaced tee's.
 
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