One Big Boiler Vs. Multiple Boilers

 
Pacowy
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Post by Pacowy » Thu. Jan. 16, 2014 8:49 am

Sting wrote: Plus if you undersize your wet system slightly - it will perform better most of the time
This is a key assumption that I don't think should be accepted without question. "Most of the time" - i.e., most of the spring and fall, parts of the winter and essentially all of the summer (if used for DHW) a hydronic system will be idling, so factors like standby losses favor a smaller system. However, "the rest of the time" is when the vast majority of the coal actually gets burned, and that's when factors like the potential efficiency advantage of a large unit with a big heat exchange area become relevant. IMO conditions that prevail "most of the time" don't necessarily tell you anything meaningful about the efficiency and/or effectiveness a system will achieve when it's actually doing the thing it's there for.

Mike


 
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Post by Sting » Thu. Jan. 16, 2014 9:59 am

If you have a million BTU load such as a church - you no longer set one big boiler - you set three sized to run in concert to carry the load. They run as necessary and the run cost is reduced
you say potato - I say potato - be careful what you think it correct
in any system that is correctly sized to the load - if there are multiple sources of heat energy - they are sized to handle the load such as above
one is never over-sized as you suggest Mike

 
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Post by Pacowy » Thu. Jan. 16, 2014 10:54 am

Sting wrote:If you have a million BTU load such as a church - you no longer set one big boiler - you set three sized to run in concert to carry the load. They run as necessary and the run cost is reduced
you say potato - I say potato - be careful what you think it correct
in any system that is correctly sized to the load - if there are multiple sources of heat energy - they are sized to handle the load such as above
one is never over-sized as you suggest Mike
I don't see how three boilers in a church relates to the OP's situation, or to the efficiency (or effectiveness) considerations faced by a homeowner trying to choose among boilers. The idea that you can compute a load and then buy the smallest boiler that will cover that load sounds great in theory, but I don't think it works very well in practice, particularly for coal equipment. There is a long list of factors that can cause coal equipment to produce less than its rated capability at any given time - batch of bad coal, heat exchange surfaces not cleaned, fuel/air mixture off, restriction of air to control excessive clinkering, etc. . On top of that are the traditional ways that heat load computations may understate the actual boiler output needed to maintain satisfactory heat - oops it's colder than the software assumed; oops the wind blowing hard sucks the heat out of my house faster than the software assumed; oops the software never asked how many people will be trying to take showers in the morning before work/school when the house is trying to come back up to temp from overnight setback; oops the software assumed I'd never want to add to my living area, heat a pool or hottub, or put a modine in the garage. And so on.

If I had a choice between matching a boiler to a computed heat load or looking at bigger boilers, I would always recommend at least considering the bigger boilers. Above and beyond the protection they provide against unforeseen, or at least unplanned, circumstances, a bigger boiler under some circumstances may promote efficiency by virtue of its larger heat exchange surface.

The idea that boilers "are never oversized" is another generalization that just doesn't fit the data. When coal was a major fuel for central heating, the Dead Men made a habit of installing boilers that look "big" by the standard you're trying to apply. While they're not here to explain themselves, you can go through numerous threads on this forum that document the above issues, and provide clarification of why more attention to actual circumstances and less deference to computer outputs are needed to provide satisfactory heat from coal.

Mike

 
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Post by franco b » Thu. Jan. 16, 2014 11:40 am

Pacowy wrote:I don't see how three boilers in a church relates to the OP's situation,
Each boiler can run at its optimal firing rate when needed. With one boiler it is better to size it so that it too is run at its optimal rate most of the time rather than long periods with too low a fire. Obviously needs careful analysis of heat load.

 
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Post by Pacowy » Thu. Jan. 16, 2014 12:20 pm

franco b wrote:
Pacowy wrote:I don't see how three boilers in a church relates to the OP's situation,
Each boiler can run at its optimal firing rate when needed. With one boiler it is better to size it so that it too is run at its optimal rate most of the time rather than long periods with too low a fire. Obviously needs careful analysis of heat load.
The post to which you are replying contained numerous examples of the practical problems - each of which has been referenced or discussed at length in multiple threads on this forum - associated with the approach you are advocating. The theory of your approach "obviously" may be "better", but to me that doesn't mean that it should be preached without reference to the real problems it has been shown to cause for real coal users in some circumstances. One of the benefits of the forum is the way it surfaces and documents real-world experiences of coal users; I don't think it helps anybody - particularly new or potential new coal users - for those experiences to be set aside in favor of an abstract theory. When the theory doesn't fit the data, you need to modify the theory, not ignore the data.

Mike

 
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Post by coalkirk » Thu. Jan. 16, 2014 1:12 pm

My experience is that many coal boilers are under rated. That is they can produce more BTU's than their listed rating. I've got to go with Sting on this one. Over sizing is wastefull most of the time. I want my boiler to run hard during extreme demand, rather be sized to just chug along under those circumstances.

 
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Post by Pacowy » Thu. Jan. 16, 2014 1:47 pm

coalkirk wrote:My experience is that many coal boilers are under rated. That is they can produce more BTU's than their listed rating. I've got to go with Sting on this one. Over sizing is wastefull most of the time. I want my boiler to run hard during extreme demand, rather be sized to just chug along under those circumstances.
I would agree that many of the "old school" ratings were pretty conservative, often due to high assumed pickup factors, but I think the opposite is true of at least some current ratings - e.g., ratings based on firing rates that are not easily sustainable in practice.

I assume that at some point you, Sting and the other proponents of your theory will explain to boiler designers that they don't need to worry about heat exchange surface area, because a little boiler is as effective as a big one. Probably you should also explain to EFM Highboy owners that they were foolish if they paid anything extra relative to a comparable regular 520, since, in the theory you've postulated, the 28+% larger heat exchange area of the Highboy does not translate to greater efficiency in extracting BTU's from combustion gases.

Mike


 
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Post by kstills » Thu. Jan. 16, 2014 3:18 pm

coalkirk wrote:My experience is that many coal boilers are under rated. That is they can produce more BTU's than their listed rating. I've got to go with Sting on this one. Over sizing is wastefull most of the time. I want my boiler to run hard during extreme demand, rather be sized to just chug along under those circumstances.
The biggest issue I see wrt this is that boiler size is not proportional to boiler price, making an oversized boiler the more sensible option.

 
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Post by Pacowy » Thu. Jan. 16, 2014 4:24 pm

kstills wrote: The biggest issue I see wrt this is that boiler size is not proportional to boiler price, making an oversized boiler the more sensible option.
:up: So the bigger heat exchange area that increases the efficiency of heat transfer to the boiler water comes at a lower incremental cost. Hmmm, that is sensible...

Mike

 
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Post by EarthWindandFire » Thu. Jan. 16, 2014 4:28 pm

I think both points of view are correct. A single large boiler is just as effective as two smaller units. The error to be made here is installing too small of a boiler. For practical purposes, a single large (oversized?) boiler may be the right choice in most applications. Personally, I would rather service and maintain a single boiler than several. However, much can be said about redundancy and nothing is more costly than a boiler down for maintenance during cold weather.

 
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Post by kstills » Thu. Jan. 16, 2014 4:31 pm

Pacowy wrote:
kstills wrote: The biggest issue I see wrt this is that boiler size is not proportional to boiler price, making an oversized boiler the more sensible option.
:up: So the bigger heat exchange area that increases the efficiency of heat transfer to the boiler water comes at a lower incremental cost. Hmmm, that is sensible...

Mike
I can be convinced that greater efficiency is realized by more surface area (especially if there is a small fan blowing on the unit......) however, in my circumstance I would prefer that I run 2 smaller units than one larger one.

But that has more to do with the mass of water I'm spinning around in this system, I think the fin and tube guys don't have the same issue that I do.

 
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Post by Pacowy » Thu. Jan. 16, 2014 4:46 pm

Sting wrote:If you have a million BTU load such as a church - you no longer set one big boiler - you set three sized to run in concert to carry the load.
As long as Rob has set this up as a separate thread, I'd note that there is no rule or standard that says "you no longer set one big boiler". While I've heard of multiple boiler arrangements being suggested, and undoubtedly some have been implemented, all of the "big load" situations with which I am personally familiar carry the load with a single unit. These include at least 4 buildings where substantial changes recently were made in the heating system, but a multiple boiler set-up was not installed. One replaced a failed giant stoker boiler with another giant stoker boiler. Another was unhappy with the inadequate output of a "professionally"-sized boiler, and replaced it with a bigger single unit. Multiple boilers likely reduce some heat losses, but I'm not aware of them taking over all of the million BTU loads.

Mike

 
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Post by Pacowy » Thu. Jan. 16, 2014 4:55 pm

EarthWindandFire wrote: The error to be made here is installing too small of a boiler.
Amen. If I'm putting $ into a heating system, I want it to supply BTU's and comfort, and not excuses about why it's not up for the job. If there also are some efficiency benefits, so much the better.

Mike

 
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Post by waldo lemieux » Thu. Jan. 16, 2014 5:49 pm

Ive never gotten a call saying "you know ,I think we could have gotten away with a smaller boiler."
Key word , gotten away with :D

 
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Post by Rob R. » Fri. Jan. 17, 2014 5:04 am

Pacowy wrote:While I've heard of multiple boiler arrangements being suggested, and undoubtedly some have been implemented
Multiple boilers are usually reserved for commercial/industrial applications, but nothing says the logic can't be applied to a large home. I have heard of a pair of EFM 520's heating a large B&B in the Adirondacks. ;) It depends on the load, size of boiler room, budget, goals, etc. It also doesn't have to be two coal units, could be a coal boiler and an oil boiler helping out 4 days per year, etc.

Another thing to consider is that some states require licensed boiler operators (commercial/institutional application) if the boiler size is over a certain number. A friend of mine is an electrician in NJ, and he recently told me about a school he was working in. I don't remember the exact figures, but I think they replaced a single 1M BTU boiler with 4 250k BTU units. A Tekmar control "stages" the boilers and brings them online as necessary...it also rotates the unit that is running 100% so the service hours are evenly distributed. A licensed boiler operator lost his job, but the school saved a lot of money and fuel.

The factory I used to work in had three boilers, and the guys usually ran two regardless of load, that way if one went down it wouldn't be an interruption in steam service. The third was brought online for additional capacity, or to take the load from another that was due for service. By having three boilers they had redundancy, ease of service, and most recently they started running just one boiler in the summer to save energy.


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