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Which boiler is more efficient?
steamfitter
Member Posts: 161
Someone recently asked me this question:
Which boiler is more efficient: a smaller boiler firing at its full capacity or a larger boiler firing at its lowest capacity? If both were putting out the same amount of heat into the same hypothetical hot water heating system.
After a long pause, I answered with the standard: depends?
I'm thinking it depends on the definition of efficiency (as in AFUE or otherwise). Depends on the details of the actual boilers and it's relation to the piping, etc, etc etc.
After more thought, I leaned towards the larger boiler running at low fire or turndown. Interesting question, I thought.
Any thoughts???
Which boiler is more efficient: a smaller boiler firing at its full capacity or a larger boiler firing at its lowest capacity? If both were putting out the same amount of heat into the same hypothetical hot water heating system.
After a long pause, I answered with the standard: depends?
I'm thinking it depends on the definition of efficiency (as in AFUE or otherwise). Depends on the details of the actual boilers and it's relation to the piping, etc, etc etc.
After more thought, I leaned towards the larger boiler running at low fire or turndown. Interesting question, I thought.
Any thoughts???
0
Comments

One answer is here the ratings for the Peerless WV/WBV boilers. Each model has several firing rates. When you reach the page, click on Product Ratings and the chart will display. The numbers speak for themselves.
http://www.peerlessboilers.com/Products/ResidentialBoilers/SeriesWBVWV™/tabid/118/Default.aspx#dnn_productratings1 

Introducing a different bunch of variables there. Leaving those ou, when I look at @steamfitter 's original question, and use the nifty table @Steamhead came up with, I find that the bigger boiler (in that series anyway!) firing at exactly the same rate as the smaller boiler  therefore the same gross BTU in  has a slightly higher efficiency than the smaller boiler. For example, a WBV03 firing at 0.95 gallons per hour shows an efficiency of 85.6 (AFUE), while the WBV04 firing at the same rate shows 87.0. Your mileage may vary... and I would bet that different boiler series from different manufactures may also vary.HVACNUT said:I'll take the larger boiler with the lower firing rate. Less short cycling and more likely to reach steady state conditions.
When I look at @HVACNUT 's comment  which is also valid  we are introducing the whole vexed set of questions revolving around matching the boiler and firing rate to the load  and that is really external to the boiler design, although I am quite convinced that it makes a much bigger difference in system efficiency than the small changes from the boiler size effect!Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England1 

Interesting comments!
Steamhead, thank you for the Peerless link! The numbers do speak for themselves. I do find it interesting that they are reasonably close to each other.
I agree with Jamie Hall's comments. The question is on the boiler itself. I do wonder if the ratings are consistent with other manufacturers. Very broad question that can easily veer off to ideas about efficiency of boilers being related to the system they're connected to.
Thanks guys!0 

Does this not depend on the boiler? If it is one with outdoor reset that controls the firing rate (typically a modcon), would not the most efficient boiler be the smallest one that could heat the house on the coldest day (which around here reliably goes below the design day temperature by 8F each year for a few hours a year). Because if you use a larger boiler than that, it will short cycle more on the days that are not so cold, such as February, March, and April this year in my part of New Jersey. I am assuming no modcon has enough turndown range for all conditions.0

To answer the direct question initially posted. The one with the smaller HX. Which would be the smaller one that is running 100% vs the one with the bigger HX running at a modulation input matching the other boilers 100% input. Gross input for the two is exactly the same. So it's a matter of how much more energy it takes to heat the boilers bigger HX at the lower modulation.0

Gordy,
That makes sense. When you explain the efficiency difference in reference to input vs. output, the smaller HEX needs less heat energy. Does this mean that the output of the boiler with the smaller HEX is guaranteed to be closer to the input? Having lesser heat loss up the chimney and thru the jacket?
If efficiency relies on the relationship (math) between the gross input vs. gross output, (putting "net output" aside) is it possible for one manufacturer to design a larger boiler with a higher rate of efficiency at 20% modulation, than a smaller boiler running at 100%?
My curiosity continues because of the slight differences in efficiency rates of the Peerless Boiler example that Steamhead posted.The numbers were very close.
I'm wondering if it is possible for the engineering (burner and HEX) of one manufacturer of modulating boilers to be superior to another manufacturer of, let's say, condensing boilers with minimum or no modulation? Will the size of the HEX, regardless of design, always be the determining factor of efficiency?0 
I should say that it is possible for a "bigger" boiler to share same size HX as a smaller one. Many mod/cons share same HX size with different gross inputs. If you look carefully at boiler line technical details you will see this.
So I guess with out some details of boiler sizes in your initial post. Some assumptions can be made.
Smaller 100k fixed gross input boiler 1.
Larger boiler at 100k at a lower modulation of boiler 2. Open ended. 100k at 80%? 50%? 20%?.
What I'm assuming you meant in your initial post is that when you said bigger boiler you mean bigger HX, and higher 100% gross input.0 
Most boiler manufacturers share like HX designs depending on type. Water tube, firetube, three pass, pin. Etc. burners seem to be close to same designs for each type of HX.
Think computers. Most off the shelf manufactors share like components in the box.
0 
Thank you!
Interesting info!0 
Large or small, if they have the same btu potential(fixed), their efficiency should be the same......no?0

Forgetting one aspect to the original question Paul. Same btu input to two different size hx's. If HX size didn't matter all boiler manufacturers would make one big one to accommodate a huge range of output sizes. There has to be a design ratio of HX to burner size that produces the best efficiency
100k heating 2.5 gallon content verses 100k heating 5 gallon content.0 
There has to be a design ratio of HX to burner size that produces the best efficiency
I notice you didn't put a punctuation mark at the end. Was that a question? Forgetting how they determined btu input Gordy? Or how they determined gas or oil's ability to produce btus?0 
Here is some specs on lochinvar mod/cons. I used lochinvar because Their specs make it easy to compare boiler sizes, and the relationship of water content, and HX surface area. Notice their efficiencies are the same across the board as to different sizes, and different designs being KBN (water tube), WH (wall hung fire tube), and KHN ( fire tube 10:1 tdr).
Also notice either water content,or HX surface area , or both change between sizes.
HTP makes it a little more difficult to find that comparison in their
Docs. However their efficiency ratings are all over the board when comparing sizes, and different lines. Like to see their relationship of water content, and surface area. I'll dig a little more.0 
Does that fall within the guidelines of the original question?0

You still have to get the most out of the btus made from the fuel once burned converted into usable energy. Remember the initial question is which if any is more efficient. So that is boiler output btus converted to usable energy.Paul48 said:There has to be a design ratio of HX to burner size that produces the best efficiency
I notice you didn't put a punctuation mark at the end. Was that a question? Forgetting how they determined btu input Gordy? Or how they determined gas or oil's ability to produce btus?
If we are talking input then both situations would be identical no question.
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You've applied much more to the question than was originally asked.0

Not really. "Depends" how much one really wants to dig into it.0

So, which would be more efficient.......a one foot piece of pipe, with 1" flames every half inch along it's length, or a 2 foot piece of pipe, with 1\2" flames every half inch along it's length? His criteria0

Here is the HTP elite line. The closest match to @steamfitter scenario is the EFT 55 with max modulation 55 k 96.3% efficiency compared to the EFT 285 with a low end modulation of 57 k with an efficiency of 95%.
Now the question becomes is the efficiency at max modulation, and minimum modulation with the same return water temp the same. We have read where lower modulation in this scenario is more efficient.0 
I think I have posted enough to digest. There is a direct relationship to HX area, water content , and burner size. Your stuck on gross input. We are talking output. So HX , and burner relationship is key to which is more efficient. Input 100k is 100k no matter if it's 100k at 100%, or 100k at 20,or 10% period...0

I'm going fishingsteamfitter said:
Someone recently asked me this question:
Which boiler is more efficient: a smaller boiler firing at its full capacity or a larger boiler firing at its lowest capacity?If both were putting out the same amount of heat into the same hypothetical hot water heating system.
After a long pause, I answered with the standard: depends?
I'm thinking it depends on the definition of efficiency (as in AFUE or otherwise). Depends on the details of the actual boilers and it's relation to the piping, etc, etc etc.
After more thought, I leaned towards the larger boiler running at low fire or turndown. Interesting question, I thought.
Any thoughts???0 
"We have read where lower modulation in this scenario is more efficient"
I thought you were talking about output. Should I make the font larger?.0 
Why would the initial post even mention efficiency if we were talking about input.......0

"Which boiler is more efficient: a smaller boiler firing at its full capacity or a larger boiler firing at its lowest capacity? If both were putting out the same amount of heat into the same hypothetical hot water heating system"
I'm confused......I thought that constituted both. Firing rated being input and heat being the output.0 
Your not taking into account that the boiler firing at its lowest modulation yet same input as the other boiler at max input has to have a bigger HX. Think about it.
If boiler A is 100k at max firing.
If boiler B is 100k at low fire 20% 5:1 tdr it would be a 500k boiler at high fire. That would have a significantly larger hx in water content and surface area.
Both boilers can not be the same size if both are putting the same amount of btus into the system, and one is firing at max modulation, and the other is firing at lowest modulation.0 
Now back to the original question...Is one more efficient than the other? Same heat applied, same heat coming out.0

i don't know of any boiler that's 100% efficient. "Same heat applied same heat coming out". Now you are rewording the post??0

Your still failing to realize that the two boilers are NOT the same size HX in the real world. I suppose you think both systems will come up to temp at the same time?
0 
Gordy......Two different boilers, same heat applied, same heat out. Rewording what? He framed the question. You applied everything else.0

Okay what ever Paul says.........0

Thank you Gordy for posting ratings and digging into the numbers.
Interesting discussion!
Learning a lot about boilers, design and theory here on heatinghelp.com!0 
Original question is "Which boiler is more efficient: a smaller boiler firing at its full capacity or a larger boiler firing at its lowest capacity? If both were putting out the same amount of heat"
I still think it is a simple question. If we have a smaller boiler is outputting it max btu's at a high fire, say 100K, and now we have a larger boiler outputting 100K but is rated at say 300K the larger boiler would be more efficient as the ratio of btu's to fireside heating surface would be greater. This is assuming all else is equal.
With that said be it a modulating boiler or an oil fired boiler changing fan speed or modulating the inducer we can control the fuel and air. With an atmospheric bgas burners we can control the fuel but really hard to control the air. With ribbon burners and lowering the input you will get to a point where the efficiency just dumps.0 
Let's keep it real life. At the same input on a bigger HX it would take longer to heat the bigger HX, and larger water content. So now you are down to I'm heating .7 gal of water in a .7 gal pot with 100k, or 4.2 gal of water in a 4.2 gal pot with 100k
The efficiency might be 94% for the 100% output on the .7 gal pot.
The efficiency might be 95% for the 20% output on the 4.2 gal pot, 6 times the volume. which will bring the water to temp first?because of the added mass of the HX, and water content of the bigger supposedly more efficient boiler will run longer to heat the water....
Incidentally a boiler with 100k input at 20% modulation would be. A 500k boiler with a 5:1 TDR.
So keeping it as real as possible since to achieve comparing a 100k 100% input boiler to a 100 k at 20% input boiler I'll use the Lochinvar KB 106 with a water content of .7 gal, and a HX surface area of 9.1 SF. Compared to the commercial line knight XL 501 with a water content of 4.2 gal, and an HX surface area of 50.8 SF.
both are water tube HXs and both 5:1 TDR. Both efficiencies are very close.
Not so easy as some would think.........1 
From one perspective it's really pretty simple  if it's condensing, and combustion is at spec, it will run at the rated efficiency regardless of hx size, or modulation stage.
High efficiency is not magic, it's whether or not the inherent heat in water vapor is captured after combustion. If your burner is out of calibration (and you're sending extra warm air and/or unburnt fuel out, efficiency will also go down. But typically the bigger culprit in lost efficiency is failure to condense.
When gas or oil burn, most of what burns is hydrogen and the result is water vapor. As anyone who's ever gotten a steam burn knows, when that steam condenses it releases a lot of heat  that's what a condensing boiler recovers.
Return water temperature is a big driver of hx efficiency. It's why a lot of combi boilers are rated at one output for space heating and a second, higher output for hot water production.
The "return" water for hot water heating is 50 to 70 degrees while the return water for space heating is warmer. In the case of baseboard or radiator heating, return water temps are sometimes close to the boiling point of water with the result that there's no condensation and even at 10% output efficiency is standard (low).0 
I don't agree with @DanInNaperville because of the way heat transfer works.
In a boiler you have the cold side (the water you are heating up) and the hot side (the hot gasses from combustion). In any heat exchanger the temperature difference (actually the log mean temperature difference) that drivers the movement of heat from the hot to the cold sides of the heat exchanger.
So say there is a modulating boiler that does not condense and is operating above the temperature that condensation would take place (not sure one exists  but bear with me) At 100% output the hot side of the heat exchanger must be some temperature above the water temperature to drive the heat transfer and the exhaust gas from the boiler will be at a particular temperature (the lower this temperature  the more efficient the boiler). Now the boiler is turned down to 25% and two things must happen. There will be less fuel and air required to produce the lower output and at the same time the hot side of the boiler must operate at a reduced temperature difference between the output water and the flue gasses. So the flue gasses will be cooler and the hot side of the heat exchanger will be cooler. The lower gas flow from the reduced power input means the gasses have "more time" to interact with the heat exchanger so can transfer more heat and efficiency will increase.
In a condensing boiler the temperature of the hot side of the boiler is everything in terms of condensing efficiency. Reduced power output means lower "hot side" temperature and dramatic improvements in efficiency  have a look at https://www.hpac.com/heating/article/20925496/maximizingsmallboilerefficiency Below the condensing temperature lower output means much greater efficiency.
Just to get this into perspective there is only about 10% difference between a boiler operating on condensing or noncondensing regime. A "more efficient" boiler has a better heat exchanger and this is true of any boiler. It is just that condensing boiler manufacturers have to go further to produce the numbers that sell their product. What really matters is what you connect the boiler to  as @DanInNaperville said
Regard John0 
We often use afue to compare efficiency and it is helpful. We also use combustion efficiency, again helpful.
But in most cases what we are looking for is the equipment that will cost less to operate. Energy Kenetics has done alot of research into afue ratings and this page, https://energykinetics.com/afue/, is what they have found.
We have installed hundreds of System 2000 boilers and an 87% afue System 2000 costs 10% to 20% less to operate than an 87% afue CI boiler.
That being said, my answer to the original question would be the smaller boiler.0
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