How accurate are the efficiency percentages claimed by the mod con boiler mfr's?

john123

Does anyone know how these 94%/95%/98%? boiler efficiencies are measured.
I am guessing it is measured under ideal conditions with a perfectly tuned boiler.

How do you think they could so it would give the best result?

I'm thinking they could perhaps run a stream of super low temp water as the return stream at a fixed temp and measure the delta t with the boiler running at full power?? But I keep hearing about 140F with a 20F delta; why does this 140/20 seem to be a usual suggested setting?

Is this something we could check ourselves easily?

Should be taking a point or two off this 95% when sizing a boiler for real world conditions?

pecmsg

THats measured and calculated under ideal conditions.

Like MPG in your car, actual results may vary.

GGross

Interesting to note that most cast iron boilers are tested under conditions that would destroy them. I have seen Viessmann's test conditions, and they are accurate, that is why they are generally under the top efficiency brands by a point or two. 95% is really where they are at when tuned well and condensing

Any condensing boiler will lose efficiency when it is not condensing though, and there is always something to be said about the overall accuracy of how we calculate efficiency. I'm sure that will come up in this thread.

Big Ed_4

It depends on the system running temperature ... A condensing boiler is designed to run at low temperature which delivers the higher efficiency . Radiant is perfect running around 120* where baseboard 160*-200*

Jamie Hall

As @pecmsg said 'your mileage may vary". The figures published are accurate, or reasonably so, under the conditions specified by the regulatory agencies. Like fuel mileage, those conditions are somewhat arbitrary and, in the real world, quite unrealistic.

Not only will the actual boiler performance vary from the published (the actual efficiency will never exceed, and may well fall as much as 10 to 15 percent short), but what you really have to concern yourself with is the overall system efficiency -- which just doesn't seem to be talked about much.

hot_rod

The return water temperature drives the boilers efficiency, they can and do reach high 90% efficiencies

Cycle efficiency and system efficiency is another discussion

Jamie Hall

hot_rod said:

The return water temperature drives the boilers efficiency, they can and do reach high 90% efficiencies

Cycle efficiency and system efficiency is another discussion

Oh quite correct. If the system which they are powering consistently has return water below 140 F, they will reach their advertised efficiency. The effect of return water temperature isn't really quite linear, but as a ballpark the efficiency will drop around 2.5 percent for every 10 degrees F the return water temperature rises above 140.

And that's why I emphasize overall system configuration. If they are powering a hot water heating system which operates at low temperature, they can achieve the advertised or close to it.

john123

@ggross: thanks for your comment; can you tell us how Viessmann did it with their test conditions?

john123

@pecmsg: thanks for your comment; that's what I think too. I don't think I have ever seen my car mileage even close to the advertised mileage. Probably the mfr is using a brand new boiler and if it doesn't give them the result they want, they are going to try another until they get the result they want. What about a boiler that's been used for one season let alone 10 seasons(half way through the life cycle)? Won't the efficiency go down (even a little) as the the little spaces in the heat exchanger get filled up with debris?

GGross

For the AFUE test its all a standard testing method that everyone follows, these are the "laboratory conditions" that you often hear about that don't occur too often in a real world setting. AFUE has the boilers run on a fixed delta T for example, something that rarely if ever happens in the real world. Posted AFUE numbers all have to come from that lab test

Internally they test based on closer to real world conditions, with boilers connected to different emitters, running under "normal" conditions (actually heating a space, allowing the burner control to function as intended etc). This has always been the basis of their boiler design, running efficiently under real world conditions, that's why they came up with Lambda pro, and their own proprietary burner controls and programming, the widely available controls/etc don't work as well outside of lab settings as they want them to. they were always more concerned that the boiler would be efficient under less than ideal conditions than having the highest AFUE number.

Hot_water_fan

First, we must acknowledge that all tests are imperfect. But there’s value in a standardized approach, even if we complain about real world results. 

The key is low return water temps and if feasible, low standby losses by running long hours. Do that, and you’ll be fine and you can forget all about the number. 

GroundUp

They have their given conditions that each unit is tested with. Measuring the fuel input compared to the BTU output, along with the flue temp and several other factors using a combustion analyzer, the efficiency can be determined. For perspective, 2 days ago I started up a new Lochinvar Noble boiler in a pole building with a radiant slab. Now this boiler is rated at 95% AFUE, but what follows are the efficiencies calculated with my combustion analyzer based on temperatures and firing rate.

10% firing rate with 56 degree supply temp- 99.6%
10% firing rate with 102 degree supply temp- 98.7%

100% firing rate with 59 degree supply temp- 96.1%
100% firing rate with 116 degree supply temp- 93.7%

Based on past boilers of the same make and model, it's not uncommon to get down into the low 90s or even high 80s when running a 180 degree supply temp with a tight delta.

hot_rod

Don’t get too hung up on the delta. If a boiler is running 40 f supply, 35f return it will be running near peak efficiency.

160 swt at an acceptable 40 delta would get you condensing also.
The key is return temperature below the fuels dewpoint allowing for it to sweat, condense.
Some big Viessmanns running up on the north slope at an oil production facility running low temperatures for freeze  protection, putting out gallons of condensate per hour.

A good visual indication of efficiency is how much water is peeing out of the condensate drain tube. Assuming it is a condensing boiler😉

john123

@GroundUp: can you measure the efficiency by just analyzing the combustion?

pedmec

the DOE test standards are done with very low return temperature water. i have seen the testing procedures and i believe the test are done with 65 degree return water as the standard return temperature. i could be wrong on the temp but i know it is very low.

GroundUp

john123 said:

@GroundUp: can you measure the efficiency by just analyzing the combustion?

The analyzer spits out an efficiency number based on what it calculates from the flue gases. Is it 100% accurate? Probably not. But it's close. The only way to be exact is measuring the fuel volume and calculating it against the BTU output

GGross

Here are some quotes from the DOE document I could find, it is 208 pages, might not be current but is at least from 2016 or so. there was a lot of discussion, and request for comment on return water temperatures. Some manufacturers wanted to test multiple return temps, others don't . starts on page 80, it might be one of the most boring documents I have ever seen, so of course I enjoy it
https://www.energy.gov/sites/default/files/2023-02/consumer-boilers-tp-fr.pdf

. Return Water Temperature
The test procedure at appendix N currently requires a nominal return water
temperature (“RWT”) of 120 °F to 124 °F for non-condensing boilers and 120 °F ± 2 °F
for condensing boilers (see section 7 of appendix N and sections 8.4.2.3 and 8.4.2.3.2 of
ANSI/ASHRAE 103-1993, which are incorporated by reference)

In response, the CA IOUs reiterated their request for DOE to review whether the
120 °F RWT requirement is appropriately representative of real-world operating
conditions. (CA IOUs, No. 20 at p. 2)

BWC supported DOE’s tentative conclusion of including the single return water
temperature specified in ANSI/ASHRAE 103-2017 for ease of comparison between
models and manufacturers.

A.O. Smith commented that the current return water temperature is representative
of an average value for the wide range of operating temperatures in the field and
indicated that requiring testing to multiple conditions may require adjustment of the
standards. A.O. Smith added that non-condensing boilers are more likely to be installed
in systems with higher supply and return water temperatures, and condensing boilers are
more likely to be installed in systems with lower temperatures. (A.O. Smith, No. 24 at p.
3–4) A.O. Smith stated that testing at multiple water temperatures would add testing
burden to a test that could already span two days to complete, and that the burden to
81
retest and rerate products would also include updating heat output ratings and safety
certifications. (Id.

NYSERDA noted that return water temperature has a significant impact on boiler
performance and urged DOE to incorporate return water temperatures that more
accurately reflect real-world conditions. NYSERDA stated that the 120 °F return water
temperature is too low, does not represent the boiler running conditions according to a
research study done by The Electric and Gas Program Administrators of Massachusetts

Jamie Hall

A very brief comment on these standard tests (whether boiler efficiency or your latest set of wheels). They are done under standard conditions -- as stated. Everybody plays by the same rules. They are, therefore, a good way to compare different models/designs/what have you. They are a miserable way to predict real world performance, and should never be used for that.

Zman

It all depends on the return water temp to the boiler. This is a huge gap in understanding in the energy code. Most codes in my area are now requiring a high efficiency boilers, yet none of them are requiring the low temp emitters that would be required to achieve high efficiency. Unless you make a very deliberate effort to design your system to compliment a high efficiency boiler, you probably wont do better than 90%.
https://www.mge.com/getattachment/Saving-Energy/For-Businesses/Tips-to-control-your-energy-use/Modulating-Condensing-Boilers/non-condensing-(1).jpg.aspx?lang=en-US&width=432&height=416&ext=.jpg

hot_rod

The combustion analyzer just tells you combustion efficiency. Next the energy needs to transfer into the water. Fouled boiler heat exchangers can limit that efficiency, which the analyzer is not reading 

Then distribution efficiency 

At days end the building efficiency drives fuel costs, regardless of how efficient the boiler and distribution is

john123

@hot rod: I am guessing most people will not pay a top price to have their mod con boiler properly cleaned every year. If that were true and people pay for the cleaning say --every few years, what would you estimate the average loss of efficiency due to the fouling of the hx, after 10 years (of a 20 year lifespan)?

ChrisJ

hot_rod said:

The combustion analyzer just tells you combustion efficiency. Next the energy needs to transfer into the water. Fouled boiler heat exchangers can limit that efficiency, which the analyzer is not reading 

Then distribution efficiency 

At days end the building efficiency drives fuel costs, regardless of how efficient the boiler and distribution is

I thought the way they calculate efficiency is by taking the theoretical flame temp and then subtracting the flue temp?

That should show a dirty heat exchanger no?

exqheat

At what temperature in a mod con does condesation stop. 127,140 ?

john123

@exqheat :see above by @zman; chart shows condensing stops at the dew point.
Question: does the dew point change dependant on the outside weather at all?

Jamie Hall

john123 said:

@exqheat :see above by @zman; chart shows condensing stops at the dew point.
Question: does the dew point change dependant on the outside weather at all?

No. The dewpoint of the exhaust gas -- unless it is diluted by a draught hood or something -- is a function of the fuel being burned, not the intake air temperature.

dronic123

@Jamie Hall : thanks for your comment (rookie question I guess)

Jamie Hall

dronic123 said:

@Jamie Hall : thanks for your comment (rookie question I guess)

Not at all. Good question, and not obvious.

exqheat

So? If a system radiation needs more than 127 degrees water temperature in the radiators, it would seem that there is no condensation of exhaust gases. This being the case, why recommend a mod con boiler?

Hot_water_fan

Binary thinking trips us up so much. Does it NEED 127F+ return temps 100% of the winter? Or 25%? Or can the supply always be high yet we use a high delta T to bring the RWT down? 

hot_rod

exqheat said:

So? If a system radiation needs more than 127 degrees water temperature in the radiators, it would seem that there is no condensation of exhaust gases. This being the case, why recommend a mod con boiler?

It's the return temperature at the boiler that determines how much it condenses.
The dewpoint of the fuel depends some want on the CO2, which is what the combustion analyzer reads to come up with efficiency, and the setting that the boiler manufacturer wants.

My Lochinvar asks for CO2 9- 10.5% on high fire NG, 8.8- 9.5% on low fire as an example.

Also some Viessmann slides explaining condensing mode.

My Lochinvar Knight allows up to a 35∆, so to obtain a 120 condensing temperature I could run a 155° SWT, return is at 120 and condensing starts.