When 95% isn't 95%

GW

Does anyone proactively explain how the 95% boilers don't operate at 95% when using the boiler? We leave the little testo slips behind and one customer was asking me the other days why it wasn't saying 95%; i explained it to him and he was fine. My fear is the irrational person that likes to cause trouble.

Out proposals state, "blabla boiler , rated at 95% AFUE"

I know some info is too much info: just curious if anyone else had had a problem with this, or if leaving it alone is best.

Harvey Ramer

Tell them it gets up to that. Depends on the operating conditions.

Noel

AFUE is different from Combustion efficiency. AFUE adds for a vent damper and also for a spark pilot, neither of which adds to combustion efficiency. AFUE assumes the boiler is 70% oversized, which also doesn't effect combustion efficiency.

Your mileage may vary.

Jamie Hall

Another way to look at it is that the rated 95% efficiency is under very specific test conditions, which involve everything from the combustion air temperature and pressures and the exhaust conditions, as well as the return water temperatures and flow rates, and further refers to steady state running. Any variation from those test conditions will affect the efficiency (it's even possible in principle to go slightly higher! But rare...), sometimes a lot. The biggest variable, of course, is return water temperature and flow.

Good luck, though. There are a lot of folks out there who expect their car to get the exact mileage ratings on the window sticker, and you can't get them to believe otherwise...

GW

Yes I think the MPG theory is good, everyone understands that. The 95% is highway driving (low temps), the City rating isn't listed.

If i had a questionable character i would cool off the boiler and print the slip when it's running lower temps.

It's just a small pain, by the time we tune the o2 the temps have climbed a bit; often times the boiler is quite hot by the time we nailed the combustion numbers. Especially if it's summer time.

Noel

AFUE counts savings achieved during the NOT RUNNING part of the duty cycle when the pilot is not lit and the damper is closed. I think people trying to measure AFUE with an analyzer miss that point. It is ANNUAL efficiency, not RUNNING efficiency.

GW

fair enough, yet we get higher numbers on the gas furnaces.

JUGHNE

FAF just about always 65-70 return temp of the fluid.

Zman

All this is true.
Just don't get started on what assumptions the Combustion Analyzer made to come to that conclusion....

RayWohlfarth

The water temperature and the combustion air temperature will vary the combustion efficiency of the boiler. The colder the water, the more efficient the boiler is. I was doing research for a customer and his boiler said, "Up to 98% efficient." Upon reading the manual, the boiler is 98% effiient at lower fire with 80 degree F water. Once the boiler water is around 170 degrees F, the efficieny is about 84-86%.

Mad Dog_2

Perfect lab conditions skewed to IDEAL results. If they wanna pursue it further, refer to manufacturer. Mad Dog

Gordy

To make it more complicated the efficiency is a moving target in respect to return temps.

HVACNUT

> @Gordy said:
> To make it more complicated the efficiency is a moving target in respect to return temps.


Agree.
Also, combustion air temperature has a big part to play.

Has the definition of AFUE changed?
Meaning, originally AFUE only pertained to warm air furnaces.
Now with mod/cons, ODR, buffer tanks, post purge of the appliance, and purging unspent BTU's to the conditioned space or indirect, is the same calculation used, or was it reworked to meet the parameters of modern equipment?

And we all know AFUE and steady state combustion efficiency are two different things.

The only boiler I've seen to consistently match AFUE to combustion efficiency is the Buderus oil fired G125BE (no longer offered in the U.S.). Rated at 90% AFUE, non condensing.
Combustion settings are based solely on a chart to increase/decrease fuel pump pressure according to the incoming (outdoor) combustion air temperature.
Dead on every time, and 0 smoke of course.

But mostly, I think they just slap those yellow stickers on anything.

captainco

AFUE += A FOO EE or Yucky. The most inaccurate rating that can be applied to anything. Yes it is a point system that includes off cycle savings. Unless it is installed outdoors in a room, a ModCom should have no off cycle losses and therefore 100% efficient when not running.

The minimum combustion losses on a ModCon are about 10% which means the maximum efficiency can only be 90%. The hotter the water temperature the lower the efficiency, especially on latent heat recovery. A ModCon operating with a 160 degrees water temperature may have a maximum efficiency around 85%.

Harvey Ramer

I've had several ModCon's that were doing 98% according to my analyzer. They were direct piped to a radiant slab.

captainco

To be 98% efficient the flue temp would have to be less than 80 degrees and the O2 would have to be 0%.

Harvey Ramer

The water temp was around 80 I believe, when the measurements were taken. I don't remember what the exact 02 number was, but it was within manufacturer specs.

captainco

Every % O2 = 1% less efficiency. I am pretty sure the O2 was at least 3% to 4%. Trying to figure out how much latent heat is recovered is not easy. According to some articles, flue gases condense 100% at 77 degrees.

GW

I see that my original post/ inquiry left the door open for some good old fashioned debate.i now see that it was a flawed question.

The real question is this- do you print the combustion report leave it? Inviting yourself to answer questions?

captainco

Some older analyzers don't even calculate efficiencies above 90%. I think the print out is too complicated for a consumer to comprehend. Heck, most of us don't know what they mean.

GW

yes I thinks it shows a bit of professionalism that the print out is there on sight through

captainco

I don't know Gary. If my doctor gave me a copy or print out of my electrocardiogram would that make me feel any better? It at least has accurate information but I don't have a clue what is means.

NYplumber

I simply explain, its like a Prius. Drive it the way it was meant to be driven and it performs. Drive it like a race car and watch the fuel rating plummet. he same goes for gas and diesel vehicles.
A few years back one of the car magazines took a BMW M3 around a race track with a Prius if I recall correctly. The M3 got a better mpg rating for the same mileage due to driving below its full power while the hybrid car got a lower rating then the sports car due to driving it out of its intended conditions.

Mark Eatherton

I always tell my students that if they can delete the percent efficiency number from their combustion tester, to do so. There's no less than 1,000 ways to get to a given percent number. I believe the Testo's allow the percent number to be eliminated. Concentrate on the critical values, and leave it at that. As far as leaving the tale of the tape in the mechanical room, how are you going to know where you are going, if you don't know where you were at??

When I was teaching at RRCC, we took a brand new Munchkin, and ran straight street water (40 degrees F) into the appliance, and for just one second, I saw 99% efficiency pop up on the analyzer... It is attainable, but the parameters to do so are not something you'd see in a residential or commercial setting, except during a cold start as others have noted. Not the "norm".

ME

GW

Very good, I have not had any complaints, but I’m always looking around the corner for the next road block

Jean-David Beyer

My house and modcon with outdoor reset puts 80F water into my slab if the outside temperature is 50F or higher; I assume the return temperature is equal or less than that.. Flue temperature was 123.6F, Inlet 69.6F (warm day last week). So I should be able to get pretty good efficiency at those times. Analyzer said efficiency was 92.1.

But I put a minimum of 120F water in the upstairs baseboards, so then the efficiency will be lower.

NY_Rob

In a retrofit application the homeowner might be fortunate to see even low 90's efficiency (except maybe for a short time during shoulder seasons).
With 15-20BTU/SqFt heatloss you can't keep up with 105F SWT once outdoor temps drop, you need higher supply temps which impacts efficiency. Maybe if all the existing radiators were replaced with Heating Edge or some other low temp radiators you'd get closer... but who does that?

I read recently that new construction codes can bring structure heatloss down to 10BTU's/SqFt. Build that house with Heating Edge radiators and a mod com and you might get mid 90's efficiency even in Jan & Feb.