condensing AFUE, what water temperature?

Chuckles

The Weil-McLain web site says the Ultra (a condensing boiler) has an AFUE of 92% but is actually up to 96% efficient for low-water-temp applications. This suggests that the official DOE AFUE of the Ultra is not measured at a low temperature. But if it is measured above 130F, shouldn't it be the same as a non-condensing boiler (80's)?

Is there a standard water temperature specified by DOE for AFUE purposes, or do condensing and non-condensing boilers get rated at different temperatures, maybe whatever the manufacturer thinks will give the best AFUE?

Will low temperatures (below 130F?) increase the efficiency above the AFUE as WM seems to be suggesting, or will temperatures above 130F reduce the efficiency below the AFUE as I have read elsewhere?

jim sokolovic

AFUE water temperatures...

The AFUE test can only be run at 120 degree water in / 140 degree water out. At this time, there are no different water temperatures that can be run for this test, whether it is a condensing boiler or not. The program that calculates the AFUE is only valid if you test at these temperatures. Higher water temps would give a higher steady-state efficiency, and lower water temps would give a lower steady-state efficiency, if you wanted to express it that way.

Chuckles

not the opposite?

> Higher water temps would give a

> higher steady-state efficiency...


I thought that for condensing boilers, it would be the opposite because they would not condense.

jim sokolovic

Sorry about that...

Higher water temps would decrease the steady-state efficiency, and lower water temps would increase the steady-state efficiency, as you had the common sense to see. That's what happens when I try to type fast with my one finger /half brain method. No more computer til Monday for me, thank you!

Mike T., Swampeast MO

AFUE

As far as I can tell the US Dept. of Energy administers the program with "how to do the test" coming from ASHRAE. Why don't they just drop the pretext and name themselves ASFHRAE for "American Society of FORCED-AIR Heating, Refrigeration, Air-Conditioning Engineers"?

The formulas are EXCEPTIONALLY complex as they try to determine efficiency of a DIGITAL (on-off only) heating plant providing heat in the real world where things are anything BUT digital...

One general problem with the rating of hydronic systems is that of distribution losses. Since AFUE is about the appliance itself, forced air receives a "bonus" in the "number" as if distribution loss is at all considered, it's with idealized ductwork. Those making forced air systems will claim that ductwork can be made that is very non-leaky and systems designed such that they don't introduce pressurization problems in the space but from what I've seen (old AND new) EXTREMELY few forced air systems achieve this. True hydronic systems on the other hand are inherently non-leaky and are incapable of introducing pressurization problems.

Strike one against hydronic systems regarding the "number". Less of that "highly efficient" hot air ever makes it to where it is supposed to be, so even a standard boiler with a lower "number" can be just as or even more efficient when placed into an operating system.

Conventional boiler efficiency as a function of supply/return temps is beyond me but in general I believe that conventional boilers have greater combustion efficiency as supply and return temps rise. There's really nothing similar to this (adjustment of supply/return temps)in the forced air furnace world, so (I suppose to avoid the possibility of "inflating" the "number" for boilers) conventional boilers seem to be tested at temperatures at which they aren't particularly efficient.

Strike two against hydronic systems regarding the "number". Good (and long-standing) design practice will allow a conventional boiler to be operating in its most efficient range most of the time.

Modulating boilers REALLY get shafted! The forced air mfgrs started complaining that the "number" wasn't high enough for their multi-stage equipment--often showing such to be less efficient than a conventional forced air furnace. Of course ASHRAE payed attention to this. So...they lumped multi-stage furnaces and modulating boilers into the same category. While the author of the changes certainly seems to know his calculus he doesn't seem to understand diddly about true proportional control. He often called two-stage furnaces "modulating"--HAH! Even a PROPERLY sized forced air furnace (sized via Manual "J"--also from ASHRAE) everything is assumed to be so oversized that a two-stage furnace is expected to be operating at low fire around 94% of the time! So...that poor TRULY modulating boiler is STILL assumed to be operating at a nearly constant rate of relatively low fire the vast majority of the time--it's not even allowed to be rated when doing what it is supposed to do--vary its fire based both on outdoor conditions AND desired comfort level indoors!

Strike three against hydronics concerning the "number". If (and likely when) forced air mfgrs. manage to make a truly modulating furnace I can nearly promise that the rules regarding the "number" will change!

Actual users of proportional boilers are reporting some extraordinary efficiencies--supported by reduction in fuel consumption.

Is is any wonder that proportional boiler mfgrs say something like, "The AFUE 'number' is X, but in actuality expect something more around Y!"

Steve Ebels

Here's an example

This is taken from Viessmann's tech sheet on their Vertomat commercial size condensing boiler.

Supply Return Efficiency

104* 86* 95.3%

158* 122* 90.4%

176* 140* 87%

Lower return water temps = higher efficiency

mph

Efficiency

I believe the efficiencies are higher with lower water temps because the higher the delta t between the water and flue gases the more energy is transfered.

Jeff

Mike T., Swampeast MO

Talking about the AFUE \"number\"

But not all are designed to produce flue gasses at temperatures below their dewpoint. It's when you start extracting latent heat from the water in the exhaust that you gain efficiency unavailable in other heating appliances.

That's why the "number" is limited to around 85 for non-condensing appliances.