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How do you calculate Water Heater Recovery Efficiency?

I am having trouble calculating water heater recovery efficiency.
Specifically, I do not know how to calculate or determine RE for a 90 or 95% Thermal Efficiency gas storage tank water heater, examples Vertex 100k unit or State GP 650 76k or 100k unit.
Since these units are over 75k BTU input, they are classified as commercial water heaters, do not have energy factors specified by law, and I am now simply trying to learn how to calculate their recovery efficiency RE.
Any help would be greatly appreciated.

Asked by Andrew Dunn
Posted Sat, 11/05/2011 - 18:23


8 Answers

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I'm confused by your question. Do you want to know the recovery rate or the water heating efficiency?

Answered by Martin Holladay, GBA Advisor
Posted Mon, 11/07/2011 - 09:52

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I am sorry you got confused.
This was my exact question, how to calculate "recovery efficiency" for residential water heaters because we are often required to list a units 2 specs, energy factor and "recovery efficiency" for residential water heaters.
Commercial water heaters are another story.
Keep in mind that the issue is that the "recovery efficiency" criteria is typically not listed, and this is why I want to learn about how it is calculated.
Recovery efficiency is also different than thermal efficiency to my knowledge.

Answered by Andrew Dunn
Posted Mon, 11/07/2011 - 11:45

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Thanks, Andrew. I guess I just displayed my ignorance.

I just did some Googling, and learned that "Recovery Efficiency ... is the energy-to-hot-water conversion ratio. For an electric water heater, the RE is 100%. For gas water heaters the conversion rate is typically 76-78%, with expensive high efficiency units reaching 94%. This is because some energy must be left in the combustion byproducts to assure good venting."

I'm not sure what the standardized test procedure is. Probably there is an ASTM procedure. Anybody have more info?

Answered by Martin Holladay, GBA Advisor
Posted Mon, 11/07/2011 - 12:34

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There may well be an ASTM procedure, and if there is, this is the logic it would follow. You'd need the gas (natural or LP) composition, plus combustion air temperature and temperature and CO2 content of the flue gas. From the heats of formation of the gas components, and assuming 100% oxidation to CO2 and H2O, you have the gross heat of combustion. From the reaction stoichiometry you calculate the percent CO2 at zero percent excess air, and from the actual CO2 content of the flue gas you calculate the excess air that gives that dilution. These calculations give you the composition of the flue gas. Simple heat balance, from inlet gas and air to flue gas at its temperature, via reaction, yields the heat removed from the flue gas, being what is absorbed by the water. Dividing heat absorbed by net heat of reaction at the standard state, gives the efficiency.

Obviously, as you absorb more of the heat of combustion out of the flue gas, increasing the efficiency, you get a cooler flue gas, and possibly condense some of the water vapor produced by combustion. Flue gas temperature and efficiency are directly related for any fuel gas.

For simple field calculations, assuming "typical" fuel gas composition and percent excess air for properly adjusted burners, the efficiency can be estimated fairly well from the flue gas temperature. The usual way is by adding or subtracting a percentage from a reference point for each so many degrees from the reference temperature. There would be a different ratio for each type of fuel. Unfortunately, I can't give any numbers, as this is not my field.

Answered by Dick Russell
Posted Tue, 11/08/2011 - 11:36

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This just in from AO Smith and Technical Support regarding the Model: Vertex - GDHE-50-NG:

This water heater is tested by AHRI as a commercial water heater because of it's input BTU rating. That is why they did not(and will not) issue the water heater an energy factory. Because of this, the heat loss of the insulation is measured in BTU/HR. The recovery efficiency of this water heater is 96%.
*This confirmed one of my questions, Recovery Efficiency here is approximate or equal to the unit's thermal efficiency.

Also from them:
I have attached at GAMA certificate for this water heater. This certificate is from the AHRI web page.

Thanks everyone for the input, and hopefully this can be useful in the future also...

Answered by Andrew Dunn
Posted Tue, 11/08/2011 - 14:55

Helpful? 0

Andrew, as you discovered, TE and RE (aka, steady state efficiency) are more or less equivalent. But EF is what you're really after. Consider that a high quality conventional non-condensing water heater has a thermal efficiency of about 80% and an EF of about 62%. It would therefore be reasonable to assume the Vertex might have an EF in the mid-70's were it actually rated. BTW, insulation (jacket) loss is not the only factor that distinguishes EF from RE. The largest difference between the two ratings is heat lost up the flue during off-cycles.

The procedure for establishing EF ratings assumes a given average ambient temperature, entering water temperature, hot water usage, and duty cycle. The true EF therefore depends greatly on actual operating conditions. So in reality, EF is just an estimate in any case. Of course this doesn't help us jump through program hoops. And you're certainly not the first person to encounter this issue.

Answered by David Butler
Posted Thu, 11/10/2011 - 02:58

Helpful? 0

Thanks for your input and detail.
Fortunately, I already understood Energy Factor and that it's calculation and test procedures are at least clearly defined in many resources.
Since it was not and can't be defined (EF) for the 100k Vertex unit, I did go searching for an estimate from manufacturers, etc.
Fortunately also, I was entirely aware that EF was the criteria to mostly consider for efficiency, if available.
*they trained VERY well at Sustainable Spaces/Recurve.
My question was about recovery efficiency as a clarification because it had seemed so vague, and previously, could not get a clear answer on whether RE and TE are similar or the same. Got that finally via AO Smith.
On another note, since the Vertex unit did qualify definitely for the federal tax credit, which requires an EF of over .8 or .82 , can't recall the exact, they seemed to clearly be able to meet this criteria always.
This would mean a higher EF than quoted in your message, which was what I got from engineering and support over the past 2-3 years. Any thoughts on this?
Thanks always for your feedback and support!
-Andrew Dunn

Answered by Andrew Dunn
Posted Thu, 11/10/2011 - 10:56

Helpful? 0

Some of the RESNET folks made a simple spreadsheet to help with the calculations. Do a google search for Commercial Hot Water EF Calculator.xls it comes up on the 4th page. Based on a 3 bedroom house the GDHE-50 has a .73 EF and the GPHE-50 is .74.

Answered by Richard Kinkel
Posted Fri, 11/11/2011 - 19:51

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