BiTe IT and SiT on IT! (PAH)

Dave Yates (PAH)

BT (from BiTe) = Bathing Temperature - as adjusted by your customer. 106 is the human threshold for pain, so if they don't know or it's a new as-yet unbuilt home, pick one between 102 & 106 F.

IT in both cases equals the COLDEST inlet temp you'll see during the year. Typically 40 F no matter where you live - including warm places like FLA. (I use 35 F in my design work.)

SiT = Storage Temp.

So, the formula looks like this: (BT minus IT) divided by (ST minus IT). The answer will be the percentage of hot water in the delivery stream.

All that remains, is to determine how long the use lasts and GPM flow rates, which can be ascertained by timing how long it takes to fill a one- or two-gallon bucket.

Then multiply GPM by the percentage of hot water (great for tankless type sizing) and then times the minutes of continued usage (great for tank-type water heater sizing)

For tankless units, you'll need to know the net Btu input and the required temp rise from that 40 F inlet temp - lets say 120 F is your target. That's an 80 degree rise. If water is 8.3 Lbs per gallon, that's going to require 664 Btu's net input for every GPM of hot water required.

Suppose you find your customer needs 4.5 GPM of hot water per minute. 4.5 x 664 = 2988 Btu's x 60 minutes = 179,280. Your GROSS input needs to be close to 220,000 Btu's if that's an 82% NET efficiency appliance. Many charts included with manufacturers' tankless literature utilize a 45-degree rise for projecting GPM rates. 85 F water doesn't meet my customers' definition of hot water, much less allow for mixing down with cold so they have adequate pressure (or flow if it's a scald-guard unit that must see flow on both sides).

Design for design conditions or they'll have designs on your wallet! Do the math or you take the bath. (Excerpt from my presentation on the Comfortech Roadshow)

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Larry Weingarten

Dave...

...this is great information, but from the title I assumed you were either talking politics or food ;~)

Yours, Larry

Dave Yates (PAH)

or... something worse!

Thanks Larry. The best way for me to remember things is to make up a phrase or series of words I can remember. The first time I needed that formula was shortly after water heater manufacturers dropped from 140 F to 120 F and derated the inputs to lower the risk of scalding and meet new federal energy guidelines - suddenly we were getting complaints that folks were running out of hot water.

So.... knowing 1 Btu raises 1 Lb of water 1 degree F, the rest became a math game.

With the advent of tankless water heaters becoming all the rage following the tripple whammy tank styles got hammered with these past 18-months (FVIR, steel prices & new insulation to meet yet another fed energy eff rise), and the wild claims being touted by a number of manufacturers, it was time to get out the old formulas and dust off the cobwebs.

That's what led to this article: http://www.contractormag.com/articles/column.cfm?columnid=356

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Plumdog_2

Right on Dave

The current "Energy Factor Rating" that applies to water heaters is a generalized overall energy usage figure which supposedly factors in all aspects of the appliance and it's usage and how energy is consumed by it. My two 40 gal gas tank-type heaters have a Energy Factor Rating of .56; some tankless on demand type heaters are claiming .85. This rating is not a gas usage efficiency but more of an abstract rating. I would like to know how the testing is done and what criteria are used.
I have tried the math on the output claims and have also come out with less substantial figures than the specs.
With all the high science available today it seems amazing that no Independent Test is available to substantiate the various claims and to measure performance for comparison, so the consumer and the professional alike can compare apples with apples and leave out the ridiculous and misleading marketing claims. How about a "Hydronic Dynamometer" that various gas water heating appliances could be hooked up to for direct comparison? Performance testing side by side like the auto magazines.