How do I calculate (PAH)

hot_rod

Does anybody know how to calculate stand by heat loss for oil, gas, electric and indirect water heaters???

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Unknown

Boy, you pick some tough ones.

Try this idea on. For a gas water heater, run it up to temp, and then clock the gas meter for a week, and end after a run cycle, to have the same water temperature. Of course you can't use it then.

Your milage may vary with chimney and weather conditions, as well as air temps.

Just divide BTUHs by time, seems to me.

Scary to think about what size numbers you may get.

Noel

hot_rod

easy part / hard part

The easy part (it seems to me) is the calculation for jacket heat losses using conventional heat loss calculations. IE: delta T betwen interior & exterior multiplied by R or U values & square footage of the surface. So far so good eh?

How do I deal with that steel flue tube running up the middle???

The project I'm working on requires knowing these numbers.

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Duncan_3

Not really.

But ASHRAE might.

I'd go with Noel's idea if I thought gas meters were accurate, which I don't. Even so, Noel's way would probably be your most accurate. Of course, you'd have to have a dedicated meter - or turn off all the other gas appliances.

For an electric, I'd calculate the area of the inner tank, and use Q = U x A x dT. Then convert btus to watts if you needed it in percent.

You might do the same for an indirect. Or use the manufacturer's claims of temperature loss. That doesn't take into account the boiler standby losses, which ultimately fires the indirect.

This is all compared to how many gallons of water are kept at what temperature, surrounded by what ambient air temperature.

And do you want losses to the inside of the building envelope to count as standby losses?

What exactly are you trying to figure out, Dave? How much it loses to the outside, or how much it loses to the utility room, or both?

Alan R. Mercurio

Hi Dave

I'm not 100% sure this is what you're looking for? but I got it out of my Bock manual.

STANDBY LOSS FORMULA

1 Gallons in tank X 8.25 X 90 o F = Stored Heat

2. Stored Heat X % Standby Loss X Hours on standby = Standby BTU

3. Standby BTU X $ per gallon = Standby Cost Per Day
138,500 BTU

Example:

(32 X 8.25 X 90 o F) X (.03) X 23.2 = 16,537 BTUs

22,049.28 x 1.10 = $.175 per day x 365 = $63.91 per yr
138,500

Your friend in the industry,

Alan R. Mercurio

Oil Tech Talk

Eric Taylor_3

Here's a thought.....

You know how many gallons are stored, and you know the temp, so therefore you know how much energy is stored in the tank.

If they rate jacket losses with *F/hr, you should be able to turn that into Btuh if you solve for stored energy as a function of temp.

Calculating accurate flue losses would be next to impossible given that every situation is different, but you can measure the total loss by tracking how fast the internal temp drops after the unit shuts off, solve for Btuh and subtract the calculated loss from the jacket.

PS-- I'm the same Eric Taylor as the Mtechnology guy. I just got cable at home. 2.5 Megabits per sec. !!!!!

Wheeeeeeeee

Unknown

Dave I beleive if you

get in touch with A.O. Smith Water Heater company they did a comparison a while back on systems and standby lossses. I was looking around the office for it but I can not find it. Give them a call they may be able to give it to you.

I also seem to remember the GRI (Gas Research Institute) people having something.

hot_rod

The tough parameter...

is going to be the flue gas passageway losses on the oil and gas heaters. The U value of the steel is pretty obvious, but what's going to require a SWAG is the CFH factor. As with any hydronic heating questions, the only correct answer is "It depends..."

If the appliance is connected to a really high stack, and there is adequate combustion and ventilation air available, then the standby stack loss could be significant. (140 deg F in the tank, 70 deg F air moving up through the flue gas passages being drawn by natural draft.)

I'm sure there have been some scientific studies done to determine it, but again, are their field parameters the same as yours? I've seen some water heaters with so much draft through them that I was amazed that the pilot could stay lit.

Once you figure out how many CFH you're pulling up the stack during standby, you could calculate the energy loss based on delta T and .018 btu/cu ft/degree F rise per hour.

It is calcuable, but you're gonna need a LOT of data to make it accurate.

ME

hot_rod

Here's the puzzle I'm trying to solve.

Stand by heat loss comparison for the full range of water heaters at 120 and 140 degrees to compare the increased cost incurred by storing at 140 degrees F (or higher).

The operation math for production and use wasn't that difficult to determine, but I can't find any info on stand by losses.

Draft will vary (unless set up in a lab), but I would assume there have been studies performed by manufacturers or the DOE. No luck finding any so far.

We're removing an oil fired water heater next week and installing an indirect in its place. Still working, so I might just set that rascal up at the shop & experiment. Three gas water heaters in the apartment/office building that I can play with too, but sitting around waiting for a water heater to cycle is not really very scientific as the draft will never be a constant.

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Mr. Bill

Dave if you're bored

follow this link

http://www.eren.doe.gov/buildings/appliance_standards/residential/waterheat_0300_r.html

hot_rod

Jumpin cat fish!

There goes my chances of relaxing on this day. No Super Bowl - just a big fat headache from trying to comprehend some of those mathematical formulas!!!

Looks like the flue losses represent about 3%, but that's from a brief skimming of several hundred pages & hardly etched in stone.

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