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# Constants and variables

Member Posts: 540
especially when comparing the gpm of a tankless verse an indirect.

Example: Tankless manuf. states @ 65* temp rise the equipment will produce 5.1 gpm, @ 115* tap temp. Compare that to an indirect 60 gal. @ 170000 btu, 200* boiler water @ 115* domestic temps is rated @ 355gph giving 5.9 gpm.

But the reality is that the indirect performs much better than the numbers say. This makes it very difficult to prove when comparing numbers to a homeowner.

Not a good thing with the domestic loads of most of the houses I'm doing requiring sometimes double, the btu's of the heating plant.

• Member Posts: 540
Looking for an equation

or equations, to determine the output capacity of an indirect.The constants that are given by most manufacturers are 50* incoming water @ 200* boiler side water temps and X amount of btu will provide xx gpm.

But these constants, the 50 and 200 may not exist. Looking for a way to prove actual capacity of a givin indirect with varying variables.

Example: 100000 btu's 50* incoming water and 160* boiler side temps with 7 gpm tub faucet. How many gpm will actually be available. Also how to factor in the gain in capacity with say, 125* stored water temps verses 160* stored water temps.

Thanks.

• Member Posts: 5

i was just thinking the same thing this morning after seeing an article about upsizing an indirect based on keeping boiler supply down in the condensing range. Unfortunately, i don't have the answer and think it's dependent on the actual surface area of the coil. We know the q=550*gpm*deltaT must equal out to q=U*A*deltaT Trying to study the Viesmann chart for different temps to figure out if the equation holds true but the U it seems would differ among manufacturers and the deltaT changes during recovery. Maybe i'm not looking at this as simply as i could be?
For the water to water side, just make 500gpmdeltaT=500gpmdaltaT if you want to figure differences from the 50 degree incoming figured.
• Posts: 0
The only thing constant...

is CHANGE itself.

I'm certain the manufacturer has all this data, its just a matter of whether or no they're willing to share it with the general public. It is easy for thier competiton to take those numbers and turn them against the manufacturer.

Wouldn't it be great if there were a given standard that all products could be compared to/against?

Sounds like a job for ASHRAE MAN (Tim Doran?)

ME
• Member Posts: 5

actually, i think it was Mark's article that i read. Maybe you can share the data with us???
• Member Posts: 1,790
It is frustrating

Bob, I have shared your frustration when trying to compare indirect tanks.

I just wish the manufacturers made the information available at all. In reality it is difficult to get even a pressure drop curve.

I was once told by the technical support at a major indirect manufacturer that the double-wall coil had twice the surface area because there were two walls. Let's say, after that,I'm a little skeptical.

I wish every manufacturer would print a Technical Data Manual like Viessmann does. Check the curves starting on page 12. That is what I wish we had available on the internet from every tank manufactured today.
• Member Posts: 1,790
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