Configuration advise for 2 Tankless heaters in a special application

[Jim Schafer]

Hi All,

I operate a small Textile mill at my farm. We process raw animal fiber into yarn and other products. A big part of what we do is wash raw wool. We use water that is 160*. This temperature is the most efficient for getting the lanolin away from the fiber. Other fibers are washed at lower temps (120 to 140*) Currently I am using a Takagi T-M32 which is 240,000 btu. In the winter my ground water is between 50 and 54*. We go through about 500 gallons of heated water per 8 hour shift. We need as much as 110* of temperature rise(which, in my opinion, is a lot)

At these temperatures, understandably, I can get about 3 gpm of 160* hot water out of the T-M32. I'd like to raise that to 4 or 5. I have about 10 gpm available (before heating).

I have purchased an additional heater, a Takagi AT-h3-DV-p (the T-M32's have been discontinued) it is rated at 199,000 btu. I use a dedicated propane tank for fuel so I will have enough gas to provide fuel to both units.

My question is (after that long winded intro), for my application, should I run the units in parallel as master and slave units, or in series, with the first one set to preheat to 110* and the other set to heat to 160* ?

Thanks in advance for any info you might be able to share.

 

[Reach4]

Probably series.

 

[Sponsor]

 

[Dana]

Depends on the actual flow requirements. The heat exchangers have substantial pumping head, as well as a minimum water pressure. Putting them in series could introduce a much larger and potentially problematic pressure drop at higher flow, whereas in parallel the net pressure drop will be lower. Ergo, putting them in parallel is probably going to be the least risk.

 

[Reach4]

Putting them in series could introduce a much larger and potentially problematic pressure drop at higher flow, whereas in parallel the net pressure drop will be lower.

Good thinking, as usual.

Regular water heaters have low pressure drop, and are usually put in series. That avoids one carrying a lot more flow than the other. High pressure drops in the tankless WHs would solve the sharing/balance problem, and cause the flows to be similar. As a downside, the minimum flow would roughly double. I think that minimum flow thing would probably not be a problem for this application.

 

[Jadnashua]

That option is usually shown in the installation manuals. From what I remember, generally, they run them in series. Some units throttle the volume down to try to maintain the temperature rise based on the incoming water temp. If you set the first one to an easily attainable value where it won't try to throttle the volume down, then the rise to the needed temperature means the second one in line will also be able to go full throttle to achieve maximum volume. Running them in parallel may be harder to get them balanced. Those that can be run in series 'talk' to each other to maximize the throughput. If yours do not have that functionality, then it's a tossup which would work best. It's likely to be harder in parallel with different burners as the smaller one would drop the temp of the combined output when running flat out when their outputs mixed since its temperature rise would not be as high in the winter.

 

[Dana]

Even without throttling down to keep the temperature high, the pumping head through a tankless heat exchanger is considerable at high flow, so if higher flow rates is the goal, it becomes self-limiting to put two in series.

 

[Jadnashua]

The Takagi manual shows each unit connected in parallel, and, depending on the model, you can install as many as four together with the built-in logic. It may or may not work properly with two different models. If you need more, it can support as many as 20, but requires a separate controller(s). You need to verify the models you are thinking about support this function, and can work together. Essentially, one of them gets set as the master, and the other(s) become slaves, allowing them to each adjust themselves. The diagram is on page 24.

At a 120-degree temperature rise, the 199K unit is good for about 3gpm output, so since you want more, even two may not suffice.

http://www.takagi.com/media/56599/2000535218.pdf