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# maximum flow

Member Posts: 20
Hi guys. I need some answers from a pros. Assuming I have a boiler 100,000 BTU input and an indirect water heater 98% efficiency.

What is the maximum flow in gallons/min can I have trough the indirect water heater to have endless hot water? Please explain.

Thanks

• Member Posts: 20
maximum flow?

Hi guys. I need some answers from a pros. Assuming I have a boiler 100,000 BTU input and an indirect water heater 98% efficiency.

What is the maximum flow in gallons/min can I have trough the indirect water heater to have endless hot water? Please explain.

Thanks
• Member Posts: 6,232
Is this a \"TRICK' '?' ?

there are no known machines in earths atmosphere that are more than60 percent efficent mechanically....unless it is operating in negative gravity and in a controlled environment such as a vacume. is this a science experiment?
• Member Posts: 20
maximum flow? - no trick

No there is no trick. This is a regular question. The high efficiency boilder has 95% and heat exchanger's transfer rate is 98% (For example Ergomax). So i need to know what maximum flow can handle the boiler to have endless hot water if the boiler is 100,000 BTU
• Member Posts: 435
it depends

walter, it depends on your flow rate out of the hot water tank, and the temp of the water coming in, and the temp you have set for the temp leaving, ....

all the DHWT that i have used, always have a chart answering your question, kinda....

as an example, an Allied 80 gal, single coil will do:
226 gals/hr based on - boiler temp of 200*, temp leaving HWT @ 115*, temp entering tank at 50* and the thermal input of the boiler water is 129,000 BTU/HR

hope this helps!

leo g

• Member Posts: 549
\"Efficiency\"...

...is a poor term to use with a heat exchanger, which is all an indirect is. You can have a really crappy design, but keep increasing the surface area until the desired level of heat exchange shows-up. You can make the HX the size of a boxcar for domestic hot water service, and get a 1* approach, if you want to. But they'll cost so much that you'll never be able to sell one.

With the low water temps you'll run with a very high effiency boiler, the LMTD (log of the mean temperature difference) will drop, and that number is a HUGE factor in the size of any given heat exchanger. To make up for a low LMTD value, the surface area will have to increase. If the homeowner wants 130*F DHW, then there's no way the boiler outlet/HX inlet water temp can be less than 131*F. Even with that, the HX will be gigantic. 140*F would be the real world absolute minimum.

How many GPMs of domestic hot water do you need? What are the inlet/outlet domestic water temps you're dealing with? If you know the output BTU and outlet water temp from the boiler, you can figure the maximum available DHW supply. Even with a 100% eff boiler that would give you 100,000 BTU/hr output, that BTU is the limiting factor. If the incoming city water is 40*F, and being heated to 130*F, that's a 90*F delta-T. 100,000 / 90 = 1,111.11 lbs of water per hour. To get GPM, 1,111.11 / 500 = 2.22 GPM.

You can play with the temps and GPM, getting more flow and lower delta-T, or increase the delta-T and drop the flow, but the boiler output will be the ceiling.
• Member Posts: 6,232
i looked some stuff up for you

plate exchanger E 23? flow rates...is this the heat exchanger that comes with the unions already on them?
• Member Posts: 6,232
example

tony clears some of the field for you. here is a bit of info a shower valve is say limited to about 2.8 gallons a min flow ... does that help?
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