Tankless heater flow rate vs. pex elbows


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Old 03-01-20, 11:58 AM
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Tankless heater flow rate vs. pex elbows

Thank you in advance for your replies to this question.

Last week I had a brand new AO Smith ATI-540 installed, and I notice that when I turn on the hot water at two bathroom sinks and then in the shower, the streams coming out of the two sinks visibly sags. Note that the same experiment with only cold water does not produce the same result.

So I'm wondering if anyone here can provide a candid answer as to whether all these 90-degree elbows in the PEX going in and out of the water heater may be causing a lower-than-advertised flow rate.

Thanks again.

 
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03-01-20, 02:55 PM
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I doubt the 90's are affecting your flow as you describe, I'm sure they are having some effect, just not the cause of the large drop at the sinks/shower. I would suspect that the new on-demand water heater has a flow restrictor that is preventing the water from going too fast past the heat exchanger. If the flow is too fast the heater cannot heat the water enough and you end up with lukewarm water which is no fun in the shower. I see on-demand heaters do this all the time, but typically they are point of use heaters.

Is that 3/4" PEX or 1/2"?
 
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Old 03-01-20, 02:36 PM
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90° fittings are the worst for flow. The mass of water heading down the pipe must almost slam to a stop, then get going in another direction. It's particularly worse for PEX as it's fittings go inside the nominal pipe diameter and restrict the flow. In the picture below you can see how much smaller the inside a PEX fitting can be, much less than the pipe's diameter.

 
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Old 03-01-20, 02:55 PM
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I doubt the 90's are affecting your flow as you describe, I'm sure they are having some effect, just not the cause of the large drop at the sinks/shower. I would suspect that the new on-demand water heater has a flow restrictor that is preventing the water from going too fast past the heat exchanger. If the flow is too fast the heater cannot heat the water enough and you end up with lukewarm water which is no fun in the shower. I see on-demand heaters do this all the time, but typically they are point of use heaters.

Is that 3/4" PEX or 1/2"?
 
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Old 03-01-20, 04:16 PM
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Thanks, gents. The pex you see is 1/2 inch. The tankless unit replaced a 40 gallon heater hooked up to 1/2 inch copper. So no change there ...

​​​​​​As a point of interest, the tub faucet max flow is 5.3 gpm and the sink faucets are each 1.2 gpm. Max flow on the ATI-540 is supposed to be 10gpm. I wonder if what I'm observing is simply a result of running three faucets in the same bathroom...
 
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Old 03-12-20, 12:45 PM
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To anyone who may be interested: a closer examination of the user's manual that came with the ATI-540 revealed that the flow rate very much depends on the temperature of the water coming into the house, as well as on the temperature that the heater is set to. See attached screen shot. So, if the temperature of the water supply is 70 degrees, the ATI-540 can produce hot water at the advertised rate of 10 GPM. But, if the water supply is at 40 degrees (and here in Rhode Island, it's probably somewhat colder than that), then the rate drops to slightly more than 6 GPM. These rates apply IF the water heater is set to 100 degrees. The rates drop if the heater is set any hotter. My heater is set to the standard 120 degrees, so even if the water supply was 70 degrees, my max flow would be slightly less than 8 GPM.

In conclusion, these observations would have been very interesting to consider prior to spending $3400 on getting this unit installed. Shame on me!
 
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Old 03-12-20, 04:27 PM
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That is how on demand heaters operate. They all suffer from that limitation. It's one of the trade-offs for the energy saving.
 
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Old 03-13-20, 05:13 AM
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The graph from the AOSmith manual shown above contains an error in the sentence at the bottom. The maximum flow rate at 8 GPM occurs at set temperature of 120 degrees, not 130 degrees. At 130 degrees set temperature, the maximum flow rate is less than 6.3 GPM.
 
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Old 03-16-20, 10:07 AM
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I suspect that any flow issues aren't so much because of the 90 degree ells, but more because of the 1/2" pipe entering and leaving the heater. Water heaters of all types typically are fed with 3/4" pipe and the downside 3/4" branch is split and reduced to 1/2" to feed separate hot water needs.
 
 

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