Source strangulation for an instant water heater?

  • #76
OmCheeto
Gold Member
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... Three different kinds of copper K, L, M with pressure drops of the order of 45% higher in K compared to M. Looks like the type of copper will have to be discovered too.
According to wiki, the three types of copper pipe have the same exterior diameter, but due to their varying thicknesses, they have different inner diameters, which accounts for the different pressure drops for pipes of the same exterior size. [ref: wiki]

I'm certainly learning a lot more about piping than I had intended.

ps. For future reference, Jonathan212's assertion is correct, and is based on the chart and equation on page 79 of the Copper Tube Handbook pdf mentioned earlier in post #69.

Table 14.6 is based on the Hazen-Williams formula​
P = 4.52*(Q^1.85)/(C^1.85 * d^4.87)​
Where:​
P = friction loss, psi per linear foot​
Q = flow, g.p.m.​
d = average I.D., in inches​
C = constant, 150​

and per wiki:
sizeODID
KLM
1/25/8
0.528​
0.545​
0.569​
3/47/8
0.745​
0.785​
0.811​
 
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  • #77
15
3
Think of Pressure this way. You have the self car wash down the street, how big is the hose? More pressure smaller hose, BUT it will not make enough of a difference to go from 3/4 to half. Bigger pipe means more volume for the house and means water will reach farther and supply more fixtures bathrooms, kitchens and so on sufficiently. And those that do not believe me, I used to be a plumber :P
 
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  • #78
JBA
Science Advisor
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That is the exact reverse of the reality that the smaller hose, which can be rated for a desired higher pressure supply and flow and still be flexible, is the result of the higher pressure requirement, not the converse.
 
  • #79
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Sorry I'm late to this, I had read it on my tablet and it was too hard to respond there. I may have missed it somewhere, but I really think the solution is far simpler than where these posts have headed.

I really think that trying to work to increase pressure/flow at the input is barking up the wrong tree. From the OP: "probably as it senses low inlet pressure." - The heater sense flow, not pressure. There was a chart in post #62 that showed the turn on/turn off points to be at 2.3 and 1.8 L/min. So once you are drawing hot water, you need to keep the flow above 1.8L/min, or the heater will shut back off. As the OP mentioned the problem is worse in warm weather, this makes sense, the user adjusts the faucet for a hot shower, but that takes less draw from the hot side in the mix, since the cold side is warmer in warm weather. This was touched on in post #5, but seems to have gotten lost.

The simple solution is to run a separate hot water faucet as you shower, to bring the total flow of hot water higher than 1.8L/min. Somewhat wasteful yes, but that's the simple solution.

My shower is ~ 4L/min total flow. OP should measure theirs. This was asked in post #37, but was rebuffed by OP. I guess maybe they don't want help?

I think another, more complex and costly solution was mentioned, to add a small-ish tank heater at the output, to buffer the on-demand heater. But that will also go cold eventually w/o enough flow to trigger the demand heater on.

 
  • #80
15
3
You could put a insulated loop in with a Grudfos pump. That might work.
 
  • #81
1
1
Keep it simple... Chances are, the shower head is where flow is restricted. Just use a shower head that allows more flow. You may test it by removing the shower head from the hose and try if the heater shuts down or not. If not, the shower head is the culprit.
 
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