You are assuming that the restriction always has a 20 foot pressure drop. An ideal orifice at steady state flow is the only thing that will act that way. The OP is about increasing flow in a real system, and I suspect if he actually measured the time of the transient it would be somewhat less than a few seconds.
Rhetorical questions:
What is the pressure drop when the flow is zero? What is it at 5 gpm? 6 gpm?
What happens to the pressure drop with respect to flow rate if the ideal orifice is replaced by a spaghetti of small diameter tubing? If the pressure drop at 10 gpm is twice the 5 gpm and at 20 gpm it’s three times the 10 gpm?
As said before, the pressure drop at a flow rate of zero is zero. You can roughly assume that the frictional losses increase with the square of the average velocity in feet/sec. A more accurate formula is:
hf = f X L/D X V^2/2g
where hf is the friction loss in feet
L=pipe length in feet
D= average inside diameter of the pipe in feet
g= gravitational constant (32.174 ft/sec^2)
f= friction factor
The friction factor is a dimensionless number determined experimentally for pipes of varying composition. For turbulent flow (Reynolds number above 4000) the friction factor can be determined by a formula opriginally developed by C. F. Colebrook in 1939. If you want the formula let me know, I’m not very good at typing in formulae so you may want to find it on your own.
Non turbulent flow (Reynolds number below 2000) is not relevent in this case. It only applies to extremely viscous liquids or extremely slow flow rates. There are some interesting things that happen in that boundary region between Reynolds numbers of 2000 and 4000, but only to fluid dynamicists, and neither of us is one.
Mine has none. The water flows in, goes through what appears to pipes akin to a radiator, and goes out at 140˚f. There is no reservoir.
Sediment? Possible. VERY possible. Bad valve? Also quite possible. The house is about to go on the market. It bothers me, but may make a House Inspector unhappy. The valve and stem stuff I can do with care and no leaks. Sediment in the lines? Nope.
If I think about it ( I am not at the house now ), the sink is roughly 10 feet from the tankless heater. Some bends, sure. And- the piping to that sink and only that sink is below the level of where the hot water tank USED to be. The sediment levels in the water are such that I split / cracked 2 tanks in 10 years with sediment cake build-up inside the glass tank. That, coupled with 2 adults and 2 teens needing a shower, made a tankless a no-brainer.
I will work on the plumbing at the sink level. Fairly cheap to do, and the first thing to approach. I’m not gonna go desoldering pipes and replacing them.
The air in the lines? Wouldn’t air rise? Shouldn’t I have this problem up 2 flights in the powder room in the master bedroom?
ETA: I think Cheesesteak nailed it. I’ll chime back in after some light repairs !!
There is a butterfly valve (or some kind of valve) in the heater that measures the water flow going through it and turns the burner/heater on and off. When the hot water tap is just opened to a trickle my on-demand heater won’t turn on and I just get cold water; the tap has to be open a certain amount to trip the valve. However there is no noticable pressure change when that happens so I doubt that is the problem here.
Heated water precipitates dissolved solids .
AIUI, tankless manufacturers recommend a de-liming or de-scaling on a (perhaps) yearly basis using a mild acid like household vinegar. Refer to the owner’s manual if you are blessed with its possession.
As noted, air eventually goes into the water even when deliberately trapped and would only be evident in new installation not so designed.
You guys are all spending too much time in theory and not enough time playing with plumbing parts.
I have seen the exact same thing in my house several times over the years.
The water pressure regulator at the service entrance is failing. It is allowing the pressure to creep up while there is no flow, and then the pressure falls once the flow starts.
for about $10 you can buy a pressure gauge that screws onto a hose bib and verify this.
Screw on the pressure gauge have someone turn on a faucet elsewhere and note the pressure with the water flowing. Then have them turn off the faucet, and note the pressure. Then leave it overnight, and note the pressure first thing in the AM before turning on the bathroom faucet. It is going to be higher in the AM then it was when the water was running the night before.
IME water pressure regulators are only good for about 5-10 years before they start acting up.
I’m with Cheesesteak and InLucemEdita. I think it is the faucet itself. Many single knob mixing valves are designed to balance the pressure between hot and cold to prevent scalding when the cold water pressure drops.
The valve may not be able to distinguish between a faucet turned all the way to hot, and a drop in cold water pressure. The valve has a thermostatic valve in it, which might take a few seconds to react, just as the op is experiencing.
Even if it is just a standard faucet, it wouldn’t be surprising that it might not work properly when turned to an extreme setting.
When my tankless water heater (Bosch) was installed, I was told that the hot water flow is restricted. If I turn the little dial on the heater to its maximum point, (given the current temperature that would be enough to melt my skin) I lose a significant amout of the water pressure. The water heater only produces 2.4 gallons of hot water per minute (I think). So if I increase the heat, I am taking in more hot water at a lower flow rate. If I turn it even slightly to cold, the flow rate increases since I am letting in more of that high flow cold water.
A knowledgable tankless installer would either plumb in tees prior to in/out isolation valving or use unions in lieu of valving so the unit can be freed and connected to independent circulation.
Neptunian Slug's post highlights the fact that "instant" hot water can't happen in real time and may shed light on the OP as well as some mfg's use of reservoir to compensate.
