The downstairs bathroom- physically LOWER than the heater unit by a few feet- does something annoying as heck. When I turn the faucet to 100% hot, the pressure drops significantly. That is to say, it is strong for few seconds then lowers in pressure. This is with nothing else on in the house.
If I edge the single-knob faucet towards cold even a bit, the pressure jumps up. The bathroom piping is from 1968 but the sink and hoses are 5 years old. Why, when the water coming in is not even hot yet, does it do this?
Remember, it takes 30-60 seconds for an on-demand hot water heater to make hot water and get it down the pipes.
I don’t think it has anything to do with the heat. The heater is restricting the flow rate, either intentionally to raise the maximum temperature or incidentially by too small piping or pipe bends.
When the faucet is off, the pressure in the hot water pipe all the way to the faucet is the same as the cold. When you open the hot, that pressure gives a higher flow rate momentarily until the pressure drop caused by water flowing through the heater restriction reduces the flow.
Since the cold doesn’t have the restriction, it flows at the higher rate continously.
They have a heater chamber of finite volume, some quite large - depending on the manufacturer.
Also, all the theories of “restrictions” in the pipes don’t make any sense. The OPs description indicates a non-linear element in the plumbing system, like trapped air or an anti-hammer bladder.
If this is the only place in the house where this happens, it would seem to point to a problem with the hot water piping rather than something with the heater. Some sort of restriction may make sense. The static pressure in the restricted pipe will be equal to the cold water line pressure (approximately) even if the pressure under flow is much lower. When the tap is opened, there will initially be a high flow rate that drops off rapidly as the pressure at the tap decreases to the dynamic pressure.
I think there’s something wrong / loose in your faucet. When you go 100% hot, the faucet valves are lined up a very specific way, and there is a restriction. When you move slightly away from 100% hot, the valves change the way they’re lined up slightly, and the restriction is eliminated.
Test. Turn off the cold water inlet to the faucet, and see how the pressure responds to different positions on the valve. If you get the same low pressure / high pressure jump by moving slightly towards cold, you’ll know its a valve problem.
I was just waiting for you to chime in, just to see if you could resist your obnoxious pedantry.
I guess not.
For the sake of this discussion, water is incompressible. It is only significantly compressible in the realm of extremely high pressures, like those experienced in explosions.
From my Fluid Mechanics text: “all fluids have some degree of compressibility”. Also, “pressure variations in a compressible fluid are usually very small because of the small unit weights and the small differences of elevation being considered in hydraulic calculations”.
So you’re both right.
The OP’s problem seems to me not one of pressure, but of volume. Hence my suggestion of a restriction, blockage, etc. in the line.
It may be possible that the heater itself has some kind of restriciton associated with it - something which holds the water back. Or not, if the problem doesn’t exist anywhere else on that same line.
Sounds to me something like an accidental crimping or crushing of the line, some crud keeping the valve from opening fully, something like that.
Inaccurate? Only in the sense that F=MA is inaccurate because one hasn’t taken relativistic effects into account.
And, I didn’t set out to bait you, I just realized after I posted that you would probably come by to point out that what I said was not strictly true, while at the same time not contributing any information to the discussion.
If your tankless is the highest point in the system and was very recently installed, there could be an air pocket. Usage always entrains such air eventually. This is why bladder arrestors are used versus high point air stubs.
It is more likely mineral deposit as Ogre says, especially if you have unsoftened well water, and compounded by alterations to the lines by addition of the heater. Cheesesteak’s valve test is also a good idea to prove corrosion/ obstruction in a single handle operator.
“…it takes 30-60 seconds for an on-demand hot water heater to make hot water and get it down the pipes.” This seems excessive.