water-heater
I have an inline water heater. Once it gets hot it will stay that way for 20+ minutes at a time easily.
When the shower gets used it feels like their is a floor in how cold the water will get – if I turn the cold up past a certain point it runs ice cold.
This waterheater has been in a while. I've already flushed the lines with vinegar and tripple checked the gas and gas regulator.
What more should I look at? My guess would be the water flow control mechanism but I really don't know.
Many homes suffer from low pressure/flow issues for various reasons. This regular drop in pressure/flow seems to indicate an external problem, such as a large water user (commercial or industrial) using water from your water main line for some automated process.
Also, check with your water distribution entity to see if they have changed the way they regulate or maintain water delivery pressures (most use pumps).
As for a fix, if one of the aforementioned causes turn out to be true, it may take some diplomatic activism (or maybe some not so diplomatic activism).
It's not exactly like mine, but it looks like it's probably an overpressure (no temperature) type valve (aka pressure relief valve), and it should not be dripping unless you have overpressure - so a constant drip means it's likely in need of replacement. (Or possibly just cleaning, if you get lucky; don't expect to get lucky.)
Indeed, it looks very much like this adjustable one.
The line on the cold water line could be intended to feed a hose bibb.
Given that it's probably intended to relieve pressure events before the P/T on the tank so that the P/T on the tank does not become leaky through frequent activation, you probably could replace it with a pressure tank.
Best Answer
Tankless/on-demand (is this the same as inline?) water heaters only heat if the water flow rate is above some minimum, typically around 0.5 gallons per minute. As the water temperature is made more cold, less and less hot water is going into the mix. At some point, the hot water flow decreases below the threshold and the water heater turns off, yielding cold water flowering through the hot water pipes.
Your water heater may have an adjustable flow rate threshold, and if so, it could be decreased. Another solution would be to decrease the temperature setpoint to be closer to the desired final temperature (though you may need the hotter water for dish or clothes washers).