There are pressure issues that are often overlooked when there is an inadequate amount of hot water in a tankless system.
The complaint may be something like: “we just had a new tankless installed and have no hot water.”
High pressure causes high velocity and hence poor exchange of heat, which is why someone may complain of not having hot water. The outlet pipe or hot side of the tankless may feel cool to the touch when under flow and burner is firing.
In order to rule out pressure as the issue, follow the steps below.
Purchase a pressure gauge with a hose-bib attachment for quick connection to an outside hose connection or an inside hose connection anywhere in the domestic system (be it hot or cold water)
Turn on the valve and check the pressure. If the pressure is over 60-65 PSI, you’ll need to install a whole house reducing valve
One way to tell if you have high pressure (beyond 60-65 PSI) is to listen to the pipes as you open and close faucets. If they are noisy or fluttering when at low flow conditions, you should definitely check the pressure
Remember, the closer you live to a water tower, the higher the probability that your pressure is too high. 80 PSI should be you maximum pressure number, though this does not necessarily mean that you want numbers in the 80s.
If needed, add a Watts or Equal whole house reducing valve and set to 60 PSI when water is flowing.