Our home was built in 2013 with a GE gas tankless hot water heater. Over the past year the hot water pressure has dropped considerably while the cold water pressure remains fine. I don’t know where to start.
The usual suspects
- Heat traps on the inlet and the outlet of newer water tanks are heat traps. They are very similar to check valves but they serve a different purpose, they're supposed to stop flow from heat rising in the pipes. Sometimes they are of a ball type and sometimes they are a flapper type. They have a significant amount of restriction when they are new and can build up a lot of debris especially with hard water.
ball type heat traps https://www.waterheaterhub.com/water-heater-heat-traps/
- Globe valve Globe valves are an older style low flow valve. They are good for hose bibs and are good for throttling flow. Many faucets employ the same technology. They are not good for full flow applications like a main shut off.
Globe valve anatomyhttps://en.m.wikipedia.org/wiki/Globe_valve
- Thermostatic mixing valve if equipped they tend to bugger up at times. They are significantly more expensive than the other potential problems. To test it, close the water supply and drain the pressure. Remove the mixed output from the house plumbing system and temporarily pipe it to a pail. Run the water timed to see how long it takes to fill half the pail. Remove the hot side and repeat the process. caution the water from the hot side of a mixing valve can be as high as 180°F compare the results. A significant difference will suggest that the mixing valve is faulty. A minor measurable restriction is expected.
Thermostatic mixing valve hot/cold on sides, mixed on bottom, adjustment on top. www.amazon.ca
These are the best places to start your search
Heat traps are available on Amazon and globe valves should be replaced with ball valves also available on Amazon.
In my opinion, if your water is quite hard, your exchanger got clogged (with limestone) and need to be replaced if it's indirect heat (main closed loop trough the burning chamber and plate exchanger to DHW) or just have the water heater single exchanger (if it's direct hot water) cleaned with acid.
The last time my hot water pressure dropped was because of clogged heat exchanger.