No or low water on start up

[Michael Burgan]

i have a rheem RTGH-84DVLN natural gas tankless water heater installed back in 2011 that I’ve been having a problem with over the last few weeks. When I turn on the hot water at any fixture the pressure is really low for a second or two until the unit fires up then the pressure will return to normal, there had also been several occasions when I turned on the hot water where there was no water at all. When this happens I have to turn the water on and off a few times until the water comes back and the unit starts up.

There hasn’t been any change to the plumbing in the house in over 2 years when I remodeled my bathroom, and I descale the unit at least once a year myself with vinegar and a 1/6 hp pump, when I went to do this back in January I had a hard time getting the vinegar to flow through the unit. I had to turn the cold water side on for a second in order to get the vinegar to start pumping through the unit. I would like to resolve this issue before it completely fails leaving me with no hot water, Any help with this matter would be greatly appreciated.

 

[Tankless Tech]

I am not familiar with your area. how hard is the water? more info is helpful.
Would also like to know the process you use to descale. If you try pumping vinegar or any liquid through the unit itself and no other piping and it has a hard time getting through it sounds like your heat exchanger might be clogged with scale.

 

[Sponsor]

 

[Tankless Tech]

Also, Do you have a thermal expansion tank installed anywhere in your home?

 

[Michael Burgan]

I turn off the gas, unplug the control box, turn off both hot and cold water valves, remove the filter to check for debris while the unit drains, reinstall the filter, hook up the hose from the pump to the cold water side and the return hose on the hot water side, open both maintenance valves, fill the bucket up with vinegar, and turn the pump on. I don’t really think the heat exchanger is clogged because when I do get it flowing it does flow well, i has to open the cold water valve for a quick second before the pump started circulating the vinegar through. It’s almost like there’s a valve or something in the unit that’s not opening preventing the water from flowing through the unit. Even when I get that low hot water flow problem it’s still enough pressure to turn the unit on but it don’t turn on right away, once it does fire up I get full pressure again.

Also I don’t have a thermal expansion tank or hard water.

 

[Tankless Tech]

try taking the water inlet filter out and examining / cleaning it