No hot water from kitchen tap

trouble shooting section will help https://sahotwater.com.au/specifications/Rinnai/Service%20manual/smV1616W_V2018W_V2020W.pdf

If it is hot in the kitchen, then once the water has reached the shower it should be hot. The leads me to believe that the issue is with your shower mixer unit. Nearly all are thermostatically controlled.

Or, if the heater is downstairs and the bathroom is upstairs, you don't have enough hot water pressure. You need a good plumber with a pressure tester (called a manometer).

ON max water will not flow until the water is hot enough. give it a little time to react.

Some taps are designed to release excess pressure but it could just be a dodgy tap washer that leaks when the pressure of hot water in pipe work builds. Change the tap washer in your tap, see if it fixes it, if not it's just the design.

If some taps have really hot and others luke warm, then you might have defective mixing valve, check valve, or crossover on single handled faucet.
http://waterheatertimer.org/Crossover.html

Crossover can also cause stacking event. Stacking could be part of puzzle.
http://waterheatertimer.org/Stacking.html

Once crossover or check valve or mixing valve problem is solved, if you have really hot water to some taps, then turn down thermostat to avoid scalding.
http://waterheatertimer.org/How-to-adjust-temperature-on-water-heater.html

Add a comment for more help.

Ask the plumber who did the mods. to come a check that he hasn't crossed a hot/cold pipe. Really sounds like he has, this would cause hot water to mix with cold. Is the water hotter further away from where he did his work? This would be a clue. Easy way to check is to turn off the water to the hot water system, should be a valve on the middle pipe going in to the heater. This should turn off all water to all your hot taps. If you still have water at any hot tap or shower then the pipes have been crossed.

Run through the basic checks

• Power lead from the heater plugged in a socket turned on.
• Gas on to house
• All valves below heater in on position.
• If they have controllers in the house turn on (key pads marked with Rinnai logo in kitchen or bathrooms usually)

There is a turbine that spins in the heater when a hot tap is run. If the heater has been sat for some time not operating the turbine might have stuck. Run the hot tap and keep it running, using a hammer give the centre pipe connection under the heater a firm hit a few times(not too hard). This might unstick the turbine.

Usually when you are having a problem at only one fixture, and not any others it is a sign that you have a faulty fixture. Hope this helps.

you can get it off the rinnai web site they have a spot on there to get the internet manuel,, but from what you are saying you may need to adjust the thermostat, or the temp, remember even though you will have endless hot water it still takes the smae amount of timed for the hot water to get to your faucet,, not only that make sure your shower heads aren't holding back to much water pressure, if you don't have enough water going thru the unit it will not stay lit, if will come on and go off, judt to dstisfy the unit,, make sure the filter is clear, on the cold side there is a cap screw if you take it off there is a screen in side you just pull it out and clean it and reinstal it,,

We had the same problem - the hot water would only be hot for 3 -5 minutes and shut off, then we had to turn the water off, wait a few minutes, and do it again. VERY FRUSTRATING.

Solution: The Rinnai serviceman showed me the heat-exchanger blower/fan: It was clogged with dust and moths.

Another clue: Our Rinnai was extremely noisy when the heat came on. After the fan was cleaned and replaced, it is nearly silent.

If you are reasonably machanically inclined:
TURN THE BREAKER / POWER OFF to the unit.

The blower-fan is located in the lower central area of the unit (at least on our unit). If you draw imaginary north-south, east-west lines through the center of the unit, the fan is in the lower-left quadrant. There is a spring clamp securing the black rubber accordian-like tube to the fan. There are two screws that secure the fan to the unit. there are two plugs that connect to the fan, as well as a third air-flow sensor that should be removed. then you can look at the fan unit, as well as the grill on the underside of the heat-exchanger that should be cleaned.

My thought was to go out and get a simple bug-screen, and place it over the air-input outlet vent going outside, but the repair guy said that although it may sound good, if that screen ever gets clogged, or reduces airflow to the Rinnai, then the unit will work harder, or even shut off, giving the same problem,