We have installed a rinnai water heater in the basement. Since it takes a little time before the hot water arrives in the kitchen or upstairs in the shower, we are wasting qite a bit of potable water every day. What is the best/most energy efficient way of avoiding the waste of a precious resource like water? Installing an electric water heater? Many thanks for and best regards. Bettina

1) Guesses: Sounds like a bad single-handled faucet cartridge, or low water flow at faucet, or clogged water filter.
Replace faucet cartridge, clean the aerator, and/or clean sediment out of pipes below sink, and clean water filter at tankless unit each month.

2) For exact measure of problem:
Typical faucet can have 2 gallon per minute flow rate.
2 gpm flow rate means faucet fills 1 gallon container in 30 seconds. Test faucet using 1 gallon container and a cooking timer to get flow rate.
Clean aerator to improve flow rate. Compare kitchen flow rate with other faucets in home.
http://waterheatertimer.org/Low-hot-water-pressure.html

Most homes have 1/2" diameter pipe running to kitchen faucet.
100 feet of 1/2" diameter pipe holds 1.2 gallons of water. This assumes no sediment build-up inside pipes.
http://waterheatertimer.org/Water-heater-recirculation-system.html

Assuming only hot is turned on: If faucet is 100 feet away from water heater, and pipe diameter is unobstructed 1/2" diameter, and faucet has 2 gallon per minute water flow, then hot water should arrive in 36 seconds.
50 feet away, and hot water should arrive in 18 seconds.

If kitchen has single handled faucet, and the handle is raised in the middle, then equal amounts of hot and cold water are drawn at same time. This doubles amount of time for hot water to arrive.

If single handle faucet has a bad cartridge, then faucet can have crossover.
Crossover means that cold water can enter the faucet even if faucet is turned to hot.
Bad single-handled faucet cartridge can delay or stop the arrival of hot water.
http://waterheatertimer.org/Crossover.html

Tankless require maintenance including cleaning combustion parts yearly, de-lime heat exchanger frequently, and clean water filter each month.
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf
http://waterheatertimer.org/pdf/Bosch_Overcoming_fluctuating_temps.pdf
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

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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.

hi there should be control pads in the condo for seting the water temp ?

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,,

You need a recirculating line. You will have to hire a plumber to run a water line from the farthest fixture or to the fixture you are concerned about on the hot side and return it back to the water heater. this will cause hot water to constantly run throught the hot water line (via the recirculating line) and when you want to draw water from the fixture, you will already have hot water right at the line.
I know this works with standard tank water heaters, but it is a lil trickier with rinnai units(but very possible) if your plumber pleads the 5th and tries to sell you another unit call the Rinnai manufacturerand ask them how.

What type of pipes are install on this heater?

sounds like the water line is frozen before it gets to heater. You can take a blow dryer and try to thaw the line.You should be able to hook up a hose on the hot side of water heater. turn valve off so it wont flow into house.You can turn on hot water valve on in kitchen but you will have to thaw cold and hot line. Make sure you know where water shut off is to the house just in case a line breaks.

I think the dip tube in your water heater has failed, this is a plastic tube that goes in the cold side of your tank and forces incoming cold water to the bottom of the tank allowing hot water out at the top. When this tube deteiorates the incoming cold water mixes at the top of the tank causing the problem you have. the dip tube runs about 5 bucks but takes someone with pluming skills to install.

The only way that I have found to work is to place a small electric water heater close to the point of use. Yes this does defeat some of the efficiency of having a tankless water heater but if you figure electric rates are now cheaper than gas rates in some areas, it may save you money.

Another alternative is to put a domestic circulator in-line and recirc the domestic water either all the time or install a timer.

None of these options are "low-buck" options.

15' of 1/2" copper holds less than 1 gallon of water so something is wrong with the heater if it takes 3 gallons to get hot water to the faucet.

It sounds like the Rinnai units are working correctly but you've got some plumbing problems.  Any tankless heater has a minimum water flow required to keep the burners going.  If the burners were going with very little water flowing through them to carry the heat away, the heat exchanger could overheat and fail.  The solution is generally to the electronic controls to set the desired temperature and then just use hot water.  Once you start mixing cold water in, you actually reduce the flow of hot water and the burners turn off.  If you are right near the cut off point, the burners will cycle on and off resulting in those very annoying hot-cold-hot-cold showers.  
The fact that you've got those very loud noises indicates some other sort of plumbing problem.  Hooking up two tankless units in series is much more complicated. In heavy draw situations, the second unit is asked to finish heating the water up to temperature when the first unit couldn't quite get it there. Now you're putting hot water into a heater and heating it more.  You have a lot more issues with fluctuating temperatures and pressures within the pipes which could conceivable cause these horrible noises.  Everything's got to be set up right to for things to function properly and it's easy to get something wrong.
From your description, it sounds like a better solution would have been to mount the two heaters in different places. e.g. One in the basement to serve basement and first floor faucets.  The other on the second or third floor to serve those two floors.  Since these units are direct vent (don't use inside air), they can even be mounted in a closed closet as long as the vent can get to the outside.  With this setup you avoid the complexities of a series installation and you also have a much shorter wait time (and therefore less waste) for the fixtures on the upper floors.