What are your cold water temps?

Something that can be a problem is when the "cold" water isn't very cold and so it requires very little hot water to make it 105.

Result can be is it's not enough hot water flow to keep the water heater going.

Also look for a water heater that has a low minimum flow rate.

just turn down the target temp on the heater .

rinnai has some of the lowest flow requirements in the industry.

Tankless water heaters don't perform well with super low flow shower heads. You need a shower head that provides a flow of around 2 GPM. I'll bet your's is delivering less than 1 GPM. It may be clogged with debris, or it could be an overactive flow restrictor. Try cleaning it and remove or drill a 1/8" hole through the flow restrictor.

A shower head that is really low flow can cause back pressure. The valve can deliver plenty of hot and cold, but the restricted flow in the shower head is not allowing you to adjust to 50-50 hot and cold. The hot water pressure and flow are quite a bit less than the cold, due to the tankless water heater. So what you have is all hot or all cold, nothing inbetween. Say you're running the shower and it's hot. The tankless water heater is working fine to deliver the hot, and you have the valve turned all the way to hot, so no cold is mixing in. As soon as you start to mix in some cold, the excess pressure knocks out the hot completely. It literally pushes the hot water out of the mixing valve and back into the pipes. The tankless water heater senses the lack of flow and shuts off. Now you can't even find the hot water when you turn the valve back to hot, because the tankless heater lets cold water through when it's below the minimum flow rate. After a few seconds, the hot water returns, but now you're all hot again. You can't just set the valve half-way, because the cold pressure is greater than the hot, and the back pressure from the weak shower head is also greater than the hot.

Try to just remove the shower head completely and see if you can get the right temperature mix coming out of the unobstructed shower arm pipe.

"Tankless water heaters don't perform well with super low flow shower heads. You need a shower head that provides a flow of around 2 GPM. I'll bet your's is delivering less than 1 GPM. It may be clogged with debris, or it could be an overactive flow restrictor. Try cleaning it and remove or drill a 1/8" hole through the flow restrictor."

While I see where you are going with this and have occasionally drilled a hole in a flow restrictor I want to offer a bit more detail. I know US Rinnai specs very well. It being a Brazillian model I do not know the specs there, but let's see where this goes.

Tankless water heaters have both a minimum flow rate, .4gpm on mine and a minimum firing rate, 10kbtu on mine. My Rinnai will hold hot water operation down to .25 gpm. I can also read the actual flow through the water heater down to .1gpm on the touch pad display as well as read the output temperature on the unit. If your unit has a display you should be able to get the same info out of the unit. My water heater/shower combination works perfectly. I have a 2.5 gpm shower head. With an output of 120* and a shower temp of about 106-8 I am flowing 2.1 gallons of hot mixing with .4 of cold to arrive at my comfort temperature.
In my experience they are rock solid on temp output to within +/-2*. It is possible to see a surge in temps. I would say that could be due to a partially plugged heat exchanger, fluctuating cold water supply pressure or as noted, your use temp is set to high and you are running the unit at its minimum flows. If you are used to tank water heaters the answer to any hot water supply problem is to turn the temp up. Frequently, the answer with tankless is to turn it down so you improve the H/C mix balance. It is possible you may have to re-build the valve. The comment about pressures is correct. You will loose a couple pounds of pressure across the water heater but with a steady supply pressure it shouldn't make a difference.

Did you guys notice this is a new hous with a new valve and most likely a new heater ???

I don't think it is a problem of rate of flow. Because the brand he has mentioned here woks pretty good with low flow rates. Check the temperature settings of your heater, maybe the problem lies there. Has your water heater been like this since it's installation or is it a new problem. If it has started just now then something has happened to it while working. Maybe you should call a mechanic.

This post was edited by DwayneHutchins on Mon, Feb 24, 14 at 0:54