First floor raditors get hot but second floor raditors get warm at the bottom only i have a circular hot water system

You either have the thermostat in upside down or you have an air pocket and the system needs to "burp". Sometimes this can be done by warming the engine and letting it cool down.

The anitfreeze might not be good enough and the coolant is frozen or slushly so it does'nt circulate. The therostate can be stuck or the raditor fans could be bad. With the gauge hot is the raditor hot too? All over? The raditor can be partly plugged. The raditor needs to be brimming full and the recovery coolant tank needs to bein the hot to cold range.. Water is'nt the only thing that's in the coolant as without antifreeze it'll freeze and not circulate.

First make sure the cooling system is full. This might require letting it cool and filling it more than once though the main raditor cap. Then it needs looked over for leaks. If it won;t stay full it'll overheat. The hose from the recovery tank to the raditor connection at the cap is a problem spot and it the raditor can't **** the coolant from the tank back into the raditor the engine will soon get low. If these things are alright then you neeed to watch it heat up, and see what's happening. The engine should worm up and as it does the upper raditor hose will warm and but hot but not right away. The side of the raditor should start to warm up soon after that, Touching the raditor it should warm up evenly from top to bottom if it doesn;t it's clogged. If the hose and engine get hot and the pressure inside the hoses rise but the raditor says cool then the therostate is sticking and needs to be changed. If the whole radidor gets hot till everythings too hot and the fan does'nt come on then you need to test the fan and it's fuses and connections. The cooling fan will be comanded on if you unplug the coolant temperture senser that's pluged into the side of the therosate hoseing as it's where the upper hose connects to the engine. If the fan seems to be coming on and hot air is coming off the back of the fan and it's still getting too hot and you've decided that the whole radidor is getting evenly hot then the engine may have an issue. Exhaust clogged up ( cat canvertor, muffer bent pipes) The check engine light might be on and the reasons it's on need to be examinded.

If water pump leaking replace both, if not and truck is over heating start with thermostat.

1 Inspect Hose Ruptured, Cracked or Leaking Raditor Hose. 2 Inspect Head Gasket Leaking Head Gasket. 3 Inspect Thermostat Thermostat Stuck Closed. 4 Inspect Raditor Cap Worn or Damaged Raditor Cap. 5 Inspect Raditor Raditor Fins Obstructed. 6 Inspect Belt Missing or Broken Water Pump Belt. 7 Inspect Heater Core Heater Core May Be Leaking Antifreeze/Coolant into The Vehicles Floor Area. 8 Inspect Intake Manifold Gasket Leaking, Worn, or Damaged Intake Manifold Gasket. 9 Inspect Cooling Fan Switch - Raditor Faulty Raditor Cooling Fan Sensor/Switch. 9 Inspect Water Pump Leaking Water Pump. 9 Inspect Raditor Fan Motor Faulty Raditor Cooling Fan Motor. 9 Inspect Relay - Raditor Cooling Fan Motor Faulty Cooling Fan Relay. 9 Inspect Fan Blade Broken, Missing, or Faulty Fan Blade(s).

Open the freezer door, remove the bottom tray, smash the ice on the floor with your fist to remove it. Now get a glass of hot water and pour it into the trough which is at the bottom of the back wall of the freezer. Allow a few minutes for the ice to melt then remove it with an ice pick or scrrewdriver. Once the ice is out, pour more hot water into the trough. Now take a long tube ( an old washer fill hose works perfectly) and stick one end in the hole in the trough. Now fill your mouth with warm water and blow it very hard through the tube. Once the drain is clear, pour some hot water and a very little amount of bleach through the hole.

Is the floor warm just in certain spots or all over. If just in a certain spot you may have a hot water line leaking in floor.

try this, set the bottom therostat if it is electric , 10 degrees hoter than the top is set , if gas have no ideas

have you flushed out the radiator ? get some radiator flush and flush it out. There could be a blockage in the radiator.

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.