The heater is only kicking on low as the wate demand goes up the temperature of the water drops because the heater is not kicking up like it used to. The heater is a little over a year old.

If your Bosch Highflow 21e hot water system is experiencing fluctuations in water temperature while showering, it could be caused by a few different factors. Here are some possible reasons for this issue:

1. Flow rate too high: The hot water system may not be able to keep up with the demand if the showerhead or tap is allowing too much water to flow through. This can cause the water temperature to drop suddenly. Try reducing the flow rate by turning down the tap or using a low-flow showerhead.
2. Water pressure fluctuations: If the water pressure in your home fluctuates, it can cause the hot water temperature to vary. This can be caused by various factors, such as other appliances using water at the same time, or changes in municipal water supply pressure. Installing a pressure regulator or a flow restrictor may help to stabilize water pressure.
3. Blocked or dirty water filter: A clogged or dirty water filter in the hot water system can restrict water flow, causing the water temperature to fluctuate. Check the filter and clean or replace it if necessary.
4. Malfunctioning thermostatic mixing valve: The thermostatic mixing valve (TMV) regulates the water temperature in the hot water system. If it's faulty or dirty, it can cause temperature fluctuations. A licensed plumber or technician can inspect the valve and replace it if necessary.
5. Faulty temperature sensor or control board: If the temperature sensor or control board in the hot water system is faulty, it may not be able to maintain a consistent water temperature. A technician can diagnose the issue and repair or replace the faulty component.

If you're unsure about how to diagnose or fix the issue with your hot water system, it's best to contact a licensed plumber or technician for assistance.

Gas or electric?
Electric water heater ... then test for crossover, check for water leak, replace thermostats, test elements
Gas water heater ... test for crossover, check for water leak
http://waterheatertimer.org/Crossover.html

http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

http://waterheatertimer.org/How-to-test-water-heater-element.html

water leak: turn off all taps and put ear against pipe and listen for water leak... check if water meter advances when all water taps are off.

Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

There is an inlet screen on demand water heaters. Look for that and clean it. The tubes in the heat exchanger could be clogged. The manual will tell how to clean it. This is very involved and uses acid. It can be dangerous, so I recommend you have it done.

You probably need commercial 150gallon gas water heater.
Or three 80 gallon tanks, depending on number of people.

80 gallon residential gas heater is rated for low-demand 6-8 person household.
80 gallon residential electric is rated for low-demand 5 person household

80 gallon is not correct for 14-28 people in 7 apartments
http://waterheatertimer.org/Whatsize2.html
http://waterheatertimer.org/Two-water-heaters.html

Call local plumber, or local code department.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Question is not clear.
Add a comment and say what you want to do.

You have 240Volt electric water heater.
Lower thermostat is set as low as it can go at 77.
But you want water temperature set to 66?
First thing to remember is that water heater thermostat settings are approximate since they read temperatures through the steel tank wall.
Second thing is that thermostats for electric water heater are generic, and are not made for lower temperature range settings. This is probably since most people complain that water is not hot enough.
I'm looking at a generic lower thermostat from Lowes right now. The low temperature setting is 90 degrees F.
Looking at some older thermostats from a drawer right now: one does not have a lower temperature mark, the other is a Reliance with low temp of 95, the old craftmaster goes down to 80.
So I don't even have a thermostat that goes as low as the one you have, except for gas water heater thermostats that traditionally can be set to 50 degrees for vacation mode to prevent freezing.
More thermostats can be seen on following link:
http://waterheatertimer.org/How-to-adjust-temperature-on-water-heater.html

The only thing you can do is get a couple of unit heaters and point them in the direction of your water heater,and wate for it to un thaw.

it is probably a burnt element.

This sounds like a common problem with tankless type water heaters. The modulating burner has a set Btu range it can operate within, a maximum and minimum. With very low flow rates and very low temperature rises, the burner has trouble maintaining a steady output temperature because the btu load drops below the heater's minimum input. Typically the machine will overheat the water beyond it's setpoint, the burner will cut out, and then re-ignite once the output temperature has dropped low enough.
The solution to stopping this behavior is to simply increase the water flow through the device (i.e. higher flowing fixture or larger pump). This type of problem is exaserbated with recirculation systems that bring a very high water temperature to the unit.
Higher end tankless brands have a lower minimum input btu and thus fewer problems with this.

Someone has not sized the system correctly. The water heater will heat a certain GPM a certain temperature rise. Most of those "generalizations" from the manufacturers are referencing the lowest of all the flow requirements like 2 lavatories. The truth is, don't use other fixtures while showering or get a bigger system. Lots of information related to sizing the tankless system at http://www.mechanicalengineeronline.com/Mini_Sites/Tankless_Water_Heaters_Ebook/Tankless_Water_Heaters.htm Have a good one. PS:Properly sized On-Demand water heaters are actually really good. Be prepared for the day your water heater quits.

An on-demand, or tankless, water heater is a system for heating water as it passes through the pipes, usually very near the point of use. Traditional tank water heaters bring the water to approximately 140 degrees Fahrenheit (60 C), in order to ensure the water supply lasts long after the heater turns off. Since this is obviously far too hot to be pumped out of a shower of faucet, cold water is added to the mix in the pipes. Coupled with the loss of thermal energy over time, the average tank heater has an efficiency of approximately 80%. On-demand water heaters have a number of advantages over their tank-using counterparts. While tanks often degrade over time and begin leaking, the apparatus used in an on-demand water heater is much less susceptible to the burdens of age, and will often outlive the house they're installed in. The amount of energy used is a fraction of that used in a tank water heater, due to increased efficiency and a much smaller window during which a heating source is in use. While tank hot water heaters can, and often do, run out of hot water after a number of showers, on-demand heaters provide a never-ending supply of hot water, ensuring that warm water is always available when needed. A tankless water heater can cost between $500 and $1500, and they most commonly use either electric or natural gas energy to heat the water. Most mid-sized households report an energy savings of approximately 25-45% for a gas or propane heater, and up to 50% with an electric heater, over using traditional tank heaters. Water is usually set to a maximum temperature of 120 degrees Fahrenheit (49 C), though many heaters allow for custom setting of the maximum temperature via remote control. Outflow rates range from 8-14 gallons (30-53 litres) per minute. Installation for most tankless water heaters is incredibly easy, involving simple plumbing. In the case of electric heaters, all that is required for a fuel source is to plug the heater into an outlet and begin heating your water. Some modern on-demand water heaters can incorporate a solar preheating system. This system uses a traditional solar heating system and tank to bring the water nearer to the desired temperature, saving substantial energy use in the final on-demand heating. A solar preheating system costs between $500 and $800.