Water heater heats but not to same temperature

You will need to source a user manual - the specifications state the temperature as adjustable but I have no details as to how.
Max 160, min 120 degrees F.

In hard water areas the temperature should be below 140 to avoid limescale formation.

Hear is a list of possible causes for running out of hot water.

• Water temperature is set too low. Increase the temperature to approximately 125-130 F.
• Reduced the water flow when the water filter or fixture aerator are clogged.
• If there is a plumbing crossover, cold water is mixing with the hot water. One of the reasons for the plumbing crossover is if the single-lever mixing valve was installed, and it failed.
• Gas valve does not provide enough pressure; either due to the low gas pressure or if it is not open fully. Is the gas line sized correctly?
• Sediment and limescale build-up inside the heat exchanger. You might want to use the descaling procedure and flush all the deposits. The scale deposits act as an insulator preventing the heat exchanger to transfer the heat to water.
• Is venting system clean of the debris and does it provide enough fresh air for proper combustion?

Is tankless computer older than 5 years? Imagine how well a 5-year old laptop computer works.

Setting temperature to 140 degrees might be the basic problem:
Read following resource:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html#temperature

I doubt this Takagi computer unit was maintained as shown in product manual, and that heat exchanger was not delimed each year as shown in product manual. So the heat exchanger is probably heavily coated with un-removeable scale... since yearly deliming will not fully remove scale on tankless units, the tankless computer is designed to sprial downward in efficiency despite glossy brochures that promise endless and effortless hot water for 20 years ... as if any computer will last 20 years:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html#de-lime

Since you are not getting error code, the problem might be crossover, or some other plumbing problem:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html#no-error
http://waterheatertimer.org/Crossover.html

I always recommend replacing tankless computer with tank-type water heater for $385 for same-day do-it-yourself solution and to save the endless expenses that arise with computer-driven water heaters.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

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

Probably the factory-installed heat trap nipples are clogged.
Open following links.
http://waterheatertimer.org/Low-hot-water-pressure.html
http://waterheatertimer.org/Low-hot-water-pressure.html#trap
Noisy whistling and humming and low pressure are common problem with heat trap nipples and inserts from factory.
Replace heat-trap nipples with pex-lined nipples, or remove heat trap inserts with screwdriver or needle nose... careful not to break insert so it falls into tank ... which can then get lodged in hot water line elbow or under-sink shut off.
Factory-installed heat traps are part of overall strategy so manufactures can claim more efficient water heaters, without requiring customers to any thinking.
Other strategies include foam insulation instead of fiberglass (so that wires cannot be replaced on electric units), reducing factory-set temperature from previous 130-135 to 100-120, and re-estimating downward the hot water use for average family.
So some of the strategies are useful, while others problematic or imaginary.
Call manufacturer to pay-for or provide new pex-line nipples. Warranty for fist year covers Labor and parts, and after 1st year, warranty covers parts
http://waterheatertimer.org/Water-heater-manufacturers.html
Save $10 a month in future replacement costs: maintain water heater so it lasts longer:
http://waterheatertimer.org/How-to-maintain-water-heater.html

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

I recommend safety changes to prevent ruptured water heater and assorted leaks.
135 degree hot water.
Maybe replace TP valve if it is dripping.
Drip pan under water heater with 3/4" drain line to outside.
Expansion tank on incoming cold water line to absorb pressure build-up, and prolong life of tank and plumbing.

Here's why:

1) Yes you can have scalding hot water.

2) I recommend 135 degrees since that will burn the evil right out of ya. And generally kill clothes washing bacteria.
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html

3) 150 is dangerous and can seriously burn skin in 1.5 seconds. Not safe around children and elderly.

4) Temperature settings on electric water heater are approximate, since thermostats are calibrated approximately, and read temperature through tank wall.

5) Also if you want to increase amount of hot water for high-consumption household, then setting tank temperature upward is one solution since more cold water is mixed with hot, so amount of hot inside tank goes further.

6) High temperature can cause TP valve to release water.
TP valve is pressure-temperature safety valve. Do NOT cap off.
Rating should be 125 psi and 180 degrees ... but all things water heater are approximate, and TP valves need replacement now and then.
It is not recommended by water heater manufactures to use 150 psi TP valve when water heater came with 125 psi. Rating shows in small print on end of TP. New TP valves sold at local hardware. Let tank cool before installing new one.

7) Higher temperature and high pressure can rupture tank.
Pressure over 80 psi shortens life of water heaters and plumbing fixtures. Put BTG100 pressure gauge on water heater drain valve to check pressure.

8) You need a drip pan with 3/4" drain line to outdoors, or to drain vent line, to prevent flood caused by ruptured water heater.
Turn water heater off when going on vacation.
Inspect water heater each 6 months.
Drip pan available at local home center or hardware:
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html#drip

9) Heated water expands and puts pressure on tank and pipes.
Water does not compress... so when water gets hotter it expands against tank wall and against pipes.
Especially in closed system (check valve present) when system is without expansion tank.
Expansion tank is installed on incoming cold water line.
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html#expansion

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

Reliance is AO Smith company.
http://www.reliancewaterheaters.com/prod/gas.html
http://waterheatertimer.org/Water-heater-manufacturers.html

1) Reliance does not post the 501 manual any more and I've written and asked for that manual, and they sent generic manual. Model 501 is both gas and electric.

2) That tells me the old energy factor is probably lower than .54 approximately, but it will be even less because energy factors are for new water heaters in perfect operating order under ideal conditions.

3) Energy factor is a measure of how efficiently the water heater turns energy BTUs into hot water.
Gas water heaters have higher energy factor because gas burner has more BTU punch than electric element, and can heat water faster.

4) However, energy factor is not measure of actual cost. It is merely a way to compare giant water heaters with smaller heaters, and gas with electric, without considering cost of heater, installation cost, repair costs, parts costs, life expectation of unit, disposal/recycling cost, or replacement cost.

5) You can improve your true energy factor by buying small cheap heater and maintaining it yourself so it lasts many years beyond manufacturer's warranty.
http://waterheatertimer.org/Replace-anode-rod.html

6) If someone buys a bigger water heater because it has better energy factor, it will not pay back the higher purchase cost. Same size to same size, the difference with higher energy factor is a few dollars per year.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

7) If you have sediment inside of tank, the energy factor is out the window.
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

Add a comment for more free help.
Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Many times there are no numbers on thermostat since temperature settings are approximate.
Link below shows step-by-step temperature adjustment for typical tank-type gas and electric water heaters. Residential electric tanks can usually be set from 90 to 150, and gas from 50 to 150.
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html
Use cooking thermometer at water heater drain valve for more precise reading.

If water heater used to be hotter, look at diagrams following link:
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

Other water heater tips and tricks:
http://waterheatertimer.org/Replace-anode-rod.html
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html

If you have tankless water heater, consult manual.
Changing temperature on tankless can lead to problems.
Tankless should be de-limed each year to maintain efficiency. U-tube search 'delime tankless'
Read the manual for yearly maintenance requirement.
All costs are energy, and tankless are maintenance heavy.

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.