How do i reset my boss therm

Check for a green wire near the top right of your machine. It would loop back to itself with a little black heat shrink on it. This should be far away from the heat exchanger. if its near it, I would recommend pulling it out of the wire clips, and relocating it to behind the control unit on the right side of the machine. Keep it away from heat....its a resistor, and will pop random A3 codes if the heat increases its resistance. Reset your code and you'll be good to go.

It tells you that your water heater called for heat for more than an hr and never got hot enough to meet the demand.
CAUSES

-bad sensors
-no hot water from boiler
-bad thermal transfer
-low delta T across water heater

It tells you that your water heater called for heat for more than an hr and never got hot enough to meet the demand. multiple scenarios can cause this
CAUSES

-bad sensors
-no hot water from boiler
-bad thermal transfer
-low delta T across water heater

It tells you that your water heater called for heat for more than an hr and never got hot enough to meet the demand.
CAUSES
-bad sensors
-no hot water from boiler
-bad thermal transfer
-low delta T across water heater

Absolutely.

Er3 means that the water is being heated for more than 1 hour without reaching the set temperature.
Need new aquastat.

This requires a little math....

BTU - amount of heat it takes to raise one pound of water 1 degree F.

I gallon of water = about 8.54 pounds

91 gallons of water = about 777 pounds = 777 pounds x 70 degree rise = 54,390 BTU.

1 Therm (what the gas company measuers in) is 100,000 btus

1 therm costs about $.45 (45 cents) depending on where you are

54,390/100,000 x $.45 = about $.25 per hour

This cost can vary somewhat depending on the inlet water temperature, gas cost, and the efficiency of the hot water heater

You seem to have a burned out heater elemt. Most water heaters have 2 e.ements. If the upper one bruns out, it will not heat at all. If you have a cheap heater built by State, you probably have a bad lower thermostat. It will over heat and trip the protection device. If you reset the red button on the upper thermostat and it gets too hot (check in about 2 hours) then replace the lower therm. You could still have a burned out upper element.