British Thermal Unit (Btu)

       The British thermal unit (symbol BTU or Btu) is a traditional unit of energy equal to about 1,055 joules. It is approximately the amount of energy needed to heat 1 pound (0.454kg) of water (exactly one tenth of a UK gallon or around 0.1198 US gallons) from 39 to 40°F (3.8 to 4.4°C). The unit is most often used in the power , steam generation , heating and air conditioning industries. In scientific context the BTU has largely been replaced by the SI unit of energy , the joule , though it may be used as a measure of agricultural energy production (BTU/kg). It is still used unofficially in metric English-speaking countries (such as Canada) and remains the standard unit of classification for air conditioning units manufactured and sold in many non-English-speaking metric countries.
      In north America , the term "BTU" is used to describe the heat value (energy content) of fuels and also to describe the power of heating and cooling systems , such as furnaces , stoves , barbecue grills and air conditioners. When used as a unit of power , BTU per hour (BTU/h) is the correct unit , though this is often abbreviated to just "BTU".
      The unit MBTU was defined as one thousand BTU , presumably from the Roman numeral system where "M" stands for one thousand (1,000). This is easily confused with the SI mega (M) prefix , which multiplies by a factor of one million (1,000,000). To avoid confusion many companies and engineers use MMBTU to represent one million BTU. Alternatively a therm is used representing 100,000 or 105 BTU and a quad as 1015 BTU.       
              

Aquastat

     An aquastat is a device used in hydronic heating systems for controlling water temperature. To prevent the boiler from firing too often , aquastats have a high limit temperature and a low limit. If the thermostat is calling for heat , the boiler will fire until the high limit is reached , then shut off (even if the thermostat is still calling for heat). The boiler will re-fire if the boiler water temperature drops below a range around the high limit. The high limit exists for the sake of efficiency and safety. The boiler will also fire (regardless of thermostat state) when the boiler water temperature goes below a range around the low limit , ensuring that the boiler water temperature remains above a certain point. The low limit is intended for tankless domestic hot water , it ensures that boiler water is always warm enough to heat the domestic hot water. Many aquastats also have a "diff" control which determines the size of the range around the "low" and/or "high" controls.
            Aquastst is a registered Trademark of Honeywell International Inc.