I have a heat exchanger used for domestic hot water and need to calculate BTU usage for billing purposes. I have a supply and return temp sensor and a meter that pulses a dry contact every 100 gallons. I have these three signals tied into a programmable controller. The display needs to output the total BTU's consumed and will be reset each month so that a bill can be generated for BTU usage. Can someone help me out with this calculation? Thanks for your help.