How can I guesstimate the overall efficiency of my DHW system? I have a fairly new cold start oil boiler and a 30 gallon Superstore tank that sits about 50 feet from the boiler, with insulated pipes in between. We don't use a lot of hot water, but oil use has been very constant at about 28 gallons per month, which seems high to me. That is almost $100 a month at current oil prices. A friend with a similar system but a larger family says he uses only 10 gallons per month. Interestingly, 28 gallons a month is almost exactly what my brother’s always-hot oil system used when he left it running for six months while the house was empty.
I have been told by several people to install an electric water heater and use it in place of the oil, at least during the non-Winter months. "Your electric bill will not increase by $100", they say, which seems true. Electricity here is 16 cents per kwh, or about $47 per million BTU. If my oil system is only delivers 50 percent overall (71,000 BTU per gallon), oil at $3.35 would likewise be $47 per million BTU.
So it comes down to a question of standby losses in the boiler and piping. Does it seem likely the oil system loses fully half the heat available in the oil it burns?