Q: Since my hot water tank is heated by propane gas and since I cannot wrap it, how may I make it more energy efficient? I have placed insulation wrap on the hot water pipe that leads out of the tank. What else may I do? – Barbara Waters, Millinocket

A: Gas-fired water heaters contain a flue through which the combustion gases pass, giving up their energy to the surrounding water. Under steady state conditions, about 70 percent of the energy in the propane will be transferred to the water. When using 100 gallons of hot water per day, the efficiency drops to about 60 percent; at 50 gallons per day, the efficiency drops to about 54 percent. If no water is used at all, with the cost of propane at $2.19 per gallon, the annual standby cost will be about $150.

An electric water heater (with no central flue to carry away energy when the unit is in standby) will cost a bit less than $100 per year. This assumes the electric unit has the extra wrap. Gas units are now available that heat water “on demand,” or with no standby loss. But these units cost nearly $1,000.

Gas water heater wraps are not recommended for three reasons:

. The wrap may interfere with air flow to the combustion zone.

. Room air flow into the draft hood may be limited.

. The great inefficiency of a gas-fired unit is the open flue. This loss is not addressed by the wrap.

At 60 percent efficiency and with $2 per gallon propane, the cost per unit of useful energy delivered into the water is about $36 per million Btu. With electric unit, at nearly 100 percent efficiency, the cost is about $45 per million Btu.

But none of this helps Barbara Waters. The average life of a water heater is about 14 years. When the unit needs to be replaced, she may, if her household is small, decide on an electric unit, or an on-demand unit, if the household is large. But the price of gas and electricity change.

Answer provided by Dick Hill, retired emeritus professor of engineering at the University of Maine.