My circa 1989 natural gas water heater is clearly on its last legs, and am looking to replace it.
Over e-mail I was rough quoted a price for a power vent model. The plumber took a look and noticed my current unit is garavity (chimney) vent unit.
He said that a gravity vent unit is significantly cheaper (he gave me both prices), and not that much less efficient. Plus it has less to go wrong (no blower, sensor, etc).
The only reason I hesitate is that at some point I may want to repalce my circa 1975 furnace. Not sure what the max efficiency of a gravity vent furnace is.
Does it make sense to have a chimney for the H2O heater and a non chminey furnace?
The usual efficiency for gravity vented appliances is 80%.
Here’s what I would do:
Gather you old gas bills from the summer (when the furnace was off) and determine the average amount of gas the water heater uses per month.
Gather you old gas bills from the winter (when the furnace was on) and determine the average amount of gas the furnace and water heater uses per month. Subtract the amount you calculated in step 1 and then you have your average amount of gas the furnace uses per month.
Multiply the average monthly amount of gas used by each appliance by the efficiency of that applaince. Take result and divide it by the efficiency of the new appliance. That will be the average monthly amount of gas used by the new appliance.
For instance, say your water heater uses 1000 CF of gas in the summer. If the efficiency of the water heater is 80%, then your number is 1000*0.80 or 800. If the new water heater is 90% efficient, then take 800 and divide it by 0.90 which is almost 889. So in the summer you would expect to use 889 CF per month, a saving of 111 CF per month.
Do the same with the furnace to find out how much gas you will save during the winter. Figure out the cost savings and see if it makes sense to buy the higher efficiency units.