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Following up on "Which water heater types do you recommend?" and "Are tankless water heaters worth it in cold climates", is there a way to calculate the point at which installing a tankless water heater makes economic sense? Given the cost of purchase and installation, it should be possible to determine the break-even point, factoring in the amount of hot water used and the temperature change required.

Is there a way to estimate hot water usage separately from cold water? If I have a gas furnace and gas hot water heater currently, can the gas usage by each be separated?

Community
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kmm
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You get the bast cost saving when you have to replace your tank or boiler anyway for another reason. Remember that for some of the year, the lost heat from the tank may be of value to you depending on how the tank is sited and how cold and long your winters are.

Also are you having to use more air con due to the tank, if so the air con savings may be well worth while.

Walker
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With electric, standby losses on tanks should be minimal or otherwise not important, especially if your water heater is indoors in cold climates. (Any tiny amount of heat loss is going to be heating your home, and if you're in a cold climate, that's not a bad thing.) Of course, moving from an electric tank to a gas tankless would save money, but moving from an electric tank to electric tankless is a waste of money.

With NG, however, standby losses are much more significant... and most of your losses are through the flue. Since you have NG, yes, the tankless can save you money. It has nothing to do with the amount of hot water used nor the temperature change from the incoming cold water to the outgoing hot water, however. The savings are the removal of standby losses, which are based upon flue losses, tank surface area & insulation, and the temperature difference between the heated water and the ambient air temperature.

A word of caution, however: they will scale up quicker. If you live in an area with hard water, get a water softener or flush it with vinegar/CLR/LimeAway/etc. every year.

Michael
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Assuming you live in California, your gas usage in the hottest months of the summer wil be almost all for water heating (aside from any gas cooking you do).

Also assuming your hot water use will not increase because you never run out of hot water, you could lookup the Energy Factor for your old and proposed new tankless water heaters here:

http://downloads.energystar.gov/bi/qplist/High-Efficiency%20Gas%20Storage%20Water%20Heaters%20Product%20List.pdf?dbeb-f042

and compute the gas savings and thus the dollar savings. If we assume your old water heater EF is 0.6 and the tankless is 0.85 and your old monthly gas bill is $10, the new bill would be computed as follows.

Your old water heater delivered $6 of hot water and wasted $4. To generate $6 of hot water with the tankless, you need $6/0.85 = $7.06 of gas instead of $10. Thus your month savings would be $2.94

The new tankless will cost around $1500 to $2000 if you pay for installation.

Philip Ngai
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