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...I'll bet it's cost-prohibitive to put in an older type gas water heater because there is no flue...

I have no idea why you might want to install an older style gas water heater, but those available today probably can be exhausted through a sidewall using plastic pipe.

The old style gas water heaters that used a flue were ~60% efficient, with the lost heat allowing the exhaust gases to be hot/light enough to rise out the flue passively. The modern high efficiency gas water heaters (~90%) extract so much heat from the exhaust that it's not light enough to rapidly rise through a flue, so they need to be force-vented through a PVC pipe, usually out a wall. Even at only 60% efficient, the gas still heats your water for much less than electricity in most parts of the US. So, if someone had a flue (house from 1980s or earlier), then it might be worth taking a closer look at if the cost of the heater itself is that much less than a high efficiency water heater.

Note that I didn't say I *wanted to* install an older style gas water heater, but that I thought I *couldn't* because it would be crazy to add a flue.

Subsequent research has shown that a heat pump water heater should save enough over a regular electric to pay for the difference in three years. So, I don't think I'll go with a gas water heater and add the extra plumbing for gas and exhaust.
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