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I was surprised to learn when I moved to a cold climate that some gas water heaters can operate without electricity. Modern ones use a small piezoelectric igni

I was surprised to learn when I moved to a cold climate that some gas water heaters can operate without electricity. Modern ones use a small piezoelectric igniter to light a pilot light. Probably obvious in retrospect, but took a bit of brain rewiring to realize no-power != no-hot-water given I had mostly dealt with areas with electric water heaters with fairly small tanks up till then.

adolph 6y ago
I’ve been wrapping my head around it too as that’s how our hot water heater works even though the gas-using furnace doesn’t work in a blackout.
- ashtonkem 6y ago
  I understand your surprise, but the difference is understandable when you think about how each system needs to operate. It's a matter of thermal mass, location, and complexity. Both a hot water and a forced air furnace work by burning natural gas to heat something up until it's hot enough. In both cases the combustion is separated from whatever its supposed to heat by a chunk of steel, and the gas is simply burned in open air without any extra compression. Ventilation is usually done via convection, which works without power. So far, so good; both cases just need a small amount of power to open a valve and kick off combustion. The similarities largely end there though. In the case of a hot water heater, the flame is heating a big slab of metal under 40+ gallons of water. It's very hard for this slab to overheat and get damaged, as water is both thermally dense and a great conductor. There's also no need to move the heat around; the flame is literally right underneath the thing that needs to be heated. This proximity also makes the control logic extremely simple; you turn on the flame when the tank is too cold and turn it back off when it isn't. This consumes a tiny amount of power, and you can use the heat generated by the pilot light to keep such a circuit alive. A furnace on the other hand has an issue. While burning gas in a furnace is easy, getting that heat back up to where you need it isn't. Air is a pretty terrible thermal conductor, and without some cooling that combustion chamber is going to overheat and get damaged. You need to move a lot of air past the combustion chamber to get the heat away before that happens, which means you need a powerful fan to get that air moving. These fans consume a lot more power than you can reasonably generate locally, about 75-400W per hour, with an extra spike needed to get the fan moving in the first place. You also have the issue that it's not possible to measure the outcome of the furnace at the furnace; which means you're going to need relatively complex remote sensing and control devices, which only adds to the system's total power needs.

adolph 6y ago

I’ve been wrapping my head around it too as that’s how our hot water heater works even though the gas-using furnace doesn’t work in a blackout.

ashtonkem 6y ago
I understand your surprise, but the difference is understandable when you think about how each system needs to operate. It's a matter of thermal mass, location, and complexity. Both a hot water and a forced air furnace work by burning natural gas to heat something up until it's hot enough. In both cases the combustion is separated from whatever its supposed to heat by a chunk of steel, and the gas is simply burned in open air without any extra compression. Ventilation is usually done via convection, which works without power. So far, so good; both cases just need a small amount of power to open a valve and kick off combustion. The similarities largely end there though. In the case of a hot water heater, the flame is heating a big slab of metal under 40+ gallons of water. It's very hard for this slab to overheat and get damaged, as water is both thermally dense and a great conductor. There's also no need to move the heat around; the flame is literally right underneath the thing that needs to be heated. This proximity also makes the control logic extremely simple; you turn on the flame when the tank is too cold and turn it back off when it isn't. This consumes a tiny amount of power, and you can use the heat generated by the pilot light to keep such a circuit alive. A furnace on the other hand has an issue. While burning gas in a furnace is easy, getting that heat back up to where you need it isn't. Air is a pretty terrible thermal conductor, and without some cooling that combustion chamber is going to overheat and get damaged. You need to move a lot of air past the combustion chamber to get the heat away before that happens, which means you need a powerful fan to get that air moving. These fans consume a lot more power than you can reasonably generate locally, about 75-400W per hour, with an extra spike needed to get the fan moving in the first place. You also have the issue that it's not possible to measure the outcome of the furnace at the furnace; which means you're going to need relatively complex remote sensing and control devices, which only adds to the system's total power needs.