Jadnashua
Retired Defense Industry Engineer xxx
There are two general methods to heat water for domestic use: a tank, or a tank-less system. They each have their advantages and disadvantages. I’m only going to briefly discuss tanks, but spend more time talking about tank-less systems from an energy and performance viewpoint.
Tanks
There are three commonly used tank types: electrically heated, burner based (natural gas, propane, or oil), or indirectly heated from a boiler. The main advantage of tanks is that they are well-known, been around for a long time, fairly simple, and until you run them out, maximum volume of hot water at any volume your piping can provide. The disadvantages are: they are fairly large, especially if you need a lot of hot water quick, the quality of the insulation and the type of tank design will have some standby losses as heat is lost to the room, and they have a limited lifetime. That lifetime can vary considerably depending often, on the luck of the draw, but design can improve things a bit.
Tank-less
The allure of a tankless system is that they promise an unending supply of hot water and have essentially no standby losses. They generally are a lot smaller than a tank, and most of them are designed to hang on a wall. They are usually quite a bit more expensive to install and may require upgrades to the infrastructure, either electrical capacity or gas supply. They are more complicated than a typical tank-type water heater, but their heat exchanger and controls are often designed to be modular and replaceable. While there are parts on a tank type water heater that can be replaced, they often last the life of the tank, which usually rusts out and thus requires replacing the whole thing.
General
With a tank-type, you have a relatively small heat source because you tend to have a long time to recover. If you need lots of water constantly, there are high capacity burners that can aid, but the more common thing is to use a larger tank, especially in a residential situation. That may not be sufficient in a commercial situation, and there they may use both high capacity burners, larger tanks, and maybe multiple units (like say in a restaurant that is washing dishes or a spa that is filling lots of tubs or running showers all day long).
With a tank-less system, you need a big enough heat source to raise the temperature of the water enough to be useful as it passes by. This is much harder to achieve with electricity than with a flame based burner. There are two factors that determine how much you can raise the temperature in a tank-less system: how big the heat source is, and how large the flow of water being used is. It should be fairly obvious that if you have a candle and wave your hand over it, you can barely feel the heat from it. Try that with a blowtorch, and you’d definitely notice! Throw in the speed with which you move your hand, though, and even with a blowtorch, if you moved fast enough, you may not burn yourself, implying you did not get much heat transfer. That example vastly simplifies the issue with a tank-less system: to get usable temperature rise, you either run the water though the thing very slowly, or you need a really big burner.
So, how much heat does it take to warm water? Being backwards from the rest of the world, we use British Themal Units (BTUs) to measure the output of our heat sources and degrees Fahrenheit to measure temperature along with pounds and gallons. A few definitions and conversion factors are needed to go much further:
Tanks
There are three commonly used tank types: electrically heated, burner based (natural gas, propane, or oil), or indirectly heated from a boiler. The main advantage of tanks is that they are well-known, been around for a long time, fairly simple, and until you run them out, maximum volume of hot water at any volume your piping can provide. The disadvantages are: they are fairly large, especially if you need a lot of hot water quick, the quality of the insulation and the type of tank design will have some standby losses as heat is lost to the room, and they have a limited lifetime. That lifetime can vary considerably depending often, on the luck of the draw, but design can improve things a bit.
Tank-less
The allure of a tankless system is that they promise an unending supply of hot water and have essentially no standby losses. They generally are a lot smaller than a tank, and most of them are designed to hang on a wall. They are usually quite a bit more expensive to install and may require upgrades to the infrastructure, either electrical capacity or gas supply. They are more complicated than a typical tank-type water heater, but their heat exchanger and controls are often designed to be modular and replaceable. While there are parts on a tank type water heater that can be replaced, they often last the life of the tank, which usually rusts out and thus requires replacing the whole thing.
General
With a tank-type, you have a relatively small heat source because you tend to have a long time to recover. If you need lots of water constantly, there are high capacity burners that can aid, but the more common thing is to use a larger tank, especially in a residential situation. That may not be sufficient in a commercial situation, and there they may use both high capacity burners, larger tanks, and maybe multiple units (like say in a restaurant that is washing dishes or a spa that is filling lots of tubs or running showers all day long).
With a tank-less system, you need a big enough heat source to raise the temperature of the water enough to be useful as it passes by. This is much harder to achieve with electricity than with a flame based burner. There are two factors that determine how much you can raise the temperature in a tank-less system: how big the heat source is, and how large the flow of water being used is. It should be fairly obvious that if you have a candle and wave your hand over it, you can barely feel the heat from it. Try that with a blowtorch, and you’d definitely notice! Throw in the speed with which you move your hand, though, and even with a blowtorch, if you moved fast enough, you may not burn yourself, implying you did not get much heat transfer. That example vastly simplifies the issue with a tank-less system: to get usable temperature rise, you either run the water though the thing very slowly, or you need a really big burner.
So, how much heat does it take to warm water? Being backwards from the rest of the world, we use British Themal Units (BTUs) to measure the output of our heat sources and degrees Fahrenheit to measure temperature along with pounds and gallons. A few definitions and conversion factors are needed to go much further:
- BTU – the energy required to raise one pound of water one degree F.
- Weight of one gallon of water (at standard temperature – it gets lighter as it is heated and heavier as it gets colder, but for our purposes, we’ll assume it stays the same weight) - 8.345404 pounds…we’ll round that off to 8.35 pounds for our discussion
- BTUs -> Watts – 1 BTU=0.2930711W (this will be enlightening when comparing a burner to an electrical tank-less)
- Volume of a gallon of water – 231cuin