Understanding the Difference Between Indirect and Direct-Fire Water Heaters

When it comes time to purchase a new water heater, you have a variety of different options to choose from. While most homes still use traditional tank water heaters, many people are now opting for tankless units instead as they are less prone to leaking, use much less energy and have a longer lifespan. However, if your home has a radiant heating system, then you may definitely want to consider opting for an indirect water heater instead of a direct-fire unit. To understand why, today we’re going to look at the differences between direct-fire and indirect water heaters and how each one works.

Direct-Fire vs. Indirect Water Heaters

All gas and oil water heaters are direct-fire units. This means that they burn fuel to produce heat, and this heat is then used to provide hot water. Indirect water heaters are different in that they work passively and don’t actually produce any heat on their own. Instead, the hot water produced by a boiler is used to then heat water inside a storage tank.

How Does a Direct-Fire Water Heater Work?

In a traditional water heater, the gas or oil burner sits directly underneath the hot water storage tank. The flames produced by the burner heat the bottom of the steel tank, and this heat is then transferred to the water inside the tank. As the gas or oil is burned, extremely hot combustion fumes are produced. A pipe runs up through the middle of the tank so that the combustion fumes can be vented outside. The fumes cause the flue itself to become hot, and this heat is also transferred from the flue into the water.

Gas tankless water heaters are also direct-fire units, but they work slightly differently than traditional gas water heaters. Tankless units don’t store water and instead only heat on demand when hot water is called for. Inside the tankless water heater is a metal heat exchanger similar to the one found in a gas furnace. The hot combustion fumes travel from the burner through the heat exchanger, which almost instantly causes the metal to become quite hot. Cold water then flows over the metal heat exchanger, and the heat energy instantly causes the water to heat up.

How Indirect Water Heaters Work

An indirect water heater is really nothing more than an insulated storage tank that is connected to a boiler in a closed loop. An indirect water heater doesn’t have any heating elements of its own unlike a direct-fire unit as all of the heat is supplied from the boiler instead.

Whenever the boiler drops below a certain temperature, it kicks on and begins to circulate hot water to the indirect water heater tank. This hot water travels through a pipe that runs up through the tank and then back to the boiler.

The hot water from the boiler doesn’t actually mix with the water in the tank. Instead, the water from the boiler simply heats the metal pipe which in turn disperses the heat to raise the temperature of the water in the tank. Once inside the tank, the pipe forms a coil so that it can more effectively and evenly disperse the heat. This coil is essentially another type of heat exchanger that works by absorbing heat from one source (the water from the boiler) and transferring it to another (the water in the tank).

Benefits of Using an Indirect Water Heater

The biggest benefit of using an indirect water heater is that it is by far more energy efficient than any other type of domestic water heater. Unlike any other type of unit, using an indirect water heater won’t cost you a cent during the colder months when your boiler is running to heat your home. In fact, an indirect water heater will actually work to reduce how often the boiler needs to run during the winter.

In a normal hydronic or radiant heating system, the boiler constantly circulates hot water to all of the radiators or underfloor heating coils through a closed loop. This means that most boilers don’t actually store hot water as the water is instead constantly circulated throughout the system whenever the boiler is running. With an indirect water heater, some of the heat energy the boiler produces is essentially stored inside of the water heater tank. This means that the boiler typically won’t need to run as long since the heat energy from the water in the tank will help to heat the water in the radiant heating system more quickly.

Some energy will be used during the warmer months when your heating is off since the boiler will need to run occasionally to supply hot water to the indirect water heater. Nonetheless, an indirect water heater will always be much cheaper to use when considering the whole year since it won’t use any additional energy whenever your heating is on.

Another factor that makes indirect water heaters much cheaper to operate is that modern boilers tend to be much more energy efficient than any type of water heater. A new high-efficiency boiler will usually be around 96% efficient, which means that it directly converts 96% of the energy it uses into heat. On the other hand, gas tankless water heaters are often only around 80% efficient and traditional water heaters can be as low as 60 to 70% efficient. This means that even if your boiler does need to run occasionally, it will still use less energy than a normal water heat would due to its higher efficiency.

The fact that indirect water heaters don’t have any heating elements or nearly as many components as other types of water heaters also mean that there is less that can break down. As a result, your repair needs will generally always be much less when using an indirect water heater. This same reason is also why indirect water heaters typically have a much longer lifespan since there are fewer things that can fail.

All this being said, indirect water heaters do have a few drawbacks as well. For starters, they tend to be quite a bit more expensive to install than other units. Another issue is that an indirect water heater will put added stress on your boiler and potentially shorten its lifespan since it will also need to run during the warmer months. Indirect water heaters also heat more slowly since they work passively instead of directly heating the water. This means that you will typically need to wait longer for your water to reheat after you’ve used all of your hot water up than you would with a direct-fire unit.

At IT Landes, our team of licensed plumbers can help you decide which type of water heater is best for your home and also ensure that it is installed correctly. We also specialize in water heater repairs and maintenance as well as a full range of other plumbing, heating and cooling services for both residential and commercial customers. If you have any questions about indirect water heaters or need to schedule any service in the Harleysville area, give us a call today.