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Is Furnace and Water Heater the Same?

Furnaces, water heaters, boilers – there are so many different appliances that can be used to heat up your house. Are they all the same?

The main difference between a furnace and a water heater is that the latter provides hot water, while a furnace heats up the house by warming the air. Water heaters can be used to heat the house through radiant heat; however, such heating systems are not as common.

What Is the Difference Between a Heater and a Furnace?

In a nutshell, water heaters do not heat your home, but they can provide hot water. Furnaces, in their turn, heat your house by warming up the air.

There are electric and gas water heaters. Depending on the type, it would be either the heating element or the gas burner that is responsible for heating up the cold water. 

The models that have a tank heat water continuously – the warm H2O rises to the top of the tank and then gets dispensed from the top. Tankless units are able to provide hot water on demand.

When it comes to furnaces, they can also be either electric or gas appliances. The unit sucks the cool air into the system, warms it up, and then pushes the hot air back into the room.

A boiler is an appliance that uses water to heat a house. The steam and the hot water travel through pipes to radiators or through radiant heat pipes installed underneath the floor.

Read: What Is Water Heater Heat Pump? Everything You Need To Know

Do All Heaters Have a Furnace?

All furnaces are heaters, but not every heater is a furnace.

A heater is a broad term that is used to describe any appliance that produces heat. Space heaters, boilers, and even electric blankets and hand-warmers are, basically, heaters.

Space heaters, for example, do not have a furnace. However, furnaces can work together with your HVAC system and a boiler.

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Read: How Much Does It Cost To Move A Furnace And Water Heater?

Are Water Heater and Furnace Connected?

Yes, your water heater and furnace can be connected in some cases.

In an immersed coil installation, for example, your water heater might have a coil-type heat exchanger that gets heated up by the furnace. 

Is Gas Water Heater Connected to Furnace?

If you have a gas water heater and a gas furnace, then the appliances might be sharing one exhaust vent through which the combustion gases escape the house.

Such a combination is relatively common for furnaces with a standard efficiency (around 80%).

Read: Does HVAC Include Water Heater?

How Does a Gas Hot Water Furnace Work?

A gas hot water furnace or a boiler is part of the central heating system. 

Essentially, the system is a collection of pipes that have water permanently sealed in them. The same water ends up circulating around the house every single day.

The gas enters the boiler via a pipe. The boiler then burns the gas that heats up the heat exchanger which transfers the heat to the water.

A pump pushes the water through the system. It flows through a closed loop and as the heated water passes the radiators, it gives off a certain amount of heat.

As a result, the water that returns to the boiler is cold again.

Usually, such systems come with a thermostat that ‘tells’ the boiler when to stop heating the water. The cycle is going to be repeated until the temperature in the rooms reaches the preferred setting.

The exhaust gases from the gas hot water furnace escape from the house via a flue and get dispersed outdoors.

Read: Ideal Temperature Of The Water Heater

Can I Use a Hot Water Heater to Heat My House?

A water heater can be used not only to produce hot tap water but to also heat your house.

But such a system would require a high-efficiency water heater and fan coil. Moreover, you should either live in a mild climate or have a house that is very well insulated.

Finally, such a system can be efficient only if you need less than around 75.000 BTU/h of heating.

The way the system works is quite simple. The hot water heats the fan coils of the house’s heating system and the heat is then distributed through the ducts.

Even though water heaters are rarely used to heat the house, you can use a tankless model for radiant heat, so that the heated water can flow directly through the floor tubing.

Tip: know your hot water needs, before choosing a heat exchanger for the system.

Read: The Best Temperature For The Water Heater And Why?

Do You Need a Furnace to Heat Water?

Some water heaters don’t have the heating element directly in the tank. They use the heat from a furnace (or a boiler) and are connected to it via a pipe.

Such systems certainly do need a furnace to heat the water up, however, the majority of water heaters are able to work independently as they have their own heating mechanism.

Does an HVAC System Use Water?

A regular HVAC system does not use water. A typical AC needs only air and refrigerant, while a furnace needs air and gas.

However, there are hydronic HVAC systems available. These units do not heat or cool air directly, they have water that acts as a heat exchange medium.

Such systems have a few significant benefits:

  • The piping requires less space than typical air ducts
  • It is easier to pump water to the upper floors, than to blow air vertically
  • The system doesn’t have air ducts, so dust and mold won’t get spread through the HVAC system

Why Is There a Water Line to My Furnace?

If you have a highly efficient furnace, then the water line coming out of the unit is the condensate line.

Furnaces with an AFUE higher than 90% have a secondary heat exchanger. This helps extract even more heat from the combustion gases to make sure that no heat goes to waste.

The secondary heater extracts so much heat that a part of the gases turns into liquid (condensate). The water gets drained out of the unit via a PVC pipe (the condensate line).

The line is connected directly to the HVAC unit and terminates outdoors. Some condensate lines have a small pump that helps the water run through the pipe more quickly, but a lot of condensate lines work on gravity alone.

Does a Furnace Create Water?

Conventional furnaces do not create water. However, if you have a highly efficient condensing furnace, then the unit will be producing water.

Such units have a secondary heat exchanger that receives the combustion gases once they have already gone through the primary heat exchanger. 

Even more heat will get extracted from the gases. As a result, they will cool to a point where the gases get condensed into water and carbon dioxide

It is relatively easy to figure out whether you have a conventional or a condensing furnace. Take a look at the exhaust pipe – if it’s a PVC pipe, then you have a highly efficient condensing furnace.

How Do You Add Water to a Furnace?

  1. Open the shut-off valves to the radiators and close all of the air valves.
  2. Open the water supply shut-off valve (on the tank). Water will start entering the system.
  3. Open the air valve on the radiator that is closest to the boiler.
  4. Wait until the water starts coming out of the air valve and close it.

Repeat the previous steps on the remaining radiators.