How to choose the best hot water heater for your home

With a range of hot water heating options available, what's your best bet? Learn more about combi boilers and DHW tanks, to help find out what would work best for your family.

Vitcocell 300-V DHW tank (with boiler)

Let's start with a combi system

A combi boiler, short for “combination,” offers both hot water and space heating. A combi boiler contains a plate heat exchanger, which heats water on demand. As the heat exchanger heats up the water, the boiler supplies it to your shower, sink, or other areas of the house. Because it heats water on demand, there is no storage tank like you might see in your basement now. This lack of hot water storage means that the boiler must run every time hot water is needed. A combi boiler is small – it's typically wall-mounted, and requires a very small amount of space. Want to save the basement for a game room? Then install a combi boiler in the closet to open up the floorplan for activities!

Boiler and separate DHW tank

On the other side of the spectrum is the more traditional option of a boiler and separate DHW (domestic hot water) storage tank. The water inside the tank is heated by the boiler, where it is stored at a heated temperature until it is used. Unlike the combi system which fires every time you turn the hot water on, a DHW tank will maintain the heat, and will typically only fire the boiler after about half the tank has been used. This can save energy, and also increase the overall lifespan of your boiler, since it doesn't need to fire each time hot water is needed. In terms of space, this boiler and tank system is closer to what you're likely used to seeing in a basement or utility room, like the example shown above. The boiler takes up some space on the floor or wall (non-combi systems are also available as wall-mounted units), and you will also need approximately 24 x 24 inches (60 x 60 cm) of floorspace for the tank.

Image: FamVeld /

Hot water storage systems

A hot water storage system like a DHW tank allows more usable hot water to be available than a combi system. When hot water is available from the DHW tank, that water can be used, i.e. to fill up a bathtub, in a much shorter time than the 10-12 minutes it would take with a combi boiler system. The reason for this is down to the limits of a combi boiler's flow rate – it can only heat so much water at a time, regardless of how much water can move through your plumbing fixtures. This also applies to taking multiple showers at once. Because of the DHW tank's stored hot water, taking two showers at once is possible. Because a combi boiler makes the hot water as it is used, the situation may occur where the combi system is only able to keep up with a single shower at a time.

Now, what would happen if you fill your DHW tank with hot water, but the dishes are done, showers are taken, and there's no need for hot water for hours or more – wouldn't that waste the energy used to heat the water? Surprisingly, the answer is no! Hot water stored in a quality DHW tank will maintain its temperature for some time thanks to high levels of insulation, meaning that temperature loss is minimal, only about 0.7º per hour with a 42 gallon Vitocell tank. In simpler terms, the boiler only has to run two “recovery cycles” per day to maintain that hot water for immediate use.

Vitodens 100-W B1KE Series: A wall-mounted combi boiler

So what's the main takeaway?

It all depends on your individual needs. If space is the most important factor to you, then a combi boiler may be the winner. But if efficiency and flow rates of your hot water take precedence, then having a boiler and tank might be your best bet. Do some research to discover your options, find a trusted contractor (or three!) to get quotes and professional recommendations, and weigh your options to determine what will work best for your whole family for years to come. Please feel free to contact us for more information.

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