Single appliance approach resolves issues and presents opportunities – Part II

By Cliff McNeill

As discussed in Part I (Mechanical Business January/February 2023, online at, the advantages of a single appliance approach, combined with a price point that is close enough to compete against the mass production of the forced air furnace industry, sounds like the magic bullet. But there is always a but. We need to understand what a combi-boiler can and cannot do and set the expectations.

Let’s take a quick look at what exactly a combi-boiler is. A combi-boiler is a boiler that incorporates a heat exchanger to produce domestic hot water without the use of a storage tank. In the modern version of this, this is typically a plate exchanger with a flow sensor and a diverting valve.

When the homeowner uses hot water, the combi-boiler senses the flow of water and operates to provide hot water for the homeowner. However, unlike a direct fired water heater, it requires a minimum flow rate in order to operate and has a maximum flow rate it can deliver. This limits the number of fixtures it can provide hot water to simultaneously, but if the flow rate is within the right range, it can provide it continuously.

Not a new idea

Combi-boilers have been around for decades but were abandoned as the design was considered inefficient. The original combi-boiler was typically a gas- or oil-fired cast iron boiler with a “tankless coil” inserted into an opening in the cast iron. When domestic hot water was needed, domestic water would flow through the tankless coil and be instantly heated and delivered to the fixtures with no minimum flow rate required and no delay except for the time to flow from the boiler to the fixture. They were considered inefficient because in order to do what they needed to do, these boilers remained hot 24 hours a day, 365 days a year and were constantly losing heat through their venting systems.

Challenges to be aware of

Back to the present, here are some of the challenges with modern combi-boilers when using them to replace a typical forced air furnace and water heater combination.

Manufacturers are coming to the market with floor standing combi-boilers.

The first challenge is that most combi-boilers are typically installed on the wall. This means there must be a wall suitable to mount the boiler to, or you need to construct a wall or similar structure to mount it to and this can add cost. However, some manufacturers are coming to the market with floor standing combi-boilers. You can now use the floor space previously occupied by the water heater to place the combi-boiler.

The second challenge combi-boilers face is they need to do two significantly different jobs. Unlike a furnace that is sized for the heating load of the home and a water heater that is sized for the hot water needs of the home, a combi-boiler must be sized to handle each of these.

Since a combi-boiler is essentially an on-demand water heater for heating domestic hot water, it needs a relatively large BTU input to heat a reasonable amount of hot water. Reasonable being anywhere from three to four gallons per minute. In Canada, this typically means an input of between 150,000 and 200,000 BTUs. However, the heating load of a typical Canadian home is about half or less than half of that.

This means the combi-boiler will typically be two to three times as large as it needs to be to heat the home because it is sized to meet the needs of the home’s domestic hot water. This can lead to the combi-boiler short cycling during a call for space heating and this is hard on equipment, inefficient and affects the service life of the components in the boiler. Most combi-boiler manufacturers anticipated this when designing the controls for this type of application and offer a setting that limits the firing rate of the boiler when doing space heating. This means the full capacity of the combi-boiler is available for hot water heating and you can match the output of the boiler to the space heating load to limit short cycling.

The third challenge that faces combi-boilers is that they typically can’t do two things at once. They are either providing hot water for showers and fixtures or providing heat for space heating but not both at the same time. This is where the homeowner needs to understand what is possible and what to expect.

The final challenge for the combi-boiler is homes that have demands larger than four gallons per minute, which is more hot water than the combi can produce. In this case, the better option to meet the homeowner’s expectations is the high-efficiency boiler, air handler and indirect domestic hot water tank sized for that larger load. This option was discussed in Part I.

Wit that being said, there are several products in the form of venting systems, air handlers, high-efficiency boilers and combi-boilers that have come to the market in the last number of years that are opening up more and more doors to compete for systems that have typically been dominated by the furnace industry.

Manufacturers have given the industry the products and the tools. Now we need to take advantage of the opportunities.

Cliff McNeill is with Equipco in outside sales for Southern Alberta. He is also Equipco’s in-house technical specialist in hydronic heating and engineer-focused products. McNeill can be reached at [email protected].









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