Combi Boilers vs. Solo Boilers for Domestic Water

Written By: Tito Melendez, Triangle Tube Technical Sales Support, Mid-Atlantic Region

When considering what type of boiler is best suited for the job, there are several things to consider. First, let’s talk about the differences between a solo and a combi boiler.

Combi Boilers
A combi boiler is an appliance that produces both hydronic heat and domestic hot water. This type of setup is usually considered a very efficient way to achieve both heat and hot water using a single appliance. Combi boilers have a secondary heat exchanger which transfers heat energy between the hydronic side of the boiler to the incoming domestic water. While the boiler will run continually in domestic hot water mode as long as there is a demand,  the total amount of hot water is limited by the boiler's net output and capability of the secondary heat exchanger.
Many times, a contractor may choose to install a combi due to its small size and foot-print compared to a separate boiler and hot water tank. In my experience, I’ve seen factors that are overlooked when making this decision.  One factor includes the incoming water temperature during the coldest time of the year. Colder incoming water requires more heat to reach the desired domestic water temperature, reducing the water temp output. It is important when comparing combi boilers to compare different manufacturers by the same temperature rise so you are comparing apples to apples.
If you’re considering installing a combi when a tank-style water heater already exists in the home,  walk through the home to inspect the type of fixtures in the home.  Have a conversation with the homeowner.  Homeowners often think combi boilers can provide “infinite” hot water. They actually provide “endless” hot water up to a certain flowrate. If the demand for hot water is greater than what the combi is capable of, the hot water flow rate will decrease. If you spot what I call a “car wash” shower, one that has multiple body sprays, a large rain head, or a jacuzzi style soaking tub, the contractor should probably reconsider a combi boiler. Another consideration is the number of simultaneous showers the homeowner will take and appliances such as multiple dishwashers or washing machines. Check our the Triangle Tube Product Selector tool on the website.  It’s a great resource for finding the best fit for your install.  

Solo Boilers
A Solo boiler paired with a Smart 316 indirect water heater requires more mechanical room real estate than a combi boiler but can meet a higher hot water demand due to its high recovery rate. Boilers need to be properly sized to meet the greater of either the heating requirements or Indirect’s required BTU input to achieve the expected gpm output and storage availability of the tank. Using an Indirect water heater also gives you the ability to recirculate hot water with the addition of a recirculating pumpIf you need hot water immediately available in situations where the fixture is far from the mechanical room, a solo boiler with a separate indirect tank is the best solution. It may be possible to do this with a combi boiler, but the setup will be more complex and often will require a small storage tank thus taking up more space in the mechanical room.
Using proper pump sizing on the hydronic side of the indirect tank as well as properly sized piping is crucial for optimum performance. The unique design of the Smart 316 offers lower head loss compared to other tanks on the market today, requiring smaller pumps.  Another beneficial feature includes the polyurethane foam insulation which allows it to have a temperature loss of less than ½ a degree per hour.
Triangle Tube boilers include a sensor that can be used in place of the already built-in aquastat.  This allows the installer to set output temperatures through the boiler's software while displaying the indirect tank’s target temperature.

The recent upgrades to the Smart 316 stainless steel tank include an upgraded welding process with acid pickling which increases the life of the tank.  This is important as it will require less maintenance.

When considering whether to install a Combi boiler or a Solo boiler with an Indirect tank, it is important to take the time to evaluate all scenarios. It is not wise to simply replace boilers and tanks based on what is currently existing. 

Triangle Tube provides many charts in our manuals to help determine the best fit for the job.  There is also a growing library of YouTube videos and literature to support your needs. Triangle Tube’s robust technical support is here for you. Contact our in-house phone tech team via phone or email.  Our regional field technicians and sales managers are also part of our support team. The contact information is located here.

With the cold weather breathing over our shoulders I wish you all a happy, healthy, and prosperous season. We are all in this together!


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