Indirect Water Heater vs Combi Boiler

The rise of the tankless water heater in the plumbing world has translated into technical advances that allows customers to have the option for a combination of both a boiler and tankless water heater.  This appliance has commonly been called a Combi boiler. With the rise of something new always comes the desire to be on the cutting edge of technology and efficiency. However, we as the experts need to be able to tell our customers the advantages and disadvantages of the technology. We're often asked if we perfer combi solutions or heating boiler only - but we can recommend both depending on the situation.

Combi boilers and Heat only boilers with indirect water heaters have their own pros and cons. See these benefits in the chart below.



Combi Boilers

Space Saving

Space heating oversized for space.  Could cause need for Buffer tank in heating side or more frequent service.

Unending hot water

GPM limited by incoming water temperature and BTU input

Doesn’t heat tank when not in use

Will need more frequent service on boiler heat exchanger and brazed plate heat exchanger.

More efficient way to heat water than tank type water heater

Complications caused by hot water recirculation if used.

Heat only with Indirect Tank

Space heating can be sized to space need

Takes more floor space

Tank can run out of hot water, but recovery can be faster than a typical water heater

GPM is not limited by the BTU of the boiler

Doesn’t cause short cycling of boiler which typically means less frequent service needs.  Sometimes recovery time can be 3-4 times faster than a typical tank water heater.

Keeps tank warm all the time but typically has superior insulation value to a tank type water heater

Hot water recirculation can easily be added to system

More efficient way to heat water than tank type water heater

Combi boilers have their place and are a great solution when they are operated within their limitations. To select the use of a Combi boiler is more than a question of what is the largest one I can install to get the most amount of hot water I can. The reality of tankless water heating is that it is an endless supply but not an endless amount. That means in Michigan where our ground water temperature is between 40-50 degrees that we are limited to between 2.5 – 5.2 GPM supplied at a 70-degree rise. The GPM is dependent on the model.  A typical shower will run around 2 GPM. So that means on a 3 full bath house that a combi boiler may not be the best option unless they know they can only run about 2 showers at a time without possible fluctuation. If they have a large soaking tub that has a filler that is 5 GPM or larger a combi boiler may not be the best option. Having a large filler on a 60- or 80-gallon soaking tub is not uncommon because the fill time may be over 10 minutes if it isn’t.

Indirect tanks can be sized anywhere from 30 to around 120 gallons.  A lot of the times no change in boiler size is needed to heat these tanks unlike a Combi boiler application. Indirect tanks have a max input BTU rating however it doesn’t need to have the max input to heat the water.  A typical 40-gallon water heater has a BTU input of around 35-40MBH and a 75 gallon can have around a 75MBH input.  Most situations that need around a 100MBH boiler if you install a 40-gallon water heater your recovery could be around twice as fast as a typical water heater. Some 40-gallon indirect tanks can take up to around 135MBH. A common mistake made when using an indirect tank is to upsize the boiler to reach the rated recovery rate. This is not necessary most of the time. To make sure though an evaluation of the domestic hot water usage may be necessary. Calculation of domestic hot water usage is whole other subject that needs its own article. 

Knowing the pros, cons, and limitations help you educate your customers so they get the system that best fits what their desires and needs are.

Contact the B-Y hydronics specialist with any questions on which solution would be best for your job. 

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