Can I use a single water heater for hot water and heating?

The Apollo HydroHeat system was designed to effectively use domestic hot water for heating purposes. This system gained popularity in my locality as it was marketed by the gas company as a convenient solution to retrofit existing homes for gas use.

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(Diagram from user manual)

The innovative approach of this system involved a typical domestic hot water heater, albeit with a significantly higher BTU rating to accommodate the increased demands. A defining feature of the Apollo system was its unique side loop—a set of additional water connectors on the side of the heater that allowed for the circulation of hot water via a fan coil equipped with a pump.

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To prevent legionella growth and maintain adequate heating performance, the hot water heater needed to be set to a substantially high temperature to ensure that the returned water would still be hot enough. This setup necessitated the installation of a tempering valve, which mixed in cold water for safe domestic supply. Unfortunately, many systems were configured without this valve, leading to potential risks.

At some point, Apollo ceased the sale of their system—likely due to various code and safety concerns. However, side loop hot water heaters and fan coils are still commonly available in the market.

One significant challenge encountered with such systems is that traditional heating boiler temperatures hover around 180°F. Unfortunately, standard hot water heaters are incapable of reaching these temperatures. Without the addition of a tempering valve, the temperature must be limited to approximately 120°F for safety reasons, which ultimately restricts the overall heating capability.

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