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Heating Plant Boiler Retrofits (Design)

The premise of condensing technology is to have low return water temperatures and as the low temperature water passes through the boiler, it reduces the temperature of the flue gas much lower than older non-condensing boilers. As the flue gas temperature drops below 130F, water will begin to condense out of the flue gas. A non-condensing boiler is not designed to handle this corrosive flue gas condensate which would cause degradation of the heat exchanger. Where as condensing boilers commonly have stainless steel heat exchangers designed to withstand condensate formation.

Typically, boiler plant distribution piping and terminal units such as baseboard radiators were previously designed to operate at high temperatures, constant flow, and a 20 degree ∆T. The result is that without altering the distribution piping, the retrofit boiler plant has restricted operating conditions. For the majority of the heating season the system is required to operate high temperatures and the return water temperature will be above 130F. This will limit the effectiveness of retrofitting with condensing boilers.

If return water temperatures are above 130F, there is no difference in efficiency between near-condensing and condensing boilers. However, the shoulder seasons (Spring and Fall) present some opportunity to use condensing technology. The heating system can utilize a temperature reset strategy to reduce the system temperature during mild weather. As the system temperature is lowered the efficiency of condensing boilers increases.

Given there are only limited times during the heating season that the condensing boilers provide improved efficiency it doesn’t merit installing a full boiler plant with multiple condensing boilers. The increased cost of the initial installation would not payback. The energy savings usually would not be enough to justify the more costly boilers over the lifespan of the system.

To realize the benefits of a condensing boiler in the shoulder seasons and the low initial cost of near-condensing, it’s a good strategy to install one condensing boiler and the remainder near-condensing. Multiple boilers piped in a primary secondary arrangement is a common retrofit strategy because the new boilers are hydronically separated from the existing system. The concept is to completely separate the primary and secondary circuits. Therefore, the secondary circuit pump head has no effect on the primary and vise-versa. This setup also makes it simple to prioritize which boilers operate under what conditions. The system controller can set the condensing boiler as the Priority 1 boiler when return water temperatures are below 140F and similarly set the near-condensing boilers as Priority 1 when the return water is above 140F. This promotes good boiler rotation and ensures the system is operating at its highest efficiency.

To conclude, heating plants retrofit with a lead condensing boiler combined with near-condensing boilers can provide optimal energy savings without the premium price tag.