Design Considerations for High Efficiency Condensing Boiler Systems

In part 1 of this blog, we discovered and defined a High Efficiency Condensing Boiler (HECB). Current efficiency standards require all boilers to follow suite with high efficiency energy consumption standards. Soon standard efficiency (less/equal than 85%) boilers (SEB) will be obsolete, so two questions we should be asking:

1) How to design hydronic systems for high efficiency boilers?

2) How to integrate HECB into SEB systems?

In Part 2 we will discuss these questions, and the first question has direct answers.

To answer the first question, the designer needs to ensure that the return water temperature is at or below the condensing temperature of the flue ((130 ֯ F)). This can be done by various methods:

A) For new Systems: Design with lower building supply temperatures, thus lowering your return water temperature. In the southeast US, many new buildings are well heated with 130F supply water.

B) Increase you ΔT to (30 ֯ F or 40 ֯ F). This allows for the lowering the return water temperatures to the condensing boilers.(Be careful that too large a delta T does not require an increase in your size of water heating coils.)

C) Incorporate outdoor air reset. Equipping the boiler with a sensor that detects outdoor air temperature. The building supply water temperature can be lowered during lower heat load periods, which allow more hours of operation in the condensing mode.

The second question is more complicated. Any time a boiler retrofit is incorporated, you must consider the existing available space, piping, heat exchangers, coils, flues, etc. Depending on the age of the existing boiler to be replaced, the heat load should be reevaluated to determine if the system was initially oversized(HINT: It was!). If there are improvements to the envelope, then the system will be oversized. All three of the previous options should be considered to squeeze as much energy out of your system as possible. One change that is often overlooked, but would need to take place, is the flue venting. Typically, SEB are vented with Category I type B-venting. HECB require Cat II or Cat IV venting that can withstand the acidic condensate the occurs in the flue gases usually constructed of stainless steel or Poly Propylene. Some boiler and water heater manufacturers have literature that allows for the use of PVC or CPVC. We never recommend PVC or CPVC because they are not UL-Listed flue vent materials and are not worth the risk, especially considering PolyPro is of similar cost. See UL 1738 for more information on CAT II, III, and VI venting.

Tune in later for “PART 3 – Kids These Days are Talking Turn – Down: Is Higher Really Better?”!