Energy Efficient Hot Water Boiler Plant Design Part 5: Fan Coils, Pumps & Flow Control

By Chad Edmondson

So you’ve decided to use condensing boilers on your next project – GREAT!  Just remember that this decision will impact other aspects of your mechanical design. 

Condensing boilers do not operate properly in systems that are designed around traditional non-condensing technology.  What do we mean by “traditional, non-condensing technology”?  Systems with much higher return water temperatures.

In the past most boiler plants were designed for supply water temperatures of 180 degrees or even higher to the fan coils.  In order to keep the supply water temperature well above condensing levels, return water temperatures were also very high, with a supply/return Delta T of only about 20 degrees or so.   Water velocities were typically high, and heat transfer surfaces inside the coil could afford to be smaller because of the high water temperature. 

All this is not the case with condensing boilers, which require return water temperatures that are 130 degrees or lower in order to condense (120 degrees is a good design point rule of thumb).  Getting return water temperatures down to condensing levels requires that supply temperatures also be significantly lower – in most cases no higher than 140 degrees although the actual design temperature might be higher.  Remember, the more water vapor your boilers condense the more efficiently they operate, so the lower the temperature (supply and return) the better.  

Adjusting Your Fan Coil Design

Fan coils operating at these lower temperatures and ∆Ts will require increased heat transfer surface to extract then required amount of heat from the supply water to maintain space temperature set points.  Engineers who take the time to adjust their coil design (and they better!) will find that they might need additional rows of coils to maintain sufficient heat transfer. 

Of course, when it comes to heat transfer, one change begets another.  Increased surface area inside the fan coil means increased static head.  Your fan might need to be larger with a higher horsepower.  On the bright side, the system is always going to be running at part load (remember-- condensing boilers operate more efficiently at part load!) so pump energy consumption will be lower as a result of lower flows.

Proper Flow Control

Lower flows necessitate another important design adjustment:  Flow control.  Standard pressure dependent control valves are not suitable for condensing boiler systems because they cannot maintain stable flow rates under fluctuating pressures.  And pressures fluctuate widely in a condensing boiler system as a result of increased part load operation and, more often than not, the variable speed pumps that typically accompany these high performance systems.  Therefore pressure independent control valves (PICVs) should be specified.  These valves are designed to automatically reposition internally whenever there is a change in demand for the space and maintain accurate flow control down to very small load.

More Boilers at Part Load

Finally, in addition to these minor adjustments, condensing boiler systems should be designed with multiple boilers that operate simultaneously at part load whenever and for as long possible.  Condensing boiler efficiency drops significantly as load increases.  However, the long-term operational savings has proven that this additional investment in equipment (if properly designed!) is totally worth it.

Click here to watch our YouTube video on Condensing Boiler Technology.

Download this document on Condensing & Non-Condensing Boilers/Water Heaters.  This is a simple 2-Page "cheatsheet" that covers important topics like condensation, temperatures, venting, piping, and boiler materials.