A number of different systems were evaluated. Each was rated for their positives and negatives regarding energy efficiency, space restrictions, and ease of installation. Rinnai ductless heaters emerged as the front runner in initial cost as well as operational cost.

The largest obstacle for the project team was finding a way to gain approval to use a single heater for a multiple room apartment when the applicable codes and standards were written around conventional ducted or distributed systems to provide heat in every space.

The design team met with the U.S. Department of Housing and Urban Development (HUD) to set up the specifications for performance as well as a modeling and testing protocol to show how the Rinnai heaters could meet these parameters. The parameters included maintaining an average bedroom temperature at a minimum of 68°F at three feet above the floor, and an average main room temperature at a maximum of 74° at five feet above the floor (the objective being that the bedroom would be within 6° of the living room where the Rinnai unit was installed). Also, the diversity of the temperatures within the various spaces was to be no more than 3°.

Computational fluid dynamic (CFD) modeling was utilized to model three different heating scenarios, each with two occupants — one standing in the main room and one in the bedroom. Two of the scenarios used different Rinnai heaters as the heat source, while the third used the existing electric baseboard heat.

The CFD modeling showed the temperature differences throughout the apartment and demonstrated that the Rinnai Heater 263 met all the temperature requirements. The average bedroom temperature at three feet above the floor was 67.2°, and the average temperature of the living room at five feet above the floor was 73.2°, a difference in temperature of 6°.

To ensure the Rinnai units were capable of evenly heating apartments containing one or more bedrooms and one or more exterior walls, “log tag” data collectors were installed. The log tags were mounted in various locations to continuously measure the room temperature of each space over a three month period from January to March.

Outdoor air temperatures also were monitored to evaluate significant rises and drops in temperature. Prototype or test installations were completed and field tested to verify the CFD modeling.

The results of the log tag study showed the single bedroom apartment with a Rinnai prototype installed had a more uniform temperature profile than the corresponding space with the existing electric baseboard heat. The unit with the Rinnai unit held the temperature of the adjoining bedroom to within the 6° allowed range. Even with the bedroom door closed, the living room and bedroom’s temperature diversity was within the specified 3°.

The electric heat temperature profile showed inconsistent spikes and drops, whereas the profile of the gas-fired Rinnai units appeared more consistent and smooth. The log tag study verified the CFD modeling and confirmed that all the HUD specified parameters would be met with the installation of the Rinnai units.

Load calculations were completed to size the appropriate unit for each apartment. The new heating system design placed the Rinnai units below the window in the main living room. To obtain optimal circulation of air throughout the apartment, the door to the adjoining bedroom was undercut and a transfer grille installed above the door.

Additionally, fan-powered AirShare units were available for larger spaces, or where appropriately sized gravity transfer grilles would not fit. The results over the past four years have been significant. The average yearly savings in energy costs since the installation of the Rinnai heaters has been $31,500.

The total energy savings over the first five years since switching from electric heat to gas-fired Rinnai heaters was more than $206,128. The project construction cost was $150,000, approximately $1,000 per unit. The units paid for themselves in 4.75 years based on energy savings.

Publication date: 05/07/2012