— By Richard Boothman —
Increasing winter heating efficiency.
Restaurants, retail outlets and shopping centers face plenty of efficiency challenges with heating during the winter. There are, however, easy tweaks, procedures at startup and specialized equipment that can save energy, reduce costs and improve the lifespan of the HVAC system. While oversizing and under-sizing are also common problems, there are ways to minimize their negative effects.
A key step to take early on is to make sure the building is pressurized so that it’s not pulling out more conditioned air than it’s putting in. Set the volume of the unit, the volume of the extraction and the exhaust and make sure that it’s maintained. Outside air is expensive to condition because it’s coming in raw, so any leakage has a big effect on efficiency, whether it be in the summer or winter. Some units can actually monitor building pressure and adjust fan speeds through a variable frequency drive (VFD).
During wintertime, in most parts of the country, the equipment will be operating in heating mode. Standard gas-fired heating equipment operates at an 80% to 82% efficiency level. This is a cutoff point for most heat exchangers. Once more efficiency is gained, the products of combustion will cool to such a point that they will start to condense. Also, a large amount of the heating load will be dedicated to heating up outside air that is being used for ventilation purposes.
At the design stage, a building owner should work with the design professional to specify high efficiency gas furnaces that will operate at 90% efficiency, at the least. The condensate produced from the flue is handled with a collector and drain. Lowering the outside air load can be accomplished by using an energy recovery device such as a heat recovery wheel. Here, air that has cost a significant sum to condition is passed through a heat exchanger whereby it exchanges heat to the cold, incoming air. Significant energy savings can be realized.
Under-sizing equipment will most certainly be a problem as the machine simply won’t be able to maintain conditions at design. Oversizing in heating can be overcome if the gas furnace has a good turndown capability. Any piece of equipment designed with a ventilation requirement will need to have a good turndown. Oversizing is also a problem in cooling as without a modulating control on the compressor, the unit will simply not run long enough to achieve dehumidification. The result is a cold, clammy space.
Flow and Reduced Load
Decoupling the outside air ventilation load from the space load by employing a Dedicated Outside Air System (DOAS) can often help reduce energy consumption. These machines focus purely on the outside air load so the terminal units and space conditioning units are able to focus only on the interior load, which is what they were designed for. In the cold season, it’s often desirable for the DOAS to supply air below the dew point’s set point to further remove latent load from those space conditioning units. This not only reduces load and energy consumption with those units, but can increase indoor air quality and decrease maintenance by minimizing condensate in drain pans, which can be a source of biological growth.
Finally, everything about HVAC is flow — the air side, gas side and water-cooler side. It’s all about flow. Without good flow, the unit runs longer and harder. And that adds to energy consumption. The best way to maintain proper flow is to make sure the filters and the ducts are clean and unrestricted.
Conditioning cold winter air is expensive. But with a few commonsense adjustments during startup and a willingness to invest in the right equipment, spaces can be kept warm in February without breaking the budget. While heating spaces in the wintertime, and trying to be efficient in the process, has been going on forever, a new development has designers thinking about indoor air quality and ventilation in an unprecedented light. COVID-19, the global pandemic that has shut down restaurants, office buildings and shopping centers across the world has forced the industry to consider safety alongside efficiency. Here are six tips to create safer commercial, hospitality and retail spaces:
- Ensuring you are providing at least the building code for the outside air requirement: possibly reducing the coverage of individual units and breaking the space down into smaller sections so as to decrease contamination. We have seen an increase in demand for ionization products that are installed in the equipment.
- Ensure proper exhaust air recirculation rates: According to ASHRI 1060 certification, the proper term is Exhaust Air Transfer Ratio (EATR). An HVAC technician should be able to compare air pressure at the return air inlet and the supply air discharge of the energy recovery wheel. The recommendation is to maintain a higher pressure in the supply air section than the exhaust air section. This should reduce the amount of EATR to 3% or less.
- Do not bypass or disable the energy recovery system: This could result in negative outcomes such as reduced outdoor ventilation and changes in indoor humidity. This question has been asked by those concerned with the current pandemic, but energy recovery systems affect just 3% or less of the incoming air.
- Consider increasing ventilation over building code minimum: It is possible to increase ventilation flow through the DOAS and energy recovery system while maintaining the benefits of energy recovery and humidity control while reducing the chance of airborne infections.
- Clean the energy-recovery wheel: The energy-recovery wheel can be removed and cleaned as needed. We recommend using a mild non-acid cleaner. An additional step using a 3% mixture of hydrogen peroxide will help calm the fears of those concerned with the transfer of viruses.
- Even during these challenging times, a DOAS unit is a cost-effective way to improve air quality. Inherently safe energy-recovery systems make them even more attractive for contractors and building owners.
In this new world created by the pandemic, the HVAC industry will continue to develop ways to create ever healthier indoor spaces for the people who inhabit them. Increasing heating efficiency in the winter is always a worthwhile goal, and with advanced planning, a willingness to embrace developments in technology and a commitment to maintenance, it can certainly be achieved.
— Richard Boothman is director of North American sales at Modine.