Strategy for existing buildings with steam boiler systems in extremely cold and dry climate zones (6A, 6B, 7, 8)

Step 1 - Load Reduction Strategies

For an existing building in an extremely cold and dry climate zone that uses a steam boiler system for building heat, domestic hot water, and sterilization, the priority is to reduce the load on the boiler system. You'll need to move away from fuel-fired equipment, of course, in order to truly decarbonize, but let's focus first on reduction - which has the dual benefit of both lowering your carbon emissions today and reducing the load that you need to convert to another source.

1. Reduce internal cooling loads and minimize reheat through variable air volume ventilation and retrocommissioning. The most cost-effective thing you can do is use less energy - whether fuel-based or electric. Retrocommissioning, as we use it in this guidebook, is a catch-all term for getting your house in order and making everything work at its most efficient. This is often referred to as "blocking and tackling." We have guidance for you in the VAV and Retrocommissioning sections. But, there's a prerequisite for most of these strategies - you really need to have a modern, programmable, DDC control system. Pneumatics and legacy controls just don't offer the capability that you need to optimize your building systems' performance. A major controls upgrade can be a major investment, but properly implemented, it pays dividends quickly and for years to come.
2. Implement exhaust air energy recovery. Outdoor air ventilation presents both heating and humidification loads, while both heat and humidity are exhausted from the building. Energy recovery systems can recover both heat and moisture from the exhaust air stream, though moisture recovery is limited to systems (such as energy wheels) where the potential for cross-contamination is acceptable. Where energy wheels not permitted, run-around coils or air-to-air heat exchangers can recover sensible energy, reducing the load on the heating plant. Consider using the exhaust air as a source of heat beyond simple air-to-air recovery, using heat pumps to strip more heat from the exhaust air.

Step 2 - Heat Recovery and Hydronic Heating

Convert to hydronic heating. Most hospitals that rely on steam heating plants distribute heating energy via a medium- or high-pressure steam network. that makes sense for a gas-fired steam boiler, but it's pretty much a dead-end for decarbonized heating. Low-carbon heating sources are almost exclusively low-temperature and rely on water for transport through the building. So, while converting to hydronic distribution does not, by itself, reduce carbon, it is necessary to enable the low-carbon future. Unfortunately, it's one of the most expensive aspects of decarbonizing a heating plant. Hydronic conversion will likely be done in concert with one or more of the following:

1. Electrify your heating plant with heat pumps and heat recovery chillers. When we talk about electrification, we do not mean a one-to-one replacement of steam boilers with electric boilers. In fact, unless you're in a location with a very clean power grid, a straight conversion would actually increase carbon emissions. The benefit of electrification comes through the use of heat pumps, which use electricity to move existing heat rather than make new heat. Heat pumps and heat recovery chillers are essentially the same thing - they move heat from where we don't need it to where we do need it, by raising it to a useful temperature. In the case of heat recovery chillers, we're moving heat out of the chilled water system (making it colder) and into a heating hot water system (making it warmer). Heat pumps may transfer heat from outdoor air, from a thermal storage tank, from a lake, or from the ground. Because they are transferring heat rather than creating it, they are 2.5 to 5 times as "efficient" as electric heat.
2. Use heat pumps to replace gas-fired boilers for domestic hot water. Heat pumps can also be used to heat water for domestic use - either as part of a heating hot water system, or as a dedicated water heating system.
3. Use thermal energy storage with renewable solar thermal energy and heat recovery for load shifting and handling nighttime heating challenges. When the outdoor temperature dips below about 30 degF (for most hospitals), there won't be enough heat in the building to overcome the losses of the building envelope and ventilation losses. You'll need to supplement heat recovery with another source. Thermal storage is one way to manage that load, and there are numerous options for thermal energy storage, including stratified tanks, phase change systems that make ice or that use a custom-blended material, and using the earth itself as a storage medium through ground-source heat pump or borehole thermal energy storage systems. Consider other sources of heat: plumbing wastewater; constant cooling loads such as data centers, medical equipment, or kitchen refrigeration condensers; exhaust air - any reliable stream of air or water that is warmer than outdoor air can be a source of heat. In some cases fuel-fired hydronic boilers may still be required to meet cold-weather loads, as a supplement to heat recovery.
4. Convert to electric or adiabatic humidification. Humidification loads can be significant in cold, dry climates. Energy recovery from exhaust air is the first step. In the absence of a steam boiler, electric steam generators can provide a relatively economical source for humidification, but ASHRAE 170 recently approved the use of adiabatic humidifiers, which do not require a heating source, other than the air itself. That's an important point about adiabatic humidification - it reduces the air temperature. In warm, dry weather, the cooling effect is beneficial, but in cold weather, additional heat is needed to prevent over-cooling.
5. Convert to electric steam generation for sterilization. While electric generation is not necessarily lower carbon than gas-fired boilers, they may make sense if your only steam load is sterilization. They have the added benefit of reduced maintenance and will ride the curve of the grid as it decarbonizes.
6. Consider solar thermal steam generators. Solar thermal steam generators can use solar energy to store heat in a thermal fluid, which is then used to create steam. While obviously a zero-carbon heating source, these systems can be costly to design and install.