This month, I am continuing my series on commissioning “simple” HVAC equipment with hydronic unit heaters. These are about as ubiquitous and low-tech as any equipment in a new building and, therefore, are prone to being nearly ignored by designers, contractors, and commissioning professionals.

Although simple, unit heaters are typically very important and used where the risk of freeze damage is high (e.g., vestibules, utility rooms, stairwells, loading docks). In addition to these devices needing to provide heat upon demand during cold weather, the design team may specify controls intended to minimize energy consumption when heating is not needed. Therefore, the commissioning professional must take them seriously during design review, controls integration, balancing, and testing.

Unit heater physical configurations are, by definition, almost universally the same: one fan drawing air from the conditioned space and pushing it through a heating coil back into the conditioned space. This month we are talking only about hydronic unit heaters, i.e., those piped to the building’s heating hot water system.

The unit heater elements that change from project-to-project are the controls. The figures illustrate three of the most common control schemes for hot water unit heaters.

Figure 1 is the simplest with a thermostat turning the unit heater fan on upon a call for heating and off when the thermostat is satisfied. The coil piping is open to the hydronic system and, if there is hot water flowing, the unit heater will deliver heat when the fan is on. If there is no hot water flow, the fan will run continuously without adding any heat to the space.

Figure 2 is the same as Figure 1 with the addition of an aquastat on the hot water return pipe. The aquastat senses whether there is hot water flowing through the unit heater coil and passes the thermostat call for heat to the fan only if the pipe temperature is above a certain set point. In this way, the fan only operates if there is a chance for the unit heater to actually heat the space. It is slightly more energy efficient than Figure 1.

Figure 3 is one of the most energy-efficient approaches to controlling hydronic unit heaters. A two-position control valve on the hot water pipe only opens upon a call for heat from the thermostat. Once hot water flow is sensed by the aquastat, a signal from the aquastat starts the fan. Of course, there could be other wiring strategies for these same components. The key to this strategy is only flowing hot water through the coil when it is needed, thus reducing both a negligible amount of pump energy and a significant amount of unnecessary radiation heat transfer from the coil. 

The commissioning professional needs to understand the intended unit heater operation during the design phase, verifying that the designers unambiguously define the control components and sequence of operation. Clearly, there are first-cost, performance, and operational cost implications with each of these three options. The commissioning professional will most likely be the team member to review these pros and cons with the owner. This can be a very brief conversation, but the design phase is the time to have it, not after the project is complete and the owner realizes these “little” items are not what the owner expected. ES