Hydronic Circulators

Mon, July 07, 2016

Don't Just "Go with the Flow"

Most modern hydronic heating systems use circulating pumps to move water through the piping system, to distribute the energy in the water to the radiation that heats the space. In previous articles, we have discussed one of the formulas we use most often when evaluating and designing hydronic heating systems: BTU/hr = GPM x Delta T x 500. This formula gives us a way of calculating the proper flow to deliver heat to a system. We have also discussed head loss calculations and means of estimating head losses. What we haven't discussed is why it is important to have the correct flow? Unfortunately, pumps don't know how much they are supposed to pump. They provide as much flow as the system will accept, until friction losses limit their capacity. With constant speed pumps, that can be anywhere along the pump curve. To understand the importance of proper flow, it is important to understand a little about how the emitters work. After a designer has done a heat loss calculation, they must selects radiation, and they should specify three pieces of information: Output (BTU/hr.), Supply Temperature, and Differential Temperature. Then, using the formula above, design flow is determined. What happens then if the flow is wrong? If the flow is too low, the differential temperature across the emitter is too large, and the average temperature of the emitter can be too low to provide enough heat for the space. Worse-case, the flow could be so low that it would be 'laminar', or without turbulence and be unable to give off much heat. There are also problems associated with flow being too high. If the fluid velocity is above 8 ft./sec. or so, noise and erosion can be an issue. If the delta T across the system is too short, the average temperature of the radiation will be above what the designer expected, and the space could overheat, and/or cycle excessively. When a pump is providing too much flow, it is using more energy than it needs to. The inefficiencies created by incorrect flow can go back to the boiler plant where, as we have discussed in earlier articles, boiler efficiency is tied to return water temperature, and combustion efficiency is affected by firing cycle time. Flow sensitive boilers can be harmed, or be unable to sustain firing without adequate flow. The 'packaged' pump that comes with many boilers may, or may not be correctly sized for the actual piping arrangement on the job. Residential circulators are often arbitrarily selected, and 'one size fits all' in nature. It is important to properly select your circulators, and if possible, verify that the flow is correct. Some pump manufacturers are able to provide multi-speed, and variable speed pumps that can be hard-set for a discrete speed, or controlled to a process variable to maintain proper flow. Variable speed pumps can even be used for injection mixing to provide for setpoint control, and outdoor reset. Historically, one of the appeals of hydronic heating systems is their forgiving nature. That is partly because if people got enough heat, they were happy. This forgiving nature often leads to heating systems with oversized components and inefficient operation. Now, with rising heating costs, systems are designed to maximize both comfort and efficiency. Properly sized and controlled circulators which are individually selected for their application and location in the system play a key role in attaining these goals.