Low-mass heat emitters are essential to the future of hydronic heating.

Figure 1. Photos by John Siegenthaler, P.E.

Each time I sit down to write a column for Supply House Times, I ponder over what technically oriented information about hydronic systems would be helpful to wholesalers. As much as I like to write about the design aspects of hydronics, I understand that not every wholesaler designs systems. However, every wholesaler involved in selling hardware for hydronic systems should be interested in design trends. What concepts will be used in future hydronic systems? How will these systems differ from current or “accepted” practice? What kind of equipment will wholesalers be selling to facilitate these systems? Forewarned is forearmed ...

This month I want to focus on the thermal mass of hydronic heat emitters. Thermal mass refers to the amount of heat the heat emitter must absorb to raise its temperature 1º F. 

Figure 2. A comparison of thermal mass for several common hydronic heat emitters, each having the same heat transfer capability.

Heated floor slabs have very high thermal mass. It takes a lot of heat just to warm the slabs to the point where they can begin releasing heat to the space. This has both good and bad implications. On the plus side, a high-mass heated floor slab allows a space such as a garage to restore comfort very quickly following a period when large overhead doors are open and cold outside air pours in. On the negative side, high-mass heat emitters are very slow to react to desired temperature changes or unexpected interior heat gains.

Some hydronic heat emitters have very low thermal mass. Examples include fin-tube baseboard and compact panel radiators. Such emitters contain minimal amounts of metal and water and can respond to calls for “heat on” vs. “heat off” in a fraction of the time required by a heated floor slab. Think minutes rather than hours.

The coming generation of low-energy-use houses, especially those designed for significant passive solar heat gain, will require heat emitters that can respond quickly. They need to reach normal operating temperatures quickly when “turned on.” They also need to stop emitting heat quickly when

“turned off.” The key to fast response is low thermal mass.

Figure 3. Internal construction of the micro-fan enhanced low-mass panel radiator from Figure 1. Courtesy of JAGA North America.

Super Heavyweight To Flyweight

I recently compared the thermal mass of several hydronic heat emitters. They included heated-floor slabs, cast-iron radiators, tube-and-plate radiant floors, tube-and-plate radiant ceilings, compact-panel radiators, and low-mass panel radiators. An example of each is shown in Figure 1.

To keep it a fair comparison, I calculated the thermal mass of each heat emitter based on it providing 1,000 Btu/hr output while operating with an average water temperature of 110º F, and an assumed room air temperature of 70º F. The low average water temperature reflects favorable operating conditions for contemporary hydronic heat sources such as condensing boilers, heat pumps and solar collectors. Figure 2 shows the results.

This graph shows a huge difference in thermal mass. The low thermal-mass panel radiator has less than 0.5% of the thermal mass of the 4-inch heated floor slab for equivalent output at the same operating conditions. It achieves this low mass by using a fin-tube element with very little water content. These panels can warm up quickly when necessary, such as during recovery from a setback condition. Just as importantly, they can stop heat output quickly when solar energy, or another internal heat gain, presents itself.

Figure 3 shows the internal detail of a very low-mass panel radiator. This product is the “low H2O” panel from JAGA North America. It uses an internal rack of “microfans” sitting above a large fin-tube element to enhance convective heat transfer at low water temperatures.  The microfans only draw about 1 watt of electrical power each at full speed. Their speed is controlled by a small circuit board and temperature sensor supplied with the radiator. The microfans boost low-temperature convective heat output by up to 250%. These panels can operate at water temperatures down to 95º F making them ideal matches for systems using mod/con boilers, solar collectors or hydronic heat pumps.

Figure 4. Wilo Stratos PICO circulator.

From Fast Emitters To A Simple System

The combination of low-mass panel radiators supplied by a simple home run distribution system powered by a variable-speed pressure-regulated circulator is an ideal approach for the coming generation of low-energy-use houses. One example of such a system, with optional heat input from a solar collector array, is shown in Figure 4. (See below for pdf). This system provides both space heating and domestic hot water.