A modern approach to a traditional means of hydronic heating.

Figure 1. Photo credit: Emerson Swan

Some of you probably remember the 1970s commercial that asked the rhetorical question: What’s more American than baseball, hot dogs, apple pie and Chevrolet?

If the focus were narrowed to hydronic heating, the all-American classic heat emitter would be fin-tube baseboard. Originally developed in the 1930s as a substitute for cast-iron radiators, fin-tube baseboard has not changed much in several decades. Residential-grade baseboard is now a commodity product in the hydronics market. When paired with a higher temperature boiler, it’s hard to beat from the standpoint of dollars per Btu/hr. output.

I’ve learned some things about the appeal of baseboard. In one situation I reluctantly presented it as a lower-cost “downgrade” to panel radiators. I was then promptly informed that the look of baseboard was very acceptable, but there was no way those panel radiators were going on a certain person’s walls. Even fancy Euro-style panel rads got rebuffed in favor of common baseboard.

People gravitate toward familiar products. Those who’ve grown up in the northeastern United States recognize baseboard in homes and other buildings. Although few think that fin-tube baseboard enhances the aesthetics of a room, most will tolerate its presence because they understand what it is and why it’s there.

Aesthetic issues aside, traditional baseboard makes the most sense when combined with a conventional boiler, one that cranks out water in the range of 170 to 200˚F under design load conditions. Such water temperatures allow baseboard to heat average rooms without need of installing the product around the entire perimeter of the room.

Traditional baseboard trades low material content and installed cost for the availability of higher water temperature. In the days when fuel oil cost 50 cents per gallon, this was acceptable. With the average price of fuel oil now hovering around $4 per gallon, it’s harder to justify.

The widespread availability of mod/con boilers, along with renewable energy heat sources such as solar thermal collectors and heat pumps, has pushed the hydronic heating industry toward lower-temperature distribution systems. Low water temperatures are a prerequisite to achieving high thermal efficiency with such heat sources. We discussed this back in the June 2011 Hydronics Workshop column, titled “Lowered expectations.” (You can find this in the archives atwww.PMmag.com.) 

This leaves traditional fin-tube baseboard in a difficult spot. Choosing it as a heat emitter and sizing it around design water temperatures of 170˚ to 200˚ usually limits the thermal efficiency of a mod/con boiler to the upper 80s under design load. Progressive designers striving for high levels of energy efficiency often see this as a significant handicap, and thus view baseboard as an outdated product not deserving of use in modern systems.

Figure 2. Photo credit: Emerson Swan

New kid in town

A couple of years ago,Al Hansonfrom manufacturers rep firm Emerson Swan Co. sent me information on a new type of fin-tube baseboard. It was called Heating Edge, made by Smith’s Environmental Products, and had an external form factor very similar to traditional residential-class baseboard (about 3 in. wide and 7 in. tall), as seen inFigure 1.

However, what’s “under the hood” of Heating Edge baseboard is very different, as seen inFigure 2.Heating Edge baseboard has much larger fins than traditional baseboard. The fin area is about three times larger, almost filling the entire space within the enclosure. It has two 3/4-in. copper tubes running through those fins. The tubes can either be piped for parallel flow, counter flow or in series. In the latter case, the hottest water flows down the upper tube, makes a U-turn at the end and flows back along the lower tube.

Assuming anaveragewater temperature of 110˚, Heating Edge baseboard releases about 290 Btu/hr./ft. when the two pipes are configured for parallel flow, and the total flow rate through the element is 1 gpm (0.5 gpm through each tube). This increases to about 345 Btu/hr./ft. with a total flow rate of 4 gpm (e.g., 2 gpm per tube). Both of these ratings include the 15% heating effect factor that is often added to the tested thermal performance of baseboard.

If the two tubes are configured for series flow (hot along top and return along bottom), the output at 1 gpm flow rate drops about 10%.

Consider a 12-ft.-by-16-ft. room in a well-insulated building with a design heating load of 15 Btu/hr./ft2. The room’s design heating load is 2,880 Btu/hr. This could be handled by a 10-foot length of Heating Edge baseboard operating at anaverage water temperature of 110˚and 1 gpm flow rate. To produce equivalent output, a 10-ft. length of conventional residential baseboard would require an average water temperature of about 150˚.

Figure 3.

Keeping it simple

The schematic inFigure 3shows one way to configure Heating Edge baseboard within a modern residential system that also supplies domestic hot water.

The “anchor component” in this system is a well-insulated heating appliance that offers several features, including:
  • A modulating gas burner and internal condensing heat exchanger;

  • Plenty of thermal mass (water) to stabilize burner operation, even with extensive zoning;

  • A drainback-protected solar thermal subsystem;

  • An instantaneous DHW-generating subsystem using a stainless-steel heat exchanger; and

  •   A self-contained captive air volume that serves as both an expansion tank and drainback reservoir.

The solar thermal subsystem adds heat to the lower portion of the storage tank whenever possible. It uses drainback freeze protection. When the collectors are a few degrees warmer than the tank, the collector circulator operates to create flow through the collector array. When the collector temperature cools relative to the tank temperature, this circulator turns off, and all water in the collectors and external piping flow back into the tank.

A captive air volume at the top of the tank, under slight positive pressure, provides both drainback space and an expansion volume for the system. On sunny days the collectors may keep the tank well above the required temperature for either space heating or domestic hot water. Both situations are addressed through the use of mixing devices.

When there’s a draw for domestic hot water, a flow switch, set for 0.5 gpm, turns on a low-power, variable-speed circulator that moves hot water from the top of the storage tank and through the primary side of a stainless-steel, brazed-plate heat exchanger. Cold domestic water flows through the other side of this heat exchanger and is instantly heated. The speed of this circulator is controlled based on the temperature of the leaving domestic hot water. If the leaving water temperature drops, the circulator speeds up to increase the flow of hot water through the primary side of the heat exchanger, and vice versa.

Another option would be to use a fixed-speed circulator in combination with an anti-scald-rated thermostatic mixing valve on the domestic side of the heat exchanger (shown in breakout detail in Figure 3).

A single ECM-based, pressure-regulated circulator provides flow to a homerun distribution system for space heating. Each length of Heating Edge baseboard is supplied by its own 1/2-in. PEX or PEX-AL-PEX supply and return tube. With good piping design, this circulator could supply the entire distribution system under design load conditions using no more than 40 watts of electrical power.

Each baseboard has an adjustable thermostatic radiator valve that monitors room temperature and adjusts flow rate as needed to maintain that temperature. No wires, no batteries, no transformers, no programming - just simple, effective and reliable room-by-room temperature control.

The three-way motorized mixing valve operates using outdoor reset control to provide the optimum supply water temperature to the baseboards.

Coming of age

Before learning of the Heating Edge product, I witnessed very little evolution of fin-tube baseboard to meet the reduced water temperature requirements of current and future hydronic systems. I viewed fin-tube baseboard as a commodity product with an ever-declining share of the new systems market. A relic of the days when high-temperature water and low installed cost always trumped fuel efficiency.

It’s reassuring to see progressive thinkers willing to retool to provide a product that’s in sync with the direction of modern hydronic heating. I certainly recommend you keep this product as part of your design repertoire.