Hitting a home run
Your solar combisystem will benefit from this easy-to-install heating system design.
Figure 1. Photo courtesy of Caleffi North America
After reading my Solar Design Notebook in pme’s October issue, perhaps you formed an opinion about what’s the “best” heat emitter for use with solar thermal collectors. That’s fine, but remember no heat emitter can deliver optimal performance without an equally well-thought-out distribution system.
Although many potential piping layouts could serve your purpose, one stands out as the simplest, easiest to install and literally most flexible approach. I refer to it as a home-run distribution system. An example of such a system using panel radiators as heat emitters is shown in Figure 1.
Home-run distribution systems start with a manifold station. Usually it’s the same type of manifold station that would be used in a radiant floor heating system. In Figure 1, the manifold station is shown in a recessed wall mounting. It also can be mounted horizontally under the floor provided it remains accessible.
Paired runs of 1/2-in. PEX or PEX-AL-PEX tubing provide the supply and return from the manifold station to each heat emitter. The flexibility of this tubing allows it to be routed through most framing cavities much like an electrical cable. This is particularly nice in a retrofit situation where the use of rigid tubing would otherwise require some “Sawzall surgery” to walls, ceilings, etc.
Figure 2 shows a home-run system in schematic form. It adds a thermal storage tank as the heat source and a variable-speed pressure-regulated circulator.
Variable-speed pressure-regulated circulators have been in use for more than a decade in Europe and are now available in North America from companies including Grundfos, Wilo, Taco and Xylem. These circulators all use microprocessor-controlled, electronically commutated motors that can operate over a wide range of speeds and in different control modes depending on the application.
For a home-run system, the circulator would be set to operate in a “constant differential pressure” mode. As such, it varies its speed whenever necessary to maintain a constant (installer-set) differential pressure between its inlet and outlet ports. The relationship between the pump curves at different speeds and the various system head loss curves (depending on which zones are active) is shown in Figure 3.
Notice how the yellow dots, which represent hydraulic equilibrium between a given system head loss curve and a given pump curve, all stay on the same horizontal line. This line represents the differential pressure across the manifold of a home-run circuit under design load conditions. Because these points stay on a horizontal line, the differential pressure across the manifold remains constant at this setting regardless of which zones are active. This ensures stable flow rates in all zones at all times.
At full speed, circulators with ECM motors operate on about 50% of the electrical wattage required by standard hydronic circulators of equal capacity. This characteristic, in combination with “intelligent” speed control, delivers annual electrical energy savings of 60% or higher relative to standard wet-rotor circulators. These circulators are quickly raising the performance bar in all types of hydronic systems. Their energy-saving characteristics make them particularly attractive for solar thermal applications where minimizing electrical energy use is a key design goal.
The combination of a home-run distribution system, heat emitters equipped with thermostatic radiator valves and a variable-speed pressure-regulated circulator is a simple yet elegant subsystem for pairing with solar thermal collectors, as well as an auxiliary boiler for those days when the sun isn’t shining so brightly.
The thermostatic radiator valves shown on each panel radiator in Figure 2 constantly monitor the air temperature of the room in which the panel is located. If that temperature drops 1º F or more below the TRV’s temperature setting, the valve’s stem slowly begins opening to allow increased water flow through that panel. This causes a very slight drop in the “hydraulic resistance” of the distribution system, a change quickly detected by the pressure-regulated circulator. Within a few seconds the circulator responds by increasing its speed until the differential pressure it was previously operating at is restored.
The big picture
Now that we’ve discussed several of the state-of-the-art components and design concepts, let’s put them together into a system that leverages their individual qualities. A schematic of one system that does this is shown in Figure 4.
The main component in this system is a well-insulated storage tank with an integral modulating gas burner and internal condensing heat exchanger. The burner fires to keep the water at the top of the tank at a sufficient temperature to provide domestic hot water. Typically this temperature is in the range of 110º to 125ºF.
A drainback-protected solar thermal subsystem is seen on the left side of the system. When the collectors are a few degrees warmer than the water near the bottom of the tank, the collector circulator runs 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 flows back into the tank.
The piping connections on the tank allow the solar subsystem to “work on” the water in the lower (and cooler) portion of the tank. The hottest water remains stratified near the top of the tank where it is available to supply either space heating or domestic water heating.
No antifreeze is required with this design. There also is no need for a heat exchanger between the collectors and the storage tank. These characteristics reduce cost and increase the thermal performance of the solar collectors. The same water that flows through the collectors also flows through the heating distribution system. The system is completely “closed” from the atmosphere and as such protected from oxygen-based corrosion.
The captive air volume at the top of the tank is under slight positive pressure. This air space provides both drainback space and an expansion volume for the system. Assuming proper installation, most of the air initially occupying this space never leaves the system. The oxygen molecules in the initial air charge quickly react with any ferrous metal in the system to form a thin and inconsequential oxide film. The remaining “air” is mostly nitrogen, which is an inert gas and doesn’t cause corrosion. Any air molecules dissolved in the water supplied to the distribution system are gathered by the air separator downstream of the tank and returned back to the air space.
The water in the tank serves three purposes. First, it provides thermal storage for the solar collectors. Second, it provides thermal mass to buffer the highly zoned space heating distribution system. The latter function protects the burner against short operating cycles, which if present would lower efficiency and increase maintenance. Short-cycle protection is very important in systems with extensive zoning. Finally, the thermal mass of the stored water stabilizes domestic water heating.
Domestic water is heated as it is needed. A flow switch detects whenever domestic water is required at a flow rate at or above 0.5 gpm. Under this condition, the switch turns on a small circulator that moves heated water from the top of the thermal storage tank through the primary side of a stainless-steel heat exchanger. Cold water is instantaneously heated as it passes through the other side of the heat exchanger.
An antiscald-rated thermostatic mixing valve protects against high domestic water temperatures when the tank is at an elevated temperature. The latter condition is likely, especially at the end of a sunny and warm day. For the fastest possible response, the piping between the thermal storage tank and heat exchanger should be short and fully insulated. Combination isolation/flushing valves should be installed on the domestic water inlet and outlet of this heat exchanger. They allow the heat exchanger to be isolated from the system and periodically flushed if necessary to remove scale.
A single variable-speed pressure-regulated circulator provides flow to the home-run distribution system for space heating. Each panel radiator has been sized for the maximum heating load of the room it serves based on a supply temperature of 120º. With good piping design, this circulator could supply the entire distribution system in a typical 2,500-sq.-ft. house using no more than 40 watts of electrical power under design load conditions.
Each panel radiator has a thermostatic radiator valve that adjusts the flow rate through its associate panel to maintain the desired room temperature setting.
The three-way motorized mixing valve upstream of the manifold station does two things. First, it acts as a temperature-limiting device to protect against what could be high temperature water in the storage tank after a sunny spring or fall day. Second, it provides outdoor reset control of supply water temperature to the heat emitters to stabilize room temperature for optimum comfort.
Figure 5 (on the following page) shows an alternative system design based on an auxiliary boiler that’s separate from the storage tank. The functionality of this system is almost identical to that of the system in Figure 4. Because the boiler is external to the tank, two additional circulators are required. One circulator provides flow through the boiler and another provides flow in the “primary” circuit from the tank, past the boiler and through the closely spaced tees that interface to the space heating subsystem. These circulators also can be ECM-based, but operate at a fixed speed that provides adequate flow through the boiler and within the primary circuit.
That’s a taste of what modern hydronics heating technology can provide to those designing solar thermal combisystems. When you consider the details, you’ll likely conclude modern hydronics technology is the “glue” that holds nearly all thermally based renewable energy systems together.
It’s also a canvas upon which creative designers can develop unique, reliable and highly energy-efficient heating systems.