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Part II■—Drainback Systems

Tom Lane & Ken Olson

©2001 Tom Lane & Ken Olson

Whether your climate freezes one day of the year or every day of the year, your solar water heating system must have 100 percent fail-safe freeze protection. If your freeze protection depends on either human intervention or the availability of electricity, you are living in the shadow of Murphy's law.

Two types of solar hot water systems are most appropriate for freezing climates—drainback systems and closed loop antifreeze systems. Solar Hot Water: A Primer (HP 84) covered the fundamentals of solar water heating systems. Solar Hot Water for Cold Climates (HP 85) covered the principles and components of the closed loop, antifreeze-type, solar water heating system.

In this article, you will learn the inner workings and components that make up the drainback solar water heating system. Some wrenches (system installers) of the frigid north have experienced drainback system failures due to flaws in design or installation. This article presents a culmination of knowledge and experience for freeze-free operation in cold climates. The drainback system is not to be confused with the strongly unrecommended draindown system mentioned in HP84.

Draindown systems rely on an electrically operated draindown valve to fill and drain the solar collectors with domestic tap water under household pressure. In general, draindown systems have had a poor performance history, with frequent freeze-related failures due to hard water, valve malfunctions, and other drain-related failures. By comparison, a drainback system uses a pool of unpressurized water that is completely separate from the pressurized domestic water in the solar storage tank.

Drainback Fundamentals

In a drainback system, the collector fluid, typically

The heart of a drainback system—the raervoirt^lc demineralized water, is circulated from a reservoir tank stores the unpressurized water from the flat plate .. ... . .. . , , , , collectors that ''drains back" whenever the circulating through the solar collectors whenever useful heat can pump shuts off be collected. When the pump shuts off, the water drains

Drain Back Tank Homemade

back by gravity to the reservoir tank. The collectors and all other piping exposed to potential freezing conditions are left empty.

The positive drain of collectors and supply/return piping is the key to the system's freeze protection strategy. This is an unpressurized closed loop—it is closed to renewed supplies of corrosive oxygen associated with

Below the drainback tank, a Ruud solar storage tank with an integral wrap-around heat exchanger stores the potable hot water.

From reservoir tank

Plumbing for hot and cold potable water

Below the drainback tank, a Ruud solar storage tank with an integral wrap-around heat exchanger stores the potable hot water.

From reservoir tank

Plumbing for hot and cold potable water

Homebrew Insulated Hot Water Tank

replenishing water (see Rust Never Sleeps, HP84, page 49). Domestic water is heated by the collector fluid via a heat exchanger, which transfers the heat to water in an insulated storage tank for domestic use.

Drainback System Components

The principle components of a drainback system include solar collector(s), circulating pump or pumps, 80 or 120 gallon (300 or 450 l) storage tank, differential control with sensors, heat exchanger, and reservoir tank. You may also include optional features such as temperature gauges, sight gauge, and flow meter.

Some drainback systems do without a storage tank. Known as "whole house drainback systems," they have a very large reservoir tank that has the heat exchanger submerged inside of it. The heat exchanger is plumbed in series between the cold supply and the house DHW plumbing, and often an on-demand water heater designed to boost preheated water if needed. Frequently, large, single tank systems are used with two internal heat exchangers, one for DHW and the other for space heating. Let's take a closer look at the function, considerations, and recommendations for each of these components in a drainback system.

Collectors & Piping Must Drain

Flat plate solar thermal collectors are the most appropriate type of collector used with drainback systems. They are well suited to low temperature applications (under 150°F; 66°C), such as domestic water heating. They are constructed with parallel tubes connected at top and bottom by header manifolds. Multiple collectors are plumbed in parallel with each other. Flat plate collectors and parallel plumbing have been well described in the solar thermal articles in HP84 and HP85.

The critical aspect in a drainback system is that the collectors and associated plumbing must always be able to freely drain by gravity when the pump shuts off. Collectors must be mounted facing true south (true north for locations in the southern hemisphere), with the parallel tubes (risers) of the absorber plate oriented vertically at a minimum 25 degree (approximately 5 inches rise per 12 inches run) tilt from horizontal. The risers should be a minimum of 1/2 inch ID. Never mount the collectors with the riser tubes oriented horizontally (parallel to the roof eaves). The top and bottom headers of the collector must be 1 inch. The collector's headers must be level and 1-1/2 inch headers must be used if more than three collectors are connected together in parallel.

All piping to and from the collectors must have at least a 10 degree slope (approximately 2 inches rise per 12 inches run) and must be at least 3/4 inch. Smaller pipes

Flat Plate Solar Collectors

Flat Plate Solar Collectors

Temperature Sensor

Note: collectors and piping must have a continuous slope in order to drain.

Boiler Drain:

For filling the system

Static Water Level

Temperature Sensor

Analog

Typical Drainback System

Note: collectors and piping must have a continuous slope in order to drain.

Boiler Drain:

For filling the system

Boiler Drain:

For filling the system

Static Water Level

Analog

Tempering Valve:

Limits hot to house

Differential Control: Triggers pump based on temperature differential

Pump: High head, AC

may freeze due to inadequate draining. All pipes in exterior or unconditioned spaces should be sloped a minimum of 15 degrees (approximately 3 inches rise per 12 inches run). The greatest slope possible is encouraged on the hot return line from the collectors. This facilitates air travel up the pipe so the collectors may drain. Two 45 degree fittings are recommended in place of a single 90 degree elbow to assist drainage in all exterior and unconditioned spaces. All pipes should be supported at least every 4 feet (1.2 m) to prevent sagging, which may contribute to inadequate draining.

Reservoir Tank

The drainback reservoir tank holds the collector fluid. It can be installed at any height above the solar storage tank, but must be within a conditioned or freeze-free environment. The reservoir tank is typically mounted on a strong shelf above the storage tank. It may also be placed on the second floor of a two-story home to reduce static head by decreasing the distance the pump must lift the fluid to the collector.

In the tax credit era, it was possible to buy an insulated copper drainback reservoir tank with a sight glass. This is a clear glass tube on the side of the drainback reservoir that enables you to view the water level before and after the pump starts up. The sight glass allows you to monitor the level of fluid so you can ensure that the fluid is always above the circulating pump, and so you can detect fluid loss from leakage.

A collector manufacturer, SunEarth, is making a copper drainback reservoir tank that is not equipped with a

Hot to House

Boiler Drain:

For filling the system

Tempering Valve:

Limits hot to house

Hot to House

Differential Control: Triggers pump based on temperature differential

Component Boiler Tempering Line

Pump: High head, AC

sight glass. Alternatively, you can simply buy a 10 or 20 gallon (38 or 76 l) conventional water heater to serve as a reservoir. Mobile home parts distributors sell these tanks inexpensively. Install the reservoir tank on the return line from the collector above the heat exchanger and storage tank. The fluid returning from the collector must enter the airspace at the top of the resevoir tank. This allows air to move up into the collectors and piping so they can drain. Caution! If you are using a conventional water heater as drainback resevoir, it is very important to remove the dip tube from the cold inlet port. Otherwise the dip tube will impede draining of the collectors by preventing air from rising up to the collectors.

You may use a clear flow meter as a sight glass. It should be on the feed side line to the collector located at the same exact height as the top of the drainback reservoir tank. Water will seek its own level after the pump cuts off, and it should be near the top 1/4 inch of the flow meter. Flow meters are available in 2 gallon (7.6 l) per minute (gpm) to 16 gpm (60 lpm) sizes from Letro (LE-LD359B or LE-LDF359T) or Blue & White (F-450LHB). Contractor prices are about US$40 each.

A true drainback system is an unpressurized closed loop. Some early drainback system designs incorporated vacuum breakers and automatic air vents in the collector loop. These features are mistakenly borrowed from the open loop, draindown system design. This atmospheric venting of drainback systems is unnecessary, often problematic, and not recommended.

Controller & Sensors

The controller turns the circulating pump on and off. It is a differential type control, identical to that used in closed loop antifreeze systems. A thermistor-type sensor is fastened to the collector outlet pipe. Another is fastened directly to the solar storage tank wall, under the insulation, at the bottom of the tank. The sensors are wired to the controller with #18 (0.8 mm2), two-wire, stranded PVC double jacket exterior wire.

When the outlet temperature of the collectors is 20°F (11°C) greater than the bottom of the solar storage tank, the controller activates the circulating pump. The controller shuts the circulating pump off when this temperature differential is reduced to 5°F (2.8°C), and the system drains back by gravity to the reservoir tank.

Many controls have adjustable differential settings. Differential controls are also equipped with a high limit cut-out feature. If the temperature of the solar storage tank reaches a preset high limit, usually 180°F (82°C), the controller turns the circulating pump off, and the fluid drains back to the reservoir tank. High quality differential controls are manufactured by Heliotrope Thermal and Goldline Controls.

A typical differential pump controller— the Heliotrope Delta-T.

A typical differential pump controller— the Heliotrope Delta-T.

Delta Differential
The Taco 009BF is a typical high head AC centrifugal pump appropriate for use in drainback systems.

Circulating Pumps

Drainback systems use a high head, AC centrifugal circulating pump operated by a differential controller. Centrifugal circulating pumps are most commonly used in solar hot water systems and hydronic heating systems. These are appropriate because of their efficiency, reliability, low maintenance, and low energy consumption. They also permit backflow of fluid when not operating. This is a critical aspect of their function in a drainback system.

Head is the pressure that a pump must overcome to circulate the collector fluid. Static head is the pressure a pump must overcome to lift the water to the highest point in the system. The static head is equal to the vertical distance between the lowest water level and the highest point in the system. The lowest water level occurs when the pump has lifted water to the highest point in the system and before return flow has begun. Dynamic head, or friction head, takes into account the pressure a pump must develop in order to overcome the frictional resistance of the fluid moving through the pipes and fittings.

In the case of the drainback system, the circulating pump must start up at full speed and develop full head in order to lift the water to the highest point in the system and overcome the static head of the system. Once the fluid begins to fall down the return pipe, the syphon effect reduces the static head to a minimum, and the flow rate increases.

Side-Arm Heat Exchanger

Drain Riser

Pump:

High head, AC

Drain

Pump:

High head, AC

Drain

This high head requirement generally precludes the use of a DC circulating pump powered directly by a PV module because the system will not fill until sufficient pressure is developed. So AC powered circulating pumps are most common. A battery powered DC circulating pump is an option, but not readily available in high head models.

The Grundfos UP 26-96BF, which can lift to a maximum head of 30 feet (9 m) of static head, or the TACO 009 BF, which can lift to a maximum head of 35 feet (10.7 m), are the two most commonly used pumps. The TACO 009 BF draws the least power. You can anticipate that these pumps should last fifteen years in a drainback system. They are lubricated by the fluid they pump.

The circulating pump should be located a minimum of 4 feet (1.2 m) below the lowest water level in the drainback reservoir to avoid sucking air into the pump. The formation of air bubbles on the suction side of the pump impeller, called cavitation, increases the load on the circulating pump. It can reduce or stop fluid movement altogether, and will burn out the pump motor. It is also preferable that the pump be mounted vertically on the cool side of the heat exchanger. Operating at a lower temperature may increase the life of the circulating pump.

Heat Exchangers & Tanks

The heat exchanger transfers heat from the collector loop to the domestic water loop. A good heat exchanger will have a large surface area, and be constructed of a highly conductive material such as copper.

Heat exchangers may be integral to the tank's wall, or have a single or double wall between the two fluids. Of course, a single wall is more effective, but it may not be code compliant. Even using water as the heat transfer fluid, safety-minded mechanical inspectors recognize that a nonpotable heat transfer fluid might be used in the future. This would put occupants at risk of contamination of the potable water supply.

Heat exchangers may also be internal (within the storage tank), or external, as a separate component. An external heat exchanger, often referred to as as a side-arm heat exchanger, allows easier access for servicing (cleaning and descaling) if you add cut-off valves and a means of access.

A separate circulating pump on the water side of the side-arm heat exchanger is recommended. If you rely only on thermosyphon flow on the water side, it will be 30 to 40 percent less efficient than a double pumped system, and 20 percent less than a tank with a built in heat exchanger. When using an external heat exchanger, the two fluids should flow through it in opposite directions. This optimizes heat exchanger effectiveness by maintaining the maximum average temperature differential between the two fluids.

Cutaway of a Stone Lined Tank with Integrated Heat Exchanger

Hot Out

Insulation

Backup Heat:

Electric shown

Backup Heat:

Electric shown

Hot Out

Insulation

Double Linedtank

Cold In

Heat Exchanger:

Single or double wall, removable

Stone Lining:

1/2 inch cement

Cold In

Heat Exchanger:

Single or double wall, removable

Stone Lining:

1/2 inch cement

Pink Fairy Armadillo
An internal, integrated (and removable), fin-type heat exchanger.

System Sizing & Costs

System sizing is determined by the daily volume of water usage, required temperature rise, and the local climate. Performance will be affected by the size of solar collector and the size of the solar storage tank. A solar contractor in your area may have the means of providing a detailed performance analysis, which is beyond the scope of this article. Here are some useful guidelines to get you in the ballpark.

As of 2001, there are only two brands of closed loop tanks with integrated heat exchangers that wrap around the outside of the tank wall (making it scale proof), and are foam-insulated in place—Rheem and Ruud. These are both made by Rheem Manufacturing. Rheem is one of the largest manufacturers of hot water heaters, and makes an 80 gallon (300 l) tank with a wrap-around copper heat exchanger. They are commonly available nationwide and generally adequate for a family of four. Ask for the 80 gallon Solar HE (heat exchanger) model. I hope that manufacturers will bring back the 100 and 120 gallon (380 and 450 l) tanks with heat exchangers that were available during the tax credit era of the '70s and '80s.

Always bring the collector return line into the top side heat exchange opening on the Rheem/Ruud tanks, and out the bottom to return to the collectors. This configuration ensures that you are supplying the collectors with the lowest temperature possible, and is 20 percent more effective than plumbing it in reverse.

Vaughn Corp., a small company in Salisbury, Massachusetts, makes 65, 80, and 120 gallon (250, 300, and 450 l) tanks (trade name Sepco) with bolt-on, removable, single or double wall, vented heat exchangers. The heat exchanger is located at the bottom of the tank and is finned to offer greater surface area, which increases heat exchange effectiveness.

Unlike the Rheem/Ruud glass lined tanks, Sepco tanks are stone lined. These tanks typically last much longer than a standard glass lined tank. The stone (1/2 inch cement) lining makes them heavy, so you will need two people and an appliance dolly to move them into place. Do not use the less expensive, higher performing, single wall heat exchanger unless you check your local codes for compliance, even with water as the heat transfer fluid.

The first step is to determine your hot water needs. Widely accepted estimates for residential hot water consumption are 20 gallons (75 l) per day for the first two people and 15 gallons (55 l) per day for each additional person. Size your tank according to your daily hot water requirement. Tanks come in standard sizes, so don't fret over trying to be too exact. Just get the available tank size closest to your estimate.

The second step is to determine the square footage of solar collector, based on the size of your solar storage tank. You can use the following generally accepted rules of thumb offered by AAA Solar, for each of four climatic regions of the U.S.

• Sunbelt: 1 square foot (0.09 m2) of collector per 2 gallons (7.6 l) of storage.

• Southeast and Mountain states: 1 square foot of collector per 1.5 gallons (5.7 l).

• Midwest and Atlantic states: 1 square foot of collector per 1.0 gallons (3.8 l).

• New England and the Northwest: 1 square foot of collector per 0.75 gallons (2.8 l).

Here's an example. A household of four will consume an estimated 70 gallons (265 l) per day, so use an 80 gallon (300 l) solar storage tank. Based on your location, your collector area will be on the order of 40 square feet (3.7 m2) in Arizona, 55 square feet (5.1 m2) in Georgia, 80 square feet (7.4 m2) in Kansas, and 106 square feet (9.8 m2) in New Hampshire.

Typical Drainback System Sizing & Costs

Gallons HE Tank

Collector Area (square feet)

Number of People

FSEC BTU Rating*

WH Per Day

Cost (US$)

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