Simple Drainback Thermosiphon Combi-system Design.
The economics of solar hot water are improved when domestic hot water heating is combined
with other applications such as radiant floor heat, snow melt, hot tub or pool heating. The
solar combi-system’s costs are reduced and performance enhanced by plumbing the solar heat
exchange tank in thermosiphon with a standard hot water backup system. Load side heat
exchange designs such as the Simple Drainback are particularly well adapted for thermosiphon
and combi-system use.
Below are images of the Simple Drainback system plumbed in thermosiphon with a standard
hot water tank. The system on the right is a test system, with flow meters and temperature
sensors attached. This test system records and reports the effectiveness of the solar system
and thermosiphoning between tanks. Both systems are combi-system plumbed.
Exhibit #1- Simple Drainback tanks plumbed in thermosiphon.
LOAD SIDE HEAT EXCHANGE SYSTEMS
Freeze protected solar thermal systems are divided into two camps: drainback and glycol
systems. Glycol systems feature ‘collector side’ heat exchange. Drainback systems are ‘load
side’ designs. While both designs offer freeze protection, the drainback system’s load side
design offers simplicity and flexibility not found in a glycol system’s collector side design. Load
side systems are easily adapted for thermosiphon and combi-system designs.
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Exhibit #2 below illustrates collector side verses load side heat exchange combi-system designs.
In this example, a domestic hot water (DHW) system is combined with a radiant application.
For the radiant application, additional storage is depicted.
Shown at the top of Exhibit #2 is a classic
supply-side, or collector side heat
exchange glycol system. Collector fluid is
circulated inside a small heat exchange
coil at the bottom of a tank. The heat
exchanger at the bottom of a storage
tank transfers heat from the collector
fluid inside of the coil to the potable
water in the tank. Glycol based collector
fluid in the collector loop is pressurized,
and an expansion tank is necessary to
accommodate changes in system
pressure. The glycol solution in the
collector fluid provides freeze protection.
Overheat protection afforded by heat
dumps or radiators. Exhibit #2- Load and Supply side Heat Exchange
In contrast, a drainback’s load side heat
exchange system features solar fluid filling the tank, with potable water remaining inside the
heat exchange loop. The Simple Drainback’s heat exchange coil is located at the top of the
tank. Drainback systems utilize on/off pump control strategies plus gravity to protect collectors
from both freezing and overheating.
Drainback systems are sometimes described as closed loop unpressurized, or atmospheric
systems. The collector loop in a drainback system is filled with tap water at atmospheric
pressure before the collector loop is sealed. Even though this system is referred to as
atmospheric or unpressurized, in actual practice there are variable pressures within the solar
loop as heated water expands and contracts. Observed pressures in the Simple Drainback
system range from 3-5 pounds in normal operations to 14 pounds in a rare steam back event.
The addition of an expansion tank is avoided by leaving 2-3 gallons of air in the top of the
Simple Drainback tank. This air space allows for thermal expansion and provides an air break to
facilitate drainback of the collector loop when the collector loop pump stops.
Many drainback systems are “two tank, two pump” systems. One pump moves water from the
drainback tank to the collector, while a second pump moves water between the drainback and
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potable water tank. A typical drainback tank may hold 15 gallons of solar fluid, and have 15’ of
heat exchange coiled in that tank.
The Simple Drainback is a single tank, single pump load side heat exchange design. Instead of
holding 80 gallons of domestic hot water, the Simple Drainback holds 80 gallons worth of BTUs
in the tank. The Simple Drainback’s 80 gallon tank is filled with 80 gallons of tap water collector
fluid. The 80 gallons of collector fluid in the tank is heated to 170° F by the sun. A 100’
spiraled copper heat exchange coil is located at the top of a tall, cylindrical tank. Potable water
is heated as it passes one time through the heat exchange coil, that coil bathing inside the top-
of-tank high temperature solar heated water. The stored BTUs in the 80 gallon tank provide 80
gallons of 120° F water.
An inherent advantage of load side heat exchange tanks is stratification. Temperature
stratification within a tall, narrow tank enhances the heat exchanger’s effectiveness. In the
Simple Drainback system, it is not uncommon to see top of tank temperatures fully 50° F above
bottom of tank temperatures, especially when cold tap water passes through the heat
exchanger. In this case, cold from the tap water in the heat exchanger transfers and drops
inside the tank, leaving the heat exchanger bathed in hot water at the top of the tank.
Enhanced tank stratification is a unique characteristic of load side designs. Exhibit #3 below
shows the Simple Drainback (left) with a typical two tank drainback system (right).
Exhibit #3- Simple Drainback and Two Tank Drainback Systems
Load side heat exchange systems are sometimes referred to as instantaneous or one-pass
systems because potable water is heated as is passes through the heat exchange coil. The
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effectiveness of the 100’ single wall spiraled copper heat exchange coil is demonstrated in
Exhibit #4 below. The graph on the left reports the temperature of water exiting the Simple
Drainback heat exchanger at a flow rate of 1 gallon per minute (gpm) given top of tank
temperatures. The temperature difference between water exiting the heat exchanger and top
of tank temperature at 1 gpm flow is approximately 1.1° F. On the right is water temperature
leaving the heat exchanger at a flow rate of 3 gallons of water per minute, with the
temperature difference being approximately 8.9° F between top of tank and exiting water.
Exhibit #4- Water temperatures leaving the heat exchanger given 1 gpm and 3 gpm flows.
Design differences between the Simple Drainback’s load side design and a glycol system are
profound. Simple Drainback’s design simplicity translates to lower cost equipment, lower cost
installation, ease of maintenance, and longer life. Importantly, the fundamental difference in
design allows the Simple Drainback to be plumbed in thermosiphon, and employed in
combination with radiant floor, snow melt, pool or hot tub heating applications.
THERMOSIPHONING-LOW COST THERMAL STORAGE
Mating a solar system with a backup hot water system is traditionally done in one of two ways.
The most common connection has the solar system acting as hot water “pre-feed” for the
regular water heater. The concept is that hot water flowing from the solar system into the
standard hot water heater will not require heating, thus saving energy. A second approach is to
mechanically move hot water from the solar system to the domestic system using pumps and
controllers. This second strategy provides the dual benefits of reducing standby energy losses
and increasing solar storage. The tradeoff for these advantages is the cost of a pump and
An alternative to joining solar and standard hot water heaters is a thermosiphon design. The
trick is to plumb the systems such that the two tanks circulate hot water via thermosiphoning
without pumping. Joining solar hot water tank with an existing hot water system using
thermosiphon design extends solar storage without the expense or complication of circulation
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pumps and controls. The result is a low cost, elegantly simple and highly functional solar hot
water system with integrated hot water backup.
Thermosiphon design follows a basic principle of fluid dynamics, that hot water rises and cold
water falls. Introducing heat causes water molecules to become excited and expand. Heated
water becomes less dense and comparatively lighter than dense heavier cold water.
Thermosiphoning harnesses the decreased density and buoyancy of heated water to naturally
induce fluid movement.
Plumbing the Simple Drainback to thermosiphon with an electric, gas or propane tank involves
joining the two tanks such that the cleanout of the backup tank is above the inlet to the heat
exchanger, and preventing reverse thermosiphoning via a check valve. Exhibit #5 below shows
the connection between the bottom of backup tank cleanout and the Simple Drainback’s heat
exchanger, along with the cold water feed tee, check valve, union and shut-off valve.
Exhibit #5- Bottom of tank thermosiphon plumbing
The steps involved are basic. Remove the existing cleanout from the backup tank and replace
with a nipple, union and cleanout valve. This combination will allow removal of the solar tank
without compromising the cleanout function.
The check valve should be placed prior to the cold water feed to prevent cold water from
entering the storage tank. Be sure to orient the valve in the correct direction! When installing
the check valve, you may find it best to slightly tilt that valve to the side to encourage easier
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operation. Blowing through the cleanout will reveal the pressures required to open that valve
when upright compared to when tilted. Tilting eases the opening operation without changing
the closing function.
The cold water feed is joined to the thermosiphon supply line with a sweat tee.
In the image above left, temperature dials ($18) and temperature sensor wells for a data logger
were added. The image on the right shows another view of the cold feed, with the check valve
immediately prior to the cold feed tee.
The top of tank assembly for the thermosiphon loop is as straightforward as the bottom. There
should be one complication, however, the cold water feed by-pass loop. The cold water by-
pass loop is installed so that the solar tank can be disabled and cold water can flow directly into
the backup hot water source.
Below in Exhibit #6 is the top of tank thermosiphon plumbing, including solar bypass and
tempering valve. In the example below on the left, the cold water bypass is open, with the solar
tank and the cold feed to the tempering valve closed. Also shown on the test system are
optional flow meters, temperature sensors and temperature dials.
Exhibit #6, Top of tank thermosiphon plumbing with solar bypass and tempering valve.
My first thermosiphon system was powered by a single set of 30 evacuated tubes connected to
an 80 gallon Simple Drainback tank plumbed in thermosiphon to a 50 gallon propane backup
tank. After the first April day of operation, the system was producing 145° F water at the tap!
High temperatures are exactly why a tempering (anti-scald) valve is highly recommended.
Thermosiphon plumbing is simple in concept, and simple is execution. Anecdotal reports and
experience suggest this method of plumbing is highly effective, allowing the backup supply of
fuel to be shut off several months per year.
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COMBI-SYSTEM: DHW PLUS OTHER USES OF HOT WATER
Solar combi-systems add function to hot water systems, increasing system effectiveness while
reducing the cost of delivered energy. Examples of combi-system use include in-floor radiant
heat, snow melt, pool heating, or spa heating. Combi-system plumbing can also be used to as a
conduit to provide heat to a solar thermal system. Combi-system plumbing allows wood stoves
to supply heat to a solar tank.
Adding BTUs to a system requires additional collectors. The addition of collectors drives total
costs up, but at a proportionally lower rate. (More on cost later.) Adding storage is a function
of design. Additional storage is beneficial for larger systems. Additional BTUs could be stored
in a pool, a spa, or under in the floor.
Combi-system plumbing for a load side heat exchanger is as easy as tapping into the tank for
supply and return. Exhibit #7 shows the supply and return ports on a Simple Drainback combi-
Exhibit #7- Combi-system supply and return
The supply for the combi-system involves adding a fitting to the electric element port. Electric
element threads are not NPT (National Pipe Thread) compatible. The straight threads of that
port require a specially threaded fitting that supports a high temperature gasket. These are not
fittings you’ll find at the local Home Depot or Lowes. It’s advisable to have this fitting supplied
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by the factory. Once connected to the tank, solar fluid can be drawn directly from the tank, and
directed according to need. An alternative to electric element port access is top-of-tank access.
The electric element port on a Simple Drainback tank is located below the heat exchanger.
Accessing hot water at this location (bottom center of the tank) will result in lower
temperatures than accessing at the top of the tank. However, when pumping, hotter water
from the top of the tank will be drawn down and mixed with cooler water in the center of the
tank. For many combi-system applications, lower temperature water (110-125° F) is desirable.
Solar systems are very capable of producing an abundant supply of water at this temperature.
The combi-system return can be either at the top or the bottom of the tank. The return shown
in Exhibit #7 is at the bottom of the tank. Plumbing the combi-system return port is identical to
plumbing the backup tank’s thermosiphon loop feed. The cleanout is removed and replaced
with an assembly that includes a tee for the combi-system’s return. Returning to the bottom of
the tank is preferred, as cold water at the bottom of the tank is pumped to the collector loop.
Lower water temperatures at the collector lead to enhanced collector performance.
ADVANTAGES AND DISADVANTAGES OF THERMOSIPHONING COMBI-SYSTEMS
Load side heat exchange systems enjoy several advantages over collector side systems,
including fewer and less complicated components, faster installation, less maintenance and
greater operational efficiency. For the handyman installer, load side systems offer the
advantage of not requiring a glycol pump station to charge the collector loop. In essence, load
side systems represent a reasonable choice for handyman self-installers.
Solar thermal systems must functionally and economically compete against a host of
alternatives. Thermosiphoning combi-systems offer competitive advantages due to their low-
cost extension of the core functions of solar thermal.
Installing a solar thermal system involves what accountants refer to as sunk costs. Sunk costs
are the baseline costs of the system, including a heat exchange tank, collectors and collector
loop. Extending the collector capability and adding a few extra connections represent the
incremental costs associated with a thermosiphoning combi-system.
Adding thermosiphon capability fully utilizes low-cost backup sources of hot water, allowing
them to serve as additional solar storage. Costs associated with maintaining a readily available
supply of hot water in the backup tank are eliminated several months, and minimized for the
balance of the year.
Disadvantages of these systems include space requirements and cost. A solar tank is two feet
in diameter and six feet tall. A backup tank will require roughly the same amount of space. In
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addition to square footage required for tanks, there needs to be room for plumbing associated
with the combi-system. In addition to tank space, a good deal of shade free space is required
for an adequately sized collector system. A single 30 evacuated tube collector is 8’ long and 6’
deep. A single collector of this size will provide domestic hot water supply. For additional BTU
production, additional collectors of this size are required. Shade free, accessible collector array
space close to the tanks is an issue to consider.
Costs associated with thermosiphoning combi-systems are partially offset by tax and other
incentives. It would not be unreasonable to expect such a system to cost $7,500 to $9,000, or
more for larger or complex systems.
Like all forms of energy, solar thermal must undergo continuous improvement to achieve lower
costs and greater efficiencies. Thermosiphoning combi-systems accomplish both. And, if
you’ve ever been in a house or shop with radiant floor heat, you appreciate the intangible
benefits that a solar thermal system can offer.
Exhibit #8- Thermosiphoning combi-systems with insulating wrap.
P.S. Most photographs shown were incomplete, uninsulated systems. Final product images
were omitted. Exhibit #8 shows two more examples, the system on the right with final
insulating wrap on both the backup and the Simple Drainback tank.
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