There are several fundamental concepts that need to be understood prior to designing a hydronic distribution system. This section presents them. It also provides references to previous issues of idronics™ in which the concept is discussed in more detail.
Open Hydronic Systems
Any hydronic system in which the water contacts the atmosphere outside the system, even through a small opening, is an “open” hydronic system. An example is shown in Figure 2-1.
Open hydronic systems have several characteristics that must be respected.
First, the point at which the water contacts the atmosphere will remain at atmospheric pressure at all times. This is not necessarily a problem in systems where piping only rises a few feet above the level where water contacts the atmosphere. However, the pressure in all piping above the water line level will be under negative pressure relative to the atmosphere, as shown in Figure 2-2.
If the sub-atmospheric (e.g., negative) pressure of the water drops to or below the water’s vapor pressure, immediate boiling occurs. The vapor pressure of water is highly dependent on its temperature. A commonly known correlation is a vapor pressure of 14.7 psi absolute pressure at a temperature of 212ºF (or 100ºC). The earth’s atmosphere exerts an absolute pressure of approximately 14.7 psia at sea level, and thus an open container of water at sea level will boil if the water reaches 212ºF.
Water will boil at temperatures well below 212ºF under negative pressure. Figure 2-3 shows the vapor pressure of water over a range of absolute pressures, as well as the more commonly used “gauge” pressure scale.
Boiling must be avoided in hydronic systems. This requires that the combination of temperature and pressure of the water remain safely within the green shaded area of Figure 2-3. It is prudent to provide a margin of safety against the water approaching the blue vapor pressure curve in Figure 2-3, on the basis of temperature or pressure.
This generally requires that any open-loop hydronic system not have piping that is routed more than a few feet above the water line in the system. Ideally, most, if not all of the piping in an open system should be below the water line.
Corrosion Potential In Open Systems
Another concern with open hydronic systems is the ability of water to reabsorb oxygen molecules from the location where the water in the system contacts the atmosphere. As water cools, its ability to absorb these gases increases. As water is heated, its ability to hold oxygen and nitrogen molecules in solution with H20 molecules decreases. The repetitive heating and cooling of water in a hydronic system, in combination with the water/ atmosphere interface, allows the water to transport oxygen molecules throughout the system. If this oxygen contacts ferrous materials, such as steel or cast iron, corrosion will occur. Over time, this corrosion can cause premature failure of any ferrous metal components, such as boilers, circulators, valves, panel radiators or expansion tanks.
Thus, ferrous metal components should never be used in open hydronic systems. Components made of stainless steel, copper, brass, bronze or engineered polymers such as PEX are generally required in open systems.
Closed Hydronic Systems
Any system in which all contact between the fluid within the system and the atmosphere is blocked is classified as a “closed” hydronic system. The majority of residential and commercial hydronic systems currently used in North America fall into this category.
Because they are sealed from the atmosphere, properly designed and maintained closed hydronic systems experience very little entry of oxygen, and thus have far lower potential for oxygen-based corrosion than do open systems.
To avoid the previously described oxygen-based corrosion, boilers made of carbon steel or cast iron should only be used in closed systems. Likewise, steel panel radiators, steel expansion tanks, cast iron circulators, or any valves and fittings made of steel or cast iron should only be used in closed systems.
Closed hydronic systems can operate at positive pressure relative to the atmosphere. Many residential and light commercial hydronic systems operate with positive pressures in the range of 5 to 25 psi in the lower portion of the system.
In large industrial hydronic systems, the ability to operate at positive pressures allows water temperatures well above 212ºF. Although possible, most of the modern hydronic systems used in residential and light commercial buildings seldom need to (or should) operate at temperatures exceeding 200ºF. Even lower water temperatures are preferred when they are compatible with the system’s heat emitters.
All closed hydronic systems containing any source of heat generation must be equipped with a pressure relief valve. This valve automatically opens at a specific pressure rating to prevent any higher pressure from developing within the system. Most residential and light commercial hydronic heating systems are equipped with pressure relief valves that open at 30 psi pressure.
Heat Source Characteristics
There are hundreds of devices available that can serve as a heat source for a hydronic system. They include boilers fueled by natural gas, fuel oil, propane, wood and electricity. They also include a range of hydronic heat pumps that gather low temperature heat from air or water, and transfer that heat to a stream of water at higher temperatures. Both flat plate and evacuated tube solar thermal collectors can also serve as hydronic heat sources.
Some hydronic systems also use two or more independently operated heat sources. For example, a residential system may use a boiler in combination with an air-to-water heat pump to provide space heating, cooling and domestic hot water.
From the standpoint of compatibility with specific distribution systems, heat sources can be classified as follows:
1. High thermal mass versus low thermal mass
2. High flow resistance versus low flow resistance
3. Conventional versus condensing
High Thermal Mass vs. Low Thermal Mass Heat Sources
A high thermal mass heat source contains significantly more metal or water relative to a low thermal mass heat source. Examples of high thermal mass heat sources include cast iron boilers, steel fire-tube boilers, and high water-content, “tank-type” modulating/condensing hydronic heating appliances.
High thermal mass provides stability in a system that contains many zones. It protects the heat source from short cycling under partial load conditions. For many common applications, one can treat high thermal mass heat sources as “self-buffering.” This term implies that no additional buffer tank is needed
Other hydronic heat sources contain minimal amounts of metal and water. They can be categorized as low thermal mass heat sources. Examples include modulating/ condensing boilers with compact heat exchangers, electric boilers, and hydronic heat pumps with coaxial heat exchangers.
Low thermal mass heat sources can quickly heat up to normal operating temperatures when called to operate. Their low thermal mass also implies that very little residual heat remains in the heat source following shutdown. These heat sources are relatively light in comparison to high mass cast iron and steel boilers, and in some cases can be mounted to a wall rather than placed on a concrete floor.
Low thermal mass heat sources are not self-buffering. When combined with zoned distribution systems, they often require use of an external buffer tank to add stabilizing thermal mass to the system, and prevent undesirable short cycling. Figure 2-6 shows an example of this approach in which a low thermal mass water-to-water heat pump is connected to a buffer tank in a system that has several zones.
High Flow Resistance vs. Low Flow Resistance Heat Sources
Many heat sources that would be categorized as high thermal mass also have low flow resistance. This is beneficial because it reduces the electrical power required by the circulator that creates flow through the heat source. Low flow resistance heat sources typically do not require a dedicated heat source circulator. Conversely, many low thermal mass heat sources have higher flow resistance, and often require a dedicated circulator.
Figure 2-7 illustrates the differences between high flow resistance heat sources and low flow resistance heat sources. The head loss versus flow rate curves (shown in blue) represent this effect. The steeper blue curve represents a high flow resistance heat source. The shallow blue curve is for a low flow resistance heat source.
The red curves are pump curves for two different circulators. The higher pump curve is for a more powerful circulator, while the lower red curve is for a lower power circulator.
Assume that the objective is to create a flow of 10 gallons per minute (gpm) through the heat source. Figure 2-7 shows the pump curve of the more powerful circulator intersecting the head loss curve of the high flow resistance heat source at a corresponding flow rate of 10 gpm. Notice, however, that the same 10 gpm flow rate can be achieved using a lower power circulator in combination with the low flow resistance heat source. Lower power circulators have the potential to save hundreds of dollars in operating cost over the life of the system, and thus are preferred whenever they can create the necessary flow rate.
The high flow resistance of some heat sources requires that they be connected to the distribution system in a manner that uses a separate circulator just to create the necessary flow through the heat source. Figure 2-8 illustrates how this can be done using either closely spaced tees or a hydraulic separator.
Both of these piping methods provide hydraulic separation between the dedicated heat source circulator and any other circulators in the system.
See idronics™ #15 provides more information about primary/secondary piping options.
Heat Emitter Characteristics
As is true with heat sources, hydronic distribution systems must also be well matched with the system’s heat emitters. This matching requires knowledge of the heat emitter characteristics.
One characteristic that plays a critical role is the thermal mass of the heat emitter(s) used in the system. Some hydronic heat emitters have very high thermal mass. An example would be a 4-inch-thick heated concrete floor. Other heater emitters have very little thermal mass, an example of which would be a fin-tube baseboard convector. Figure 2-9 compares the thermal mass of several common hydronic heat emitters.
Notice that the thermal mass of the 4-inch-thick heated concrete floor is over 100 times that of the low mass panel radiator.
High Mass Heat Emitters
High thermal mass heat emitters can store large amounts of heat in the materials present between the water flowing through them, and the room to which they are delivering heat. This characteristic can create significant differences between the time and rate at which heat is absorbed into the heat emitter from the water passing through it, and the time and rate at which that heat is delivered to the space being heated. The greater the thermal mass of the heat emitter(s), the greater the thermal lag effect can be.
This can cause significant over-shooting and undershooting of the desired room air temperature. Figure 2-10 illustrates an example.
Consider an interior space heated by a high mass heat emitter such as a heated concrete slab. Assume that the desired goal is to maintain a constant interior temperature of 70ºF. On a cold winter night, there is a continuous, but slowly increasing heat input rate to the slab so that its heat output keeps pace with the heat loss of the space. By morning, the slab has been “fully charged” with heat, and its surface temperature is in the low 80ºF range.
Now assume that the heated slab just described is in a building with large south-easterly facing windows. By mid-morning, there is a significant internal heat gain caused by solar radiation through these windows. Other internal heat gains are also created by occupants and use of interior equipment. This causes interior air temperature to rise. The thermostat senses this and turns off any subsequent heat delivery to the slab when the air temperature rises 1ºF above its 70ºF setpoint.
However, the slab surface is still well above the desired 70ºF indoor temperature and continues to release heat into the space. The result is a significant temperature overshoot as the residual heat “drains” from the slab over several hours. Such overheating is undesirable. Occupants are likely to deal with it by opening windows for ventilation, or even turning on the building’s cooling system. Comfort is comprised and energy is wasted.
This thermal lag effect can be minimized by:
1. Not using high mass heat emitters in spaces that are subject to significant and/or unanticipated internal heat gains from people, sunlight or interior equipment.
2. Using outdoor reset control to keep the supply water temperature to the heat emitter just high enough to meet the current prevailing heating load. Properly applied, outdoor reset control produces nearly continuous circulation through the system’s heat emitters, and thus helps equalize the rate of heat input to the emitter with the rate the emitter releases heat to the space.
idronics™ #7 provides a detailed discussion of outdoor reset control.
In the right application, high thermal mass heat emitters can also provide beneficial effects. For example, consider the use of a heated concrete floor in a garage-type facility, such as a vehicle maintenance building, fire station or aircraft hangar. When a large door in such a facility is opened on a cold day, there is an influx of cold outside air over the surface of the heated slab. Because heat is stored in the slab, it responds with an immediate “surge” of heat output to counteract the effect of the cold air. This surge can be sustained for several minutes and helps restore comfort very quickly after the large door is closed. Thus, high thermal mass floor heating is an excellent choice in many garage-type facilities.
Low Mass Heat Emitters
Low thermal mass heat emitters such as most fin-tube baseboard convectors, panel radiators and fan-coils have the advantage of fast thermal response. Many can begin releasing heat to their respective spaces within seconds of having heated water passing through them. This allows for better matching between the building’s varying heating load and the heat output from the heat emitters. It also minimizes the potential for temperature overshoot when internal heat gains occur. Low thermal mass heat emitters store very little residual heat, and thus stop emitting heat very quickly after the flow of heated water through them stops.
Low mass heat emitters are recommended in the following situations:
1. When a building is subject to significant and unscheduled internal heat gains.
2. When a building is operated with daily temperature setback and recovery schedules.
3. When frequent changes in room air temperature are required.
4. When fast recovery to normal room temperatures following a setback is expected.
Typical Water Temperature Requirements for Heat Emitters
When selecting heat emitters, one should distinguish between “traditional” supply water temperatures and “advisable” supply water temperatures.
Traditional supply water temperatures are the result of decades of application with little motivation to consider alternatives. Many of these temperature ranges were established when fuel was inexpensive, and the principle driver of system design was minimizing installation cost.
A common example is the traditional supply water temperature for fin-tube baseboard. It is typically 180 to 200ºF under design load conditions. It is even possible find heat output ratings for fin-tube baseboard at water temperatures as high as 230ºF!
This is not to suggest that fin-tube baseboard can't be designed around lower supply water temperatures. Indeed, it can. In some buildings, it may even be possible to size fin-tube baseboard for a design load supply water temperature of perhaps 120ºF. In an average house, this would require much more linear footage of baseboard compared to sizing around a more traditional water supply temperature of 180 to 200ºF. It might be impractical from the standpoint of having sufficient wall space to mount the required length of baseboard. However, in a low heating load home, where heating requirements are often less than 1/3 that of average homes, designing a fin-tube baseboard system for such a low supply water temperature may be possible given the available wall space. Traditional design practice simply ignores the latter as a possibility.
Figure 2-11 shows a range of traditional supply water temperatures for several types of heat emitters under design load conditions.
See idronics™ #25 goes into more detail about the advantages of targeting lower supply water temperatures in hydronic systems.
Heat emitters sized for the upper end of the temperature range are difficult (sometimes impossible) to match with modern hydronic heat sources. Examples of such heat sources include mod/con boilers, solar thermal collectors, hydronic heat pumps, and systems that rely on deeply cycled thermal storage. All of the latter heat sources perform at higher efficiency when operated at relatively low water temperatures. Some, such as a typical water-to-water heat pump, cannot consistently produce supply water temperatures over 120ºF.
The limited ability, or complete inability, of some modern hydronic heat sources to consistently produce high water temperatures has serious implications for distribution systems and heat emitters.
To achieve the full performance potential of modern hydronic heat sources, distribution systems and the heat emitters they serve must be compatible with relative low supply water temperatures, even during design load conditions.
A suggested criterion that addresses this issue is to design all hydronic heating distribution systems so that they can deliver full design load output using supply water temperatures no higher than 120ºF.
Beyond the compatibility issue with modern hydronic heat sources, this criterion recognizes that well-planned and properly maintained hydronic distribution systems will last many decades, far outliving their original heat source. It also anticipates that future hydronic heat sources, whatever they might be, are more likely to be compatible with lower water temperatures. Thus, considering that a hydronic distribution system installed today is likely to have two, three, or perhaps even more heat sources connected to it over its useful service life, it is prudent to design for compatibility with those future heat sources. Simply put, designing for relatively low supply water temperatures helps “future-proof” hydronic distribution systems.
Designers should also keep in mind that heat emitters don’t necessarily have to be selected based on “traditional” operating water temperatures. For example, standard residential fin-tube baseboard, which is traditionally sized based on an average circuit temperature of 180ºF, can still operate at much lower temperatures, albeit at much lower heat outputs. Figure 2-12 shows how the heat output of standard residential fin-tube baseboard varies based on the average water temperature in the fin-tube element.
At an average water temperature of 115ºF, the baseboard is still releasing about 140 Btu/hr per foot of active element length. While this is probably not sufficient to maintain comfort in an older energy-inefficient house, it might be a possibility in an energy-efficient house, provided there is sufficient wall space to accommodate the required baseboard length.
Output derating curves are also available for other types of heat emitters. Designers should consider the possibilities before dismissing a given type of heat emitter based on “traditional” water temperature selections. In the case of fan-coils, a suggestion is to ensure that the water temperature supplied to the coil can maintain a discharge air temperature of at least 100ºF. Forced-air distribution systems operating with relatively low supply air temperatures need to be carefully designed to avoid creating drafts.
Changes In Water Density
Good hydronic system design requires an understanding of several characteristics of water. One that is especially relevant to the design of hydronic distribution systems is how the density of water varies with temperature, and the resulting implications.
The density of water varies considerably with temperature, as shown in Figure 2-13. This variation also occurs with water-based antifreeze solutions.
When water is heated, it expands and becomes less dense, and vice versa. One can also think of heated water as being “lighter” than cooler water.
The reduced density of heated water induces a tendency for it to rise upward in the system relative to the location of cooler water. This characteristic can be beneficial in some circumstances, but very undesirable in others.
Before circulators were available, the weak pressure differential caused by differences in the density of hotter and cooler water provided the sole means of moving water through early hydronic heating systems. Hot water would rise upward from a boiler and pass through heat emitters where it would cool. The cooler water, due to its increased density, would flow downward through other piping, eventually reaching the lower connection on the boiler where the cycle would repeat.
Although this effect was very useful in early hydronic systems, its presence in modern systems is usually unwelcome. Any unblocked piping circuit containing a source of heated water, as well as vertical piping, has the potential to allow a slow but persistent buoyancy-induced flow to develop that will dissipate heat from that circuit, as illustrated in Figure 2-14.
This unintentional flow has been referred to by several names, including reverse thermosiphoning, forward thermosiphoning, ghost flow and heat migration. From the standpoint of thermodynamics, it is nature’s way of increasing the entropy of the thermal energy in the system.
Forward thermosiphoning is buoyancy-induced flow through a circuit in the same direction as when the circulator is operating. Reverse thermosiphoning is buoyancy-induced flow through a circuit in the opposite direction as when the circulator is operating.
Undesirable heat dissipation caused by either forward or reverse thermosiphoning can be prevented in several ways. The most common is to install a device that blocks the flow that would otherwise develop in the piping circuit.
A common swing check valve, such as shown in Figure 2-15, can stop reverse thermosiphon flow. However, it offers very little forward opening resistance, and thus cannot block forward thermosiphon flow. This characteristic can be useful in situations where only reverse thermosiphon flow needs to be blocked.
When installing swing check valves, it is important to provide at least 12 pipe diameters of straight pipe upstream of the valve. This reduces turbulence entering the valve, and helps prevent “rattling” noises from the metal flapper inside the check valve.
It’s also important to only mount swing check valves in horizontal piping with the bonnet of the valve facing up. Swing check valves are not designed to function in vertical piping, or if mounted upside down.
When both forward thermosiphoning and reverse thermosiphoning are to be prevented, a check valve with a slight forward opening pressure is required. Two types of valves can provide this characteristic: 1) A spring-loaded check valve, and 2) A weighted plug flow check valve.
A spring-loaded check valve contains a mechanism in which a small stainless steel spring holds the “plug” of the valve against its seat. When the plug in the spring check is against the valve’s seat, no flow can pass backwards through the valve.
The internal spring in most spring-loaded check valves creates a forward “threshold pressure” of 0.3 to 0.5 psi. This is generally sufficient to prevent forward thermosiphon flows from developing in systems that are not installed in tall buildings.
Many small circulators are now supplied with internal spring-loaded check valve mechanisms. These mechanisms prevent reverse flow through an inactive zone, as well as stop forward thermosiphoning into an inactive zone circuit.
Figure 2-16 shows where a spring-loaded check valve would be installed to prevent forward thermosiphon flow between a thermal storage tank and any unblocked piping path. Because it relies on a spring rather than gravity, a springloaded check valve can be installed in any orientation. However, it should also be installed with a minimum of 12 pipe diameters of straight piping upstream of its inlet to help prevent rattling due to turbulent flow.
Air Separation Detailing
Hydronic systems perform best when the water they contain is free of air and dirt. Fortunately, eliminating both air and dirt from hydronic systems is easy with the proper hardware and correct detailing. The discussion that follows summarizes placement of air and dirt separating devices within typical hydronic systems.
Air separation can be divided into two categories:
1. Eliminating bulk air from a hydronic system when it is first filled (e.g., purging the system)
2. Providing continuous collection and elimination of small air bubbles that form when the water in the system is first heated or that may be present after a component is serviced and air has entered the system.
Bulk air removal is called “purging” the system. It is usually done by introducing water into the system at a high flow rate, and forcing it to move through one or more circuits in a specific direction. The fast-moving stream pushes air along the piping like a piston pushes gases ahead of it as it moves through a cylinder. The air, and the stream of water pushing it, exit the circuit at one or more specific locations, typically through a special valve called a “purge valve.”
The details needed for purging are based on planning where water will enter the system and how it will flow from that point through one or more piping paths, until it can exit through a purge valve placed close to the water entry point. Figure 2-17 shows this concept for a single circuit.
A typical purging valve consists of two valve mechanisms in a common body: an inline ball valve, and a side-port ball valve. The inline ball valve is closed to block flow in the piping circuit during purging. The side-port ball valve is opened during purging to provide an exit point for air and some of the water pushing it along.
To purge the system shown in Figure 2-17, the inline ball valve is closed, and the side port of the purge valve is opened. The side-port valve should also be connected to a hose that can carry the discharge to a drain or bucket. The make-up water assembly, consisting of a pressurereducing valve, backflow preventer, and isolating ball valve, is turned on to allow a rapid flow of cold water from the building’s plumbing system to enter the hydronic circuit just downstream of the purging valve. Because the inline ball in the purging valve is closed, this water is forced to flow around the entire circuit, pushing air ahead of it. Air will start exiting the side port of the purge valve as soon as water flows into the system. Within a few moments, a mixed stream of air and water will exit the side port of the purge valve. Once the exiting stream is running free of visible air, the side port of the purging valve is closed. Water will enter the system until the set pressure at the pressure-reducing valve is achieved. The inline ball within the purge valve can then be opened. The system should now be purged of bulk air.
In multiple-zone distribution systems, it is good practice to install a purging valve at the return of each zone or other branch circuit.
Although forced-water purging removes most of the bulk air initially in the system, molecules of nitrogen and oxygen remain dissolved in solution with water molecules. A modern “micro-bubble” air separator is designed to capture this dissolved air and eject it from the system.
See idronics™ #15 for a complete discussion on micro-bubble air separation and dirt separation.
Expansion Tank Placement
All closed-loop hydronic systems require an appropriately sized expansion tank. The captive air volume within this tank provides a “cushion” against which the system fluid will expand when heated. A properly sized expansion tank allows the system fluid to expand and contract while only creating minor pressure variations within the system.
Most modern systems use a diaphragm-type expansion tank. This type of expansion tank contains an elastomeric diaphragm that flexes up and down within the tank shell as water enters and leaves the tank. The diaphragm totally separates the system water from the air contained within the tank shell.
The placement of the expansion tank relative to the circulator significantly affects the pressure distribution in the hydronic system when it operates.
The point where the expansion tank connects to the circuit is called the “point of no pressure change” (PONPC). As the name suggests, the pressure at this location doesn’t change when the circulator is turned on. However, the pressure at all other locations within the system will increase or decrease depending on where the PONPC is located. The most desirable results are attained when the pressure at other locations in the system goes up when the circulator is turned on. This result is achieved when the expansion tank is located close to the inlet side of the circulator, as shown in Figure 2-18.
When the circulator is turned on, it immediately creates a pressure differential between its inlet and outlet ports. However, the pressure at the point where the expansion tank is connected to the circuit remains the same. The combination of the pressure differential across the circulator, the flow resistance of the piping, and location of the PONPC gives rise to the new dynamic pressure distribution shown by the dashed green line in Figure 2-18.
The pressure increases in nearly all parts of the circuit when the circulator is operating. This is desirable because it helps eject air from vents. It also helps keep dissolved air in solution and minimizes the potential for cavitation at the circulator inlet. The short segment of piping between the expansion tank connection point and the inlet port of the circulator experiences a very slight drop in pressure due to flow resistance in the piping. The numbers used for pressure in Figure 2-18 are only examples. The actual numbers will depend on flow rates, fluid properties and pipe sizes.
Loads Supplied Through Heat Exchangers
There are situations where a heated liquid within a hydronic system needs to pass heat to another liquid without physically contacting that other liquid. This task is routinely handled using a heat exchanger.
Common examples of where heat exchangers are used in hydronic systems include:
• Separation of domestic water from system water
• Separation of antifreeze in solar collector circuits from water in thermal storage tanks
• Separation of antifreeze from system water in snow melting systems
• Separation of swimming pool or space water from system water
The most commonly used type of heat exchanger in modern hydronic systems is called a brazed plate heat exchanger. It consists of a “stack” of specially formed stainless steel plates that have been brazed together at their perimeter. The resulting component allows one fluid to flow through all the even-numbered channels between these plates, while the other fluid flows through the odd-numbered channels. A brazed plated heat exchanger provides a large internal surface area for good heat exchange, but contains that area within a relatively small component. Figure 2-19 shows an example of a brazed plate stainless steel heat exchanger.
There is a wide variety of brazed plate heat exchangers available in North America. Most companies offering them provide sizing and selection services, including software that allows several design variables such as flow rates and entering temperatures to be simultaneously evaluated.
General application considerations for heat exchangers include:
1. All heat exchangers require a temperature difference between the entering fluid carrying heat, and the exiting fluid to which the heat is transferred. This difference, known as the “approach temperature difference,” is determined by the design of the heat exchangers and the temperature, specific heat, and flow rates of the two fluid streams exchanging heat. One can think of the approach temperature difference as the thermal penalty associated with having a heat exchanger in the path of heat flow. The smaller this temperature difference is, the lower the thermal penalty. Where energy efficiency is the primary consideration, heat exchangers should be selected for a maximum approach temperature difference of 5ºF. Even small values are possible using larger heat exchangers.
2. Be sure the material used in the heat exchanger is compatible with both liquids that will be passing through it. This is especially true for highly treated water, such as that used in swimming pools and spas.
3. Be sure the head loss (or pressure drop) of the heat exchanger is factored into the overall circuit head loss when selecting a circulator. Manufacturers generally provide head loss or pressure drop information as a function of flow rate.
4. Be sure to pipe the heat exchanger for counterflow. This means that the two fluids exchanging heat should pass through the heat exchanger in opposite directions. This increases the average temperature difference between the two sides of the heat exchangers, and maximizes the rate of heat transfer, all other conditions being equal.
5. Recognize that heat exchangers completely isolate two piping circuits. If both circuits connected to the heat exchanger are closed loops, each should be equipped with an expansion tank, pressure relief valve, air separator and fill/purging valves.
6. Any dirt or debris that lodges inside a heat exchanger will reduce its thermal and hydraulic performance. To prevent this, a high-quality dirt-separating device should be present in each fluid stream flowing into the heat exchanger. If one fluid stream is domestic water, be sure the dirt-separating device is lead-free. It is also good practice to install isolation/flushing valves on each side of a heat exchanger flow path carrying domestic water. These valves allow that side of the heat exchanger to be isolated, and provide a simple means of circulating a chemical cleaning solution through the heat exchanger to remove scaling.
Figure 2-20 shows a heat exchanger piped for counterflow, along with suggested “trim,” including a magnetic dirt separator on each fluid stream entering the heat exchanger.
