Hydronic balancing: Simplicity for control of heating and cooling systems for high-rise buildings

By Peter Biondo

Probably the single most important feature for energy efficiency and comfort for high-rise and large buildings is the balanced flow of energy through the distribution systems. With hydronic heating and cooling, water is the medium for the flow of energy. If flow rates are not balanced or regulated on mains, secondary branches and distribution risers, water will follow the path of least resistance. Unregulated water will take shortcuts throughout the piping network, resulting in uncontrolled flow rates that cause wide temperature variations throughout the building, reducing the control for comfort and increasing fuel costs.

For hydronic heating systems, circulated hot water is expected to distribute the heat evenly according to the required demand in all rooms. Unfortunately, this may not always be the case. Following the principle of lowest resistance, the heated water flows back to the boiler the shortest way. Normally, this leads to more flow (and noise) to the radiators closest to the circulation pump, so that less heated water reaches the radiators situated at a more distant point. This results in both insufficiently heated and overheated rooms. The problem is the same for convector units with fan coils for cooling or heating.

Without hydronic balancing, temperatures in the building are often uneven. The situation can worsen at small and medium loads where the same room temperature can fluctuate continuously. This leads to losses of energy efficiency. Energy costs could exceed 40% without hydronic balancing. Uneven temperatures are difficult to control, so boilers and chillers run more often. If the average temperature in a building exceeds the nominal value by 1 C, the energy consumption is increased by 6 - 10 %. In cooling systems, temperatures 1 C too low will result in an increase in energy consumption of about 15%. Without hydronic balancing, the extra money spent on high efficiency boilers, chillers and digital controls is wasted.

Besides the distribution systems, boilers and chillers also require constant flow regulation to optimize life cycle and their rated efficiency. Even with the correct size pumps and piping, flow regulation through power units requires balancing. A chiller's flow rate, for instance, has to maintain the exact correct value to keep constant water temperature close to 42 F. Underflow keeps the chiller below design output, lowers efficiency and increases the chance for freezing tubes and damaging the chiller. Overflow may cause load variations between the chiller and the water supply temperature to a point where the chiller output swings constantly and causes underflow in other chillers. Overflow could be so excessive as to reduce the unit's service life. Correct balancing will do away with these problems.

What does hydronic balancing do?

Hydronic balancing is extremely important to correct the flow of water and energy in an HVAC system. It is also a simple technology implemented with the use of calibrated balancing valves (CBVs). It

• creates comfortable indoor climates
• minimizes energy costs
• corrects the oversizing of equipment
• maximizes the life expectancy of HVAC equipment

The HVAC system is a collection of component pieces of equipment including boilers, pumps, chillers and air handling units of various types and sizes. These components are connected by a piping system. Some systems have both heating and cooling coils that require balancing to be carried out twice. A piping distribution system is created to connect all the terminal units to the main plant. Balanced flow through CBVs is used in high-rise buildings, in large apartment buildings and even across large college campuses. The same design principles apply to smaller buildings with hydronic systems.

What is good hydronic balancing?

Hydronic balancing includes the placement of calibrated balancing valves (CBVs) in the hot or chilled water closed-loop system on each piece of equipment, on large secondary branches and in the distribution risers. The function of CBVs is to set and measure flow rates. There are presetting values on CBVs that, with the help of flow charts, are calibrated before system startup and measuring. Then, while in circulation, the pressure drop across each valve is measured. The CBV recalibrates flow rate from the pressure drops across the valve, and the CBV is set precisely for the required flow rate. Electronic flow meters are specifically designed for CBVs to measure pressure drop across the valve and then to calculate the flow rate and the CBV setting. This CBV will control each portion of the total design flow rate and evenly distribute water so that heating and cooling are consistent throughout the building. Balancing, then, is something you do to improve the performance of closed-loop, forced-circulation water systems for heating and cooling.

Total balance for high-rise buildings

With CBVs it is a matter of adjusting pressure drops to get precisely the right flows at all times. In the past, when practiced at all, balancing was in most cases restricted to circuits on the distribution side. But to enable controllers to function the way they are intended to, it's necessary to balance all circuits - and to do so with accuracy. Total balance, then, implies that all circuits be balanced, from the power production units, through all interfaces between the primary and secondary circuits, through the distribution circuits and to energy output at the terminals. Otherwise the system may not be operating at its optimal level of performance and efficiency.

Circuits on a plant's production side must be balanced to get design flows through boilers and chillers and to make the production side compatible with the distribution side under all operating conditions. Circuits on the distribution side must be balanced to make at least design flows available through all controlled circuits and terminals under all operating conditions. The controlled circuits must also be balanced to create the proper working conditions for the control valves and to make primary and secondary flows compatible.

To further expand on piping details for large buildings, main supply branches from boilers and chillers usually range from 8 to 12 inches in diameter. Boilers and chillers are piped in parallel to the mains. The secondary branches or risers from the mains can run anywhere from four to six inches in diameter. Distribution branches are dropped down to two inches. And finally, piping to terminal units -- fan coils or radiators -- is usually 3/4 inch. However, the detail in the sizing of the balancing valve may not duplicate to the pipe diameter that it is installed in.

Piping is best sized for low friction head loss throughout the closed-loop system, thereby saving electrical power consumption at the pump. For each HP added to a pump, to overcome frictional head loss, $723 (12 cents a kilowatt-hour) of increased annual operating costs are incurred. All closed-loop piping systems should be designed to distribute the flow of water with no more than four feet of pressure drop per 100 feet of pipe. Pipe diameter is sized accordingly.

Sizing the valves

Achieving accuracy with the flow range of the balancing valve depends on the right selection of the CBV. Valve size, in terms of CBV, must correspond to the design flow. CBVs can be selected on gpm flow rates based on a 2.3 feet of head pressure drop. It is particularly important not to oversize them. It is a likely choice that the same size pipe diameter is selected for the balancing valve. But the better selection for the valve may be one size smaller than the pipe size. If you choose a balancing valve to match the requirements of CBV, then the valve can theoretically control flow to the design flow rate. But there is much more to it than simply matching the CBV. The most important requirement is to make sure the design flow rate is at the top 25% of the valve fully open. An open valve is particularly accurate in measurements of flow rates.

At the top, 25% presetting flow rates through CBVs are measured within 3% of accuracy. In contrast, at the lower presetting the flow error can be as high as 10 - 20% or higher. This occurs as the valve disc lowers into the restriction ring, which increases friction and turbulates random flow patterns. We've all seen water flow down drains in a whirlpool. We can see the evidence of even flow down the drain. If you stick your finger down into the whirlpool the water loses its flow characteristics, and the rate of flow is no longer even. The same thing occurs when the CBV is set too low -- accuracy is lost to the friction on the valve disc, and the measuring ability of the CBV is compromised.

Calibrated balancing valve features

CBV features are varied and unique to each manufactured product. There are two distinct varieties of CBVs: The Y- pattern, globe style valves and the ball valve style circuit setters. They vary in size from 1/2" to 12" or larger. Smaller sizes up to 2" are usually cast in bronze with female thread or sweat connections. Larger sizes from 2 1/2" to 12" are cast in iron with flanged or grooved connection.

Most CBVs have the following features:

• Pre-balance design capability for calibrating the setting before start up
• Positive shut off
• Integral valve readout ports for measurement of pressure drop

Other features to look for are

• Same side/same end measuring ports for ease in use of the measuring instrument.
• Multi-turn hand wheels for a wide range of presetting
• A built in memory stop, which allows for shut-off while calibrated setting remains locked at open
• A lockable handwheel, so that, once the calibration is set, it can't be changed
• Low minimum pressure drop
• Optional built in drains for filling, purging and draining pipe

The perils of no control

Living in a time of escalating energy costs, and with a barrel of oil costing more than $120, building management firms and owners are feeling the crunch of energy costs. The only peril I see when there is no hydronic balancing is the bad economic choice that the building owner makes about spending his money (and his tenants' dollars) on heating and cooling costs when he could have bought instead, with money well spent, a Porsche. I know pretty smart people who do that. If you're going to spend money, spend it wisely and have your tenants feel comfortable at the same time. Total hydronic balancing is the cost saving feature that everyone benefits from.

Peter Biondo is technical sales coordinator and head of design for Oventrop Commercial Solar Division. Peter has been involved in the solar heating industry over 20 years and has worked on over 3,500 solar heating systems nationwide. He carries a mechanical contractors license in the state of Arizona. He can be reached via email at peter.biondo@oventrop.com.