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Maximizing Efficiency With Hydronic Boilers

June 1, 2012
Versatile systems can play roles in many applications

Hydronic boilers enable efficient control and delivery of heat when and where it is needed.

Before the introduction of condensing-boiler technology, boilers typically operated at efficiencies of 80 to 85 percent. Today, condensing hydronic boilers can operate at efficiencies in the mid- to high 90s. Hydronic boilers can be used for either building heat or process hot-water applications.

A condensing boiler extracts latent and sensible heat from combustion exhaust, and the resulting energy savings can be dramatic. Facilities that upgrade to condensing-boiler systems from non-condensing systems or from steam to hot water via a heat exchanger can achieve net heating-energy savings of up to 50 percent when the condensing boilers are applied in systems that make proper use of outdoor reset schedules, aggressive night/weekend setback schemes, and high system-temperature differentials.

Non-modulating boilers operate at full fire at all times. Whereas older-design boilers typically are most efficient at higher firing rates, the same does not hold true for today’s modular boilers. These new designs tend to be more efficient at lower fire compared with high (or maximum) fire. The modulation helps achieve the lower firing rate. The longer a modulating boiler can run at lower fire, the more efficient it is. Also, each time a boiler cycles off and on, it loses efficiency. Modulating boilers can run longer at lower temperatures than non-modulating boilers, thus minimizing excessive cycling.

It often is advantageous to use multiple boilers in condensing applications. Multiple boilers with turndown capabilities facilitate better overall system load matching. By installing smaller and multiple condensing boilers, a facility manager can stage the boilers depending on heating load, which helps save fuel compared with a system that uses one larger boiler.

Advantages Compared With Steam Boilers
Hydronic boilers have several advantages over steam boilers. First, generating heat from steam requires a steam-to-water heat exchanger, and some heat loss occurs naturally in that transfer.

Because steam systems run at a higher temperature than hydronic systems, steam systems experience losses through steam traps, leaks, and piping. Some facilities add more insulation around steam piping. This can decrease heat loss, but does not eliminate it.

Maintaining a hydronic boiler is easier than maintaining a steam boiler. An operator must closely manage the chemical treatment in a steam boiler. As the system evaporates water into steam, the chemicals and minerals stay inside the boiler and can become highly concentrated. As a result, the pH level can spike.

A hydronic boiler has a closed-loop system, so the chemicals that are added keep circulating. Unlike in a steam system, they do not evaporate or build up. Typically, operators must evaluate the chemistry in a closed-loop system only once a year and make any necessary adjustments.

Retrofitting a Steam System to Hydronic
Most buildings using steam heating run at 15 lb of pressure. Therefore, a facility being converted from a steam system to a hydronic system can get by with a reasonably sized supply header, and the amount of condensate coming back typically will be small because steam goes out and returns as a hot liquid for the condensate. The steam does not need to be pumped. It is only necessary to pump the condensate back. The system goes from 15 lb of pressure to 0 lb of pressure, or from high to low pressure, as the need occurs through natural force.

Steam systems operate at a high heat value, so there is heat that is needed in the space of the piping system, which may result in stronger British-thermal-unit motive-force delivery. That transfer can go out to various buildings easily without the use of a pump, but the condensate must be pumped back or much more makeup water will be required. In facilities already running steam radiators or steam devices, it is not always possible to reuse the existing piping to go to a hydronic system, and a new hydronic piping system must be installed.

In some cases, a facility can use the condensate return line for the water side and pump it. The steam header is much larger than needed for hydronics, which is OK for flowing water. However, a half-inch or quarter-inch line for the steam condensate may not be large enough for a water system to come back. So, at the minimum, a facility may have to run new water pipe, and depending on the condition of the steam pipe, it may not be suitable for flowing water. It comes down to a cost/benefit analysis. A facility owner must evaluate the expected efficiency gains and environmental benefits from a hydronic system against the cost of laying new pipe.

Consider a Hybrid System
A system utilizing both condensing and non-condensing boilers is considered a hybrid system (Figure 1). This type of system is especially beneficial for hospitals, universities, and large commercial buildings as larger condensing units become available in the future. The greatest benefit of a hybrid system is its flexibility. Boilers come online as needed and are set to operate in their “sweet spots,” thereby maximizing efficiency and significantly reducing operating costs.

Hybrid systems are especially advantageous for facilities that operate in colder climates. From mid-December to mid-February in climates in which the outside-air temperature could be 0°F to -20°F, it is best for a building to run its existing non-condensing boiler. At 180°F supply-water temperature in those conditions, condensing and non-condensing boilers will have similar efficiencies. The assumption is that the non-condensing boiler is big enough to handle the load—or, if it is not, most of it—and at high temperatures, the condensing boiler can run to provide heat at peak load.

Cost Benefits of a Hydronic System
The payback on a new hydronic system typically is two to four years; however, it can be less, depending on how inefficient the existing boiler is. The payback on a hybrid system is shorter if a condensing boiler is used to supplement an existing non-condensing boiler. The energy savings for this type of retrofit typically is between 25 and 30 percent. Increases in efficiency directly correlate to the run time of the condensing boiler.

It is important to note that if condensing boilers are installed and operated at a supply-water temperature of 180°F out and 160°F back all of the time, the boiler never kicks in to condensing mode and, therefore, does not deliver the benefit of increased efficiency. This scenario is all too common. Programming the boiler system correctly is important, and it requires a paradigm shift.

Tips to Achieve Dramatic Savings
Some facilities that convert to condensing boilers can cut their energy bills in half. To do this, operators must run condensing boilers during non-peak times, use an outdoor reset schedule, oversee an aggressive night/weekend setback scheme, and capitalize on a larger system temperature differential.

To use outdoor reset, a boiler operator simply enters the outside-air temperature into the control scheme or building-management system, and the system adjusts to meet the need. When the outside-air temperature is 0°F, 180°F supply water is required to heat the building; however, when the outside-air temperature is 60°F, only 120°F supply water is needed. Figure 2 shows the linear interpolation between what the header temperature is and what is needed to heat the building.

If the supply-water temperature is kept at a constant 180°F though a building all year long, when temperatures are mild the control valve to the heating coil barely will be open. Instead of running 10 or 20 gpm of water, it may just need a “squirt.” When only tenths of a gallon of water are running, the temperature is harder to control, which is why you will see some schools in the spring or fall with their windows open, even though it is chilly outside. The building is being heated with high-temperature water, without good control, so the classrooms start to overheat. The open windows bring in cold air, but the heat still is running, which wastes money. The better solution is to set the system to 140°F supply-water temperature. This allows the system to run at a gallon-per-minute rate similar to that with 180°F supply-water temperature, but the cooler temperature provides finer control.

The night/weekend setback scheme is simple. If there are only a few people in the facility overnight or on weekends, decrease the header temperature to 10°F to 20°F less than what it normally would be. Set the system to warm up the building an hour before people typically arrive.

While the night/weekend setback scheme is logical, setting a larger system temperature differential goes against the norm for most engineers. Traditionally, a 20°F differential is accepted: water out at 180°F, returning at 160°F. The heating coils and air handler are sized to fit this traditional design. However, if the returning temperature is decreased to 140°F (while maintaining the 180°F supply temperature), the building temperature stays the same, but the system only requires half of the water flow because heat is proportional to the differential temperature and flow rate. If the heat is the same, and the differential temperature is doubled, the flow rate is cut in half.

There are some benefits to this system. First, with less flow, a smaller pump can be used, which increases energy savings. Also, if the supply water goes out at 180°F and returns at 140°F, no condensing takes place. But if the water goes out at 160°F on a reset and comes back at 120°F, the system will condense sooner with a larger delta-T. If the supply goes out at 120°F and comes back at 80°F, the system is really condensing. A larger differential temperature with a condensing boiler drives the system into condensing mode sooner, in addition to saving energy in the system because of the smaller pump.

Keep in mind that under similar conditions, a non-condensing boiler cannot get much below 160°F with a 140°F return, because, below 140°F, condensing will begin in the non-condensing boiler and destroy it.

It can be difficult to change the differential temperature in existing buildings, but in new buildings, it is simple. It is a matter of sizing heating coils or devices appropriately to handle a larger delta-T. Many engineers may think increasing delta-T from 20°F to 30°F is a great stretch, and increasing it to 40°F is really radical; however, it can be achieved easily. It requires a mindset shift. Sometimes, existing buildings can get some differential increase, but not a large one because the coil surface area may not be enough to keep a constant heat with a lower flow rate.

Advantages of Hydronic Boiler Controls
With certain hydronic boiler controls, pumps can be set to maintain a constant differential temperature. For example, if a system is set for a 40°F differential temperature, but that drops to 36°F because the heat load decreases, not as much heat is being taken out of the water and the pump slows down. When the pump slows down, the water volume decreases, which draws out more heat and returns the system to the desired 40°F delta-T.

If the speed of the pump does not change to maintain differential temperature as the load drops, a system becomes inefficient and fuel costs surge. This scenario is common. In many buildings with a system designed for a traditional 20°F temperature differential, during low load, there might be a differential of only 3°F or 4°F. As a result, condensing boilers may not be able to condense. However, slowing the pump speed saves energy while increasing boiler efficiency. In many cases, a boiler will begin cycling because of the low load. Boiler cycling decreases system efficiency, so having the right controls properly set achieves the efficiency owners expect. One control strategy is to reset the water temperature lower until a 20°F temperature differential is obtained. This is driven by the heat-transfer device needing to take more heat out to meet the demand. Another strategy is to use variable-speed pumps for the boiler and slow down the pump to maintain the 20°F differential. This will allow for a lower return-water temperature that will drive the boiler into condensing mode.

Differences in Efficiency Ratings
Efficiency ratings depend on the technology. If cold enough water is brought into any unit, it will condense. This principle holds true for even the most inefficient boiler. To compare efficiencies of different technologies and units, consult the Air-Conditioning, Heating and Refrigeration Institute (AHRI) Website at www.ahrinet.org. Products evaluated by this third-party organization are tested under the same conditions. Each product tested earns an AHRI-certified efficiency rating. This is a good resource to consult for comparison purposes. Rating variances can be attributed to differences in design, material, or a combination of the two.

All designs have a place in the market. Non-condensing units, including copper-finned or cast-iron boilers typically rate on the lower end of the efficiency range. These boilers also cost less to manufacture. Higher-efficiency units include firetube boilers, but not exclusively. Some manufacturers use a copper-fin boiler and draw as much heat as possible without condensing and add in a secondary heat exchanger (typically made out of stainless steel) and condense in that to optimize efficiency. These condensing boilers typically achieve an efficiency percentage in the low-90s.

Many owners also are concerned about emissions. Most systems today can achieve sub-30-ppm NOx, but this is not necessarily the standard offering for most manufacturers. Most manufacturers have the ability to get there, but they may have to change out their burner, blower, or other components to do so.

Decentralization of the Boiler Room
In recent years, there has been a trend towards decentralization of boiler rooms. Facilities today are opting to construct several boiler houses with small, modular units instead of one central boiler plant that runs large units. One of the primary reasons for this is that a central plant requires underground piping, which can wreak havoc if a leak occurs. If there is a leak, it may go undetected for a period of time, and after it is detected, fixing it may be difficult to do without tearing up a street or sidewalk to get to it. Also, in northern climates, glycol is added to the water in pipes running outside to keep it from freezing. If a leak occurs in one of these systems, the escaping glycol can be hazardous to the environment.

Another reason to consider decentralization is the amount of energy required to run a boiler system. A centralized boiler facility requires more water to be moved at a high enough pressure to overcome the thousands of feet or miles of piping, depending on the size of the facility. Running at a high pressure requires larger pipes to keep the friction loss low.

Alan Wedal is the commercial boiler product manager for Cleaver-Brooks. He has more than 20 years of experience in hydronic heating. For Cleaver-Brooks he is the leading product manager for the commercial line of steam and hot-water boilers. He is a member of ASHRAE, the West Michigan Society of Health Care Engineers, and the American Society of Plumbing Engineers. He has a bachelor’s degree in nuclear engineering from the University of Wisconsin and an MBA from the University of Michigan.