Maximizing Efficiency With Hydronic Boilers

June 15, 2012

Hydronic boilers enable a facility to better control the delivery of heat when and where it is needed. 

Before condensing boiler technology was introduced, boilers operated at 80% to 85% efficiency.  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 heat in addition to sensible heat from combustion exhaust, and the results can be dramatic. Some facilities experience an energy savings of up to 50% in systems that make proper use of outdoor reset schedules, aggressive night/weekend setback schemes and larger system temperature differentials.

Non-modulating (on-off) boilers operate at full fire all the time.  Whereas older design units are typically 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 to high (or max) fire.  The modulation helps achieve the lower firing rate. The longer a modular 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 also run longer at lower temperatures, which minimize excessive cycling.

It is often 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 upon heating load, which helps save fuel compared to one larger boiler. 

Hydronic Boilers are Superior to Steam 

Hydronic boilers have several natural advantages over steam boilers.  First, generating heat from steam necessitates a steam-to-water heat exchanger, and there is some heat loss in the transfer that occurs. 

Also, with steam systems, there are steam losses through steam traps, steam leaks and piping because those systems run at a higher temperature.  Some facilities add more insulation around the piping, which can decrease the heat loss, but does not eliminate it.

Maintaining a hydronic boiler is easier than maintaining a steam one.  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, nor do they build up.  Typically operators only have to evaluate the chemistry in a closed-loop system once a year and make any necessary adjustments. 

Retrofitting a Steam System to Hydronics 

Most buildings using steam heating run at 15 pounds of pressure, so a facility can get by with a reasonably sized supply header and the condensate coming back is typically much smaller because steam goes out and returns as a hot liquid for the condensate. The steam doesn’t need to be pumped.  It is only necessary to pump the condensate back.  The system goes from 15 pounds of pressure to 0 pounds of pressure, or from high to low pressure, as the need occurs just through natural force.  It is operating at a higher heat value, so there is heat that is needed in the space of that piping system, which may result in stronger BTU motive force delivery.  That transfer can go out to various buildings easily without the use of a pump, but you do have to pump the condensate back or there will be a lot more make-up water.  For facilities already running steam radiators or steam devices, sometimes it’s not possible to reuse the existing piping to go to a hydronic system.  A new hydronic piping system has to be installed.  

In some cases, a facility can use the condensate return line for the waterside and pump it. The steam header is much larger than needed for hydronics, which is okay for flowing water. However, a half-inch or quarter-inch line for the steam condensate may not be big 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 really comes down to a cost/benefit analysis.  A facility has to evaluate the expected efficiency gains and environmental benefits from a hydronic system against the cost of laying new pipe.  Keep in mind that older steam units are likely only 50% to 60% efficient, compared to a hydronic system that can achieve efficiencies in the mid to upper 90s.

Consider a Hybrid System 

Utilizing both condensing and non-condensing boilers is considered a hybrid system. This type of system will be especially beneficial to 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, when it could be 0 degrees or even minus 20 degrees outside, it’s best for a building to run its existing non-condensing boiler.  At 180 degree 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, it can handle most of it, and at high temperatures, the condensing boiler can run to provide heat at the 

Cost Benefits of a Hydronic System 

The payback on a new hydronic system is typically 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 facility buys a condensing unit to supplement its existing non-condensing one.  The energy savings for this type of retrofit is typically between 25% and 30%.  Increases in efficiency directly correlate to the run time of the the condensing boiler.  

It is important to note that if a facility puts in condensing boilers and operates them at a supply water temperature of 180 degrees out and 160 degrees back all the time, the boiler never kicks in to condensing mode where you gain the benefits with 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 see their energy bill cut in half.  In addition to running condensing boilers during non-peak times, their operator uses an outdoor reset schedule, oversees an aggressive night/weekend setback scheme, and capitalizes on a larger system temperature differential.

Using outdoor reset, a boiler operator enters the outside air temperature into the control scheme or building management system, and the system adjusts to meet the need.  When it’s 0 degrees outside, 180 degree supply water is required to heat the building; however, when it’s 60 degrees out, only 120 degree supply water is needed to heat the building.  The graph (see graph) shows the linear interpolation between what the header temperature is and what is needed to heat the building. In the spring, when it begins to get warmer outside, there is not as much loss through the building.  So, the supply water temperature can be scaled back and the same comfort level maintained, thereby saving energy.

If the supply temperature is kept at a constant 180 degree though building all year long, when it is 50 degrees out, the control valve to that heating coil is just barely cracked open because only a little heat is needed.  So, instead of running 10 or 20 gallons of water through per minute, it may just need a squirt of water.  When only tenths of a gallon of water is running, the temperature is harder to control, which is why you’ll see some schools in the spring or fall with their windows open, when it’s cold outside.  The building is being heated with high temperature water, without good control, so the classrooms start to overheat.  Open windows bring in cold air, but the heat is still running, which wastes money.  The better solution is to set the system to 140 degree supply temperature.  That way, the water, can run a similar gallon per minute compared to the 180 degree supply water temperature, except it is a little cooler, so there is 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 10 degrees to 20 degrees less than what it would normally be.  The temperature should be set 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, in the industry, a 20 degree differential is accepted – a supply temperature of 180 degrees out, returning at 160 degrees. The heating coils and air handler are sized to fit this traditional design.  However, if the returning temperature is decreased to 140 degrees (maintaining the 180 degree supply temperature out), 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 a couple of benefits to this system.  First, with less flow, a smaller pump can be used, which increases energy savings.  Also, if the supply temperature goes out at 180 degrees and comes back at 140 degrees, there is no condensing.  But if the system goes out at 160 degrees on a reset and comes back at 120 degrees, the system will condense sooner with a larger Delta-T.  If the supply temperature goes out at 120 degrees and comes back at 80 degrees, 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 due to the smaller pump. 

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

In existing buildings, changing the differential temperature can be hard to do, but in new buildings it is simple.  It is a matter of sizing the heating coils or devices appropriately to be able to handle a larger Delta-T.  Most engineers think increasing the Delta-T from 20 degrees to 30 degrees is a great stretch, and 40 degrees is really radical; however it easily can be achieved.  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 that 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 the system is set for a 40 degree differential temperature, and it drops to 36 degrees because the heat load decreases, not as much heat is being taken out of the water, so the pump slows down. The water volume decreases when the pump slows down, which draws out more heat, returning to the desired 40 degree Delta-T. 

If the speed of the pump doesn’t change to maintain the differential temperature as the load drops, the system becomes inefficient and fuel costs surge.  This scenario is common.  In many buildings with a system designed for a traditional 20 degree temperature differential, during low-load, there might be a 3 degree or 4 degree differential.  As a result, condensing boilers may not be able to condense; however, slowing the pump speed down saves energy while increasing boiler efficiency.  In a lot of cases, the boiler will begin cycling due to the low load. Boiler cycling decreases system efficiency, so having the right controls that are properly set achieves the efficiency that the owner is expecting.  One control strategy that has been shown to work is to reset the water temperature lower until a 20 degree temperature differential is obtained.  This is driven by the heat transfer device needing to take more heat out to meet the demand.  Another possible way is to use variable speed pumps for the boiler and slow down the pump to maintain the 20 degree 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 upon the technology.  If cold enough water is brought into any unit, it will condense.  This principle holds true for even the most inefficiently built boiler.  To compare efficiencies of different technologies and units, consult the Air-Conditioning, Refrigeration and Heating Institute (AHRI) site 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 rate.  This is a good resource to consult for comparison purposes.  Some models have a 92% efficiency rating, and others, such as the ClearFire CFC 2500 have an AHRI efficiency rating of 99.1%.  Rating variances can be attributed to differences in design, material, or 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 get the last little bit.  So, they don’t go all stainless in the condensing, which keeps the costs down.  These condensing boilers achieve an efficiency rating in the low 90s.

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

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 smaller, modular units, compared to 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’s 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 the 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 bigger pipes to keep the friction loss low.  

To learn more about hydronic systems and how they can help your company reduce energy costs, contact your local Cleaver-Brooks representative or visit cleaverbrooks.com. 

Alan Wedal is Product Manager for Commercial Boilers at Cleaver-Brooks. 

See article as published in the Spring Issue of HPAC Engineering.