Maximizing Efficiency With Hydronic Boilers
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
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
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
Cost Benefits of a
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
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
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
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
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
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
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
Differences in Efficiency
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
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.