Part 1: Combustion Process and Combustion Efficiency

Combustion Process and Combustion Efficiency

It takes approximately 1000 Btu’s of energy to convert one pound of water into vapor.  Think of that as the price you pay as part of the combustion process in a typical non-condensing hydronic boiler.  That’s a good starting place for this next series on Energy Efficient Hot Water Boiler Plant Design. 

So much of boiler efficiency rests in what happens during the process of combustion – and how much energy escapes through the chimney in the form of water vapor.  If the water that is produced during the combustion remains in a gaseous state, it is lost through the flue, taking precious Btus with it. If the water vapor condenses inside the boiler, these Btus stay within the system – for better or worse depending on how the boiler is designed. 

Where Does the Water Vapor Come From?

Three major things are produced during the process of combustion inside a boiler:  Water vapor (H20), Carbon Dioxide (CO2), and Heat.  This is what occurs when fuel and air are ignited inside a boiler.  About 90% of the energy produced by natural gas-fired boilers is in the form of sensible heat and about 10% is latent heat.  The latter is contained within the water vapor that is produced during combustion. 

If the boiler is designed for non-condensing operation, and the system return water temperature is sufficiently high (above 130°F), the water that is produced during combustion will remain in the form of gas and is ultimately vented from the boiler.  With it goes about 10% of the energy you have put into the system.  This is the “downside” of non-condensing boilers – the physics that keeps them from ever achieving more than about 83 – 87% efficiency.  (Note: More modern design non-condensing boilers utilize fans and smaller flues, which increases efficiencies up to about 87%). 

Of course, any boiler can condense if supply water temperatures drop below a certain point.  But if the boiler is not designed with materials that can withstand the effects of the carbonic acid that is produced when water and CO2 mix, it won’t be in service very long.

Understanding Condensing Boiler Efficiency

Non-condensing boilers once dominated the marketplace and thousands are currently in service today. Operators are careful to keep system return water temperatures above 130°F to avoid “rain in the boiler”.  However, for reasons of efficiency, more and more condensing boilers are being selected for replacement and new construction.  At JMP, we estimate that as much as 90% of new boilers purchased in our market are condensing. 

Although condensing boilers are more expensive, the amount energy they save makes them a financial no-brainer for most applications.  Remember — a 1-million Btu/hr condensing boiler produces approximately 93 pounds of water per million Btus.  Since each pound of water contains about 1000 Btus of energy that equates to 93,000 Btus.  If this water were to remaining in a gaseous state as it presumably would in a non-condensing boiler, all that energy would be lost up the chimney.   If the water condenses, than that energy remains within the system in the form of latent heat.  Therein lies the increasing popularity of condensing boilers! 

Click here to watch our YouTube video on Combustion Basics

Next up, we’ll take a more in-depth look at condensing boiler technology. 

Part 2: Golden Rules of Condensing Boiler Technology

Condensing boilers now dominate the new construction and replacement commercial markets.  Here at JMP we estimate that as much as 90% of new boilers we sell are condensing boilers.  So how did we get here and what is it about the technology that transformed the marketplace?

The trend toward condensing boilers has been evolutionary. It started with the introduction of atmospheric modulating non-condensing boilers.  Modulating fire helped reduce fuel consumption, but because the chimneys still had to be sized for full air (since they were naturally drafted) a good bit of sensible heat, along with the hidden latent heat contained within the water vapor, was still lost up the chimney.  These boilers tended to max out at efficiency around 82%.

Next came fan-assisted, non-condensing boilers.  These boilers used a modulating fan to create the necessary air flow so that air could be controlled in a way that was proportional to the gas being supplied to the boiler at any given time.  Less air lost up the chimney meant fewer Btus lost to the atmosphere.  A side benefit of fan-assisted drafts was that flues could smaller, reducing a certain amount of material cost.  

Then manufacturers began to really push the envelope on the efficiency of non-condensing boilers with other design improvements, including more efficient burners. 

But none of these designs addressed the constant loss of latent heat contained within the water vapor that escaped the chimney. For gas boilers, that tended to be about 10% of the Btus it took to drive the boiler. 

Additionally, none of these boilers could maintain peak efficiency unless they were at full load.  At full load, efficiencies maxed out around 87 to 88%, and significantly dropped as the boiler modulated down.  Any part load operation involved sacrificing efficiency – an unfortunate reality since full load design conditions are more the exception than the rule.

Finally, boiler manufacturers began to develop boilers with all the modulating components and build them so that they could safely condense and thus hold on to that elusive 10% of latent heat.  It was great news for the industry, but some changes in design practice would also need to occur in order for these boilers to reach their 94-99% efficiency in the field. 

We’re getting there.  Condensing boilers are being applied in the field with great success these days, assuming everyone observes the two Golden Rules of condensing boiler operation: 

Rule #1:  Condensing boilers must condense!  If they aren’t condensing, then they are not retaining the latent heat of combustion.  This means systems must be designed so that return water temperature is at least below 130.  The lower the return water temperature the better the efficiency. Figure 1 shows just how dramatically return water temperature impacts the efficiency of a condensing boiler. 

Figure 1


Rule #2: Condensing boilers are more efficient at part load!  The reason for this is two-fold.  As the boiler modulates toward low fire the heat exchanger capacity remains the same because the surface area has not changed.  You’re still generating heat and your still condensing.  As the fire rate increases, the amount of exhaust gas increases, which robs the boiler of some of its condensing capability. 

With condensing boilers, the more you condense, the more efficient you are.  That’s why two or three boilers at low fire are always better than one at full fire.  Figure 2 shows how much efficiency is gained by operating condensing boilers at part load.  As the graph shows, a condensing boiler at 25% fire is approximately 8% more efficient than when it is at 100% fire. 

Figure 2

Types of boiler efficiencies

Figure 3 shows the combined effect of lowering return water temperature and part load operation on a condensing boiler.

Figure 3

Part 3: Condensing Boiler Heat Exchanger Construction Considerations

We now live, work, and learn in indoor environments that are often heated using condensing boiler technology.  Condensing boilers have made a strong case for themselves in terms of both space and energy efficiency.  Of course, the latter hinges on one critical factor: the condensing boiler must condense.  And if condensation is intended as an ongoing occurrence, the correct materials and equipment must be specified to withstand the harsh effects of condensation.

The most critical surface area inside the boiler is the heat exchanger. The condensate produced in a condensing boiler is acidic (typically 3 -5 pH) and can be highly corrosive to the heat exchanger as well as other surfaces inside a boiler and the flue.  Heat exchangers should be selected based on their ability to withstand certain pH levels and water pressure.  Specifies have three choices when it comes to heat exchanger material:

Stainless Steel.  This is by far the most commonly selected heat exchanger material.  In most cases this is 316L Stainless steel, which is more resistant to atmospheric and other types of corrosion than Types 302, 304 and 304L.   Stainless Steel heat exchangers are capable of handling a wide range of pH levels and can be selected for low water volume (water-tube) boilers or high water volume (fire-tube) boilers, depending on the brand you choose.  Given the high cost of stainless steel, most of these heat exchangers are of the low water volume variety, and are used in modular boiler applications. 

Aluminum.  Aluminum heat exchangers have a lower first cost which is why you will occasionally see them used in condensing boilers.  Aluminum may have a lower pressure rating so you must make sure your design is below the maximum pressure rating of the boiler.  It is also important to note that aluminum heat exchangers can only handle a narrow pH range so specifiers need to take care that their application stays within that range.  This means that an effective chemical treatment plan is necessary to keep the pH levels neutral.  Aluminum heat exchangers are only used in low water volume applications.

Cast Iron.  Sometimes condensing boilers may be equipped with cast iron heat exchangers.  The selection will work, depending on the application, but the cast iron must be at least ¼” thick.  The surface will rust, but it will not break down, and cast iron heat exchangers will typically have a long life, regardless of water temperatures.  However, it is absolutely essential to follow strict operating and blow down procedures, otherwise you will likely void the boiler manufacturer’s warranty.

Part 4: What to Do with Boiler Condensate

A properly applied condensing boiler generates a lot of condensate – 5 gallons for every 1,000,000 BTUH input.  In a large commercial or institutional building this can easily add up to several hundred gallons per week.  But where does all that condensate go? 

You can’t put it down the drain.  First of all, it may be too hot.  Most commercial building codes do not allow water any hotter than 140°F into the sewer system.  It is also too acidic (3-5 pH).  Boiler condensate has been known to eat through concrete floors so simply directing it to a floor drain is not an option.  Besides, there are certain environmental and health risks associated with putting boiler condensate into the sewer system, not to mention strict building codes.   So between the boiler and the drain you need a neutralization kit.

What is a condensate neutralization kit?

Condensate neutralization kits are available through boiler vendors and typically consist of a plastic container filled with some sort of neutralization media and an inlet for sufficient diluting water.  It maybe limestone, marble, alkaline chips or other material containing calcium carbonate. The neutralization media increases the pH of the condensate so that the acid levels are lowered to a point that is suitable for drain disposal. 

The neutralization system should be added in the drainpipe between the boiler and the building drain.  It should be piped with either PVC, plastic or cast iron piping.  The condensate will flow into the neutralization tank by gravity, so it needs to be installed below the outlet on the boiler or water heater.  If this is not possible, a pump may need to be installed.

If you are designing a condensing boiler system, a properly designed neutralization kit must be included in the specification, as you are the one responsible for making sure that disposal of boiler condensate is handled according to state and local codes.  This can be a prefabricated kit from a vendor or one that you design yourself from assembled materials and components.  The option is yours – just don’t forget include a neutralization kit into the plans!

Part 5: Fan Coils, Pumps & Flow Control

So you’ve decided to use condensing boilers on your next project – GREAT!  Just remember that this decision will impact other aspects of your mechanical design. 

Condensing boilers do not operate properly in systems that are designed around traditional non-condensing technology.  What do we mean by “traditional, non-condensing technology”?  Systems with much higher return water temperatures.

In the past most boiler plants were designed for supply water temperatures of 180 degrees or even higher to the fan coils.  In order to keep the supply water temperature well above condensing levels, return water temperatures were also very high, with a supply/return Delta T of only about 20 degrees or so.   Water velocities were typically high, and heat transfer surfaces inside the coil could afford to be smaller because of the high water temperature. 

All this is not the case with condensing boilers, which require return water temperatures that are 130 degrees or lower in order to condense (120 degrees is a good design point rule of thumb).  Getting return water temperatures down to condensing levels requires that supply temperatures also be significantly lower – in most cases no higher than 140 degrees although the actual design temperature might be higher.  Remember, the more water vapor your boilers condense the more efficiently they operate, so the lower the temperature (supply and return) the better.  

Adjusting Your Fan Coil Design

Fan coils operating at these lower temperatures and ∆Ts will require increased heat transfer surface to extract then required amount of heat from the supply water to maintain space temperature set points.  Engineers who take the time to adjust their coil design (and they better!) will find that they might need additional rows of coils to maintain sufficient heat transfer. 

Of course, when it comes to heat transfer, one change begets another.  Increased surface area inside the fan coil means increased static head.  Your fan might need to be larger with a higher horsepower.  On the bright side, the system is always going to be running at part load (remember– condensing boilers operate more efficiently at part load!) so pump energy consumption will be lower as a result of lower flows.

Proper Flow Control

Lower flows necessitate another important design adjustment:  Flow control.  Standard pressure dependent control valves are not suitable for condensing boiler systems because they cannot maintain stable flow rates under fluctuating pressures.  And pressures fluctuate widely in a condensing boiler system as a result of increased part load operation and, more often than not, the variable speed pumps that typically accompany these high performance systems.  Therefore pressure independent control valves (PICVs) should be specified.  These valves are designed to automatically reposition internally whenever there is a change in demand for the space and maintain accurate flow control down to very small load.

More Boilers at Part Load

Finally, in addition to these minor adjustments, condensing boiler systems should be designed with multiple boilers that operate simultaneously at part load whenever and for as long possible.  Condensing boiler efficiency drops significantly as load increases.  However, the long-term operational savings has proven that this additional investment in equipment (if properly designed!) is totally worth it.

Part 6: Best Applications for Condensing Boilers

Condensing boiler applications dovetail nicely with most of today’s energy efficient HVAC strategies, as well as ASHRAE 90.1 – 2010 and 2013 requirements.  New and existing boiler plants are ripe with opportunities for condensing technology and can offer payback in as little as 2-3 years.   

Here we’ve listed some of the top applications for condensing boilers and why they are a good choice in each situation.

Water Source Heat Pumps (WSHP)

WSHPs have become the “go-to” solution for many multi-tenant applications, as well as new multi-zone school designs.  WSHP take advantage of the heating and cooling requirements of each space in the entire building by recovering otherwise wasted energy in some spaces and utilizing it elsewhere in the system. Such systems are comprised of highly efficient packaged reverse cycle heat pump units interconnected by way of a water loop. Each unit satisfies the air comfort requirements of its corresponding zone. In cold weather, the heat pump removes heat from the water loop via the unit’s refrigerant-to-water coaxial heat exchanger and transfers it to the air.  Excess heat is rejected via either a cooling tower or a closed-circuit cooler.

Because WSHPs operate at low temperatures, typically 60 to 90 degrees, they are ideal for use with condensing boilers.  They not only operate more efficiently, they are not susceptible to the same maintenance issues as non-condensing boilers in the same WSHP applications.  We have seen many WSHPs installed with traditional non-condensing boilers that are plagued with maintenance issues because the lower temperature water produced by the heat pumps causes condensation in the boilers.  The only way to make this work is to install a bypass line with either a circuit setter or thermostatic control valve between the heat rejector and the boiler (Figure 1) that allows boiler supply water to mix with boiler return water to keep it above 130 degrees.

It’s simpler and more efficient to install a condensing boiler and take advantage of the low return water temperature.  WSHP not only operate at ideally low temperatures, they are almost always operating at part load, which further enhances the efficiency of a condensing boiler. 

Figure 1

This system does not have a condensing boiler – but it should! Notice the bypass line located between the supply and return line to the boiler. This is a necessary complication for non-condensing boilers used in WSHP applications. The bypass line is necessary to mix hot supply water with return water in order to keep it above condensing temperatures. Hydronic Heating with Outside Air Reset 

Many hydronic heating (and cooling) systems incorporate temperature reset as a means to save energy.  In fact, reset is now a requirement of ASHRAE 90.1 – 2010 and 2013.  The only exceptions are when the application of reset interferes with the operation of heating, cooling, humidifying or dehumidifying systems, or variable flow pumping is used as an alternative method to save energy. 

Systems with temperature reset are excellent application for condensing boilers. 

While these systems may be designed to supply 180-degree water, reset control means that they will be frequently be operating below 180 degrees.  Consider what happens when the temperature outdoors reaches 50 degrees during the heating season, as it frequently does in our own southeastern part of the country.  Under these conditions the system will automatically reset according to a specified schedule, probably down to around 130 degree supply water.  By the time that water makes it back to the boiler, it’s probably going to be 110 or 120 degrees – well below condensing threshold.  That’s a problem with non-condensing boilers, but great news if condensing boilers have been used.  With condensing boilers there’s no need to install any extra piping or valves to protect the boiler from the low temperature water.  Moreover, condensing boilers will operate more efficiently at part-load conditions.

Hybrid Solution for Reset System Retrofits

A single condensing boiler retrofit is a perfect solution for a system that currently incorporates two or more non-condensing boilers operating on a reset schedule.  In all likelihood the system is already experiencing maintenance issues due to rain in the boilers.  Replacing just one of the non-condensing boilers with a condensing boiler increases efficiency of the system and eliminates these maintenance issues.  The condensing boiler would be set up as the lead boiler and would operate all time, usually at part load conditions.  (Figure 2) The only time the other boiler(s) would come on is when outdoor air temperature reaches peak conditions and the supply water temperatures would be above 160 or so. 

Figure 2

This “hybridized” solution has relatively low upfront cost with excellent potential for payback and reduced maintenance.

Dual Temperature or “2-Pipe” Systems

There was a time when HVAC systems were designed to heat or cool with no in between.  These very basic systems use a single set of pipes going out to the system and the water is either heated via the boiler or cooled by the chiller depending on the season.  (Figure 3)

Figure 3

We still encounter these systems, and they are an excellent application for condensing boilers because they facilitate the transition time between heating and cooling mode.  With non-condensing boilers, switching from heating to cooling or vice versa, is a tedious task that often takes a number of days.  Time and care must be taken to avoid sending over-heated water to the chillers or cold water to the non-condensing boilers.  However, if the boilers are designed for condensing, then you are mostly likely operating at lower temperatures while in heating mode, so you don’t have to wait as long (if at all) for the water to cool before you switch the chillers on.  If you are switching from cooling mode to heating, there is no time lost, as the boilers will condense as they were designed and efficiency only gets better. 

Radiant Heating Applications 

Radiant heat applications, where there is hot water piping embedded in the concrete or just below the floor in a building, is another great application for condensing boilers.  These systems typically operate within a range of 85 to 125-degree supply temperatures, thus they are always condensing and operate part load most of the time.

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