Consulting engineers have a range of choices today when it comes to meeting the demands of their clients. Some customers are looking to employ the most advanced boiler technology with features like modulating burners, condensing heat exchangers, cascading options, and better heat emitters like in-floor radiant. Other customers are simply looking for heat and are not concerned with anything but meeting minimum building code requirements.

Many building owners understand and seek the benefits of modern technology, so it should be the engineer’s objective to design the heating system (and select the boilers at the heart of it) that meets or exceeds the owner’s expectations for sustained energy efficiency over the decades-long life that today’s modern boilers can provide.

Boiler System Design Considerations

Today, it seems the preferred design is condensing/modulating boiler technology. Standard efficiency non-condensing boilers are typically manufactured with cast iron sections, and operate with some sort of double or triple acting aquastat, which means the target temperature is high limit or low limit. These boilers have one output: 100% capacity. When there is a signal-call for heat, the boiler burner ignites and the boiler operates in between low limit (140°F) and high limit (180°F), typically. 

Many cast iron boilers have limitations to the temperature of the system return water, resulting in high supply temperature system design. In contrast, there are cast iron boilers on the market today that are made from a flexible cast iron and can handle these low return water temperatures. They therefore can utilize lower target temperatures, reducing energy consumption and saving money without the upfront cost of the most advanced technology.

By transitioning to a condensing/modulating high efficiency boiler, other limitations of the cast iron boilers are resolved. Boilers condense when the flue gases within the boiler are at their dew point. This means the system return water temperature must be below 140°F for a boiler to condense. When this happens, latent heat is captured from the condensation, increasing efficiency another 10% compared to a standard efficiency boiler. Modulation allows the boiler to produce only the output required to satisfy demand. So if there is a small load (one zone calling for heat), the boiler will not operate at 100% output like a conventional cast iron boiler. But will operate as low as 20% or a 5:1 turndown ratio for a single boiler or 40:1 in a cascade package system.

Another design consideration should be the system design temperature. Conventionally, the industry has designed convectors/baseboard heating systems to respond to coldest day conditions — i.e. 180°F on a 0°F day in New England. By simply adding more surface area (emitters), the design temperature may be reduced. Commonly, in-floor radiant heat is designed at 100°F — much lower than baseboard — because the entire floor is utilized as a large mass emitter. Baseboard, hydro-coils, and radiators could all be designed and operated comfortably at lower supply temperatures than the standard 180°F.

Some manufacturers provide outputs of their emitters at a variety of supply temperatures, usually 180°F, 160°F, or even as low as 140°F. For every 3°F that the supply temperature is reduced, there is a 1% saving in fuel. Utilizing 160°F and 140°F supply water encourages condensing to happen in a boiler much more frequently than just in the swing months of spring and fall, thus reducing energy consumption and again saving money for the building owner or occupant.

Design Differences: Non-Condensing vs. Condensing

Much like lower supply temperature, there is one major difference here. With a conventional boiler, the installer wants to be sure to protect the cast iron heat exchanger from thermal shock. This is when extreme cold return water enters the hot cast iron sections of the boiler. Expanding or contracting too quickly may cause the cast iron sections to crack and leak. With condensing boilers, installers are encouraged to bring low return water temperature back to the boiler to encourage the condensing function/mode.

Venting is another consideration. Standard efficiency equipment is usually chimney vented or requires additional components (e.g., a power venter) that increase upfront and overall maintenance costs. Condensing boilers are commonly direct vented. Today, it is common to build a new building without a chimney. In this instance, a condensing boiler is the perfect solution for the building needs.

It is also important to consider control strategy: sometimes saving energy is as simple as updating an old control (aquastat) to a more modern device, like a weather-dependent outdoor reset control. It has actually become almost a standard for condensing boilers to have outdoor reset/modulation capabilities. Let’s go back to the design temperatures: if the system is designed at 180°F on a 0°F winter day, then we do not need to provide 180°F water to the heating system on a 50°F day. Outdoor reset considers the outdoor temperature when trying to meet the demand of the system and will produce as low of a supply temperature as possible in relation to the current outdoor temperature and the design parameters. This allows the boilers burner to run in condensing mode much more frequently, resulting in further energy efficiency and fuel savings.

Design & installation Mistakes

A common mistake is failing to follow manufacturers’ instructional literature. Each manufacturer has its own heat exchanger design/material and restrictions. It is important to consider this when selecting a piping schematic for the installation. Cast iron boilers typically were large mass, high volume vessels that could be piped in a supply/return header configuration. Condensing boilers have much less mass and volume, requiring primary secondary piping, or the use of a low loss header, allowing a hydraulic separation between boiler and system flow.

It is extremely important that the boiler(s) be sized correctly, and especially important that they do not get oversized! The installer needs to match both the heat loads and the emitter (radiator) requirements to achieve optimal efficiency. Also, bear in mind that electricity + gas + water can produce hazardous results when a unit is improperly installed. Manufacturers’ products vary somewhat. All technicians should take manufacturers’ training classes on the products they install to be sure that all safety precautions in gas supply, combustion, and venting are observed.

Condensing Boiler Options

Boiler selection in buildings requiring one, two, even three million Btus often offer the engineer options to consider two attractive condensing options: a single high mass boiler or a combination of low mass boilers chained together — known as a cascade package. Sometimes the Btu outputs, turn down ratings, and efficiencies are the same, so the next question is how much floor space is available in the building.

In deciding between a low or high mass boiler, the engineer should be looking to meet or exceed the customer’s expectations. He should therefore examine real-world conditions and decide which boiler type will provide superior thermal efficiency in each specific application. Sometimes the recovery time of a system is more important. A control with setback function could be saving energy at times when the building is unoccupied, but at the other end of that spectrum, the system needs to be able to recover quickly. Low mass boilers are credited with having quick recovery times, which is an aspect of their low water volume.

If the engineer overlooks real product differences, plus installation and operational costs, he could end up with a boiler plant that is not adequately sized for the building’s heating needs, thus minimizing the efficiency.  

Upfront Costs and Payback

Choosing a boiler that will operate at a high level of efficiency at the system design requirements will maximize the project investment. Today’s condensing high mass boilers may come with a higher upfront purchase cost, depending on the competition, but that higher purchase price will be offset by greater efficiency gains and hence more fuel savings over a longer service life. With abundant natural gas now available from North American sources, fuel savings can be considerable, making payback on the boiler purchase a matter of less than one-sixth the duration in years of the boiler’s life.

For example, a condensing high mass boiler may cost 10% to 20% more than a comparable low mass boiler in an application requiring six to seven million Btus. This building may have an annual expected fuel bill of $60,000. But factoring a conservative fuel cost savings of around $2,000 a year using the high mass boiler, this yields a payback of $10,000 in five years’ time. Any property business owner will understand that kind of math.

Design Advantages

High mass boilers provide some additional benefits. One advantage is that no boiler pump is required. This alone can be a substantial installation and usage saving. Usually, a high mass boiler can work with a variety of fuels — natural gas, LP gas, or fuel oil. Sometimes they can even work with a combination of the two. Dual-fuel burners are installed in hospitals and schools where a “no heat” service call is simply unacceptable. Some municipalities use dual-fuel equipment in an effort to balance the budget. As their gas consumption goes up, sometimes the price can, too. So when the gas usage is exceeding the monthly budget, they flip a switch and start burning the oil that was bought out of peak season at a much more competitive price.

Another feature of the high mass, high efficiency boilers is dual return ports that keep differing water temperatures separated. Most boilers, low or high mass, have single in and out ports for supply and return. On these dual return boilers there is a supply port and two return ports. With multiple system water temperatures returning through the boiler due to system equipment features that may be present (e.g., different distribution units like fan coils and in-floor radiant or preheat for ventilation air), water temperature, because of the mix, gets raised depending on the supply and return flow rates. With dual ports, differing return water temperatures remain separated instead of blended, maintaining a lower return water temperature and increasing the boiler’s overall efficiency.

Equipment and Ratios

Low mass boilers require decoupling from the system and use of a dedicated boiler pump matching minimum flow requirements through the boiler. This need increases low mass installation and servicing costs. With a three million Btu boiler at a 20°F Delta T there are between 200 and 300 gal of water flowing through the boiler per minute. That is a significant amount and requires a large dedicated pump capable of pumping that amount of water volume, which of course consumes a lot of electrical energy. Also, not only is there a need for a dedicated boiler pump but also isolation valves — 3- and 4-in valves to allow service and maintenance or exchange in case of a replacement need. Every time the boiler fires, there is electrical consumption on the pump side, too. Engineers should recognize these costs and the differences between low and high mass: with a high mass boiler, there is no dedicated pump. It instead utilizes the main loop pump.

Low mass boilers do typically have higher modulation ratios, smaller footprints in the mechanical room, and free redundancy built into every system. So if you have a cascade of eight commercial condensing boilers with 2,664 MBH output and one is down for service, there is still suitable heating with 2,331 MBH that can be delivered to the system. If the one high mass boiler is down for service, the building is down until service is complete. Another benefit to the cascade packages is the flexibility in venting, whether it is simply a factor of the material used or the flexibility in where the termination is installed. Each unit could be vented individually or cascaded together for one penetration in the building envelope.

Condensing boilers’ flue gas condensate is corrosive by nature and requires periodic cleaning. Low mass boilers have very tight waterways and a lot of restriction, so they require more frequent cleaning. High mass boilers’ design, in general, provides wide open flue passages, which are capable of self-cleaning to a certain extent. On the water side, low mass boilers with smaller water passages are susceptible to scale buildup. Even a small amount of scale buildup can affect efficiency. The high mass design has a lot more surface to contain scale, and since a lot of it does not come in contact with the heating surface where scale tends to start, overall scale buildup is less. 

Mixed Plants

Both high mass and low mass boilers are being installed in what is now known as mixed plants. There are a few advanced controls on the market that can handle up to four boilers and an indirect water heater, no matter if the heat source is a condensing, non-condensing, natural gas, LP gas, or oil boiler. This is all accomplished in one centrally located module. Mixed plants are typically retrofit applications where a combination of new and old boiler technology is utilized to meet the growing demands of the building’s occupants. The mixed plant concept permits hybrid boiler setups with supplemental conventional boilers for greater efficiency along with lower first costs.

To cite one such installation as an example, the mechanical plant for the Russia Wharf project along Atlantic Avenue in downtown Boston employed six high mass boilers with two conventional cast iron boilers for heating a 32-story high-rise that achieved LEED Platinum certification  (Figure 1). The high mass boilers serve most of the load while the conventional boilers supplement the hot water loop for the coldest part of the heating season. The developer, Boston Properties, estimates that the high mass boilers saved over $16,000 in energy costs in the first year alone.


Engineers designing today’s advanced boiler systems have choices to make in terms of utilizing the latest technology. Modulating boilers, condensing heat exchangers, and boilers in cascade are all attractive options to employ. Depending on the application, low mass boilers can be the better solution. They are less expensive than high mass, have a smaller footprint, and are better suited for retrofit situations. But when it comes to overall efficiency and energy savings potential, high mass boilers are more efficient (manufacturers’ published curves support this) and they save building owners on fuel bills, and when in operation electricity costs as well. Design engineers should consider all of these factors. Most importantly, keep in mind the customer’s specific needs when choosing a boiler type that will also allow you to design for efficiency and long-term use.