This past winter, a friend of mine was called out to an emergency no-heat situation at a local bank. What he discovered onsite is only one example that supports my opinion: Sealed combustion appliances are superior to atmospheric units.
It was very cold the day Jeff (my buddy) was called out, and the night before had been even colder. Temps had dropped to -25 F overnight and hovered around -5 F throughout the day. As soon as he stepped off the basement steps and heard the carpet squish under his boots, he knew something in the bank’s circa-1980s hydronic system had frozen and burst.
He sloshed across the floor to the mechanical room door, opening it to find ice hanging from the boiler’s support structure and a ruptured, frozen pump.
The boiler supplies hot water to a large hydronic fan-coil unit. When he opened the air handler, wrestling the sheet metal free of the ice formed inside, he found that the copper coil had burst in several places.
Jeff believes the catastrophic system failure was caused by the combustion air supply penetration in the wall. It was sized (read: likely oversized) for the 700 MBH cast-iron boiler. When the boiler was running — which had no doubt been most of the night — frigid outdoor air was being drawn directly into the boiler room. The louver likely froze open or moisture in the pneumatic control line froze, locking the louvers open.
After hours of exposure to sub-zero temperatures, the copper coil in the air handler burst. The half-inch makeup water line couldn’t keep up with the rate the coil was leaking water, tripping the boiler’s low-water cutoff. The situation didn’t go downhill once the boiler quit running — it fell off a cliff. The pump froze solid, pipes burst and the boiler room turned into a hockey rink.
Jeff arrived before any boiler sections cracked, but that was the only silver lining. After assessing the damage, he brought temporary heat into the boiler room. This unlocked the makeup air louvers. More temporary heat was supplied to the bank until Jeff could get the parts needed to bring the system back online, including a big end-suction pump and hot water coil.
It ran effectively for the remainder of the heating season, but he left no doubt in the minds of bank managers that the system must be replaced before this coming heating season.
At first, the customer cringed at the thought of replacing the old boiler and pneumatic controls. Jeff eventually succeeded in convincing the bank’s management that the best answer was a condensing boiler, whether he was hired for the retrofit or not. Sealed combustion was a must. The big wall penetration needed to be sealed.
This true tale is one great example of why I prefer sealed combustion boilers over atmospherically vented models, especially in parts of the country that routinely see sub-zero temperatures. Even in milder climates, I think the list of condensing boiler benefits far outweighs the small cost premium and requirement for greater diligence in service and maintenance.
The list goes on
So why else do I think condensing boilers beat conventional boilers in 95% of applications?
Mod-con boilers are typically equipped with sophisticated onboard controls that allow for fine-tuning system performance. For example, if a building has a snowmelt loop, radiant zones, secondary heat via a fan coil, and a domestic hot water tank, I can not only prioritize the zone(s) I want, but I can even assign different supply water temps to each of those zones. Try that with a conventional boiler.
It’s also far easier to daisy-chain multiple condensing boilers via their onboard controls. Communication with a building automation system is usually simpler, too.
The result is a system that adapts in real time to changing conditions, maximizing comfort and efficiency. Some boiler models also allow remote access and diagnostics, which simplifies service and gives service professionals the ability to troubleshoot or make adjustments without a site visit:
Responsive outdoor reset: While outdoor reset (ODR) controls can be retrofitted to atmospheric boilers, mod-cons are built to take full advantage of ODR. By adjusting the boiler’s supply water temperature based on outdoor conditions (setting the ODR curve), mod-cons produce lower water temperatures for much of the heating season, exactly the condition that promotes flue gas condensation and improved efficiency.
This constant, or near-constant, modulation helps maintain consistent temperatures throughout the building, avoiding the hot-cold cycles associated with less responsive systems. It also eliminates or reduces the wear a boiler endures from short cycling.
Some high-efficiency boilers even forgo the need for ODR altogether, through rate-based temperature reset, thanks to logic built into the boiler’s control.
Efficiency gains: Perhaps the most obvious benefit of a mod-con boiler (or at least the benefit that’s easiest to explain to a customer) is the ability to achieve a significantly higher Annual Fuel Utilization Efficiency — often 95% or higher — compared to the 80% to 84% typically offered by atmospheric models. Over the course of a heating season, this adds up to real savings on fuel bills.
Turndown ratio and load matching: One of the most underrated features of a mod-con boiler is its turndown ratio. This is the ability to adjust its firing rate to match the actual, real-time heating load. For example, a boiler with a 10:1 turndown ratio can operate anywhere between 10% and 100% of its capacity.
This (again) avoids short cycling, reduces wear and tear, and allows the system to run continuously at low fire when demand is minimal.
Atmospheric boilers, on the other hand, operate in a simple on/off mode, which is inherently less efficient and often leads to temperature swings within the occupied space.
When cascading multiple modulating boilers together on a single system, this benefit is multiplied. If a system has four 200 MBH boilers, each with 10:1 turndown, the turndown ratio of the system is effectively 40:1. The system can fire at inputs anywhere between 20,000 BTU/hour and 800,000 BTU/hour.
Quieter operation and space savings: Because mod-con boilers use variable-speed fans and sealed combustion, they operate significantly quieter than their atmospheric counterparts. The reduced burner noise and lower flue gas velocities result in a quieter mechanical room.
Modern condensing boilers are more compact and lighter than steel or cast-iron boilers. Residential models can often be wall-mounted, freeing up valuable floor space in mechanical rooms or basements. This flexibility is a real advantage in renovations or tight installations, and it can simplify piping layouts, especially in multi-boiler systems.
Combustion air and venting: Jeff’s experience at the bank this past winter should be illustration enough that sealed combustion is easier to accommodate than atmospheric. The same applies to the flue.
Condensing boilers can be vented through smaller, lighter plastic pipe (so long as you’re using manufacturer-approved venting material). Poly vent material lasts longer and can be terminated at almost any place in a wall or ceiling, with greater allowable runs than metallic vent pipe on a conventional boiler.
Plus, various terminations are available for plastic vent pipe, including concentric terminations, allowing the installer to provide combustion air and venting through a single wall or roof penetration.
Anyway, I think I’ve clarified why I hold this opinion. Yes, atmospheric boilers still have a place in certain applications. That said, if you’re not already installing mod-cons, I’d suggest getting trained on a quality unit, either by the manufacturer or the local rep.
Mod-cons can be a little trickier to install, and annual maintenance is more important. At the end of the day, if you don’t take the initiative to learn to install high-efficiency boilers, you’ll be left in the dust. The day is coming when conventional boilers will be regulated out of existence. When that day comes is anyone’s guess, but I’m willing to bet it happens on a national level in the next decade.