For the past few decades, an increasingly popular space heating option is a system with a modulating condensing (mod-con) boiler. Because these boilers can potentially have a high efficiency (90-95% or higher), they are often promoted by state and utility subsidy programs.
In a well-designed system, the boiler’s efficiency can hit or even exceed its nameplate AFUE. But as installed, most fall well short of their AFUE test numbers and often suffer an abbreviated lifespan. Efficiency problems and lifespan-crippling sizing errors could be avoided with a modest amount of analysis.
With a bit of simple math, the risk of ending up with a modulating condensing boiler that neither modulates nor condenses can be avoided. This math is by no means a substitute for a hydronic system design, but it’s very useful. Don’t just leave the design of your system to your HVAC contractor; be proactive, and pay attention to the details of any contractor’s proposal.
To improve combustion efficiency, mod-con boilers take advantage of the fact that a major combustion byproduct of hydrocarbons is water vapor. Water as vapor contains latent energy (the “heat of vaporization”), which is about 970 BTU/pound. When water vapor condenses back to a liquid, it releases that heat.
With condensing equipment the key is to operate the appliance so that the water vapor in the exhaust condenses on the heat exchanger (not in the flue, and not outdoors) so that the heat of vaporization is delivered to the heating system water.
Without condensation of flue gases, the highest combustion efficiency that a gas or propane boiler can achieve is about 88%. Most non-condensing gas boilers are set up to run at about 82-86% efficiency. They are tuned to a lower combustion efficiency to avoid excessive flue condensation or damage to the boiler from the acidic condensate, and as a result give up even more source fuel heat to the atmosphere.
Condensing boilers are designed with materials tolerant of the condensate, but must be operated at a sufficiently low temperature to maximize condensation.
At typical air/fuel mixtures, the dew point of natural gas exhaust is in the low 130s Fahrenheit. No materials conduct heat perfectly. Films of gas next to the heat exchanger insulate the main exhaust, and films of water films on the water side impede the heat exchange as well. Usually, the entering water temperature (EWT) at the boiler needs to be or lower before condensation occurs on the heat exchanger. But below about 125°F EWT, efficiency climbs rapidly, flattening out as the EWT drops to 100°F or lower.
Most mod-con boilers are designed so that the highest efficiency is at the low end of the firing range, below which point insulating laminar flows on the exhaust side impede heat exchange even further. At lowest fire and a 100°F EWT, most condensing boilers are operating at 94-95% combustion efficiency, but some are a bit higher. To operate with maximum condensing efficiency, these boilers come equipped with “outdoor reset” controls that sense outdoor temperature as a proxy for heat load, and vary the operating temperature of the boiler to the lowest temperature that actually meets the load. These controls have to be set up and programmed to find the fine line between higher efficiency and not quite keeping up.
The single most common error is failing to set up the reset curve properly, but that’s easy to deal with even after the equipment is installed. Other common errors can be much thornier to fix, and are better to simply avoid.
The most common errors
Modulation ranges are not infinite. The common screw-ups to avoid are related incorrectly sizing for the minimum output capacity firing rate rather than the maximum. There are two common variations to this error:
1. Oversizing the boiler for the heat load. When the boiler is oversized for the heat load, it spends most of the heating season cycling on and off rather than ramping the system temperature and firing rate up and down in response to outdoor temperature changes with nearly continuous burns. With every burn cycle there is some fuel lost during ignition cycles, and some amount of heat extracted from the heat exchanger with every (necessary for safety) flue purge. Fewer burn cycles adds up to less fuel thrown away, higher efficiency, and less wear on the boiler.
To avoid oversizing for the heat load, the first order of business is to get a reasonably accurate load estimate using either a Manual-J type calculation (using aggressive, not conservative assumptions) or, for replacement equipment, a fuel use load analysis. The boiler at high-fire at an assumed 88% efficiency needs to cover the calculated load, but it doesn’t need to be more than 1.4 times the load to cover even the 25 year extreme temperature events.
When the 99% design load is known, you’ll want to calculate the load at the average wintertime temperature. Pull up a temperature graph for the area, zoom out to cover the 3 or 4 coldest months, and use the cursor to estimate the median wintertime outdoor temperature. If the minimum firing rate of a prospective boiler is more than the calculated load at the median wintertime outdoor temperature, it will be out of its modulation range most of the season. It’ll still heat the house, and maybe it won’t even short cycle (see problem #2), but it won’t be as comfortable or as efficient when it’s cycling on and off rather than firing nearly continuously, modulating the firing.
2. Oversizing the boiler for the radiation (and microzones). To keep the boiler from excessive cycling on calls for heat from the zones, there has to be sufficient radiation on each zone to elicit the minimum fire output of the boiler, at condensing temperatures.
The heat rate emitted by the radiators or baseboards varies with the average water temperature (AWT) and the length. might be emitting ~600 BTU/hour per foot of baseboard at 180°F AWT, but at 130°F AWT (the beginning of condensing) it’s delivering only ~250 BTU/hour per foot, and at ~120°F AWT (where it edges into the mid-90s for efficiency) only 200 BTU/hour per foot. The magic number for decent condensing efficiency is 200 BTU/hour per foot or less.
Similarly, deliver about 170 BTU/hour per square foot equivalent direct radiation (EDR) at an AWT of 180°F, but that drops to 70 BTU/hour per square foot EDR at 130°F AWT, and 50 BTU/hour per foot at 120°F AWT. With cast iron, the thermal mass will lengthen the burn cycles, but anything over 70 BTU/hour per square foot needs more analysis.
If the zone radiation can’t deliver the boiler’s minimum fire output at condensing temperatures, it may still heat the room, but the boiler begins to cycle as water temperatures drop. With more heat going into the water than is coming out, the water temperature rises, and is eventually above boiler’s outdoor reset curve temperature, at which point the burner turns off, even as the water continues to circulate. The burner re-fires only when the water temperature drops below the reset temperature. The cooler the water temperature, the greater the combustion efficiency (see Image #2, below), but at some point the losses from excess cycling exceeds any condensing efficiency gained, and high cycling rates prematurely wear out the boiler.
The more zones, the harder it is to have sufficient radiation on every zone to balance with the minimum-fire output of the boiler, and the more difficult it is to avoid short cycling. With high thermal mass radiation, micro-zoning can often work, but with low-mass emitters like fin-tube baseboard, it often requires adding a buffering thermal mass of water to extend minimum burn times.
The greater the available thermal mass, the longer it takes to raise the water temperature. With more thermal mass, the number of cycles drops. If the minimum burn times for the buffer are long enough that calls for heat from multiple zones are more or less guaranteed to overlap, modest cycling won’t reduce efficiency or boiler lifespan. At minimum burn times of less than 3 minutes or more than 5 burns per hour, the boiler is on the edge of a longevity and efficiency problem. One-minute burns and 10 burns per hour are on the edge of an efficiency and lifespan disaster.
Take a hypothetical case from a prior article. In this case, fuel use analysis of a house in Washington, D.C., projected a realistic heat load of somewhere between 29,155 BTU/hour and 31,400 BTU/hour at an outdoor temp of 20°F. The house in this example was previously heated with a cast-iron boiler with an output of 88,000 BTU/hour.
Assume that this is a two-story house with a full basement. The house is divided into three zones, with fin-tube baseboard of the following lengths:
Top floor: 70 feet
First floor: 60 feet
Basement: 15 feet
Total: 145 feet
At an AWT of 180°F, the 145 feet of baseboard could emit 87,000 BTU/hour at an AWT of 180°F, which balances reasonably with the 88,000 BTU/hour cast-iron boiler. If all zones call for heat at the same time, burn times would be quite long.
Eyeballing it on a , the average winter temperature in Washington, D.C., is in the low 40s F, or halfway between the 60°F to 65°F heating/cooling balance point and the outside design temperature. So the average seasonal heat heat load is only about 15,000 BTU/hour, half the estimated 30,000 BTU/hour heat load previously determined.
The duty cycle of the old boiler at the average winter load is in the 15-18% range. Calls for heat from the first floor and top floor would often overlap, but not always. Basement zone calls would short cycle.
Ideally, the replacement boiler would fix those problems.
At 1.4x oversizing for a 30,000 BTU/hour load, a non-modulating cast-iron boiler would have an output of about (1.4 x 30,000 BTU/hour) = 42,000 BTU/hour. The existing radiation would emit that much at an AWT of about 140°F — which is above the condensing zone, and would not need to be protected against condensation.
Operating at 180°F AWT, the smaller first-floor zone would still be emitting 36,000 BTU/hour of the 42,000 BTU/hour boiler output, and cycling on single zone calls would be reasonable utilizing just the thermal mass of the boiler. The basement zone would still cycle on its own, but it would sometimes overlap with calls from the upper floors.
So what happens if the replacement is a small mod-con boiler like the (see Image #3, below)? That boiler delivers 47,000 BTU/hour at condensing temperatures and about 43,000 BTU/hour at its maximum operating temperature (assuming high-80s efficiency). As a system it can deliver the 42,000 BTU/hour at 1.4x oversizing, but isn’t ridiculously oversized. Sound about right?
Maybe, maybe not. Let’s find out!
Testing for condition #1. The minimum fire input to the PF-50 is 16 MBH (=16,000 BTU/hour) so at 95% efficiency, its minimum fire output is 0.95 x 16,000 BTU/hour = 15,200 BTU/hour. That happens to be the approximate seasonal average. The boiler will modulate some even during the shoulder seasons, and all the time during the coldest weeks. Not ideal, but not terrible — modulating about half the time.
But can it condense?
With 15,200 BTU/hour of boiler output going into 145 feet of fin-tube radiation, that’s 15,200 BTU/hour divided by 145 feet = 105 BTU/hour per foot, which is well below the 200 BTU/hour per foot needed for condensing. So it definitely will be able to condense most of the time.
Testing for condition #2. Will it short cycle in condensing mode?
At 120°F AWT, the zones can emit:
Top floor: 70 feet x 200 BTU/hour per foot = 14,000 BTU/hour
First floor: 60 feet x 200 BTU/hour per foot = 12,000 BTU/hour
Basement: 15 feet x 200 BTU/hour per foot = 3,000 BTU/hour
The minimum-fire output is 15,200 BTU/hour, only 10% higher than the radiation is emitting, so the top floor will be fine — it’ll cycle some when it’s the only zone calling for heat, but the cycles will be long, and likely to overlap with calls from other zones.
With about 27% more heat going in than being emitted, the first floor zone would also probably be fine, but at an AWT any lower than 120°F it could hit short-cycling territory pretty quickly if it’s the only zone calling for heat. If it short cycles, the short cycling can be limited by raising the low temperature of the reset curve a bit, without taking a large hit in average combustion efficiency.
Are there more appropriate options for the example house and radiation than a PF-50? Absolutely! Many new generation mod-con boilers with fire-tube heat exchangers or dual heat exchangers can modulate efficiently over a wider range than the PF-50. The lower modulation range would fix both common errors (Error #1 and Error #2) with more of a margin. Here are examples of these new generation boilers:Boiler Model Min. input Max. inputNTI Trinity TX51 7,100 BTU/h 57,000 BTU/hNavien NHB 80 8,000 BTU/h 80,000 BTU/hHTP UFT-80W 8,000 BTU/h 80,000 BTU/hLochinvar CDN040 9,000 BTU/h 40,000 BTU/hIBC HC 13-50 13,000 BTU/h 50,000 BTU/hNote that the first three boilers have considerably more output at maximum fire than the 42,000 BTU/hour necessary for the 1.4x oversizing factor, yet they still have a very low minimum output — literally half (or less) that of the PF-50. Those are suitable solutions for 19 out of 20 homes in the U.S. with hydronic heating systems, and better candidates for systems that are broken up into smaller zones.
The basement is again hopeless on its own, of course. If it short cycles on basement calls one could add another 40 to 50 feet of baseboard at a lower cost than a buffer tank, but it’s probably not going to be worth the expense and effort since the long cycles from other floors means that basement calls usually overlap with calls from the other zones.
Success! From the simple-math sizing, the PF-50 makes it using the existing radiation. With some tweaking, monitoring, and fine-tuning of the reset curve, the boiler can probably come close to hitting its AFUE numbers. But breaking it up into smaller zones would clearly be a mistake, since it’s already coming close to short cycling on zone calls.
The sad reality
Most homes in the U.S. have true heat loads in the 20,000 to 35,000 BTU/hour range. While there are numerous boilers with comparable ratings (minimum and maximum output) to a PF-50, there are more 100,000 to 120,000 BTU/hour mod-con boilers being installed in those homes than 50,000 BTU/hour boilers, and that’s a shame. Oversized boilers cost more up front, cost more to operate, and don’t last as long as when the sizing is correctly proportioned to both the load and the radiation.
Unless they start with a careful heat load analysis, some installers would be inclined to install the PF-80 and still worry that with “only” 75,000 BTU/hour of output it wouldn’t deliver as much heat as the boiler it just replaced, and might fall short. Others would simply insist on a boiler with at least as much output as the old boiler, installing something as big as the PF-110 “just to be sure.” Either one of those would be a mistake, failing both Test #1 and Test #2.
The minimum-fire output of the PF-80 is about 19,000 BTU/hour, well over half the design output. That means it would only be modulating during the coldest weather, and would be prone to short-cycling on the zone calls at condensing temperatures.
The minimum output of the PF-110 is about 26,000 BTU/hour, which is fully 85% of load and about twice the amount of heat that either of the two main zones can emit at condensing temperatures. This guarantees that it will never operate above 90% combustion efficiency without short-cycling, and would only modulate during the coldest hours of the coldest days. This is more commonly seen than a right-sized boiler — it’s the rule rather than the exception.
Appropriate additions of thermal mass can mitigate short cycling, but it won’t magically make an oversized boiler right sized. Adding large buffer tanks re-invents the high-mass boiler, which is more conveniently done with a condensing hot water heater. There are many ways to screw up hydronic system designs beyond mere sizing, but unless sizing is right, the system can never be optimal.
Dana Dorsett has lifelong interests in energy policy, building science, and home efficiency. He is currently an electrical engineer in Massachusetts.