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Sizing a Modulating Condensing Boiler

You can use simple math to avoid common boiler sizing errors

Posted on May 24 2016 by Dana Dorsett

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 AFUEAnnual Fuel Utilization Efficiency. Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment. . 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(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractor; be proactive, and pay attention to the details of any contractor’s proposal.

Modulating-condensing basics

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 BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /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 exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. (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 125°F to 127°F 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 WeatherSpark 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. Typical fin-tube baseboard 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, cast-iron radiators 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 massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. 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.

An example

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 Weatherspark graph, 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 pointBalance point is the outdoor temperature at which the amount of heating provided by an air source heat pump just equals the amount of heat lost from the house. Below this point, supplementary heat (typically inefficient electric resistance heat or “strip heat”) is required. Typical balance point temperatures are in the range of 27 - 35 degrees Fahrenheit. and the outside design temperatureReasonably expected minimum (or maximum) temperature for a particular area; used to size heating and cooling equipment. Often, design temperatures are further defined as the X% temperature, meaning that it is the temperature that is exceeded X% of the time (for example, the 1% design temperature is that temperature that is exceeded, on average, 1% of the time, or 87.6 hours of the year).. 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 Peerless PureFire PF-50 (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. input

NTI Trinity TX51         7,100 BTU/h     57,000 BTU/h
Navien NHB 80           8,000 BTU/h     80,000 BTU/h
HTP UFT-80W             8,000 BTU/h     80,000 BTU/h
Lochinvar CDN040       9,000 BTU/h     40,000 BTU/h
IBC HC 13-50           13,000 BTU/h     50,000 BTU/h

Note 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.

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Image Credits:

  1. Image #1:
  2. Image #2: GBA
  3. Image #3: Peerless

May 25, 2016 7:09 AM ET

Thanks, Dana, for this important article
by Martin Holladay

This is the type of useful article which will stand as a reference piece to help GBA readers for years. I expect that this topic will come up in future questions on our Q&A page --- and now, we'll be able to link to this article.

Don't be discouraged by the paucity of comments. While most Americans use furnaces, not boilers, readers in New England are grateful for your advice on boilers and hydronic systems.

May 25, 2016 8:26 AM ET

What, me worry? :-) (Not a bit!)
by D Dorsett

Given that fewer than 25% of homes in the US are heated with hydronic systems, and only a bit more than half of those use fuels that can take advantage of condensing efficiency, and only about 3-4% of those boilers are going to be replaced in in a given year, this is by definition a somewhat esoteric topic. I view it as something akin to a cheat-sheet on a textbook chapter- a quick way to avoid egregious sizing errors that lead to higher cost, lower comfort & efficiency. In my limited experience in the field an on web forums I'd hazard that MOST condensing boilers currently installed are oversized for both the load and the radiation.

On another forum I recently read of a ~3300' house in PA with an 80,000-200,000 BTU/hr mod-con ( Dunkirk 95M-200 : ) installed 7-8 years ago that is short cycling on zone calls even at 160F+ output. The design heat load on that place is probably about 50K, and it's broken up into four zones, two of which are radiant slabs, the other fin tube baseboard. The tested AFUE is 95% for that unit, but there is no way it is getting more than 85% as-used AFUE. It literally never modulates OR condenses.

The homeowner isn't interested in replacing the boiler, is currently dealing with leaks in one of the radiant slab zones, and will be installing alternate radiation in that zone. Unfortunately it isn't physically possible to install enough radiation to suppress the short-cycling, and with the lower mass radiation the short cycling will be worse than it already is. If he's lucky the short-cycling beast will go another decade before succumbing, but I wouldn't bet the farm on that. Another 5 might even be optimistic. Had the homeowner run a heat load analysis and done the napkin math in this piece a right-sized boiler would last 2x as long, and the difference in operating cost over that 20 years would more than pay for the right-sized replacement. I suspect there were even state & local subsidies paid for installing that "96% efficiency " boiler, a total waste. This house would have done fine with a boiler with an 80,000 BTU/hr MAXIMUM input rather than an 80K minimum.

I WISH this was just an outlier, but unfortunately it's not. The design heat loads of average homes in climate zones 5-7 is under 50,000 BTU/hr, but there appear to be far more 100K-150K (or bigger) mod-cons in service than 50K-80K versions. With hydronic boilers bigger is definitely NOT better, and hopefully having a dumb-math crib sheet like this will help people avoid falling into that all too common trap. With condensing gas furnaces the output heat exchanger is built into the equipment and they will at least condense at any oversizing factor. Not so with boilers.

May 25, 2016 11:01 PM ET

Edited May 25, 2016 11:11 PM ET.

Spot on
by Eric Sandeen

I had to convince my installer to go with a 60MBTU/hr boiler, not 110, when his own heat loss calc showed 48MBTU/hr. To be fair, we have an indirect water heater attached as well. But the 60 has been fine.
Setting the reset curve was definitely an issue as well; the installer left with it set to whatever default was, and it was not at all correct. With some math & monitoring, I was able to get it tuned pretty well.
Speaking of monitoring, some boilers have ModBus interfaces which let you monitor them on an ongoing basis; I don't know if this would be of general use to many homeowners but attached are some fun examples from my boiler.

grafana-boiler.png scatter-boiler.png

May 26, 2016 12:17 AM ET

Edited May 26, 2016 12:18 AM ET.

Mod Con Boliers
by Michael Clarke

Great piece. Here in New Zealand it's impossible to get low output mod/con LPG boilers. Our house (270m²) has a heat requirement of 10kW at 0°C, but generally, because it is a modern build, only one zone is needed. This zone on the cold side of the house (South West) has a modulated demand of < 2kW at 5°C. Currently the hydronics are heated by an early (2011) 18kW Navien with a modulation ratio of 2:1 - all that was available at the time. Veissman, a German manufacturer make a really nice model range in the 300-W series, with one model able to modulate between 1.9 - 19kW. Large modulation ratios are essential to getting maximum efficiency from these boilers. Also, most manufacturers can supply 'system' and 'combi' units, with the 'combi' units able to supply domestic hot water as the primary function, then reverting back to central heating demand after the call for DHW is satisfied. As a rule, these 'combi' units have DHW heating outputs way above what is required for hydronics, and as such, I believe, are not suitable for hydronic UFH. There are so many efficiency gains to had by correct sizing, model choice and high modulation ratios, that they should not be ignored.

May 26, 2016 10:23 AM ET

Edited May 26, 2016 10:29 AM ET.

They're out there, but you may have to hunt (@ Michael Clarke)
by Dana Dorsett

The current model Navien NHB-80 dual heat exchanger boiler can modulate down to about 2.2 kw output. They make a smaller version too (the NHB-55) , but it has the same minimum firing rate, so the smaller size doesn't give it a modulating advantage.

The fire-tube HTP UFT-080 can go that low too. It is manufactured by the Korean company Kiturami, who exports it under their own nameplate as "HomSys" see: I'm not sure if there is an LPG version available, or whether there is a way for a kiwi to buy it without going grey market.

Low mass combi systems rarely modulate below ~3.5-4 kw out, most are much higher. Navien makes at least one that runs that low. It is sold in the US under the model name NCB-180, and it comes in an LPG version.

Eric: The notion that you would have to upsize a boiler to manage the domestic hot water is rarely true, but apparently many installers in the US seem to believe (against all evidence) that the size must increase when the hot water load is added. It's generally better to size the indirect for the domestic hot water load (typically the biggest tub you have to fill) and the boiler to the space heating load. Zoned as a priority zone, a 60K condensing boiler delivers heat to the indirect about 2x as fast as a typical non-condensing 50 gallon standalone hot water heater. Suppressing space heating zones during the abbreviated recovery period for the indirect will never impinge on comfort unless one opted to schedule an "endless shower" for the coldest hours of the coldest nights of the year.

Default values on the reset curve are almost never a good fit, but with a fuel use analysis from a prior boiler (or from the trend line of the fuel use you nicely graphed out of the Solo-60), it's possible to find reasonable starting points and tweak it in over time.

Modulation has limits. For very low load homes or for micro-zoned homes it's often easier & better to use a condensing tank type hot water heater, utilizing the thermal mass of the water in the tank to suppress short cycling.

May 26, 2016 11:09 AM ET

Tweaking the curve
by Eric Sandeen

@Dana - it sure seems like with a season's worth of data from the boiler (FWIW, heating degree days were gleaned directly from the outdoor reset temps, and heating therms were separated out from DHW therms) I should be able to set the "perfect" curve. I'll have to think about that some more in anticipation of next winter.
For non-geek homeowners, though, I wonder if there's any efficient method for an installer to get a better curve than default without requiring too many visits. I don't think your article elaborated on setting up the curve; maybe another article for another day?

May 26, 2016 1:19 PM ET

Setting up the reset curve is outside the scope of this article.
by Dana Dorsett

To max out the efficiency of the curve you would need to estimate out the 99% design water temperature requirements on a room-by-room basis for the available radiation in those rooms. The room with the highest water temp requirement at the 99% outside design temp defines that point on the curve. If one room needs 130F water to keep up at design temperatures and all others only need 110F water, it either needs to be set to 130F at the design temperature or add enough radiation to that room to bring it's water temp requirements in line with the others.

Then estimate at which outdoor temperature at which the min-fire output equals the whole house load, and the water temperature required by the radiation to deliver that heat, which defines a second point on the curve. You also have to keep in mind at what water temp the boiler begins to short-cycle, and establish a low-limit on the water temp there independent of load.

Very few mod-con installers will even take a stab at it, let alone make multiple site visits to tweak it in for you. (Hell, it's hard enough to find one who will even size it correctly!) Better installers are willing to take a first cut estimate on programming the curve, and maybe talk you through how to adjust it over the phone if it's not quite keeping up or starting to short-cycle.

I'm not sure it's worth another blog article, but maybe. Different manufacturers have different approaches to outdoor reset, and the programming conventions vary (a lot). Some even include non-linear curves to compensate for non-linearities of heat emitter output with temperature at low temp. (Fin tube baseboard is very non-linear in output below 110F water temps, but tall radiators and radiant slabs are fairly linear even in the 90s F water temps.)

As with anything else, if you want the most out of it you have to read the manual and do some experimenting. It's usually better to start with tweaking in the water temp at the cold outdoor temp end of the curve first, dropping it 5F at a time until it's not keeping up overnight, then bumping it up a degree at a time until it keeps up with the setpoint. This is easy to do early in the heating season when the nights are getting colder, but may need some fine tuning at mid-winter. Finding the right water temp at the warmer weather end of the curve is a bit squishier, and more prone to being skewed by day to day differences in solar gain, etc.

May 27, 2016 1:37 PM ET

Interesting but complicated
by Antonio Oliver

Very good read, Dana. But far too complicated for most. Having lived with hydronic heat the last 6 winters, I know it's not easy to get everything optimized. It get's even worse when there are not enough zones--that is the temp near your thermostat is not representing all areas within the zone it controls. I do have a couple easy questions: (1) Is there a maximum ratio of heat load requirement of smallest zone to lowest output of boiler that you would recommend? (2) Assuming someone got everything correct, do we know how long we can expect these mod-con boilers to last? I've had one the last three winters, and it beats the heck out of the old inefficient 1954 cast iron dinosaur it replaced, and the new boiler is probably oversized. I tweaked the curve many times the first winter and could never quite get it right, but a lot of that has to do with heat loss rates being so different in different parts of this old house--one room toasty, another okay under a blanket.

May 27, 2016 2:36 PM ET

There are no simple answers. (@ Antonio Oliver)
by Dana Dorsett

Far too complicated? And this was about the 4th draft, AFTER simplifying it! :-)

To anwer your questions:

1) It's not really the ratio of the heat load of the zone to the min-fire output that's important, but rather the ratio of the RADIATION output (at condensing temps) to min-fire output of the boiler. Even in a micro-zoned system you want the smallest zone radiation to be able to emit the lion's share of the min-fire boiler output to limit the cycling. There's no one answer, but if the zone radiation can't deliver at least 2/3 of the min-fire output at condensing temps it's short-cycling potential is pretty high, unless you add thermal mass.

When you micro-zone a house you can sometimes do just fine with under-radiated zones as long as the boiler isn't oversized for the whole house load by utilizing a thermally massive hydraulic separator (eg Boiler Buddy ) or some other buffering technique. When a boiler is right-sized for the whole house load the calls for heat from different zones will usually overlap, and the boiler will modulate as calls for heat from different zones come & go. By having the thermal mass of a high-volume hydraulic separator or buffer tank involved with every burn it establishes a minimum burn time, increasing the frequency and likelihood of zone calls overlapping. The napkin math analysis of how that works goes like this:

It takes 1 BTU to raise a pound of water 1 degree F. Most mod-cons won't turn off the burner until the water is 5-7F over the set point on the reset curve, and doesn't re-fire until the return water temp has dropped something like 10F from when it turned the burner off. A gallon of water weighs 8.34lbs, so to raise the temp of a 20 gallon buffer 10F takes 8.34lbs/gal x 20 gal x 10F= 1668 BTU. If the min-fire output of the boiler is 15,200 BTU/hr (like the PF-50 example in the blog article), the minimum burn time is then 1668 / 15,200= 0.11 hours, or about 6.5 minutes, even if the radiation isn't emitting anything. If the zone radiation is emitting say, 2500 BTU/hr then there is only (15,200 -2500=) 12,700 BTU/hr, and the burn time is roughly 1668 / 12,700= 0.13 hours or close to 8 minutes.

In reality even fin-tube has some water volume and other thermal mass as does the boiler itself, and you could add that all up for a first-order approximation of the burn times. It's never quite as simple as the napkin-math analysis, but that's the gist of it.

If you have old fashioned high volume cast iron boilers it has quite a bit of thermal mass in both the water and the iron. The specific heat of cast iron is about 0.11 BTU/lb-degree-F, so it takes ~9.1 lbs of cast radiator to add up to the thermal mass of a pound of water (or ~76 lbs of cast iron to be equivalent to a gallon of water), but if you've ever had to move them you'll know those suckers can be HEAVY. It adds up. Even 5" x 20" SunRad type radiators (popular well into the 1950s, and hopefully in your house) weigh ~5 lbs per section (dry), and contain ~1.25 lbs of water per section, so the cast iron itself is about 30% of the thermal mass of the radiator, and it's several times the thermal mass of fin-tube baseboard of equivalent output. Many of those houses can be safely micro-zoned without buffering, but you really need to run the math (at least the napkin-math) to get a handle on it. Minimum burn times based on the zone's thermal mass much shorter than 3 minutes run a short cycling risk.

2) Properly sized and maintained a mod-con with a stainless steel heat exchanger should go for at least a couple decades. Some of the aluminum HX versions are more sensitive to system water chemistry, and have higher failure rates. Some of the more recent stainless fire-tube HX designs look like they should go the distance without a lot of TLC, unlike many of their water-tube cousins. We'll see.

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