Lindy Coils

By Luke Wonnell

If you own a Scotch Marine or cast iron sectional boiler in the NYC metro area, there’s an excellent chance you have one or more Lindy coils installed in the boiler, which provide all the domestic hot water for your entire building on an instantaneous basis.  Depending on the size of your boiler, there are Lindy coils available that can produce up to 10,000 gallons per hour of domestic hot water – that’s 167 gallons per minute – amazing!

Very simply, Lindy coils are bolt-on removable heat exchangers which allow the potable domestic water to pickup heat from the non-potable water/steam inside the boiler.  NYC boiler codes require the use of a mixing valve downstream of the Lindy coil, because the exiting water is well above scalding temperatures.  For Scotch Marine type boilers, the Lindy coil will typically be installed on the top of the front tubesheet (left photo below) while for cast iron sectional boilers, the Lindy coils are installed in one or more of the intermediate sections (right photo below).

There are many different configurations of Lindy coils available – the image below shows just a few of the available configurations:


Boiler Re-tubing: On Site Work

Below are pictures of a recent boiler tube replacement in Brooklyn. During this project we Torched and cut 25 old, corroded bottom boiler tubes (picture 1). Once cut, these tubes are removed from the boiler and the boiler room itself. The boiler vessel and tube drum are then flushed clean. Twenty five new U.S steel boiler tubes are then installed and rolled tight for a proper seal with the front and back tube sheets (picture 2). The boiler is filled with water, recharged with our inhibitor for proper chemical protection (a necessity with new steel and fresh make up water now inside the boiler), and the boiler doors are re-gasketed and checked for leaks. In addition to the replacing the boiler tubes, we also installed a new Dry Oven and interior insulating refractory wall (picture 3 and 4).


Heat Flux and Boiling Regimes: Part 3

In an ideal world, all the waterside metal surfaces inside your steam boiler would be exactly 86°F / 30°C above the boiling point when the burner is firing. This would result in perfect boiling everywhere throughout your boiler, maximizing the operating efficiency.  While this isn’t fully achievable in the real world, the manufacturer typically designs the boiler for Region I / II operation by providing at least 5 ft2 of heat transfer surface area per Boiler Horsepower. This ensures sufficient heat transfer surface area inside the boiler to avoid Region III / IV boiling.

By performing routine preventative maintenance (annual fireside cleaning, periodic waterside cleaning, and annual burner overhaul/tune-up) you will keep your steam boiler operating in tip-top shape and avoid Region III / IV boiling. Call your Account Manager at the Metro Group today to learn more about our preventative maintenance services!


Heat Flux and Boiling Regimes: Part 2

What’s really interesting is that if you were to crank up the burner to 11 at Step 5 and allow the surface temperature of the bottom of the pot to continue increasing, the heat flux through the bottom of the pan absolutely falls off the cliff.  That’s peculiar… despite the fact we’ve added more heat, the heat transfer has dropped – how could this be?

In this scenario, the water at the bottom of the pot enters the dreaded Region III (Transition Boiling in ORANGE) or even worse the insidious Region IV (Film Boiling in RED).  In these boiling regimes, large vapor bubbles form sporadically and cling to the bottom of the pot, acting as an insulating barrier.  This “film” of vapor makes it MUCH more difficult for the pot to transfer heat from the burner to the water.

You desperately want to avoid Region III or Region IV boiling inside your steam boiler at all costs.  The unpredictable formation of this vapor “film” causes localized hot spots on the metal surfaces which will cause surface cracks and lead to premature failure.  Region III/IV boiling will occur when years of corrosion/erosion of the tubes thins out the material to a point where the local surface temperature overheats.  It will also occur if a burner is not tuned properly and is overfiring for the rated capacity of the appliance (e.g. a burner is firing at 100HP for a 90HP boiler).  Finally, fouled fireside or waterside conditions can also contribute to this scenario.  If you notice unusually high flue gas temperatures, there’s a good chance parts of your boiler are operating in Region III and/or IV.

Heat Flux and the Boiling Regimes: Part 1

By Luke Wonnell

Besides being a wicked awesome band name, the title of this article refers to some 201 level Thermodynamic principles that have a significant impact on the performance of your steam boiler. “Heat flux” defines how much heat transfer occurs through a given material’s surface area and is expressed in (Btu/hr-in2 or W/m2).  Think of the bottom of a pot as the given surface area… the heat flux simply defines how many Btu’s/hr will be transferred through the bottom surface of the pot.

What’s amazing about heat flux is it’s not inherent to a specific material… it’s not like Cast Iron has one particular heat flux value and Carbon Steel has another different heat flux value. In fact, the heat flux value (vertical y-axis) changes in very interesting ways as the surface temperature of the metal that’s in contact with the water increases (left to right on the horizontal x-axis) as shown in the diagram below:


Let’s walk through this diagram by using a real-world example of boiling a pot of water on the stove for Thursday night spaghetti dinner with the family:

Step 1 – After turning on the burner, the tap water (starting at approximately 60⁰F) in the pot slowly increases in temperature until it hits the boiling point (212⁰F). By the way… don’t watch this part, otherwise Steps 2 through 5 will never happen.

Step 2 – Once the boiling point is reached (212⁰F), the water at the bottom of the pot enters Region I (Natural Convection Boiling in YELLOW) but there are no bubbles forming yet.  The warm water towards the bottom of the pot circulates towards the top and the cooler water falls to the bottom.

Step 3 – Once the surface temperature of the bottom of the pot reaches 253⁰F [41⁰F / 5⁰C above the boiling temperature], the water enters Region II (Nucleate Boiling in GREEN) and you will see tiny bubbles forming on the bottom of the pot and rising to the surface.

Step 4 – As the surface temperature of the bottom of the pot continues to increase, you will see (and hear) more bubbles and larger bubbles rising to the surface.

Step 5 – Once the surface temperature of the bottom of the pot reaches 298⁰F [86⁰F / 30⁰C above the boiling temperature], you have reached perfect boiling where you are most efficiently conducting thermal energy through the bottom of the pot.  Engineers like to call this the critical heat flux, but I prefer “maximum boilage”.  This is typically when you throw in the pasta and turn off the burner.

For an excellent video demonstration of the different boiling regimes, visit:

What Exactly is a British Thermal Unit (BTU)?

What Exactly is a British Thermal Unit (BTU)?

By Luke Wonnell

Oh the Imperial system… home to such gems as hectares, stones, bushels and pints. Personally I’m a fan of that last one, especially when served cold.  The Imperial system is very finicky: 12 inches to a foot, water freezes at 32⁰F, boils at 212⁰F, and the water used to fill a pint glass weighs 1.08lbs. Meanwhile the metric system is so neat and tidy: 100 centimeters to a meter, water freezes at 0⁰C, boils at 100⁰C, and the water used to fill a 1meter x 1meter x 1meter box weighs exactly 1000 kg – amazing!

Since we all deal with heating equipment here in the United States, we’re used to seeing BTUs on product rating labels and literature, but what exactly is a British Thermal Unit (BTU) anyway? Well, the technical definition is the amount of thermal energy needed to raise the temperature of 1 lb. of water by 1⁰F, but for a better hands-on example, it just so happens that striking a match is about 1 BTU! Imagine striking a match and holding it under a pint glass filled with water (about 1 lb.) until it burns out – I think we can agree the match will be able to increase the temperature of the water by 1⁰F.

Now let’s continue this example by relating common boiler capacities to their equivalent #MATCHES/Hour, #MATCHES/Minute and #MATCHES/Second:

Hydronic Boiler Capacity # Matches / Hour # Matches / Minute # Matches / Second
300,000 BTU/Hr 300,000 5,000 83
500,000 BTU/Hr 500,000 8,333 139
750,000 BTU/Hr 750,000 12,500 208
1,000,000 BTU/Hr 1,000,000 16,667 278
1,500,000 BTU/Hr 1,500,000 25,000 417
2,000,000 BTU/Hr 2,000,000 33,333 556
3,000,000 BTU/Hr 3,000,000 50,000 833
4,000,000 BTU/Hr 4,000,000 66,667 1,111

Even a relatively small commercial boiler (300,000 BTU/Hr) would be the equivalent energy of striking 83 matches every second!  Just remember the next time you strike a match to start a campfire or light a fire cracker you are holding 1 British Thermal Unit in your hand!


Updated HPD Heat Season Requirements

Updated HPD Heat Season Requirements

By Luke Wonnell

On Monday, October 2nd, 2017, NYC Housing Preservation & Development released new requirements which were made effective for the start of the 2017-2018 heat season.  This release included specific changes to the nighttime heating requirements:

  • The law requires that from October 1 to May 31:
    • Between 6:00 A.M. and 10:00 P.M., inside temperatures are maintained at a minimum of 68 degrees Fahrenheit when the outdoor temperature falls below 55 degrees.
    • Between 10:00 P.M. and 6:00 A.M., indoor temperatures must be maintained at a minimum of 62 degrees, regardless of the outdoor temperature. (The nighttime requirement used to only be 55⁰F minimum indoor temperature when the outside temperature dropped below 40⁰F).


You need to make sure your heating system maintains a minimum indoor temperature of at least 62⁰F between 10:00 P.M. and 6:00 A.M. regardless of the outdoor temperature! If you need assistance getting your boiler system to perform to these updated heating requirements, call your Account Manager at the Metro Group today!

Or contact us here:

For more detailed information on the new heat season requirements, visit the address below:

Boiler Horsepower (BHP)

What Exactly is Boiler Horsepower (BHP)?

By Luke Wonnell

In the United States, the output capacity of steam boilers is expressed as Boiler Horsepower (BHP) whereas hydronic boilers are typically sized based on their input capacity in Btu’s/hr.  There are definite differences between these two methods, but it is possible to convert between the two units of measure.

1 Boiler Horsepower (1 BHP) is defined as the capacity of a boiler to boil 34.5 lbs of liquid water completely to steam in one hour at atmospheric pressure conditions. This is sometimes abbreviated as “F&A212” which means “from & at 212⁰F” or the boiling temperature of water at atmospheric pressure.

Let’s take several common Boiler Horsepower sizes and convert to equivalent hydronic boiler capacities:

BHP Lbs. of Steam / Hr         (OUTPUT) Equivalent BTU/Hr (OUTPUT) Equivalent BTU/Hr (INPUT) @ 85% EFFICIENCY Equivalent BTU/Hr (INPUT) @ 95% EFFICIENCY
1 34.5 lbs/hr 33,479 Btu/hr 39,387 Btu/hr 35,241 Btu/hr
10 345 lbs/hr 334,788 Btu/hr 393,868 Btu/hr 352,408 Btu/hr
50 1,725 lbs/hr 1,673,940 Btu/hr 1,969,341 Btu/hr 1,762,042 Btu/hr
100 3,450 lbs/hr 3,347,880 Btu/hr 3,938,682 Btu/hr 3,524,084 Btu/hr
250 8,625 lbs/hr 8,369,700 Btu/hr 9,846,706 Btu/hr 8,810,210 Btu/hr
500 17,250 lbs/hr 16,739,400 Btu/hr 19,693,412 Btu/hr 17,620,421 Btu/hr
1000 34,500 lbs/hr 33,478,880 Btu/hr 39,386,823 Btu/hr 35,240,842 Btu/hr

As you can see above, the conversion from BHP to Btu/hr isn’t neat and tidy – you end up with lot of pesky digits which doesn’t make for appealing marketing literature.

Let’s flip this process and convert common standard efficiency hydronic boiler sizes to equivalent BHP:

Hydronic Boiler Capacity (INPUT) Hydronic Boiler Capacity (OUTPUT) @ 85% EFFICIENCY Equivalent Steam Boiler Capacity (BHP)
500,000 Btu/hr 425,000 Btu/hr 12.7 BHP
750,000 Btu/hr 637,500 Btu/hr 19.0 BHP
1,000,000 Btu/hr 850,000 Btu/hr 25.4 BHP
1,500,000 Btu/hr 1,275,000 Btu/hr 38.0 BHP
2,000,000 Btu/hr 1,700,000 Btu/hr 50.8 BHP

Finally, let’s convert high-efficiency condensing boiler sizes to equivalent BHP:

Condensing Boiler Capacity (INPUT) Condensing Boiler Capacity (OUTPUT) @ 95% EFFICIENCY Equivalent Steam Boiler Capacity (BHP)
500,000 Btu/hr 475,000 Btu/hr 14.2 BHP
750,000 Btu/hr 712,500 Btu/hr 21.3 BHP
1,000,000 Btu/hr 950,000 Btu/hr 28.4 BHP
1,500,000 Btu/hr 1,425,000 Btu/hr 42.6 BHP
2,000,000 Btu/hr 1,900,000 Btu/hr 56.8 BHP

Notice how standard efficiency hydronic boilers offer fewer BHP than their high-efficiency condensing boiler counterparts even at the same input capacity. This is because the increased efficiency of condensing boilers allows them to produce more hot water with a given amount of fuel than standard efficiency hydronic boilers.


Heating Degree Days & Fuel Consumption: Part 4

Finally, let’s recalculate the HDD data and present in a pie chart that shows each month’s expected fuel consumption as a percentage of the yearly total:


While every boiler system is unique and has its own nuances, the historical monthly fuel consumption for heating purposes SHOULD follow these charts fairly close.  If you notice a significant discrepancy between your historical fuel consumption and the pie charts, there are a number of issues that may be affecting the performance of your boiler system:

  • Fireside fouling: Soot buildup on the fireside of the boiler significantly reduces heat transfer and efficiency, resulting in abnormally high fuel consumption.
  • Lack of condensate return: Malfunctioning condensate return equipment requires more fresh water makeup to be used, resulting in higher fuel consumption.
  • Poor control strategy: Multi-boiler installations need to be sequenced/staged properly to ensure that the appropriate amount of boilers is online. Too many boilers online will result in higher fuel consumption.
  • Rich combustion: When combustion is tuned too rich (low O2%), there’s more fuel entering combustion than is required for operation, resulting in higher fuel consumption.
  • Short-cycling: If the boiler is grossly oversized and doesn’t have sufficient turndown/modulation to react to the changing loads, the boiler will cycle on and off excessively, resulting in higher fuel consumption.
  • Waterside fouling: Scale buildup on the waterside of the boiler will reduce heat transfer and efficiency, resulting in abnormally high fuel consumption.


Heating Degree Days & Fuel Consumption: Part 3

Next, let’s look at the Cumulative HDDs for Manhattan from 2014 – 2017 using 55⁰F as the Balance Point:


  • 2014 (green) was the most severe heat season, finishing up with 2883 HDDs for the year.
  • 2016 (purple) was the mildest heat season, finishing up with 2229 HDDs for the year.
  • 2017 (pink) is trending closely to 2016 and will be another relatively mild heat season.

Next, let’s look at the HDD data on a month-by-month basis instead of cumulative:

  • The coldest months with the most HDDs for each year are highlighted in (red). Your fuel consumption should be highest during this month.
  • The warmest months in the heat season with the fewest HDDs are highlighted in (green). Your fuel consumption should be lowest during this month.