Basic Steam Heat Revisited
October 8, 2007
If you live and work anywhere near the Northeast corridor, you probably see more residential steam heating systems than the entire rest of the nation combined. Nearly one-third of all Americans live there. Steam-heated homes are very common in the region. True, most new constructions takes place out in the suburbs and generally utilize H/A (Hot Air) furnaces, typically with air conditioning options included, but the core heating systems of the towns that comprise much of the region are in 50-to 100-year-old homes - the majority of which are heated by boilers, not furnaces.
The large number of steam-heating systems is unique to the region, but in no way suggests steam boilers don’t exist in San Francisco, Denver, or Chicago. The percentage of steam systems encountered will be directly proportional to the age of the neighborhood housing and severity of cold weather.
Central steam heating systems, typically coal-fired, were a huge improvement over kitchen wood and coal stoves and/or fireplace heating - and became a major source of comfort from the late 1800s until the 1940s when circulators and hot-water systems started to dominate. Older “octopus” type gravity hot air systems also competed, but home builders of that era saw the merit in one or two 2-inch pipes - over five or ten 12-inch round ducts - especially as basement space became a place of basic storage, little workshops.
With the exception of a handful of regions where active coal mines still operate, all these steamers still warm the homes and larger buildings nicely, now using gas or oil as the fuel. Before declaring steam as old-fashioned, consider the number of homes and commercial spaces built in the 1900s thru 1950s that still stand - and are steam heated. They number in the millions.
Attempts to convert steam-heated homes to more modern forms of heating generally fail to consider the parameters involved; among which we see the following problems:
• Removing all the radiation, piping, boiler, and related labor is formidable by any measure.
• Most codes allow potentially unhealthy pipe insulation to stay, especially when encapsulated properly. Removing it, as required in conversion attempts, results in a very costly containment process, placing an unnecessary financial burden on the owner.
DESIGN CONSIDERATIONSCurrent steam-replacement boiler efficiencies are well over 80 percent AFUE, running a close second to the most efficient boilers and furnaces one might consider as the basis of a conversion.
When contemplating a steam to whatever conversion, how does one hide the pre-existing steam-pipe riser holes in oak floors (and plaster walls)?
The basic comfort attained by properly designed and functional steam-heating system is legendary, and for good reason. There is no fast on-and-off basis to steam heating. It takes a while for steam to come on. It takes time to get the radiators warm; likewise, it takes time for a warm radiator to become cool again. This slow rise and fall of room temperature provides a comfort level rivaled only by significantly more expensive designs and somewhat exotic control strategies. With steam, that’s the way it is naturally.
The financial investment versus the presumed benefits derived for this type of conversion is dubious at best. The cost to simply repair or modify the steam system in place, if the contractor is experienced in steam-heating systems, is almost always the better choice than what alternatives may be suggested.
There are many types of steam systems. The most common being classified by the pressure at which they were designed to operate. The names of the three most common steam heating designs are commonly referred to as: low pressure (technically operating up to 15 pounds per inch). Generally however, low-pressure steam systems operate between 1 and 3 pounds.
The two far less common designs are vapor steam systems, operating at between 1 and 16 ounces (far less than so-called “low pressure”) and rarer yet form: high pressure, i.e., anything over 15 pounds per inch (ppi) - not infrequently used in industrial applications and typically in the 75- to 125-ppi range; well over 1,000 ppi range, as in steam turbine applications is also possible. Within these three pressure-related divisions, many subforms exist, e.g., vapor vacuum, two-pipe, etc. With its 150-year-old history, the variations are endless.
STEAM SYSTEM CONTROLSThere are three basic controls required to operate a steam boiler; those being the thermostat, pressuretrol, and the LWCO (low water cutoff).
We know what thermostats do. As the name implies, the pressuretrol senses steam pressure and is part of a circuit, typically wired in series, like a daisy chain from the power source, to the thermostat, to the pressuretrol, to the LWCO and back to the power source. To find out how each boiler is connected, a wiring diagram is key. Should any one of the parts of that circuit be opened, the burner turns off.
The pressuretrol has a small diaphragm at the base connected to an internal switch. Based upon how the installer adjusts the on and off settings of the pressuretrol determines the pressure the burner will go off at - and when it will come back on. Since virtually all typical steam systems were designed by widely accepted engineering practices of 100 or more years ago, the settings of the pressuretrol are critical to many factors of safe and satisfactory operation.
Among the more obvious items impacted by how we set the pressuretrol are overall occupant comfort, responsiveness, operation of the heating system, noise (both hissing and water hammer/banging), fuel economy, and system life cycles. Generally, the lower the steam pressuretrol setting, the better. Noise from banging, priming, and surging seen at the water column/glass, fuel waste, and comfort throughout the structure is better - the lower the pressuretrol setting.
The history of steam dates back to the mid 1800s. Back then, it was not uncommon for steam devices, boilers included, to explode. Boiler designers and manufacturers, not to mention insurance companies, determined to correct the problems and effective pressure and water level controls and safeties resulted. Pressure relief valves also known as “pop safeties” became essential and have been components of steam boilers for close to 100 years.
Like the pressuretrol, the LWCO is an on-off switch, installed to prevent the boiler from firing when low on water. Millions of homes are still heated by this comfortable heating form. Almost all American boiler makers have modern, up-to-date and highly efficient steam boilers available in sizes ranging from under 75,000 Btu up into the millions of Btu ranges.
Because of a different sizing nomenclature back when steam was developing, we use different language in power ratings. EDR (equivalent direct radiation) is still used as a sizing method with steam. Unlike hot water heating and hot air furnaces, the ubiquitous Btu is not used in sizing replacement steam boilers; the somewhat archaic EDR designation is still used. The reason? Radiators were measured in square feet of radiation, not Btu. In fact, much of the heating resulting from steam radiators is radiant, not convected! And here you thought radiant heating was new.
REPLACEMENT BOILER SIZINGIt is recommended to never size a replacement steamer using the existing boiler size as the basis. Many times domestic hot water production came off the original sizing - making it much larger than necessary. Back in the day, contractors found false comfort in oversizing everything. When fuel was cheap, no one cared about the poor efficiency that results from oversized equipment. Of some note, you cannot do a heat-load calculation to size the replacement steamer either. The reason? The size and combined EDR (the amount of radiation) of the existing radiation and connected piping is all the boiler will “see.”
It will not care about heat loads in the dead of winter. The steamer will make steam based on what is physically connected to it, not how well the house is insulated, or the window and door improvements since the structure was built back in the first half of the last century; just what is connected to the piping.
To determine the correct replacement steamer, ask your steam boiler wholesaler for a booklet that provides EDR data on various forms of radiation. This essential booklet describes the EDR ratings of almost every steam radiator. By adding the EDR ratings of all the radiation in the structure together, you can determine the total EDR output required of the boiler.
Since all steam boiler makers list the EDR of their models on the boiler plate and cut sheets, you can properly size the boiler needed - provided you meet or slightly exceed the EDR boiler rating with a simple standing radiation addition/sum. Of little significance, but so you know what EDR is: 1 square foot of radiation (EDR) equals 240 Btu.
Some final thoughts:
• Always size the replacement boiler based on EDR, not heat-load calculations.
• Be sure to pipe the boiler exactly as shown in the installation instruction manual.
• Without insulation on the steam mains, the system will never work right.
• Understand the critical nature of air venting (on the mains and radiators) before installing a new boiler.
• Be aware typical low-pressure steam systems function best at 1/2 to 2 ppi - maximum!
Also be aware the pressure gauge that comes with new steamers is totally inaccurate to this pressure range; add a 0-5 ppi gauge to the replacement boiler, as well as the code required and furnished one.
And be aware that vapor systems function best at pressures under 1 pound, and the supplied pressuretrol must be switched for the lower pressures only a “vaporstat” style pressuretrol can deliver.
With either a low pressure or vapor pressure steam system, the Honeywell L408J pressuretrol is the best choice possible, in my opinion. Just follow the manufacturer’s wiring instructions.
When the new boiler is installed, skimming and cleaning the new boiler is absolutely essential to its life span, performance and functionality.
Steam systems are not supposed to “bang and hiss.” If they do, incorrect pipe pitch may be the cause, but far more likely than not, items were inadequately addressed.
I’d also like to recommend some great books as resources for steam heating: We Got Steam Heat and The Lost Art of Steam Heating by Dan Holohan; I=B=R Guide, Residential Hydronic Heating by GAMA/I=B=R; and Linhardt’s Field Guide to Steam Heating by Patrick Linhardt.
Publication date: 10/08/2007