This article was originally published in the November/December 1993 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online November/December 1993
Downsizing Steam Systems
by Henry Gifford
Henry Gifford is president of Gifford Fuel Saving Incorporated, a heating retrofit company in New York City.
If downsizing a steam heat system can't be avoided, certain tricks of the trade can ensure that reduced capacity translates to comfort and savings.
Most steam heating systems in use today are oversized, and downsizing those systems can deliver attractive savings. An oversized steam heating system can be very expensive to operate because it is difficult to avoid overheating the building. Downsizing in this case means reducing the size (capacity) of the heat distribution system (radiators and piping), the heating boiler, or both.
Unlike systems that use circulating hot water or forced air, the temperature of steam systems cannot be adjusted. Water boils at 212deg.F. This means overcapacity on all but the coldest day of the year in a system that is sized perfectly. In the non-perfect real world, steam systems almost always have capacity far exceeding the coldest day's needs, so they are always oversized.
A heating contractor would lose his or her shirt, and possibly go out of business, for installing an undersized system that couldn't adequately heat the building. So contractors usually play it safe and sell the customer just a bit more capacity than experience indicates the building needs. Also many systems are sized by experts who work for supply houses and manufacturers with a vested interest in selling lots of capacity.
Oversizing is increased when a building is weatherized. When a heating system is sized for a building with no insulation, but then the building gets tightened up, the only effective means of temperature control in these buildings becomes the double-hung windowstat. Unfortunately, heating the cold air coming in the windows lowers relative humidity. (Health care professionals experience an influx of respiratory ailment patients at the start of the heating season in areas where these heating systems are common.)
Overcapacity also results when insulation is removed, causing the pipes to overheat the spaces around them. People also sometimes move into old factory buildings that were intended to be heated with the windows opened wide to ventilate industrial processes, and live there with the windows closed. Evidence exists that some residential buildings were also intended to be heated with the windows open for ventilation purposes.
Short of Downsizing
It's best to reduce steam heating by replacing it with circulating hot water, or if air conditioning is a must, with a blown-air heating system. Of course, replacing steam with a circulating hot water system is usually only practical with steam systems of the two-pipe variety found in larger (multifamily) and newer steam installations. (For more about two-pipe installations and other aspects of steam heat retrofits, see Home Energy on Steam Heat Retrofits above.)
Another option short of downsizing the system is installing thermostatic radiator valves on individual radiators. These simple devices promptly close the air vent when they sense warm room air. The payback of their cost can be very attractive, because they are relatively inexpensive and easy to install. However, systems are vulnerable to tampering and removal by occupants, so they are not well-suited to rental apartments. Nor do they remove the need for a boiler large enough to heat all the radiation at times such as the end of the night setback period, when all the thermostats will likely be calling for heat. Despite the drawbacks, thermostatic radiator valves are usually an effective means of reducing overheating.
Another simple trick is to plug up the air vent on an unwanted radiator with an 1/8 in. pipe plug. Air in the radiator prevents the entry of steam, preventing heating. Unfortunately, steam pressure has the capacity to compress the air and still heat the radiator despite having its vent plugged or closed with a thermostatic valve. Therefore, expect diminishing results with air binding as pressure increases past about 1.5 pounds per square inch.
When reducing the capacity of the radiation, the challenge is to reduce the capacity at the lowest cost. Usually the cost of running out and buying all new smaller radiators is prohibitively expensive. Sometimes the answer is to downsize by other means: removing radiator sections, covering radiators, shifting existing radiators, or heating with supply pipes.
Removing radiator sections
Radiators, old and new, are cast in sections. A radiator is an assembly of two end sections and a varying number of intermediate (middle) sections. They are held together with threaded rods and the seal is accomplished with pliable steel push nipples. Disassembly, removing sections, and leak-free reassembly is difficult or impossible without a supply of new push nipples. Prying the sections apart without cracking them is a major challenge. Therefore while theoretically elegant, this is not considered practical.
Convectors are basically steam pipes with many cooling fins which disperse heat, and usually hug the baseboards (see Small-Diameter Convectors p.31). Where convectors are installed, cardboard cut to size and laid on top can control heat. This trick works for both steel and cast iron convectors. The building manager or tenant can cover some or all of the radiators, taking individual room loads into account. Sheet metal covers resist tampering.
The covering prevents or reduces convective air circulation--to be distinguished from radiant heat--over the convector. This reduces room heating and keeps the convector warmer. The warmer convector will reduce the load on the boiler, causing its controls (depending on the type) to cycle less often, saving fuel. The coverings in overheated or underheated rooms can be adjusted accordingly.
Cast-iron radiators can also be covered to reduce their output. However, many occupants buy or build radiator covers to improve the look of the space. Unlike a covering simply laid on top, these covers often jacket the radiators, wasting energy because the lower heat output results in complaints of not enough heat, resulting in the heat getting turned up building-wide.
If covering radiators is considered as part of a downsizing program, the building manager should cover part of the radiator and leave part bare so it can heat the room, and turn the boiler off. Inserting a piece of reflective faced foam board or corrugated cardboard covered with aluminum foil can reduce heat loss from the radiator to the wall.
Scrapping the large radiators and replacing them with smaller ones from elsewhere within the building is another option (see Swapping Radiators p.32). Pipes can heat the spaces formerly heated by the small radiators (see the next option). Rather than playing musical radiators piecemeal, it's better to follow a plan. First make a sketch of the structure and its installed radiation. Do a heat load calculation for each room to determine the radiation needed.
Next, by cutting out paper models of all the installed radiators or convectors and moving them around on the drawing, the right fits will become apparent. In some buildings little can be done, while other buildings have all the desired sizes. In cases where everything would work if there were only a few small radiators, consider buying a few to fill in the gaps. After installation the retrofitter should send up some steam and check for leaks, and rebalance air vents after moving the radiators around.
Heat With Pipes
This option entails replacing existing vertical steam risers with a series of new exposed pipes capable of heating the building. Heating with pipes works in buildings such as rehabs where steam heat must be retained, and conservation measures like double-glazed windows and fiberglass wall insulation reduce the heating load. Ideally, the pipe sizes and quantities match heating load requirements for each room. The cost of installation can be reasonable because the installer runs a series of pipes straight up and drills for air vents on the top floor.
With this set-up, tenants never complain of water dripping through ceilings from a leaking valve packing above, and the building remains at a comfortable temperature after fine tuning by moving air vents--attached to the pipes--to balance steam distribution. Remember that any venting change affects the system as a whole, and predicting the overall effects of a change will save a lot of legwork. While all vented steam systems are highly sensitive to venting changes, unevenness in this type of system will not be offset by occupants turning off radiators.
Undersized and Just-Undersized Systems
Downsizing a steam system can solve the problem of an undersized steam boiler. While an oversized boiler wastes fuel, a boiler too small to heat all the radiation can run for days without heating the faraway radiation, breaking records for high fuel consumption. Downsizing radiation can sometimes correct this problem while avoiding the expense of changing the boiler. Be careful before condemning a boiler as undersized, as the most common causes of steam not getting to a faraway part of a system are air or water (condensate) blocking the steam. Undersized steam boilers are usually found in areas where heating contractors are more accustomed to sizing hot water heat systems, but undersized steam boilers are still rare. Checking boiler or burner input against heating load can help determine if undersizing is the culprit.
A related problem is a marginally sized boiler that can heat all the radiation, but barely. This leads to high fuel bills because the boiler spends more time running at 0% efficiency, when fuel is burning but none or few of the radiators are getting heat. Reducing the radiation load can allow the boiler to heat up the system quickly so it can turn off sooner.
Anyone involved with steam heated buildings can expect the boilers to eventually require replacement. Size calculations usually occur under the time pressure of replacing a failed boiler, in a frantic rush to keep the building from freezing. Because this allows little time to calculate radiation sizes and other aspects, boilers are typically sized by standing in front of the building and looking up! That's why it can be cost-effective to precalculate the correct size for a replacement boiler. This dictum applies to both steam and hot water boilers. The calculation could be done as part of weatherization work or an energy audit, with the results kept on file. The results can also be quite useful in determining the suitability of replacing a boiler that has not failed.
Downsizing an oversized steam system can be done the right way, or the costly way. A few guidelines can avert potential disaster or dissatisfaction.
Home Energy on Steam Heat Retrofits
The Art and Science of Balancing Single-Pipe Steam Systems
Converting Steam-Heated Apartment Buildings
Retrofitting Single-Pipe Steam Heating Systems
What about installing small-diameter copper or steel convectors on a steam system? Don't do it! The problem is that the radiation holds little heat after the steam cycles off, causing wide temperature swings. This is in contrast to hot water systems that leave gallons of hot water in the radiation during off cycles. The problem is even worse in buildings where the rest of the system is still using cast-iron radiators; the cast iron continues giving off heat for extended periods of time after the convectors stop giving off heat, causing huge differences in delivered heat. This will be the case even if the radiation has the same rated output, since the output is measured with the steam on. The ratings ignore the output after the steam turns off.
What about small cast-iron convectors? These are not as bad as copper or steel convectors, as they are heavier and so able to give off heat for longer after turning off. Also they are typically installed uncovered, so the ratio of their radiant-to-convective heat output is higher than lighter convectors. However, they are better suited to hot water systems than to steam systems due to the difficulty of getting steam and condensate to flow properly in their small internal passageways. This problem gets worse as the number of connected baseboard sections increases. Therefore small cast-iron baseboards are difficult to use on one-pipe steam heating systems and are generally not recommended.
New cast iron radiators suitable for steam are still manufactured, and in many areas used radiators can be bought for a song. However used small radiators are often difficult to find, and in some areas cost as much as new ones.
Changing radiators requires that the system be turned off; therefore large scale jobs are typically done during the summer. Plan on replacing all shutoff valves because the half of the valve that screws onto the pipe and the half that screws into the radiator are incompatible between different brands and size of valves. First disconnect all the radiators and move them to their new locations. Then replace all the shutoff valves according to their pipe sizes.
The part of the valve screwed into the radiator is difficult to remove without damaging the threads it was screwed into. Fortunately bushings are typically installed into larger tappings in radiators. It's less work to just change the bushings than to try to remove the valve parts.
The trick for removing bushings is to lay the radiator on its side, put a long pipe wrench on the bushing, and give the wrench a swift kick. Out it comes and the radiator is ready for the installation of a new bushing that fits both the radiator and the valve. Avoid using cast bushings; use the steel ones as they are not prone to cracking. The cost is generally the same. They are both sold as a black bushing. The cast ones can be identified by the rough sand texture on the unmachined surfaces, while the steel has a smooth surface throughout.
If a particular radiator insists on leaking at the thread, consider paying more money for a brass bushing. The softness of the brass allows it to conform to the shape of botched threads, sometimes sealing an otherwise hopeless leak.
Related ArticlesThe Best Boiler and Water Heating Retrofits (Lobenstein and Hewett)
Combustion Safety Checks: How Not to Kill Your Clients (deKieffer)
Condensing Furnaces: Lessons from a Utility (Beers)
Don't Force Air, Go with the Flow (Springer)
Fireplaces: Studies in Contrasts (Hayden)
Integrated Heating and Ventilation: Double Duty for Ducts (Jackson)
The Key to Persistence (Nolden)
'Read Me Your Thermostat': Short-Term Evaluation Tools (Kinney)
Remodeling Bathrooms: Let the Energy Savings Flow (Johnston)
Air Sealing in Low-Rise Buildings (Hayes)
Bright Prospects for Lighting Retrofits (Hasterok)
Controlling Recirculation Loop Heat Losses (Lobenstein)
Energy Education: A Kilowatt Is a Terrible Thing to Waste (Firari)
Energy Savings Rise High in Multifamily Buildings (Diamond)
Evaluating Ventilation in Multifamily Buildings (Hayes and Shapiro-Baruch)
Making Low-income housing Affordable: The Northgate Retrofits (Patullo)
Profiles of Multifamily Weatherization Projects: A Tale of Five Cities (Kinney, Wilson, and MacDonald)
Using Fuel Bills for a Targeted Investment (Padian)
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