Problem scale. Significant, untapped opportunities exist to improve the energy efficiency of the nation’s multifamily housing stock, which accounts annually for more than 100 million tons of carbon emissions and approximately $22 billion in energy expenditures . Most multifamily rental properties were constructed before modern building energy codes were adopted, and existing utility incentive and rebate programs often overlook the multifamily sector in deploying energy efficiency incentives and rebates. These factors combine to produce an aging building stock that has received comparatively low investment from utilities to conduct energy efficiency improvements. Harnessing opportunities to improve energy efficiency in the multifamily housing sector can provide significant benefits to multifamily residents, property owners, and local communities.
Steam heating is used heavily in multifamily houses - of the 16.7 million multifamily housing units nationwide—defined as five or more units—2.5 million are steam heated .
A 2009 NYSERDA survey of 63 arbitrarily-selected existing multifamily buildings found 46 (73%) to be heated with steam boilers, and six additional buildings (9.5%) heated with purchased steam, for a total of 82.5% of the buildings heated with steam. Of these 52 steam-heated buildings, 32 have 2-pipe distribution (62% of steam-heated buildings) and 10 have 1-pipe distribution (19% of steam-heated buildings). Most of these buildings are 20 years older or more, but the tradition of steam heat is so strong, that even relatively new buildings, as recent as five years old, have been found to be designed and built with steam boilers . 2013 report on multifamily houses in Chicago region indicate portion of steam heat ~50% .
Northeast Energy Efficiency Partnerships 2014 report on multifamily properties indicated that “… in the Northeast and Mid-Atlantic region, the small multifamily housing sector accounts for approximately 2.1 million occupied housing units out of a total of 26 million total housing units”. 69% and 39% of these buildings were built before 1980 and 1960 correspondingly, so similar statistic of ~50% or higher can be expected for steam heating throughout Northeast and Mid-Atlantic of US .
Steam heating retrofits – tentative solutions
Genuine drawback of steam heating is uneven heat distribution and overheating –it’s a reason why steam heated buildings typically pictured with open windows. Attempts to balance steam distribution system using air vents [6, 7] and Temperature Regulating Valve (TRV)  were not successful. Recently introduced “smart’ radiator cover address room overheating rather than the whole heat distribution problem , another disadvantage is switching from radiation to convection heating by air which is significant loss in efficiency and comfort.
Building envelope method is supported heavily by government incentives, but return of investment can exceed 100 years . Furthermore, it’s not unusual that homes are damaged as a result of improperly installed insulation and windows - doing the right thing the wrong way may be worse than doing nothing at all .
Today steam heating conversion into hot water heating considered the best practice. The lessons learned from successful steam to HWH retrofit project were summarized as follows: “Over the years, many of us in the New York City multifamily energy efficiency world have talked about how cool it would be to convert steam-heated buildings to hydronic heating. The problem is not one of will but one of money. Changing the boiler is not the big deal—it’s the heating distribution system that is the challenge. … Plenty of these conversions have been done in the last 20 years in buildings that were gut rehabs. These jobs did not always get the best boilers, or insulation in the walls, but they did get a more efficient heating distribution. The real challenge was to convert a building with steam heat, with tenants in place .”
Short history of vacuum heating
Back in the 1900s the conversion of single-pipe steam heating into vacuum heating on average saved 35% on fuel bills . LEED Gold Empire State Building is still heated by original vacuum heating system of 1931, like many others less noticeable. How many of modern heating systems will be around in 2100? 2150?
The main idea of vacuum heating is pulling water vapor from boiler into radiators by vacuum at 5-10 psi pressure difference instead of pushing air via air vents on radiators by steam at 1-2psi. In old vacuum systems for this purpose each radiator was connected to vacuum pump in a basement. In order to prevent hot vapor entering into vacuum pump, special valves were used on radiators in single-pipe systems, and steam traps on condensate return line from each radiator in two-pipe system. Those fault prone specialty valves/steam traps became Achilles’ heel of vacuum heating. Failure of a single one out of a hundreds causes vacuum pump overloading and deteriorate the system performance.
Hot water circulators entering into the market in 1920th reduced piping size and installation cost of hydronic systems. Vacuum heating lost the market to hydronic mostly because of maintanence cost, i.e. steam traps problems, but remains a system of choice for high-rise buildings where hydronic require mechanical floors every 15-20 floors for pumping stations.
NextGen vacuum heating
Conversion from steam heating into new vacuum heating system can provide the level of improvements similar to old results; but without steam traps it is simpler, easier, cheaper and require much less maintenance . Technology is not as trendy as solar or heat pump, but it is more practical. The approach is analogous to the fine tuning of the motor in a car instead of trashing it and buying Hybrid or Tesla.
In the winter of 2013-14, a pilot retrofit of a single-pipe steam heating system confirmed fuel gas savings for different systems used to heat the same apartment –Table 1. Energy efficiency of the original single pipe steam system (~100 years old boiler, piping and radiators) was compared to the same boiler connected to new flat panel radiators by copper/plastic lines; the same comparison was carried later on new regular steam boiler. Up to 50% in savings was demonstrated by the retrofit into a vacuum heating system and in a winter of 2014 -15 results were confirmed [14-18].
Although potential savings are quite attractive, replacing radiators and plumbing still looks risky for owners of steam heating buildings. Therefore, a study was carried out on “vacuum boost” of a single-pipe steam heating system using the existing boiler and with minimal retrofit cost and changes to the existing piping/radiators. This may allow your “old clunker” to surprise you for a long time ahead with energy efficiency and the forgotten comfort of radiant heating. The study was carried on the same boiler/single-pipe steam heating system/apartment of 2013-14 setup on second floor of two-family house. The boiler starts in a vacuum and continues to fill the system with steam until the pressuretrol cut-in setting is achieved (2 psig). It oscillates further at a positive pressure between pressuretrol cut-in and cut-out settings. When the thermostat switches the boiler off, the vacuum pump maintains a vacuum in the system for another 1.5 hours to pull more heat from the boiler. In Figure 1, the annual HDD and heating cost data are presented for both floors, starting from the winter of 2012-13. Because no changes were made to the original single-pipe heating system on first floor, it is used as a base line to compare results. The heating cost for the first floor is used as a base level to estimate the effects of the heating system retrofit on the second floor. The heating cost data, presented in Table 2, are used to estimate potential savings from “vacuum boost” of steam heating. When both apartments were heated by steam, the heating cost ratio was 1:1.19. When system was retrofitted into a two-pipe vacuum heating system, the ratio dropped to 1:0.64, which correspond to an energy efficiency increase of 46.4% ((1.19-0.64)/1.19). These results were achieved with a new boiler/radiators/piping. During the 2015-16 winter, the retrofitting of a single-pipe steam system on the second floor apartment into a steam system with a "vacuum boost dropped the ratio to 1:0.81, which corresponds to an energy efficiency increase of 32.0% ((1.19-0.0.81)/1.19). Assuming that the effect of upgrading the boiler is 16% (please, see Table 1), the effect of the
“vacuum boost” is in the range of another 16%. Please note that the calculated ratios were not affected by changing fuel gas prices.
Water lost from the new vacuum system was negligible – 1-2 gallons in winters of 2013-14 and 2014-15 (water in vapor form was evacuated by vacuum pump along with air). It turned out that water lost from the “vacuum boost” steam heating system was even less, - the same water level was kept in a boiler throughout the 2015-16 winter. pH = 8 was found in routine water analysis. The less time the vacuum pump is on, the less water loss there is. Ideally, no water will be lost from a leak tight system.
Radiators replacement. SIRA modern cast aluminum flat panel radiators were tested as a space saving, economical alternative to old cast iron ones. Pictures and IR images (40 minutes after the boiler cold started in the morning) of an old cast iron and new cast aluminum radiator are shown in Figure 2, - top and bottom, correspondingly. Cast aluminum radiators are modular (five sections were added to a standard size of twelve sections in a picture), and warranted for up to 20. The difference in IR images is quite transparent in favor of SIRA radiators
Retrofit complexity. Notorious vacuum system problem of steam traps malfunctions and painstaking maintenance can be eliminated by “vacuum boost” concept. Retrofit of an existing vacuum system would be the easiest with minor changes to piping and controls. Applying a “vacuum boost” for a two-pipe system would still require adding a vacuum pump and some changes to plumbing. Expected saving are between 32 and 46, more savings might be achieved in bigger project where effect of uneven heat distribution is more significant.