Party Walls

Installing a dedicated domestic hot water (DHW) plant is a common energy conservation measure (ECM) in the New York City multifamily market. According to Local Law 87 data, approximately 80% of the audited multifamily floor area uses steam heating boilers to produce domestic hot water.[1] A recent SWA analysis of data from steam buildings with tankless coils that implemented this ECM suggests that auditors may want to think twice about recommending this measure widely.

Two unsupported arguments are typically made in favor of installing a dedicated DHW system.

1. A new condensing boiler or water heater (we will just say “water heater” here for simplicity and to distinguish the dedicated system from the heating boiler) will operate at a very high efficiency.
2. Scotch marine steam boilers are inherently inefficient and are plagued with high standby losses. Large Scotch marine boilers fire to meet small DHW loads, and correctly sizing a new dedicated water heater will eliminate short cycling.

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Welcome back! In Part One we talked about how steam pressure gets too much attention. Controlling pressure for its own sake should never be the end goal—steam pressure is just a means to an end. In this post we’ll discuss one way that controlling steam pressure can be useful—where it can help you balance the system, control the temperature, and yes, save energy.

Two-pipe Steam

The biggest issue plaguing two-pipe steam heating systems are steam traps. Steam traps are supposed to do just that—trap steam—keeping the pressurized steam on the supply side of the system and allowing air and water (i.e., condensate) to pass through into the returns. Keeping the supplies and returns separate is critical, but steam traps are too failure prone to accomplish this reliably.

At the start of any heating cycle, the system is full of air, which must be removed for steam to enter the heaters. In most two-pipe systems, the steam pushes the air out of the heaters, through the traps, and into the return piping where it eventually exits the system through a vent in a vacuum or condensate tank. That’s what happens when the traps are working. But a failed open trap is no trap at all. It lets the steam flow into the return piping and, with pressure on both the supply and return sides, air is trapped in the system. This affects those farthest from the boiler—the heaters near the ends of the mains and on the top floors—the most.  (And with air trapped inside keeping the metal cold, are they even heaters?)

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