4 EPU‐D Installation and Operation Manual
059840‐00 Rev. A
Heat‐Timer Corp.
Detailed Operation
Overview
The EPU‐D is a microprocessor‐based control designed to manage a low‐pressure steam heating system. The
control can operate a steam boiler or a two‐way steam valve using a “cycle” concept to provide the desired
amount of heat to the building.
The “Cycle” Concept
The “cycle” concept was created by Heat‐Timer specifically to allow steam heating systems to overcome the
inabilities of standard thermostatic controls to cope with the unique challenges of low‐pressure steam heating.
Unlike water and air systems, steam systems take time to build up a “head of steam”. Moreover, after the
system starts heating up, it has momentum that takes time to dissipate. This makes it difficult to control its
temperature.
Cycle Length Based on Outdoor Temperature
By monitoring the outdoor temperature, the EPU‐D is able to anticipate the building heating needs. Each cycle
period (usually 60 minutes long, but adjustable depending on the type of radiation units) consists of a Cycle‐ON
segment and a Cycle‐OFF segment. The length of the Cycle‐ON segment will vary with the outdoor
temperature. The colder it is outdoor, the longer the Cycle‐ON segment.
Figure 2:
Example 60 Minute Cycle
The EPU‐D constantly checks the outdoor temperature by means of a solid‐state sensor located on the exterior
of the building. At the same time, it monitors the heating system of the building by means of either a heating
system sensor or reverse‐acting pressuretrol. These sensors tell the control when steam has reached the
furthest location in the building (or the hardest to heat area, see “Piping Diagrams” on page 38). Based on this
combined data, the EPU‐D sends instructions to the heating plant to control the heat level in the building.
The EPU‐D also provides two adjustable outdoor temperature cutoff settings, one for daytime operation and
one for nighttime operation. When the outdoor temperature rises above one of the cutoff settings, the EPU‐D
will not call for any heat.
