Installation on Steam Systems

1. General

This bulletin provides information on wiring the MicroTherm system to steam systems with no energy management systems and systems equipped with either an Electronic Heat Timer or a Mechanical Heat Timer. Comprehensive wiring diagrams have been provided for wiring the system in all applications.

2. Pressure Switch Wiring

A normally open pressure switch (i.e. Honeywell PA-404A) must be installed in order for the MicroTherm system to operate with steam systems.

DO NOT remove any of the existing pressure controls.

1. Install MicroTherm in accordance with the installation instructions provided in Section 3.0, Installation.
2. Install a normally open (N.O.) pressure switch sized for normal boiler operating pressure at, or near the existing pressure controls using the necessary pipe fittings and anti-siphon pigtail.
3. Wire the new pressure control to terminal J6 located in the upper right corner of the MicroTherm enclosure. Refer to the illustration below.

4. Determine the normal system operating pressure (burner OFF on pressure, ON upon pressure drop). i.e.: Burner will fire (ON) at 3 PSI and stop firing (OFF) at 5 PSI etc.

If the (ON) pressure is 3 PSI for example, set the new pressure control at 2 PSI.

5. Program the MicroTherm system for the pressure switch as follows:

- Press "Menu"
- Press the number 2 key
- Select "Sensor 2"
- Toggle up or down using the +/ - keys until the screen shows "CONTACT"
- Press "ENTER"

The system will now recognize the newly installed pressure switch as a "contact switch" which will act as the limit to interrupt the economy cycle.

3. Operation

The MicroTherm system will now function as follows:

1. Burner will stop firing (OFF) at boiler pressure control setting.
2. The MicroTherm system will begin counting (OFF) time.
3. Boiler pressure will drop and the pressure control will call for the burner to fire (ON).
4. The MicroTherm system will begin counting (ECONOMY).

If the "economy" time is too long, and the system pressure drops to the low limit set on the newly installed pressure switch, the "economy" cycle will be stopped (regardless of the remaining time) and the burner will fire (ON).

This procedure allows the newly installed pressure switch to act in the same manner as a temperature sensor and provide a similar economy override safety feature.

If the economy cycle were allowed to continue after the pressure in the steam system had dropped below the low limit setting, the burner would have to run too long to try to recover lost pressure and the MicroTherm system would provide no savings to the customer.

4. Wiring for Mechanical Heat Timer

If the steam system for which you are installing MicroTherm has a mechanical heat timer, you should wire the MicroTherm system in accordance with the wiring diagram provided below.

Notes:

1. Solder the coil wires to "Call" for heat lamp socket behind panel. This relay is normally Closed and "calls" for heat from the heat timer. This relay jumps out the low limit aquastat used for heat and hot water and allows MicroTherm to go into economy cycle. Note: If heat timer lights are neon (95V), solder wires to relay in back bottom of heat timer board. There are spare lugs an relay. Then wire relay accordingly.

2. This aquastat is the low limit for economy in "heat" mode. Mount this aquastat (i.e. Honeywell L-4006-C) on the hydronic return furthest from the boiler. Set a degrees below the heat timer aquastat.

Figure TB-2, Wiring for Mechanical Heat Timer installations

5. Wiring for Electronic Heat Timer

If the steam system to which you are installing MicroTherm has an electronic heat timer, you should wire the MicroTherm system in accordance with the wiring diagram provided below.

Notes:

1. This aquastat is the low limit for economy in heat mode. The pump terminals are used on a "call" for heat from the timer to allow this aquastat to control the low limit.

2. On "No Call" for heat, pump terminals "jump" this aquastat.

Figure TB-3, Wiring for Electronic Heat Timer Installations