Wood Heating
System Design
Control Logic
Software Design
Electrical Schematic
Simple system w/ storage
Domestic Hot Water
Heat Storage
Solar Hot Water
System Components
User Guide
Programming Guide
Failsafe Design
Sample Application
LM35 Sensor Assembly
Pinout Info
Poor Man's VS Circ
Plastic Pipe Collector
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The physical wiring of the system is a continuously evolving project. This page documents the configuration at a point in time and describes some of the circuitry involved in controlling the system.

Not all wiring is shown, and the schematic is simplified. Connectors and 12 Volt relay coils are not shown.

Complete Schematic

The image to the right will open the master schematic for the system in a new window. Each section of the schematic is described in detail on the balance of the page.

Oil Furnace Control

One goal of the system is for it to fail safely. In the original oil burner installation, the zone valve limit switches were wired in parallel and connected to a pair of contacts on the oil furnace controller. These contacts are labeled 'Heat Demand' in this schematic. The zone valve limit switches have been replaced by a relay that closes whenever there is any demand, providing equivalent functionality. There is also an additional relay in series that serves to disable the oil furnace when it is energized. This relay is managed by the TS7260 controller and a manual switch in the boiler room.

Oil Circulator Control

In order to scavenge leftover heat in the oil boiler, there must be a mechanism to force the oil boiler circulator pump to run. This is accomplished by adding a SPDT relay to the oil boiler controller. In the Normally Closed (NC) position, this relay maintains the original connection between the oil furnace's own relay and the circulator pump. When energized, this relay bypasses the built-in relay and delivers 120VAC to the circulator pump.

Baseboard thermostats and zone control

The thermostats used in this system have two separate outputs - stage 1 and stage 2. Stage 1 is the normal output that's triggered when the temperature drops two degrees below the setpoint. Stage 2 is triggered when the temperature drops an additional 2 degrees below that.

This circuit shown is used for all three heating zones. The 'Stage 1' relay is therefore a 3PDT relay that sitches all three zones simultaneously. This relay is managed by the TS7260, and is used to determine whether the system will respond to the stage 1 or stage 2 thermostat outputs.

All three zones are wired identically. Only a single zone will be described here.

LEDs in the boiler room show the state of both thermostat outputs. The selected output is routed through a second relay which allows the TS7260 to disable the zone if desired. The signal is then routed to a three-position switch in the boiler room. This switch allows the zone valve to be forced open, forced closed, or to be controlled by the thermostat ('auto' setting). Finally, the signal is used to power the zone valve motor. When open, the zone valve has a kimit switch which drives the 'demand' relay mentioned in the section on the Furnace Controller.

The selected thermostat output (stage 1 or stage 2) is also used to drive another relay indicating zone demand. The contacts from this relay are connected to the TS7260 allowing it to be aware of the zone demand.

Hot Tub / Hot Water Zone Control

These zones don't have the two stage thermostats, but they do need to be managed more intimately by the controller. In each case, the thermostat (actually, aquastat) closure drives a status LED and a demand relay as in the heat zones described above. The signal is also routed through a pair of relays controlled by the TS7260. The first relay allows the TS7260 do disable the zone. This allows the controller to delay heating the hot tub until there's excess heat available, for instance. The second relay allows the controller to force the zone valve open. This allows the DHW to be heated above the setpoint.

Heat Demand Signal

As mentioned earlier, the zone valve limit switches were originally wired in parallel and connected to the oil boiler controller. In the current implementation, the limit switches are still wired in parallel. However, they drive a relay which creates the switch closure for the oil furnace controller. Each is also wired in series with a switch in the boiler room which allows that limit switch to be disabled. The result is that opening the valve does not create a demand for the oil boiler.

This 'disable' function is primarily used for the basement zone to reduct the number of oil boiler cycles. When there's no one home, the basement zone shouldn't turn on the oil boiler if it's the only zone needing heat. However, the thermostat will still open the zone valve and heat will be provided the next time the oil boiler runs.

Oil Circulator Force and Oil Disable

This circuit shows how the oil circulator pump relay is energized. The Oil Force Relay is driven by the TS7260. When closed, it supplies 24VAC to the coil of the Oil Circulator relay.

This could be done with a single relay, but the 24VAC relay was wired into the oil boiler controller years age, and there's no pressing reason to replace it with a 12VDC relay.

The Oil Disable function in controlled by two relays and a manual switch. The relays probably should be given different names for clarity, but the coil for the 24VAC relay is shown in the schematic. That coil can be energized by either the manual switch or a 12VDC relay under the control of the TS7260. In either case, energizing the 24VAC relay prevents the oil boiler from responding to a heat demand.

Tank Zone, Tank Circulator, and Wood Recirculator Zone Valve

These three functions are quite simple. In each case, a 12VDC relay under the control of the TS7260 energizes the zone valve or circulator pump. In this case, the tank circulator is mis-drawn - it actually operates off of 120VAC, not 24VAC as shown.

Hot Tub Mode Control

In most cases, the hot tub is in 'economy mode' - only heated when there is heat available and there is no higher priority. As such, it's not normally heated when the house is being heated from the storage tank, for instance.

However, if people want to use the hot tub, there must be a way to tell the controller that the hot tub is now a priority. That's accomplished by energizing the Comfort Relay, which is an input to the TS7260. There are switches with status LEDs mounted upstairs and next to the hot tub. These switches allow the hot tub mode to be selected.

Digital Inputs

In all cases where the TS7260 needs to know the state of a heating system signal, a 24VAC relay is used, and the TS7260 digital input is wired to a set of relay contacts. Many of the relays are double pole, and the second set of contacts provide some other system function. For instance, the Demand relay has one set of contacts that provide an input to the TS7260, while the other contacts provide the demand signal for the oil boiler.