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Heating System Modes

Introduction

The controller in effect has to answer two fundamental questions:

  1. If I need heat, where do I get it from?
  2. If I have extra heat, where do I put it?

The way the system deals with these questions is through mode management. The system recognizes three 'major modes' that reflect where heat is coming from (or would come from if it was needed). There are three choices. In order of preference:

  1. Wood Mode. Heat comes from the wood boiler.
  2. Tank Mode. Heat comes from the external storage tank.
  3. Oil Mode. Heat comes from the oil boiler.

In the graph above, the five thin black lines are digital I/O. The top two are inputs: 'Demand' and 'Oil Disable'. The bottom three are the mode outputs. From top to bottom they are wood mode, oil mode, and tank mode.

The system is always in one of these major modes, and the current mode is indicated by LEDs in the boiler room and as digital outputs in the live 7260 i/o page The basic logic is as follows:

  • If the wood boiler output is hot enough to be useful, we're in wood mode.
  • Otherwise, if the storage tank is hot enough to be useful, we're in tank mode.
  • If neither the wood boiler nor the tank is hot enough, we're in oil mode.

Discussion of Graph

In the graph, the system starts out in oil mode. Near the left hand edge, 'demand' goes high and the oil furnace comes on. It heats the hot water, some scavenging goes on (not directly visible), and all is quiet until the evening of the next day.

The user throws the 'oil disable' switch (see more description of this function below). Shortly afterwards, a fire is started in the wood boiler. At the same time, the house thermostats are taken off of setback, so there is now demand for heat. As soon as the wood boiler is hot enough, the system switches to wood mode. The system stays in wood mode for several hours. The house is heated, the domestic hot water is superheated, and the external storage tank (bright green line) is heated.

Shortly after midnight, the fire dies out. An hour later, the system switches to tank mode. The house has cooled by this time, so there is demand for heat again. The system heats the house from the storage tank until demand is finally satisfied around mid-morning. In the early afternoon, the sun comes out for a while and the solar panel adds a little heat to the tank. At the end of the graph, there is no demand for heat and the system is still in tank mode.

Oil Disable

We don't want the oil boiler and the wood boiler to be on at the same time. A manual switch allows the user to disable the oil boiler. This switch disconnects the thermostat inputs from the oil boiler controller so that the oil boiler is effectively disabled. The system may decide that it's in 'oil mode' and the thermostats may call for heat, but the oil boiler will not come on. This is essential to prevent the situation where a fire has been started, the wood boiler is not up to temperature, and there is a demand for heat. Without the oil disable switch, the oil boiler would come on and the controller would have to deal with heat from two sources.

Wood Mode Operation

In wood mode, the wood boiler is operating and providing heat. We enter wood mode when the wood boiler outlet rises above a threshold temperature, and we stay in wood mode until the outlet has been below a threshold for an hour.

We also monitor the 'Oil Disable' switch. If the switch cycles from 'Auto' to 'Oil Disable', we stay in wood mode for an hour. This provides a way for the user to suppress oil burner operation while a fire is being started.

During wood mode operation, the controller is managing attempting to accomplish several objectives. Some of these are detailed in other sections. In those cases, links are provided.

  • Manage the boiler. Keep boiler inlet and outlet temperatures within desired limits if possible.
  • Deliver heat to higher priority demands first (hot water and living space).
  • Decide where to direct heat when high priority demands have been satisfied.

Tank Mode Operation

When in tank mode, the tank circulator will be turned on if there is a high-priority demand for heat. Low priority demands (basement zone, hot tub) will not be serviced. However, there are switches which allow the user to designate the basement and/or hot tub as priority demands.

Oil Mode Operation

Oil mode operation has the most complex logic. If the oil burner is activated, there are two goals that the system will attempt to accomplish:

  1. Maximize the time until the next oil demand - try to prevent the oil burner from coming on again in the near future.
  2. Ensure that there is no usable residual heat in the boiler.

These goals are accomplished by oil heat scavenging - once the priority demand has been satisfied, the oil circulator is forced on and zone valves are opened in sequence to superheat the domestic hot water tank, heat the hot tub, and finally to dump any leftover heat into the external storage tank.

Superheating the domestic hot water is described in more detail in the section on hot water management.