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Oil Boiler Heat Scavenging

One of the early goals of the hot tub project was to scavenge heat left in the oil boiler to heat the hot tub after all other demands have been satisfied and the boiler is off. This was originally accomplished using a thermal switch and relays. With the availability of the 7260 controller and the heat storage tank, the scavenge logic is more complex. As currently implemented, there are two digital inputs and two analog inputs to the 7260 that are used for heat scavenging:

  • 'Demand' digital input. High when any zone except the hot tub needs heat.
  • 'Hot tub warm' digital input. High when hot tub is at temperature.
  • 'Boiler out' analog input. Temperature of oil boiler outlet.
  • 'Tank Top' analog input. Temperature at the top of the heat storage tank.
  • 'Water Heater' analog input. Temperature at the middle of the domestic hot water tank.
These inputs are used to determine the state of three digital outputs, each of which activates a relay:
  • 'Oil Circulator Force' - forces the oil circulator on.
  • 'Tank Zone Valve' - opens tank zone valve.
  • 'DHW Zone Valve' - opens domestic hot water heater zone valve.
  • 'Hot Tub Zone Valve' - opens hot tub zone valve.

When there is demand from a 'normal' zone (heat zones or hot water), the boiler operates normally. Once there is no 'normal' demand, the boiler will shut off and the 'demand' input will go low. As long as 'demand' is low, the controller will then manage the outputs as follows:

  • If the boiler outlet is high enough to be used somewhere, the 'Oil Circulator Force' output is set high, forcing the circulator on.
  • If the boiler is more than 7 degrees above the hot water heater temperature, the DHW zone valve is opened and heat is dumped into the water heater.
  • If heat can't be dumped to the hot water heater and the Hot Tub needs heat and the boiler is above 120° then the hot tub zone valve is opened and the hot tub is heated.
  • If the water heater and the hot tub aren't being heated and the boiler is warmer than the top of the external storage tank, then heat is dumped to the external storage tank.

The graph to the right shows an oil burner cycle with scavenging. The three black lines are digital I/O. From top to bottom: Hot Water Warm, Hot Water Force, and Tank Zone Valve. In this example the hot tub was not calling for heat, so it was left off the graph. If it had needed heat, it would have been next in line after superheating the domestic hot water.

Just before 'A' on the graph, Alan starts a shower. Emergency response teams are put on high alert. Anticipating the demand, the Vermont Yankee Nuclear plant spools up to peak output, and oil prices climb worldwide.

Within minutes, the hot water tank temperature drops below 105° (point 'A') and the oil boiler starts up. Despite 120,000 BTU being dumped into the hot water tank, the temperature continues to fall, reaching a low of 108°. Note that this is the temperature at the tank midpoint - the outlet at the top is still at about 115°. No screams from the shower....

At point 'B', the demand is satisfied, and the boiler turns off. At that time, the hot water tank is at 118°. The external storage tank (bright green line) has dropped just a little - it is preheating the cold well water before it enters the bottom of the hot water tank.

Scavenging begins. The oil boiler (several hundred pounds of cast iron and water) is still at about 155°. The controller opens the hot water tank zone valve and forces the oil circulator on again. We heat the hot water tank to 123°, hopefully delaying the moment when it will be cold enough to demand heat again.

At point 'C', the boiler is not hot enough to add any more to the hot water heater. Since it's still hotter than the external storage tank, we open the tank zone valve and dump heat to the storage tank. Since the storage tank weighs over 7000 pounds, we don't raise the temperature much, but every little bit helps.

At point 'D', the oil boiler is down to the temperature of the external tank. We have nowhere else to dump heat, so scavenging is over.