Of wood boilers and controls

Discussion in 'HVAC Heating & Cooling' started by nhmaster, Dec 17, 2008.

  1. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    The subject of how to control a wood fired boiler seems to come up a lot lately. Usually the problem is that the oil or gas boiler will continue to fire even though the wood boiler is hot. The L8124 triple aquastat and the L8148 aquastat \relay are often involved in the topic because these are the two most commonly used operating controls. Let's start with the L8124.
    It is a triple aquastat. Facing head on it has three knobs with scales next to them and the words High limig. low limit and the black know says differential.
    The high limit is exactly what you think it is. It stops the burner when the set temperature is reached. The low limit starts the burner when it's set temperature is reached. The differential is the limit, plus the differential setting and that is the temperature that the burner will come back on at.
    The L8148 is a little less complicated. It only has a high limit setting. Both controls have two relays in them. One to start the burner and the other to run the circulator. So how do we make these controls behave with the wood boiler? The easiest thing to do is to set the high limit low enough so that the burner will not fire. Let's say the wood boiler does a fair jhob of maintaining 180 degrees. Then you would set the high limit to around 170. The oil burner will only come on if the temperature in the system drops below that set. This works ok but if the wood boiler is out for a long period of time and it's really cold outside then 170 may not be hot enough to heat the building. Another option is to just leave it alone. Yes the burner will fire, but only to make up the difference between the limit set and what the wood boiler is putting out. I like this option better because reqardless of the boiler temperature, the house still requires water at a certain temperature to maintain the heat. In this case the burner is only helping out a little and the bulk of the heating is being done with the wood boiler. If you want to cut the burner out of the system altogether then just set the high limit as low as it will go.
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