Aquastats, Controls, Feedback loops, Hysteresis

David Chesler

Is there a good diagram or discussion that would teach me how the various controls interact?

I have a 1955 Cape in greater Boston, Arcoliner hot water by oil, diverter-tee baseboard radiators, tankless domestic hot water, single low-voltage thermostat. The controls are pretty much original equipment. The service aquastat has a single dial. (I've been trying to tweak it higher, but not so high as to trip the high limit, so I can heat the house better on below-zero days. The system can almost keep up, which is appropriate.)

What I understand so far (please correct me if I'm wrong) is that when the aquastat is set for 180, the fire goes out when the water bath reaches 180. (Can also go out from the high limit, the big red switch upstairs, the black switch on the side of the furnace, the fuse box or any power failure, flue sensor thinking it didn't light, lack of fuel oil, or the melty thing on the basement ceiling -- but those are exceptions to normal operation.)

If the thermostat thinks the room is colder than its setting, it asks for heat. When it is asking for heat, the circulator will pump water if the water bath is over 160 (and keep pumping as long as it stays over 150), and the fire will go on when the water bath drops a few degrees below 180.

When the thermostat is not asking for heat, the furnace maintains 160 (in a certain range) to provide domestic hot water.

I understand that because it's an old-time aquastat, I can't adjust highs and lows independently.

The upstairs thermostat sometimes has a little
screw marked delay (I've used both a Bell & Howell
round classic, and an electronic) but it seems to have little hysteresis built in.

What I'm wondering is what tells the fire when to turn back on, depending on the temperature of the water and the state of the thermostat? (Like I said, it seems to be at a few degrees colder than the setting for heat, and 20 degrees for domestic hot water only.) And what tells
the circulator pump that the water has gotten too cold (from radiation, and from domestic hot water use) that it should stop pumping (at 30 degrees below the setting?) or that now the water is hot enough (at 20 degrees below the setting?) that it can start pumping?

And are there other rules that the system implements?

(I'm a software engineer who owns a soldering gun and a pipe wrench -- that can be dangerous, but I've promised my wife and my oil man and my plumber that I won't try to fix it myself when I shouldn't [any more, especially in mid-winter], I just want to stand, hands in pockets, watching the dials and flames, listening to the relays and pumps, with a better understanding.)