Coalpail.com Forum

You just summarized the topic well. My point is that you pipe each unit in to a primary loop. It's simpler and easier to control each fuel with a separate t-stat and circuit. You could set up a relay to use one unit (probably the oil one) as a booster and control the booster with a strap on open on the rise aquastat. The primary pump runs all the time.

I have two smaller gas units wired this way. One is the main unit and a relay switches on the second when there is a demand for heat and the system water is cold. The #2 unit shuts down when the system water hits a set temp, leaving only one unit firing. This save gas as now only 1/2 the btus are firing.

Right now I have the AA 130 running full time and the gas units set for 50F in case of an issue with the Axeman.

From his original post, and the photos of his plumbing, what member bowman needs is some rewiring,, not to replumb his system.

I'd take a close look at the oil boiler aquastat, and see if you can't just move the wires for the circulator pump from that aquastat to the coal boiler aquastat.. That sounds pretty simple.. and unfortunately I'm out 'on the road' and don't have an aquastat in front of me to look at and make sure this would work... I don't have the operation and the contacts on the Honeywell 8124 memorized.

Greg L

I think that the piping should be changed, but absent that remember that the aquastats each on separate 24v ac circuits. Relays or separate circuits are the answer.

gregolma wrote:I think that the piping should be changed, but absent that remember that the aquastats each on separate 24v ac circuits. Relays or separate circuits are the answer.

The poor guy has already been hacked to bits about this job! Leave him be or make a lucid suggestion to easily wire what he has.

I'm not sure how its plumbed, but I think Greg is right. I would say the thermostat to the "T" connections and the circulator to the "C" connections on the coal boilers aquastat (assuming they are both Honeywell). The only issue is where the power comes from for the thermostat. If you have a low voltage transformer on the coal boiler, you could pick it up there.

I just used the power from my oil boiler's LV transformer and moved the two thermo wires to the coal boiler as I leave my oil burner live. You won't be able to power the oil down without losing the thermostat's power however.

The situation isn't really that complicated.Two pairs of wire, ran from one boiler to another.
1st. The coal boiler needs to disable the other boiler when it is up to temp. A strap on aquastat on the supply pipe as close as possible to the coal boiler will open on temperature rise ( when the boiler is up to temp ). The 14-2 wire will break the black wire that goes to the burner (oil gun ) leaving the existing circulators , zone valves, controls all still working properly on calls for heat. Also the system will return to its original state if the coal boiler cools down.
2nd. To address the coal boiler lag on a call for heat, a pair of wires(18-2) , runs from the T-T terminals on the coal boiler to a relay that is powered by the 120volts that powers the oilgun (burner). the 18-2 goe across the n.o. contacts of the relay, completing the low voltage circuit to the T-T on the coal boiler when the oil burner controls call for heat. ( which is disabled ifcoal boiler is hot) Or just put a jumper across T-T and let the boiler cycle on high limit.
This setup or variation is reliable, easy to install, easy to understand and still allows flexibility in the hookup. It can be used for any primary/ secondary piping, series piping, parallel piping. Almost all. There are exceptions, but if you can figure those out I wouldn't have anything to type about. Scott