Secondary Boiler to Existing Forced Air Wiring Diagram

Does anyone have a wiring diagram or some instruction to connect my coal boiler to my forced air furnace? I would like to have the house wall thermostat simply control the forced air duct fan in the existing furnace with a switch to turn the propane burner on/off for backup, currently there is a red and a white wire that attach to the circuit board in the furnace. The coal boiler is pretty much self sufficient, I have it wired so the triple aquastat and the timer controls the stoker operation like the EFM instructions show except the circulator pump will be always on and not controlled by the house thermostat, I'd rather the heat exchanger stay warm and have the boiler water circulating through it all the time. On the wall thermostat there isn't a "summer mode" or "Fan only" mode, just a timer looking thing that says "longer" and a mercury bubble. It's a White-Rodgers S.P.D.T. Type 1F36-335. The furnace is a Tappan 144 and has a circuit board with quite a few unused terminals.
thanks.

I sniffed around a little but could not find the diagrams for those units. If you can scan them, that would be great. I think you are all set with the wiring as your thermostat is set to trigger it now. You just need to make the furnace think the burner is on and maybe the Temp switch (that should make w/180* water next to it). Ideally, you would want it set up so that when the coal boiler is down, the HA furnace kicks in automatically.
The schematic on your EFM would help. Wait a minute, I've got one of those! No, not the boiler, the schematic.

Still no schematics, but I thought of an option.
If you install a large centrifugal (similair to ODBs use) fan in the duct and wired it directly to your EFM's call for heat output, you could put in another thermostat for the coal boiler. The two would then run independently. By setting the coal boiler 'stat about 5* higher, your furnace would kick in and take over should there be any failures or if the fire dies on the coal boiler. The 'stat is about $25-30 and the fan maybe $70-100? That way you can take your honey on vacation with all the money you save and not worry about it.
You can tie them together electrically, but it will get complicated, especially without the electrical drawings.

The desire to control two heating units with a single room thermostat is a recurring request. While I don't have a good solution I do have a suggestion. I would look at digital thermostats designed to control two and three stage heat pumps. Multi-stage heat pumps are those that have multiple levels of additional electric resistance heat. I believe the thermostats are programmable to control when the extra heat comes on. Given this flexibility it is likely some added external relays would get you what you want.

Ok I'm getting closer to a solution. If I disconnect the "W" wire at the furnace and screw it to the "G" terminal (also on the same circuit board) the house thermostat controls the duct fan only, not the burner. I think my EFM's honeywell triple aquastat has a STDP switch built in, in which case if the aquastat gets below say, 140 degrees it can connect "W" to "W", if the temp is above 140 it will connect "W" to "G", effectively switching what the house thermostat controls. On second thought after studying the furnace controls I believe once the heat exchanger in the duct is hot the burner won't run, just the fan, so my solution might be to a problem that doesn't exist.

e.alleg wrote:so my solution might be to a problem that doesn't exist.

That's a scary thought.

I've added a hot water coil to my boiler, I believe that the terminals in the furnace are all pretty much standard.

The "R" terminal is 24V power

The "W" terminal is the Heat Control

The "G" terminal is the fan control

The "C" terminal is the common of the 24V transformer

You need to add a small spdt switch, (single pole double throw) inbetween your thermostat and the furnace. Connect the thermostat lead to the common of the switch. Connect one of the switched leads to the W or heat input. Connect the other switched lead to the G or fan lead.

To control the boiler water, connect the zone valve control wires to the G fan terminal and the other lead to the C or common of the 24V supply in the furnace. Most zone valves have a reporting switch that closes when the valve is opened. This is connected in series with the circulator pump, or better yet, controls a relay which turns on the circulator.

Hope this helps

Thank you! You know I have to laugh, I looked up the STDP switch and lo and behold: it's a toggle switch. I have about 50 of them laying around. I can officially work for the government now, I have made a simple thing overly complex. When I was a kid my dad always said "NEVER touch the thermostat" so I have kind of been afraid to touch them my whole life. Now I know he just didn't want me turning up the heat.

e.alleg wrote:On second thought after studying the furnace controls I believe once the heat exchanger in the duct is hot the burner won't run, just the fan, so my solution might be to a problem that doesn't exist.

I think that is the case here. I have the same Honeywell control as you and looked at the EFM stuff and I would reccomend this. Leave all the existing furnace stuff and thermostat alone. Wire the circulator to the EFM circulator control. Jumper the thermostat screws on the Honeywell control on the EFM. When you fire the coal boiler the circulator will start on a rise from the low setpoint. If the fire dies it will shut the circulator down on a fall below the low setpoint. This prevents the coal boiler from robbing heat from the furnace when it runs. The only thing you want to watch is the temperature switch at the top of the furnace. Most of these are adjustable and you can run the boiler at a higher temp too. Between these two adjustments you just want keep the plenum temperature above the point it fires the furnace. Without connecting the two units you should have a seamless transition from one to the other automatically.

On another note, I have the Honeywell L8124 Aquastat Tech pages. I do not see any copywrite on it. Can I post it? It describes the sequence of operations performed and in what fashion, wiring options and install info. Pretty neat the way it works.

Edit: It's a good thing I went looking for this stuff. When I went back down to put the stuff in the "library", I discovered the copper tube hot water line had spung a leak. I tried to shut it off, the valve doesnt stop the water. No water tonight. Lousy well water, eats everything.

coaledsweat wrote:On another note, I have the Honeywell L8124 Aquastat Tech pages. I do not see any copywrite on it. Can I post it?

My copy of the manual says "Copyright © 2001 Honeywell • All Rights Reserved". No need to host it on the forum. It's available on Honeywell's site for download:

https://customer.honeywell.com/honeywell/ProductI ... L8124A1007

Once on the site scroll down to the bottom download box.

Also there are 8 slightly different versions two "A" versions, "B", two "C's", "E", "G" and "L". Plus the universal service versions marketed under the "Tradeline" name.

coaledsweat wrote:Edit: It's a good thing I went looking for this stuff. When I went back down to put the stuff in the "library", I discovered the copper tube hot water line had spung a leak. I tried to shut it off, the valve doesnt stop the water. No water tonight. Lousy well water, eats everything.

Bad well water? I can sympathize with you. Replace metal pipes and valves with PVC or CPVC. Hot water tanks with stainless. Threaded transition fittings with 316 stainless. Do it once and be done with it!

Yanche wrote:

coaledsweat wrote:Edit: It's a good thing I went looking for this stuff. When I went back down to put the stuff in the "library", I discovered the copper tube hot water line had spung a leak. I tried to shut it off, the valve doesnt stop the water. No water tonight. Lousy well water, eats everything.

Bad well water? I can sympathize with you. Replace metal pipes and valves with PVC or CPVC. Hot water tanks with stainless. Threaded transition fittings with 316 stainless. Do it once and be done with it!

This water is so bad it ate the treatment system the guy before me had.

I'm fed up with it and may do a teardown. Between the code violations and lousy work and materials building it, I can't take it anymore. It is my first house, and with it I have recieved basically an advanced degree education in homes because of it. Well at least it's warm.

Yanche wrote: https://customer.honeywell.com/honeywell/ProductI ... L8124A1007

Once on the site scroll down to the bottom download box.

Chose "product data" at the top of the literature box and scroll to the bottom of page 13. That describes the rise and fall actions of the switches.