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Old 07-23-2012, 12:40 PM   #1
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New Boiler System Control with 2 stage Heat Thermostat


Its been a while since I have been on here but I always get really good help. I recently finished installing an outdoor wood gasification boiler that pumps hot water to an Air / Water heat exchanger in my forced air plenum. I need to be able to turn the fan on and not the propane along with my secondary circulator pump. I also want the themostat to be able to use second stage to turn on the propane if the temp drops off or the fire goes out. So far I have draw this diagram but I am concerned about the furnace circuit board being able to handle the load from the relay coil on the right. I will also use a thermostatic switch on the W1 circuit to prevent stage 1 from coming on when my boiler water temperature is to cold.
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Old 07-23-2012, 01:26 PM   #2
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New Boiler System Control with 2 stage Heat Thermostat


I may be going in the wrong direction here. I assumed that the thermostat worked the same in heat mode as A/C mode controlling both the fan (G) and heat (W). It looks like that since I have a propane furnace the thermostat only controls the heat (W) circuit in heat mode. The fan then comes on by itself when the temperature in the plenum is high enough. I only need to turn on the fan. If I use my first heat stage to command the fan (G) terminal, I can accomplish that. I would need to put a relay in this circuit to control a circulator pump. I still don't know if the furnace control board is capable of handling the load of the primary contacts of the relay.

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Old 07-23-2012, 01:53 PM   #3
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New Boiler System Control with 2 stage Heat Thermostat


Something like this
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Old 07-23-2012, 02:23 PM   #4
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New Boiler System Control with 2 stage Heat Thermostat


so the wood boiler is first stage with the circulator being the heat call thru the add on heating coil.consider a thrmal disc on the inlet pipe to the coil that controls the supply fan when the hot water hits it so when the sta satisfies the W1 opens and the pump shuts off.the fan on that W1 heat call will run till the inlet pipe cools off and the disc opens...now for W2 propane/furnace just wire it normal as if the wood/boiler wasn't there.the main issue is the furnace will be LO/LO MED fan as designed what speed are you going to tap for the wood boiler heat disc...just throwing this out for you to pick and chose the controlling of.the disc will be rated for an amperage to handle the fan closure...if too much might have to use a relay so the disc controls the coil within that relay.the reason for a temp.disc let the hot water control the fan cycle let the board control the circulator...with that W1 heating mode
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Old 07-23-2012, 02:47 PM   #5
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New Boiler System Control with 2 stage Heat Thermostat


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Originally Posted by biggles View Post
so the wood boiler is first stage with the circulator being the heat call thru the add on heating coil.consider a thrmal disc on the inlet pipe to the coil that controls the supply fan when the hot water hits it so when the sta satisfies the W1 opens and the pump shuts off.the fan on that W1 heat call will run till the inlet pipe cools off and the disc opens...now for W2 propane/furnace just wire it normal as if the wood/boiler wasn't there.the main issue is the furnace will be LO/LO MED fan as designed what speed are you going to tap for the wood boiler heat disc...just throwing this out for you to pick and chose the controlling of.the disc will be rated for an amperage to handle the fan closure...if too much might have to use a relay so the disc controls the coil within that relay.the reason for a temp.disc let the hot water control the fan cycle let the board control the circulator...with that W1 heating mode
If I supply power directly to the fan with a thermo disc or relay can I damage the furnace circuit board by backfeeding voltage into it?

Also, if the thermo disc is active running the fan and the second stage heat comes on to activate the propane. What happens when the furnace circuit board tries to activate the fan that already has power going to it from the thermo disc. This situation is unlikely but possible.

My wood boiler system is a little different than most. I have a 500 gallon storage tank in the basement next to my furnace. The wood boiler outside pumps water into the storage tank and is computer controlled. My secondary circulator is the one I'm trying to control and pumps from the storage tank into the heat exchanger in my forced air plenum. I have an adjustable thermostatic switch screwed into the top of my hot water storage tank I plan to put in series with the thermostat wire to prevent it from turning everything on when the water is cold. I could use the thermo disc to control the fan assuming it won't damage the circuit board but Im not sure this is necessary since my storage tank is less than a foot away from the furnace. I should get hot water almost instantly.

Last edited by hartkem; 07-23-2012 at 03:53 PM.
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Old 07-23-2012, 04:47 PM   #6
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New Boiler System Control with 2 stage Heat Thermostat


Yet another revised diagram. This version gives me the added benefit of locking stage 2 (w2) from coming on if my water temperature is high enough. I have a 25 amp relay which should be enough to run the fan and circulator pump. Wonder whats the best way to splice into the blower fan wiring.
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Old 07-25-2012, 06:36 PM   #7
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New Boiler System Control with 2 stage Heat Thermostat


Does anyone know if there would be any issue supplying power directly to the furnace fan motor with a relay? It couldn't hurt the furnace circuit board by backfeeding could it?
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Old 07-25-2012, 07:24 PM   #8
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New Boiler System Control with 2 stage Heat Thermostat


Many digital thermostats have a electric heat setting. this turns the fan on when a call for heat is made. if you have one of these thermostats, all you have to do, is enable that option.
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Old 07-25-2012, 07:42 PM   #9
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New Boiler System Control with 2 stage Heat Thermostat


Beenthere,

My thermostat does have that option and it would probably work with the first wiring diagram I posted. In that diagram the relay on the right is grounded through the furnace control board and im not sure if it can handle the current draw of the primary coil in the relay. My second diagram turns the fan on directly from the relay. I tend to prefer this option if I can confirm I won't hurt the control board.
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Old 07-25-2012, 08:02 PM   #10
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New Boiler System Control with 2 stage Heat Thermostat


Can't run loads in series. So it won't work the way you have it drawn.

You could use an R8222D relay to bring on the circ and fan.
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Old 07-25-2012, 08:15 PM   #11
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New Boiler System Control with 2 stage Heat Thermostat


why can't the loads be in series? What if I wired them in parrallel off the relay?
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Old 07-25-2012, 08:38 PM   #12
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New Boiler System Control with 2 stage Heat Thermostat


Do you have AC as well as heat?
This is going to be slow in heating.
I'd use an ambient control and a 3 way valve and have stage 1 heat control the actuator on the valve.


Last edited by Houston204; 07-25-2012 at 08:43 PM.
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Old 07-25-2012, 08:43 PM   #13
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New Boiler System Control with 2 stage Heat Thermostat


Yes, I have AC and heat
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Old 07-25-2012, 08:54 PM   #14
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New Boiler System Control with 2 stage Heat Thermostat


Because 24 volts going through a 12 ohm resistance to another load means that you only get 12 volts.
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Old 07-25-2012, 08:56 PM   #15
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New Boiler System Control with 2 stage Heat Thermostat


Yep, you have to provide common to the relay..
.


Last edited by Houston204; 07-25-2012 at 09:26 PM.
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