One last thing holding me up from having wood heat

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Feb 11, 2010
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I have almost finished installing my OWB. The last thing keeping me from it is thermostat wiring. My conventional heating system is a variable speed air handler and heat pump combo with electric coils as emergency heat. I'd like to leave that system in place functioning normally in case I'm not home and the fire burns out in my OWB. I was hoping to run a second separate thermostat for the OWB so that it kicks the fan on in the air handler and also opens a zone valve for the water/air heat exchanger in the plenum. I'd like to keep the heat pump system working normally with it's thermostat set a few degrees below the OWB thermostat. Any ideas? My thought was to set the heat pump thermostat such that the fan is set to "ON" rather than "AUTO" and have the second thermostat to be able turn it off as necessary.
 
I added a second thermostat which is connected to the AC side of the furnace control - just disconnected the power going to the compressor so it wouldn't kick on. The "fan on" was too slow on my variable speed fan so I ended up going this route.

This is my opinion - I would start by trying the AC side of your existing thermostat, disconnect the compressor power as stated before, and try that before installing another thermostat. I don't know the size of your OWB but mine only has 130 gallons and I have never had the need to run the furnace. Except that time the front door was left open for the entire day . . . .

If you ever want to run the furnace - just switch over to the HEAT side and run as normal.

You need to be aware also that if you do run out of boiler heat - using a water to air HX in the furnace plenum - and you want the furnace to start up as a backup you will be not only heating the house but ALSO the water in the boiler if your OWB pump runs continuously.
 
I added a second thermostat which is connected to the AC side of the furnace control - just disconnected the power going to the compressor so it wouldn't kick on. The "fan on" was too slow on my variable speed fan so I ended up going this route.

This is my opinion - I would start by trying the AC side of your existing thermostat, disconnect the compressor power as stated before, and try that before installing another thermostat. I don't know the size of your OWB but mine only has 130 gallons and I have never had the need to run the furnace. Except that time the front door was left open for the entire day . . . .

If you ever want to run the furnace - just switch over to the HEAT side and run as normal.

You need to be aware also that if you do run out of boiler heat - using a water to air HX in the furnace plenum - and you want the furnace to start up as a backup you will be not only heating the house but ALSO the water in the boiler if your OWB pump runs continuously.
 
I am aware that running the heat pump as backup with the water/air exchanger in the plenum will also heat the water in the OWB. That will be my freeze protection.
 
juddspaintballs said:
I have almost finished installing my OWB. The last thing keeping me from it is thermostat wiring. My conventional heating system is a variable speed air handler and heat pump combo with electric coils as emergency heat. I'd like to leave that system in place functioning normally in case I'm not home and the fire burns out in my OWB. I was hoping to run a second separate thermostat for the OWB so that it kicks the fan on in the air handler and also opens a zone valve for the water/air heat exchanger in the plenum. I'd like to keep the heat pump system working normally with it's thermostat set a few degrees below the OWB thermostat. Any ideas? My thought was to set the heat pump thermostat such that the fan is set to "ON" rather than "AUTO" and have the second thermostat to be able turn it off as necessary.

Not sure how the variable speed is controlled on your system, probably by an electronic board in the air handler. Does it run on the highest speed when you use the FAN ON function? My heat pump w/resistance backup has just the standard fixed speed 240V blower. Your blower may actually be a DC motor. You should have a wire coming from the HP tsat (terminal G on mine) that is for the FAN ON option (verify this wire turns fan on continous). Your 2nd thermostat with a 24V output could drive the coil on a DPDT relay ($15). Connect the COMMON and NO (normally open) terminals on one pole of this relay to the cut wire coming from Terminal G (FAN ON portion of your HP tstat) to wherever it goes. This way with your FAN ON on the HP tstat, the blower will actually be OFF when the room is at the set temp of the new (Wood) tstat and ON when you want it to blow air thru your HX. The second pole could power the zone valve controlling the water to the HX. I think this is the "safe" way for you to accomplish what you want since I believe your blower is probably a DC motor with speed controlled by electronics you don't want to mess with. Also you don't want to disable your compressor, heat strips, or anything else on your existing system like others may have mentioned. They may not realize that this is how the HP creates (actuallys transfers) heat from the outside air. The heat strips need to be left alone as well because the outside unit will use them for defrost mode ocassionally. I would try to find a 2nd tsat that reads the same as your original one. My 2nd is a Honeywell 5110 and a simpler version of the 5220 or 5320(?) on my heat pump. Both are the same series and look the same except for physical size. I bet your system stays within 1 degree of the set temperature just like mine does too. Because the HP's put out lukewarm air this is usually the way I've seen them set up. You will also want the 2nd tstat set up to tightly maintain the set temp. That way you can set 1 for 72 and the other for 70. House will always be 71 or 72 when OWB is hot or 69 or 70 when OWB is cold. I would caution against planning on circulating the water 24/7 thru the HX say when you are gone for week. A guy here mentioned his OWB maintained something like 80 degrees when it was 0 outside. He was hoping it would keep it just above freezing but the HX worked too well. He didn't want to guess how much fuel he wasted during that week. In your case maybe adding an aquastat (Johnson Controls A419 $50) at your OWB boiler output pipe could be used to open the zone valve to the HX to steal just enough heat to keep OWB from freezing. You would want the water thru the rest of the OWB system to circulate 24/7 I would think.
 
I think you are spot on describing my variable speed blower. I'll check when I get home today. Thanks.

I wasn't planning on 24/7 circulating when I'm not home for a week either. Just an overnight oops on occasion.
 
I don't know which terminals do what yet, but here are the terminals I have in the air handler and what's connected to them:

L (nothing)
R (red wire)
C (blue wire)
O (orange wire)
Y/Y2 (yellow wire)
Y1 (nothing)
G (green wire)
E (brown wire)
W2 (white wire)
W3 (nothing
 
G (green wire) is probably the FAN ON. Test it "safely" (ie. shut off breakers, mine has 3) and remove green wire from the terminal that goes back to the tstat. Note may have other wires here in the air handler conected to this. If you have a multimeter test to see if it is 24VAC also. Test between this wire (G) and C (blue wire) which should be the Common from the 24V transformer. You should also have 24VAC between the R (red wire) 24V hot and the C (blue wire) all the time. This is the feed from the 24V transformer.

Dont quote me but I bet the rest are as follows:

O (orange) controls the changover valve on the heat pump unit outside. ie. switches from heat to cool mode.
y (yellow) goes to the compressor contactor.
E (brown) is for the emergency heat strips
W2 should be the call for heat.
W3 (not used) would be if the heat strips were split...say one is the 5KW and the other is the 10KW...they wire both mine together too I believe.
Y1 is probably when the heat pump has 2 compressors...can have just the small one, a larger one, or both running...never seen those but read about them.
 
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