Adding a separate thermosat for wood boiler heat exhanger to existing propane furnace 2 speed fan

joepie61

I just installed a outside wood boiler and installed heat exchangers in the existing ductwork. I installed a separate thermostat and thermostat wire to control the existing propane furnace(Bryant 90i) fan. My problem is I can only get the low speed fan to run and the furnace fan will not jump up to high speed ever. So, step by step, what do I need to do to make this work? If parts are needed please specify which parts I need to buy please. What I want to accomplish is that the wood boiler thermostat will turn the fan on and off and the propane thermostat be set ten degrees lower so that it will kick on the propane furnace for heat if for some reason the wood boiler stops making heat.

Thanks much!

joepie61

The existing propane furnace is also wired and contains the A coil and control for the A/C.

Harvey Ramer

Is it an Ecm blower motor or not?

Tap2

This can all be controlled with one stat (Honeywell prestige.) You will need the 2 wire model with eim interface. You will have the option in setup to open the gap between first and second stage heat to whatever you like.

Ironman

You cannot control the furnace from two thermostats. I've installed ODWBs for years and argued til I'm blue in the face with the manufactures about their stupid drawing that shows you can.
If you attempt to use two thermostats you will have the following issues:
1. You'll cause the A/C compressor to come on when the thermostat for the ODWB calls for heat due to a back feed through the fan circuit
2. When the gas furnace thermostat calls for heat and the OBWD's thermostat is also calling, the heat produced by the furnace will be transferred to the ODWB because the hydronic coil in the plenumn will be cooler than the discharge air from the furnace. In other words, you'll be back heating the ODWB.
3. If somehow you connect power to two speeds simultaneously, you'll burnout the motor on the blower, if it's a PSC motor.

I have to disagree with the comment above about using a Honeywell Prestigue thermostat for the same reason as stated in #2 above. If the first stage isn't satisfied, when the second stage is energized, the furnace will back heat the ODWB.

Back when I added ODWBs to my offerings over ten years ago, I realized the issues involved in intragrating them with conventional furnaces and heat pump systems. Since I had a background that included commercial control work, it was relatively easy for me to design a control package that would work with almost any system with a little variation.

If you don't have a good understanding of HVAC controls and circuitry, this is not something you should attempt on your own. Get a pro that knows controls.

I'm sorry that the ODWB company has mislead you into believing that this is a DIY project.

joepie61

Thanks so much! When you buy something everybody makes it seem so easy. So what if I just disconnect the ac thermostat wires and the gas furnace thermostat wires from the board and just connect the wood boiler thermostat for the season? If the wood boiler fails for any reason the gas could be reconnected in minutes. But, how would I get the fan furnace to work at both speeds when the wood boiler thermostat calls for heat?

Ironman

How about posting some pics and a diagram of how you've got the hydronic side of this hooked up ?