Systems control question

[JRHAWK9]

I'm in the process of installing my Kuuma. I plan on having a normally open motorized damper installed on the hot air duct between the Kuuma and my existing LP furnace and also a gravity damper installed right above the A coil on my existing furnace (see attached sketch). This will limit backfeeding both the Kuuma (when AC/LP heat is on) and existing furnace (when the Kuuma is on) while providing gravity airflow on the Kuuma in case of a power failure. The HVAC guy plans on powering the 110V motorized damper by the 110V LP furnace blower motor circuit.

My plan is to manually shut off (at the thermostat) my LP furnace during winter to keep it from running while my Kuuma is. If I don't, and the furnace does kick in while I'm burning wood, the motorized damper will close and this will lead to trouble. My question is this, how can I limit my existing LP furnace to only turn on when the Kuuma is not running or cool enough? I would like to make this setup fail safe and not have to worry about someone accidentally flipping a switch and turning on the LP furnace/closing the motorized damper while heating with wood which could potentially cause damage to the Kuuma. I'm thinking it would have to be something on the 24V side that would maybe cut power to the LP thermostat while the low temp sensor is "on"?

Any insight will be greatly appreciated!
thanks!

Paul

 

[JRHAWK9]

OK, I'm just thinking out loud here.

Could I take a the same 90-294Q coil I used before and wire my 110V low limit fan switch from the Kuuma to the coil contacts. Then just take the 24V AC hot (R) and break it and send one end to the hot on the 24V side of the relay and then the other end would then go onto the NC terminal on the relay. This would give power to the thermostat until the low limit fan switch on the Kuuma connects and then my LP thermostat will be inoperable and my furnace nor AC will work until the Kuuma's low limit fan switch disconnects, which means there shouldn't be enough heat left to do any damage to the Kuuma when the powered damper closes.

See attached sketch.

Am I correct is this or way off base?

thanks!

 

[STIHLY DAN]

No you are not. The low limit will make and break several times before it burns out. I would leave the lp unit powered, set the t-stat 5 or 6 degrees less than the kumma. Then when the lp t-stat calls for heat it powers the damper, and to make sure to female or child proof the system throw in a Honeywell 6021 in the ductwork so if there is still heat in it from the kumma, the damper wont open or call the lp on, until it drops to a low enough temp. you could also install a bypass duct from the supply of the kumma to the return of the lp furnace with a normally closed damper. this would pull any heat away from the kumma if the lp is running and the kumma is not quite done.