Dual fuel HP as back up - HVAC Site

milk man · 09-24-2009, 10:42 PM

Lennox XP19 & G61MPV.

On nat. gas well, free gas for life.

HO wants HP to be back up if he has well problem and is out of town.

If the well shuts down there will be an ignition failure and furnace will lock out. I think the blower will not run either.

So if I add a relay that will switch W to Y1. The coil will be controlled by a return air duct stat set for around 60 degrees. At 60 degrees (RAT) the W call to the furnace will be cancelled and this will reset the board (I hope). The W call from the stat will be sent to the Y1 on the furnace board.

So R to W (on stat) to C (on relay) to NO (on relay) to Y1 on furnace board. Lennox's Y2 call will be initiated from the defrost board below 40 degrees and a Y2 signal will be sent to the furnace's control board normally.

beenthere · 10-31-2009, 08:15 AM

Could use a LP low pressure safety switch to control a relay.
If the gas pressure drops to X", it drops power to the relay, and the relays NC contacts close and send 24 volts to the Y1 of the furnace board.

That way, the house doesn't need to frop in temp to bring on the heat pump.

milk man · 11-01-2009, 11:47 AM

Thanks for the reply.

I don't remember exactly what I did. I did test my work by adjusting the RAD's stat. I know I ended up using only one relay.

I also wanted to catch stuff like bad igniters and pressure switch faults.

I turned the gas stop off and let the furnace lock out. (I think flame sense failure). Then simulated a temperature drop in the RAD. The HP and started and the blower ramped up. But the circuit board did not reset/clear the no flame fault.

beenthere · 11-01-2009, 12:23 PM

As long as it works.

milk man · 11-01-2009, 04:08 PM

Quote:

Originally Posted by beenthere

As long as it works.

Yes, a mark in the W column is all that counts at time.

mechanicalDvr · 11-02-2009, 11:40 AM

I would power a small RIB relay from the furnace lock out relay so as soon as the natural gas locks out the heatpump is energized. This way the house temp has very little swing.

milk man · 11-02-2009, 08:49 PM

But they want to know when it happens.