Hi HVAC Gurus,
I have a heatpump+auxiliary HVAC unit. I have a 45 kBTU pellet stove installed in my great room and for the past few years I've been running it in manual mode, adjusting the heat output based on the outdoor temperature.
I've had the W wire at the thermostat disconnected most of the time so the auxiliary won't turn on. I do this because the cheap Hunter programmable thermostat doesn't give me the flexibility I want. For instance, I cannot control the 2-degree delta it uses to decide when to activate stage-2 (aux), nor can I control the time it waits for stage-1 (heat pump) before turning on stage-2 (aux). I have a very large home and it does take a while for the heat-pump to satisfy the thermostat even when it's 40 F outside; the thermostat only waits I think 10 minutes or less before activating stage-2. As an aside, I have selected a new Honeywell thermostat that does allow the control I want, and I'll be purchasing that sometime soon.
My pellet stove has a 2-wire connection that allows thermostat control instead of manual control. It came with one of those really cheap 2-terminal mercury-bulb type thermostats. Instead I need to attach it to my main HVAC programmable thermostat, and would like it to work as stage-2 instead of the auxiliary.
Here's what I've -
Disconnected the W wire from the furnace, ran a pair of wire from the pellet stove to the furnace, connected one wire to the W wire (feeding to the thermostat), connected one wire to the R 24V wire, and finally reconnected the W wire at the thermostat. It seems to be working great.
However, I did notice something odd in the wiring at the furnace. There are two thermostat wiring cables going into the furnace; one connects to the thermostat, and the other goes outside to the heat-pump. I imagine some of the wires would be needed to signal the reversing valve and compressor for the heat-pump outside, but why do almost all of the wires seem to be connected? In particular I noticed that the W wire junction had three wires... one for the thermostat, one to the heat-pump, and one connecting to the furnace. Obviously one is needed for the thermostat and for the furnace (to activate stage-2 auxiliary), but why would a W wire need to go to the heat-pump?
Second - since the pellet stove came with one of those simple 2-terminal thermostats, I thought all the pellet stove wanted to see to activate was continuity between the two wires. So I first wired one wire to W at the thermostat, and the other to C (common/blue). However, in this configuration the pellet stove would keep cycling every 1 second from on to off when the thermostat was calling for stage-2. I thought this wire configuration is all it needed to see continuity, but I guess I'm understanding how it works wrong. When I have one wire connected to W and the other connected to R 24V, it works fine. Does someone have a thermostat signaling circuit diagram they could provide me for reference on how this works?
Thanks for everything!