What's a 2 stage heating system?


Senior Member
What's a 2 stage heating system? I'm looking to buy a new tstat and What I have her in the south is called a heat pack. It's a gas furnace and air conditioner all in one giant box that sits outside. It's gas heat and a regular old air conditioner I think. I want to make sure I order the correct HAI tstat.
Two-stage means that your furnace can operate at a reduced capacity, using less fuel with a lower blower speed, which distributes warm air, as opposed to a single stage furnace which operates only at 100% blowing out hot air.
What zack is describing is a two stage furnace, which is not exactly the same as generic two stage heat. A thermostat for two stage heat will normally bring on a second stage of heating if the setpoint is not reached after a certain time, or sometimes if the setpoint has been raised, such that the regular heating mode might take too much time (recevery from setback is a common example). A common use for two stage thermostats is when you have a heat pump with resistance heat strips for supplemental heat. When the temp outside is too cold for the heatpump to keep up, the thermostat adds the more-expensive-to-run heat strips to provide additionnal heat.

A two stage furnace is a furthur refinement of a two stage system whereby a single heat source can run at different capacities to improve effeciency. An ideal heating system would run continuously and produce only enough heat to keep up with the heat loss of the home. Frequent heating cycles (called short cycling) is ineffecient. Many times, such systems use a specific type of thermostat to detect small changes in heat load and then modulate the furnace to compensate. You can now get gas furnaces that can vary their heat output almost continuously to achieve the highest effeciency. They operate like a servo system, getting error input from the thermostat to control their output.
And then, when you move into a new house, you discover the HVAC is wired up so that heat stage 1 and heat stage 2 are jumpered together... It always runs on overdrive!

And when you go investigating, you find that the thermostat cable is one conductor short. Both the controller and heater have 2-stage support, but there are not enough wires. And then you find the cable is anchored to the wall studs in the *only* place in the entire damn house where it is just about impossible to run extra wire to without opening up drywall. I've been sorely tempted to tie new wire onto one end and give the other end The Mother Of All Tugs to see if I can break the anchors free and pull through better cable, but haven't been brave enough yet. The consequences of breaking the cable are even worse.... On the plus side, the location of the thermostat is currently the worst possible place in the house and losing wiring access would force its relocation.. aargh.

I've been meaning to switch over to something like a RCS system where the HVAC controller is right there at the heater, and the thermostat head unit is on a small 4-wire link. That way I can have as many conductors between the controller and the heater as I want.

But the geek in me wants to use a relay IO card and/or RS485 and computer control it directly. :) I have some spare soekris boxes just begging to be used for this.