Furnaces (regardless of whether or not they are standard or high efficiency) are available in single stage, two stage, and fully modulating configurations. But what does this actually mean? With a natural gas furnace, this is referring to how the valve that controls the flow of natural gas to the appliance operates, and how it affects the comfort level in your home.
Single Stage Furnaces – These furnaces should be familiar to most people, and are how most furnaces have operated for decades. Single stage just means that your furnace is on or off. The gas valve is either opened or closed, creating heat or not. It’s that simple! Typically, this allows for a degree or so of temperature fluctuation in your home as the thermostat has to drop, call for heat, then the furnace comes on at full fire, brings it up to temperature, and shuts back down.
Two Stage Furnaces – These models are very similar to a single stage units, but think of them as having a ‘high’ and a ‘low’ fire mode. When the thermostat tells the furnace that your home needs heat, it will kick on in a low fire mode, which uses less gas, and creates less heat. If the system realizes that this is not enough heat it will adjust the timing of when it needs to switch to its high fire, full on mode, and ramp up. The idea here is that unlike the single stage models, this unit will have a smaller temperature swing in your home, increasing comfort levels. Typically, these units do not cost much more than single stage units and the benefits can be noticeable.
Modulating Furnaces – These take the two stage concept up a level. All this means is that instead of a high and low fire mode, the gas valve is fully variable (meaning it has an even greater range of options). This is often marketed as the most comfortable option (referring to temperature fluctuation within the home). This is all well and good, but we really don’t sell these models unless they are specifically requested. The reason for this is simple, the cost vs. benefit does not usually make sense. Modulating furnaces are a huge leap up in price (unlike the gap between single and two stage models) for a much smaller increase in comfort.