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Modulating equipment refers to any equipment that can vary its capacity in order to meet a demanded load.
Furnaces basically modulate the burner based on the input from the thermostat. The gas valve receives a PWM(pulse width modulation) from the control board and then opens the valve to a certain point. This gives us only the btus we need.
Modulating or Inverter AC or heat pumps ramp the compressor up and down in order to meet the demand given by the thermostat. They also ramp fans and to help out with this.
Basically its "cruise control" for your heating and cooling system. Its better to cruise than to floor it and slam on the brakes.
The real deal behind this modulating mystery is all in the design. When you get a load calculation done, the btus for heating and cooling are done at what we call design temperatures. In SW Michigan we are looking at design temps of 87 outdoor in cooling and 72 degree indoor cooling, then 7 degrees for heating outdoor and indoor of 70 degrees.
Typically we are not really at design temps for most of the year, especially in winter months. So what happens when we have single stage or even two stage equipment set up for design temps? What happens between the two extremes? It basically comes down to the equipment is now oversized for the space.
Single and Two Stage Equipment Downfalls
Single stage equipment is used a lot. Mostly because is hook up and go. No complicated setups or complicated thermostats or weird control boards. Two stage equipment is more or less the same. Still used by quite a few people as well. With all of that to say, the downfall of this equipment is all in the design load of the spaces we are putting this equipment in.
As we talked about design, our btu need, in heating for example, at 7 degrees we may need all of a 60kbtu furnace. But at 20 degrees we may only 20k of that. Single stage equipment is then going to short cycle, even the two stage will short cycle on 30 degree days. Ac will be like this also, but on the other end of temperature extreme.
What is so bad about that? You might ask. Well lets talk about the science of short cycling the furnace first. When you short cycle a furnace you end up doing more damage than you think. In a short cycle, the heat exchanger fails to warm up to design temperatures before shutting off. This causes undue stress because its going on and off and expanding and contracting a lot. The other thing to consider with short cycling furnaces is that we end up killing the heat exchanger from the inside out. This is due to acids being created during the combustion process. There is a substance called mercaptan, it is the stuff that gives gas a smell. This is a sulfur compound. So not even counting anything else being brought into the furnace we will have sulfurous and sulfuric acid. These acids drop out at a higher temp like 300 degrees, so when heat exchangers never warm up we keep laying layers of acid down. This eats away and soon enough we have a heat exchanger full of pin holes. Then we have the issue of comfort. If you do not stay on long enough to get walls and objects warmed up, the space will not feel comfortable. As soon as furnace shuts down you will immediately feel the cold and the furnace will have to start again soon.
In air conditioning mode we also have issues. Short cycling ac prevents us from dehumidifying like we should. This is due to the coil not getting all the air in the space through the coil. We will end up with cold and damp conditions. Not only that but the compressor shutting on and off all the time will also bring up your energy bill.
How can modulating equipment help?
So, the best answer we have for a lot of these issues is going to be modulating equipment. Modulation in heat lets us get to low fire rates on order to keep the furnace running in times of warmer outdoor weather. This keeps walls and surfaces up to temp, therefore creating a comfortable environment. This also lets the equipment run and run for longer intervals, which will maximize longevity of the product. Modulation in cooling allows the compressor to ramp down and not be at 100% all the time. This stops the short cycling, and then increases comfort by being able to dehumidify a lot more.
