Forget Multistage, Get a VFD
An inverter and variable frequency drive (VFD), lets heat pump compressors run on minimal power. These special motors can scale up or down as needed. Think of a multi-speed motor, but instead of 3-5 speeds it has 35-70 different speeds. High-efficiency heat pumps save energy – and thereby money – by using only the power they require.
Inverters make the magic happen
They do this by using an electrical inverter and various controllers. Instead of 220 volts, the inverter enables the unit to apply 90 volts, or whatever permutation is necessary. Think of a gas pedal. Yes, you could drive in 1st gear and just floor it, but that makes no sense! The electronic inverter steps the power down to conserve energy.
A secondary benefit to the inverter technology is an ability to “clean” the power. Most people probably don’t realize that electricity can – for lack of a better term – be dirty. Dirty power, like water, wrecks the appliances it flows through. It’s one of the reasons capacitors go bad especially after lightning storms. However, our techs have yet to replace a single capacitor on an inverter system, and we suspect it is large due to the power rectifying abilities of the inverter.
VFD – lets demystify this term a bit
Drive obviously refers to the motor that powers a fan or a compressor in an HVAC system. The variable frequency part points out that such a motor can get moving with less than a full 230 volts applied. Not only that, but it can seamlessly speed up or slow down depending on the frequency of the power fed to it.
The HVAC system is the largest consumer of electricity in a home. A properly setup system that hardly ever operates at full capacity can make a huge difference in your energy consumption. Now, a “properly setup system” doesn’t just involve a shiny new HVAC system. Windows, doors, and insulation play a big role in ensuring your satisfaction with a heat pump.
This smart technology sizes itself to the home
HVAC educators have bludgeoned decades upon decades of techs with one rule: You must size air conditioners perfectly. There’s lot of good reasons and for simply cooling this rule still applies. However, when an air conditioner is reversed and is required to heat a home, the rule alters as well.
Heating a home requires 2-3 times more BTU’s simply because we’re altering the temperature in the home 2-3 times further than we do in the summer.
If y
our maximum cooling capacity is 24,000 BTUs you’ll never heat a home that needs 54,000, especially when cold weather degrades overall capacity.
From the graphic of the performance data above you can see that both the cooling capacities and the heating capacities have a range. Figure 1 shows a 3 ton system operation about 10% less than a full 36,000 BTUs. Also evident is a minimum capacity of under 3/4 ton of cooling.
While inverters and VFDs let these unit size down, they cannot go the other way. The rated capacity must supply all the wintertime heating BTUs needed.
It’s complex, but this Youtube has a great overview.