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Why Do the Lights Dim When the Heat Kicks On?

A compressor pulls about six times its running current for under a second, and the lights show it.


Your heat pump starts, and for a split second every light in the kitchen sags. Then it is over. In most houses that blink is normal. The compressor in the outdoor unit takes a big gulp of electricity to get spinning, about six times what it draws once it is running, and your wiring cannot hand over that much without the voltage sagging for a moment. The thing to watch is not whether it happens, but whether it is getting deeper or lasting longer.

A big gulp, then a sip

Every motor is hardest to start from a dead stop. Think of pushing a stalled car. The first shove takes everything you have, and keeping it rolling takes a light push. A compressor is the same. Standing still, its motor lets current pour in almost unchecked. As it spins up, that current falls fast, and in well under a second it settles at its normal draw.

Here is where the lights come in. Electricity reaches your home through wires that have a little resistance of their own, and the voltage lost along a wire grows with the current flowing through it. During the gulp, the voltage at your panel drops a few percent, then springs back. Old-style incandescent bulbs show it right away, because even a small voltage dip takes a bigger bite out of their brightness. LED bulbs vary. Some hold steady through it, and some blink more than the bulb they replaced.

If you have a mini split, or an inverter heat pump of the kind common across Europe, you will not see this at all. Its compressor ramps up gently from a crawl, so there is no gulp to begin with. A gas boiler (the usual heat source in European homes) has only a small pump and fan, so it does not do it either. This blink is mostly a story about homes with a conventional outdoor unit, which means mostly US homes.

A homeowner once called me sure the heat pump was dying because the kitchen lights blinked every time it started. The blink was over in a flash and had looked the same for years. I measured the start current and it was right where the nameplate said it should be. Nothing was wrong.

Watch the blink, not the heat pump

You cannot change how much current a compressor needs to start, and you should not open the outdoor unit or the panel. What you can do is pay attention to the pattern, because the pattern is what separates normal from trouble.

  • Note what normal looks like. A dip that is over in a blink, each time the outdoor unit starts, and that looks the same month after month, is the ordinary gulp. Remember how deep it is today so you can tell later if it changes.
  • Watch for dimming that is not tied to a start. Lights that flicker while nothing is switching on, or that dim and then brighten past normal, point to a loose connection somewhere in the house wiring, often the neutral. That is an electrician call this week, not a wait-and-see.
  • Ask the neighbors. If their lights blink when your unit starts, or yours blink when theirs does, you share a utility transformer. That part of the problem belongs to the power company, and they are the ones to call.
  • Count the starts. A healthy system starts a few times an hour at most. If yours fires up every few minutes, the frequent blinks are a symptom of short cycling, and that is worth a service call on its own.
  • At the next tune-up, ask about a soft starter. It is a small device a technician wires into the outdoor unit that ramps the compressor up instead of slamming it on, and makers claim it can trim the start current by up to 75 percent. Many people fit one for generator or solar use, and quieter starts and steadier lights come with it.

The one that did worry me was a house where the blink had grown deeper over a month, and the outdoor fan hummed a beat longer before the compressor caught. The run capacitor had weakened, so the compressor was straining to start. A new capacitor brought the blink back down to a flicker.

A blink that has deepened over weeks, or lights that nearly go dark before recovering, usually means the compressor is struggling to get going, and a weak run capacitor is the common reason. A technician can measure the start current, test the capacitor and replace it in a visit. Anything involving the panel, the meter or the service wires is work for a licensed electrician or the utility, and anything inside the outdoor unit is live electrical work for a technician.

After a mini split went in at a two-story house, the owner stood in the kitchen waiting for the lights to dip the first time it ran. Nothing happened, and he asked me whether it was even on. It was. An inverter compressor simply has no gulp.

Go deeper: inrush, wire resistance and a 3 percent dip

Compressor nameplates carry two currents. RLA, rated load amps, is the running draw. LRA, locked rotor amps, is the pull at the instant of start. A common 3-ton (10.5 kW) scroll compressor lists 17.9 A RLA and 112 A LRA, a ratio of 6.3. The inrush decays as the rotor accelerates and is gone in a fraction of a second. The energy is trivial: a full half second at 112 A and 240 V is about 0.004 kWh.

The dip is current times resistance. Take 100 ft (30 m) of #2 aluminum between the utility transformer and the panel, typical of a 100 A service: 0.293 ohms per 1,000 ft, so 0.029 ohms per conductor. Both 120 V legs carry the compressor, so each sags 112 A times 0.029 ohms, or 3.3 V, a 2.7 percent dip. In 4/0 aluminum, as on a 200 A service, the dip is 1.0 V, 0.9 percent. The transformer and the drop from the pole add their share, so the dip varies house to house.

Tungsten light output scales with current to the 6.5 power and voltage with current to the 1.9 power, so brightness goes roughly as voltage to the 3.4 power. A 2.7 percent dip takes about 9 percent off an incandescent bulb; 0.9 percent takes 3 percent. IEEE 141 recommends keeping motor-start dips to 3 to 5 percent at other equipment, and utility flicker limits built on its curve allow about 3 percent for one start an hour.

A soft starter ramps the motor over a longer interval. At the claimed 75 percent reduction the 112 A step becomes 28 A, the dip falls to 0.7 percent and the bulb dims about 2 percent. An inverter compressor starts from near zero speed, so there is no step at all. Europe's 230 V appliance standards (EN 61000-3-3 up to 16 A per phase, 61000-3-11 up to 75 A) cap the voltage changes a product may cause, and its air-to-water heat pumps are inverter driven, so the blink is rare there.

How much an old-style incandescent bulb dims at compressor start, same 112 A gulp, different wiring.


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