# Air to air heat pump makes lights dim



## Dennis Alwon (May 9, 2009)

I have had a similar problem with a house I wired 15 years ago. They never had an issue until the hvac guys installed a new heat pump. They installed soft starts and did everything possible. They final changed the unit and the problem went away.

Distance from the trany to the house can also be an issue if the wires are not sized properly. I would try getting the HVAC guys to install a soft start and see what happens.


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## JVCB (Jan 2, 2012)

The hvac company says it starts on low 11 amps and on high is around 18 amps. Is this the same as slow start? They also said that certain light bulbs and dimmer switches will make it worse. Ever heard of that? thanks


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## McClary’s Electrical (Feb 21, 2009)

JVCB said:


> I had an air to air heat pump installed for the first time in a house I built in Southern VT. Every time the heatpump comes on the lights in the house dim. I had my electrian go back through and check all connections. I have been in touch with the power company and they came in installed a larger transformer on the pole stating they have had this problem before and this helped, which it did help but did not anywhere solve the problem. There is 200 amp service going to the barn/guest house with 400mcm 75' long then 350mcm about 200' to the house from the barn. Is the 350mcm not big enough? This is what the HVAC contractor sent me
> 
> "We stopped and checked the heat pump. Things look good equipment wise. The unit is two stage and starts on low speed which runs around 11 amps. On high it was running around 18 amps. These are small numbers. You can check with the electrician and CVPS. The lights are on dimmers to begin with which will make any voltage fluctuation show up more.
> 
> ...


What size is the transformer? What size is the poco drop? How many houses on that transformer?


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## Dennis Alwon (May 9, 2009)

JVCB said:


> The hvac company says it starts on low 11 amps and on high is around 18 amps. *Is this the same as slow start? *


No... Soft start-- here is a wiki explanation



> A *motor soft starter* is a device used with AC electric motors to temporarily reduce the load and torque in the powertrain of the motor during startup. This reduces the mechanical stress on the motor and shaft, as well as the electrodynamic stresses on the attached power cables and electrical distribution network, extending the lifespan of the system.[2]
> Motor soft starters can consist of mechanical or electrical devices, or a combination of both. Mechanical soft starters include clutches and several types of couplings using a fluid, magnetic forces, or steel shot to transmit torque, similar to other forms of torque limiter. Electrical soft starters can be any control system that reduces the torque by temporarily reducing the voltage or current input, or a device that temporarily alters how the motor is connected in the electric circuit.
> Electrical soft starters can use solid state devices to control the current flow and therefore the voltage applied to the motor. They can be connected in series with the line voltage applied to the motor, or can be connected inside the delta (Δ) loop of a delta-connected motor, controlling the voltage applied to each winding. Solid state soft starters can control one or more phases of the voltage applied to the induction motor with the best results achieved by three-phase control. Typically, the voltage is controlled by reverse-parallel-connected silicon-controlled rectifiers (thyristors), but in some circumstances with three-phase control, the control elements can be a reverse-parallel-connected SCR and diode.


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## wildleg (Apr 12, 2009)

did the hvac guy check the amperage on the feeder or just the unit ? I suspect that there is more than an 18 amp draw (do you have a supplemental duct heater and other equipment coming on at the same time ? check the total draw on the feeder)


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## JVCB (Jan 2, 2012)

mcclary's electrical said:


> What size is the transformer? What size is the poco drop? How many houses on that transformer?


 I tried to get in touch with the power company today to see if they have an expert technician of sorts that could help diagnose this problem but they are closed for the day because yesterday was a holiday. I would have to ask them these questions. I know construction but nothing about electrical but since my electrician seems to be incompetent I am taking it upon myself to try to figure this problem out. I will ask this when I speak to them. Thanks


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## JVCB (Jan 2, 2012)

Dennis Alwon said:


> No... Soft start-- here is a wiki explanation


Thanks, I will check this out as an option.


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## randas (Dec 14, 2008)

Life as normal in my area. 6-8 houses on a 15kva and #1 drops for 200a services. Thats how our poco rolls. My lights dim BAD when my dryer starts.


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## JVCB (Jan 2, 2012)

wildleg said:


> did the hvac guy check the amperage on the feeder or just the unit ? I suspect that there is more than an 18 amp draw (do you have a supplemental duct heater and other equipment coming on at the same time ? check the total draw on the feeder)




I believe he checked and the total draw was a lot. 144 something he said. Can't remember the correct terminology. There is no supplemental duct heater but the fan for this unit is pretty big and I believe this is on when the heat pump comes on.


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## JVCB (Jan 2, 2012)

The unit in question is a Lennox XP21 (50w41). The hvac contractor keeps saying that lights on dimmers will show the surge a lot worse. Is this true?


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## McClary’s Electrical (Feb 21, 2009)

JVCB said:


> The unit in question is a Lennox XP21 (50w41). The hvac contractor keeps saying that lights on dimmers will show the surge a lot worse. Is this true?


 
A light on a dimmer is not 100% even when it's all the way up. But how it reacts to power sag would be a guess to me. I don't think he's right, but have no way to prove it.


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## Dennis Alwon (May 9, 2009)

JVCB said:


> The unit in question is a Lennox XP21 (50w41). The hvac contractor keeps saying that lights on dimmers will show the surge a lot worse. Is this true?


I have never heard this but it could be true. Easy enough to find out remove dimmer temporarily and see if it makes a difference.


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## oldtimer (Jun 10, 2010)

Dennis Alwon said:


> I have never heard this but it could be true. Easy enough to find out remove dimmer temporarily and see if it makes a difference.



Should the heat pump (hvac) not be on a dedicated circuit? :blink:


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## Dennis Alwon (May 9, 2009)

oldtimer said:


> Should the heat pump (hvac) not be on a dedicated circuit? :blink:


I am assuming it is but it can still dim lights on other circuits.


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## Sparky J (May 17, 2011)

Check the minimum and maximum ampacities on the nameplate of the heat pump and see what size circuit wiring, breaker, and fuses (if any at DC). What you may find is the unit was wired for the minimum size and needs a bigger circuit.
Just a suggestion.
Also check for loose connections and maybe different light bulbs in the lights with dimmers? Sometimes people do use the dollar store cheepie ones.


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## oldtimer (Jun 10, 2010)

Dennis Alwon said:


> I am assuming it is but it can still dim lights on other circuits.


 I realize that, because often when a piece of equipment, lets say with a motor, or something that draws a high current on start-up kicks in, then lights may dim momentarily.

If they remain dim while the equipment is energized, *then* you have a problem!

I am sure you know what I am trying to say Dennis! 

Maybe you can word it better!


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## Dennis Alwon (May 9, 2009)

I didn't think the OP meant that the lights stayed dim. I was assuming that they came back to full brightness after the unit started. 

If it is an underground service it may be that one conductor is going bad and causes a drain on the service. I have seen a utility wire go bad after 2 weeks in the ground. Sometimes it doesn't take long.


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## McClary’s Electrical (Feb 21, 2009)

Sparky J said:


> Check the minimum and maximum ampacities on the nameplate of the heat pump and see what size circuit wiring, breaker, and fuses (if any at DC). What you may find is the unit was wired for the minimum size and needs a bigger circuit.
> Just a suggestion.
> Also check for loose connections and maybe different light bulbs in the lights with dimmers? Sometimes people do use the dollar store cheepie ones.


 
A bigger wire to the heat pump would likely make the problem worse.


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## mbednarik (Oct 10, 2011)

We had the same problem with a geothermal heat pump. The REC here lets us put electric heat on a separate service for different rates. The only thing common was the transformer between the two services and it still dimmed the lights on the main service. I vote a soft start would stop this. Utility wouldn't do anything but give the HO a bid for a separate trans.


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## Awg-Dawg (Jan 23, 2007)

mcclary's electrical said:


> A bigger wire to the heat pump would likely make the problem worse.


 
How is that?


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## McClary’s Electrical (Feb 21, 2009)

Awg-Dawg said:


> How is that?


 
lower impedence, it's gonna actually flow even more power and dim the lights worse.


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## kbatku (Oct 18, 2011)

JVCB said:


> I believe he checked and the total draw was a lot. 144 something he said. Can't remember the correct terminology.


That is probably the Locked Rotor Amps, right?


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## JVCB (Jan 2, 2012)

I am looking into whether I can get a slow start for this unit and making sure it does not void the warranty


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## JVCB (Jan 2, 2012)

mcclary's electrical said:


> lower impedence, it's gonna actually flow even more power and dim the lights worse.


Could this possibly mean that the original wire was sized to big?


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## hardworkingstiff (Jan 22, 2007)

If it were my house, I would get my electrician to confirm what the peak amp draw was on startup. If the high load is 18-amps and you are getting a 144-amp peak amp load on startup, I'd be looking at the HVAC equipment.


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## JVCB (Jan 2, 2012)

Dennis Alwon said:


> I have had a similar problem with a house I wired 15 years ago. They never had an issue until the hvac guys installed a new heat pump. They installed soft starts and did everything possible. They final changed the unit and the problem went away.
> 
> Distance from the trany to the house can also be an issue if the wires are not sized properly. I would try getting the HVAC guys to install a soft start and see what happens.


 I had my HVAC guy look into a slow start for this unit and he found one that works with this unit and does not void the warranty. He ordered it and will install. When I read through the specs it says that it will lower the start up amps by up to 75%. It says that it is actually good for the long term life of the compressor as well. This seems like a very good option and I hope it works. Thanks for the great idea.


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## brian john (Mar 11, 2007)

Seems like I just posted this.

Light flicker (dimming and restoration of light to near pre-flicker level) of lights, usually a result of motor starting and the resulting inrush current is a byproduct of several factors and generally is difficult to over come.

AC motor loads (in particular compressors) but all motor loads have inrush currents. These inrush currents result in voltage drop in the branch circuit, panel bus, service laterals, utility transformer and possibility the utility HV feeders.

Flicker is in addition user dependent; some people are more susceptible to flicker. For some reason flicker seems to bother women more that men (based upon personal experience).

A Voltage Drop (VD) as little as 3 volts on a nominal 120 VAC system is noticeable (by me), 2.5% VD.

We have had cases were the VD was all a byproduct of primarily single phase 120 VAC loads, in these cases we were able to put the lighting on L1 and all the motor loads on L2, the HVAC compressors were something the homeowner had to live with. This minimizes the flicker.


To test for this VD and resulting flicker to determine the source of the inrush, use a min max amp clamp and min max multimeter at the main service watch the lights(it may take two workers) and correlate to the meters.

1. Is this whole house?
2. Or one or more circuits?
3. Perform the test noted above.
4. If it is a circuit measure FOP from the panel to the lamp with a min/max meter watching for flicker.
5. Try to determine what has changed in the last few months, new motors?
6. Measure FOP across the line to load on the panel with a resistive loan on the circuit cycling the resistive load


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## JVCB (Jan 2, 2012)

Follow up - The slow start was finally installed by the hvac company. Problem solved. No more lights dimming. Thanks for all the great suggestions.


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