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#18325 - 12/08/02 05:54 PM Stumped on a Heat Pump  
bobp  Offline
Joined: Oct 2002
Posts: 70
Puyallup, WA, USA
Newer home (2000). Lights dim when heat pump kicks in. There are two 200A panels. Panel A is general lighting and the gas furnace (15A), furnace can not engage when heat pump does, so it's allowed to be seperate. Panel B (not a sub-panel)is heat pump, dryer, oven etc. SE are 4/0 AL XHHW-2 for the ungrounded conductors and 2/0 XHHW-2 AL for the grounded. NOLOX on all AL connections. All neutrals are tight. Neutral and grounds are properly bonded and grounded. The heat pump has a 50A breaker in Panel B with a 60A fused disconnect at the outside unit. Wired with #8 stranded CU from panel to disconnect and same from disconnect to unit. Even if nothing in Panel B is on and only a couple of lights in Panel A are on, the lights will dim when the pump kicks on. What am I missing? Everything appears to be properly sized, connected, etc. There are no other reported or found problems in the home. It also has a large jetted tub (7 ' x 6', 30" deep), when the pump motors are turned on, it does not affect any lights or anything else. It is in Panel B (same as the heat pump). I suspect something in the pump circuit, but what?

Any ideas appreciated.

Work Gear for Electricians and the Trades

#18326 - 12/08/02 05:59 PM Re: Stumped on a Heat Pump  
Wirenuttt  Offline
Joined: Nov 2002
Posts: 267
First thing that comes to mind is the neutral being sized down to 2/0. Maybe a full size neutral

#18327 - 12/08/02 06:04 PM Re: Stumped on a Heat Pump  
bobp  Offline
Joined: Oct 2002
Posts: 70
Puyallup, WA, USA

Thought of that. But (even tho allowed by NEC) if that was the problem, wouldn't the lights dim when the jetted tub pumps are on? There are two large pumps plus htrs on this thing and it's in Panel B with the heat pump and does not affect the lights. This is why I lean to the heat pump circuit. But, been wrong before.


#18328 - 12/08/02 06:22 PM Re: Stumped on a Heat Pump  
sparky  Offline
Joined: Oct 2000
Posts: 5,311
i suspect some 'flicker factor' ,
what is the motor size and distance to the serving x-former? also KVA of X-former?

#18329 - 12/08/02 06:22 PM Re: Stumped on a Heat Pump  
ElectricAL  Offline
Joined: Oct 2001
Posts: 597
Minneapolis, MN USA

How far away is the transformer? Are there other houses connected to it also? Any with large loads?


Al Hildenbrand

#18330 - 12/08/02 06:26 PM Re: Stumped on a Heat Pump  
The Watt Doctor  Offline
Joined: Dec 2001
Posts: 435
Mont Belvieu, TX
Most air conditioning systems that I know of can have a "hard start" kit installed to keep the lights from dimming on start. I wonder if the heat pump (being an air conditioner with the "cycle" reversed) would benefit.
If you think about it. Any motor, when started is a "dead short" across the line. So, for the split second before the rotor comes up to speed, there is no difference of potential between the 2 phases. This "dead short" is what causes the lights to dim.
Well, this so called "hard start kit", which I'm sure has capacitors in it, is supposed to "relieve" that problem.

Good Luck,

[This message has been edited by The Watt Doctor (edited 12-08-2002).]

The Watt Doctor
Altura Cogen
Channelview, TX

#18331 - 12/08/02 06:55 PM Re: Stumped on a Heat Pump  
bobp  Offline
Joined: Oct 2002
Posts: 70
Puyallup, WA, USA
xformer about 50 ft from home. No other homes on transformer. Pump motor is 1/5 HP. No other large loads. I started everything on Panel B (dryer, double oven, jetted tub, vacuum system, pulled the top off the septic and got the grinder pump going, 10 HP compressor in the garage) and nothing dims the lights, except that dang heat pump. I know about the cap set up for compressor motors, hadn't thought about it, I'll look into that, might be the answer. But, back to the main question, why only the heat pump? This one has me stumped!


#18332 - 12/08/02 07:04 PM Re: Stumped on a Heat Pump  
electric-ed  Offline
Joined: Jul 2002
Posts: 175
A few questions -
Has the light dimming always occured, or has it started recently?

Does the dimming only last for a second or two while the motor is starting, or do the lights stay dim until the motor stops?

Is the dimming severe enough to affect the operation of other appliances. For example, does it cause "shrinking" of a TV picture
when the motor starts?

Some degree of dimming when a motor starts is normal and not a hazard. Dimming is due to the motor's starting inrush current, which can be from 3 to 6 times the normal running current.

If the dimming (or voltage drop) can be observed (measured) on branch circuits supplied from both panels, it indicates that the voltage drop that is causing it is occuring on the line side of the service equipment, that is, in the transformer or service drop conductors.


[This message has been edited by electric-ed (edited 12-08-2002).]

#18333 - 12/08/02 07:14 PM Re: Stumped on a Heat Pump  
maintenanceguy  Offline
Joined: Dec 2001
Posts: 300
Southern NJ, USA
A hard start kit is a capacitor with a potential relay. The cap is used only for starting and when the voltage measured across the compressor supply reaches a certain potential, it takes the cap out of the circuit. These are really used to help start a compressor that's having a hard time starting but will help current draw marginally.

It may be that this heat pump has an expansion valve that seats fully when the system shuts off. That means that the high pressure side doesn't bleed it's pressure off to the low pressure side. Of if the compressor is starting too soon after it shuts off, the pressure hasn't had time to equalize.

Starting the compressor against 250lbs pressure draws a lot of current.

A bleed type expansion valve can be added or if short cycling (starting too often) is the problem, the t-stat differential setting can be changed or a delay timer can be added to the heat pump.

If you're drawing unusually high starting current, suggest they have an hvac guy come and see what he can do.

#18334 - 12/08/02 07:42 PM Re: Stumped on a Heat Pump  
arseegee  Offline
Joined: Jan 2002
Posts: 324
Statesboro, GA USA
There is a good chance that the power company is to blame for that one. Have you checked the voltage coming into the panels?

Check it with no load first and then turn on the heat, appliances and the lights and see what kind of drop you have.

One of the POCO's I deal with will feed a 400 amp service with 4/0 triplex trenched in 300 ft to a multi tapped pad.

[This message has been edited by arseegee (edited 12-08-2002).]

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