Electrician Talk banner
1 - 10 of 10 Posts

·
Super Moderator
Retired
Joined
·
18,225 Posts
Discussion Starter · #1 ·
Whenever my heat pump (compressor) starts it slightly dims the incandescent lights for less than a second. It is minor, but I can tell inside, anytime it cycles on. It does not affect TV, clocks, stuff like that. It seems to be the high inrush current?

This has been happening since the unit was installed 10 years ago. Never paid much attention to it, as it is no inconvenience. Yes, it does have the newer style compressor. The tall type. (Scroll or Screw)? Something like that.

Should I check all connections at the panel, condenser and evaporator and fan enclosure? Any suggestions appreciated. Thanks John
 

·
Premium Member
Joined
·
20,981 Posts
That's just how they are. It has more to do with the fact that the utility feeder and the utility transformer are very minimally sized. The addition of a "hard start kit" to the heat pump will lessen this effect to some degree.
 

·
Bilge Rat
motors and controls.........
Joined
·
9,497 Posts
A 'hard start kit' will certainly help.

When a heat pump system is called to start by the thermostat, 3 motors start at the same time. The compressor, the indoor blower, and the outdoor fan. The compressor requires by far the largest surge current, but the fans count too. In addition to hard start kits I've also installed a time delay relay on the compressor contactor. 10 seconds or so will allow the fans to come up to full speed, so their surge isn't seen.

I wouldn't put the delay on the fans for two reasons. First, there are two of them, and only one compressor. Installation is simpler. Second, if the relay fails to start the compressor, you simply have non-conditioned air blowing around. If either of the fans fail, and the compressor runs.....trouble.
 

·
R.I.P. 2014
Joined
·
4,722 Posts
A 'hard start kit' will certainly help.

When a heat pump system is called to start by the thermostat, 3 motors start at the same time. The compressor, the indoor blower, and the outdoor fan. The compressor requires by far the largest surge current, but the fans count too. In addition to hard start kits I've also installed a time delay relay on the compressor contactor. 10 seconds or so will allow the fans to come up to full speed, so their surge isn't seen.

I wouldn't put the delay on the fans for two reasons. First, there are two of them, and only one compressor. Installation is simpler. Second, if the relay fails to start the compressor, you simply have non-conditioned air blowing around. If either of the fans fail, and the compressor runs.....trouble.


You can't put a time delay on a condensing unit compressor that would let the outdoor fan run as it is powered by the compressor contactor. No compressor no fan. And you can't put a fan control on a heat pump outdoor unit as in the heat mode you would never have enough head pressure to start the fan.
For the most part a scroll compressor never needs a hard start kick either.

Do you understand how a heat pump works?
 

·
Modérateur
Joined
·
8,692 Posts
I don't and am interested to learn. How does a heat pump work?
Basically the Heat Pump work the same as the central or window shaker A/C run but if need the heat it will run in " reverse " by diverting valve to change the flowage on the freon pipe.

But a catch with Heat Pump there is a archavie heel [ weak link ] is during very cold weather they dont crank out much heat so therefore they do installed back up heat it can be either electric or fuel fired furance.

with electric back up heating in there it will really screw up the load demand figures if not carefull typically AFAIK most resdential unit the back up heat can go high as 20 KW some case more. so watch out that one.

Merci, Marc
 

·
R.I.P. 2014
Joined
·
4,722 Posts
I don't and am interested to learn. How does a heat pump work?
On a call for cooling:
(The reversing valve relay is energized in cooling mode by the thermostat subbase "system" switch.)

The indoor fan starts, the compressor starts, the outdoor fan starts. The hot freon leaves the compr. goes to the outdoor coil gives off heat and condenses to liquid and goes to the thermal expansion valve indoors and flashes back to gas at the indoor coil picking up heat and taking it back outdoors. The condensate water on the indoor coil runs into a pan and is carried away via the condensate drain.

On a call for heating:
(The reversing valve relay is not energized in heating mode by the thermostat subbase "system" switch.

The indoor fans starts, the compressor starts, the outdoor fans starts. The hot freon leaves the compressor goes to the indoor coil gives off heat and condenses to liquid and goes to the thermal expansion valve outdoors and flashes back to gas at the outdoor coil picking up heat and taking it back indoors. The condensate water on the outdoor coil runs off the coil until it freezes up and blocks airflow, at which time the unit will go into the defrost mode (initiated by loss of airflow across the outdoor coil or by outdoor coil temperature). The outdoor fan is stopped, the reversing valve relay is energized, the indoor strip heat relays are energized, the hot freon is sent to the outdoor coil defrosting it (defrost is either terminated by time or by outdoor coil temperature). When defrost is terminated the reversing valve relay is deenergized, the outdoor fan contacts in the defrost board are made and the units are back in normal heating modes. If the heat discharge temperature in the indoor unit becomes too low the strip heat realys will be energized bringing on the contactors, on some units the third stage of heat on the thermostat can also bring on the strip heat relays.

I hope this shows why you can't just add fan delays and head pressure controls to a heat pump.
 

·
Registered
Joined
·
5 Posts
Heat pump fan control

A head pressure fan control can be added to a heat pump. But you must also add a relay that causes the fan control to be active only during cooling mode. I use a relay with normally closed contacts in parallel with the head pressure control. The relay coil is in most cases connected to the "O" terminal of the outdoor unit. On most, but not all, units that is the signal for cooling operation.
 

·
Proud Navy Veteran
Joined
·
1,429 Posts
Great explaination RK. But now in my case the theory is the same but more controls 4 zones with a 2 speed compressor and variable speed air handler. Carrier Infinity system installed in my house. At least that is one thing I don't have to work on for 10 years (warranty). It's nice working for a Carrier dealer.
 
1 - 10 of 10 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top