Generator Load Shed Clarification

M

marmathsen

Senior Member
Location
Seattle, Washington ...ish
Occupation
Electrical Contractor
-Equipment-
Generac Generator - 26kW Guardian G00729-0
Generac Meter/ATS - 200A RXUW200A3
Carrier 1-stage Heat pump - 25HCE430A300
Carrier Furnace - 59TN6B060C17114

I need to tie in the above heat pump into the built in load shedding of the meter/main. I'm not well versed in HVAC equipment and I was surprised to see 7 wires in the heat pump j-box (not all are shown in the diagram below).

Am I correct that I can just tie into the red 24V conductor and run it in/out of the SACM in the meter main for the load shedding? Below is a diagram of what I'm envisioning.

1773163848515.png
 
Sequence of OperationTurn on power to indoor and outdoor units. Transformer is energized.CoolingOn a call for cooling, thermostat makes circuits R–O and R–Y, and R–G.Circuit R–O energizes reversing valve, switching it to cooling position.Circuit R–Y energizes contactor, starting outdoor fan motor andcompressor circuit. R–G energizes indoor unit blower relay, startingindoor blower motor on high speed.When thermostat is satisfied, its contacts open, de–energizing contactorand blower relay. Compressor and motors should stop.If indoor unit is equipped with a time–delay relay circuit, the indoorblower will run an additional 90 seconds to increase system efficiency.HeatingOn a call for heating, thermostat makes circuits R–Y and R–G. CircuitR–Y energizes contactor, starting outdoor fan motor and compressor.Circuit R–G energizes indoor blower relay, starting blower motor onhigh speed
Click to expand...

It sounds like the yellow is the one to interrupt since it energizes the contactor in both heat and cool. Interrupting the red may or may not have the desired effect, depending on how it actually works. You could also interrupt the brn/yel, that would stop the compressor but no telling if it would make the control board unhappy in some way.
 
Birken Vogt said:
Sequence of OperationTurn on power to indoor and outdoor units. Transformer is energized.CoolingOn a call for cooling, thermostat makes circuits R–O and R–Y, and R–G.Circuit R–O energizes reversing valve, switching it to cooling position.Circuit R–Y energizes contactor, starting outdoor fan motor andcompressor circuit. R–G energizes indoor unit blower relay, startingindoor blower motor on high speed.When thermostat is satisfied, its contacts open, de–energizing contactorand blower relay. Compressor and motors should stop.If indoor unit is equipped with a time–delay relay circuit, the indoorblower will run an additional 90 seconds to increase system efficiency.HeatingOn a call for heating, thermostat makes circuits R–Y and R–G. CircuitR–Y energizes contactor, starting outdoor fan motor and compressor.Circuit R–G energizes indoor blower relay, starting blower motor onhigh speed
Click to expand...
Birken Vogt said:
It sounds like the yellow is the one to interrupt since it energizes the contactor in both heat and cool. Interrupting the red may or may not have the desired effect, depending on how it actually works. You could also interrupt the brn/yel, that would stop the compressor but no telling if it would make the control board unhappy in some way.
Click to expand...

Thank you for the info!

Where is this information from?

Rob G
Seattle
 
qcroanoke said:
We always interrupted the hot of the secondary of the control xfmr at the xfmr.
Click to expand...
hillbilly1 said:
I think the op is wanting to run the gas backup heat, instead of running the heat pump compressor. Not lock out the entire system.
Click to expand...

On fossil heat/normal AC I interrupt the compressor contactor wire because it is easy and a known quantity.

On heat pumps I agree with qcroanoke, I interrupt the red at the control xfmr at the main circuit board, usually in series with the 3 amp fuse, it kills everything. Air handler, heat strips, and compressor.

I don't want a call back because something I didn't think of has cause the system to think there is an error because one part got locked out while the rest stayed running, and also because the heat strips and compressor will both be caught by this one wire.

Normally the duration of a load shed lockout is short enough and infrequent enough that the customer would not be bothered by it anyway.
 
hillbilly1 said:
I think the op is wanting to run the gas backup heat, instead of running the heat pump compressor. Not lock out the entire system.
Click to expand...
Birken Vogt said:
On fossil heat/normal AC I interrupt the compressor contactor wire because it is easy and a known quantity.

On heat pumps I agree with qcroanoke, I interrupt the red at the control xfmr at the main circuit board, usually in series with the 3 amp fuse, it kills everything. Air handler, heat strips, and compressor.

I don't want a call back because something I didn't think of has cause the system to think there is an error because one part got locked out while the rest stayed running, and also because the heat strips and compressor will both be caught by this one wire.

Normally the duration of a load shed lockout is short enough and infrequent enough that the customer would not be bothered by it anyway.
Click to expand...

I like this approach. I think I'll do this. Thanks everyone!

Rob G
Seattle
 
marmathsen said:
I like this approach. I think I'll do this. Thanks everyone!

Rob G
Seattle
Click to expand...
To follow up on what Hillbilly said.
If your customer wants to run the emergency backup heat (gas or fuel oil not electric). This may also shutdown your indoor air handler.
Back in the day they would use one xfmr for both units.
I must say I never did do low voltage residential control wiring but it seems that was one way it was done.
 
I did not see if the OP stated it has gas or oil for backup heat, or is electric.

If it is gas or oil, I might be tempted to use the load shedding output to *close* pink to pink inside the outdoor CU. Tricking it into thinking the coil is frosted. This should cut off the compressor and kick in the backup heat source. This would take an inverting relay.

Thing is, most generators these days are 20 kw or better. Even a 14 kw can run a 5 ton compressor no problem along with other things. So it would be a rare event to actually cut off the compressor for load shedding. Load shedding is mostly done to pass plan check and make the inspector go away. So the easiest and cheapest and least failure prone way to do it is usually best. Not getting all fancy and generate a callback or 3.
 
Birken Vogt said:
If it is gas or oil, I might be tempted to use the load shedding output to *close* pink to pink inside the outdoor CU. Tricking it into thinking the coil is frosted. This should cut off the compressor and kick in the backup heat source. This would take an inverting relay.
Click to expand...
See I did not think that through enough. Actually tricking the unit into thinking it's frosted would not work. That would just reverse the refrigerant and keep the compressor going.

That is why I like to break the red at the AHU. No muss no fuss. Shut the whole thing down for the duration of the overload, after it comes back all will be well.
 
qcroanoke said:
To follow up on what Hillbilly said.
If your customer wants to run the emergency backup heat (gas or fuel oil not electric). This may also shutdown your indoor air handler.
Back in the day they would use one xfmr for both units.
I must say I never did do low voltage residential control wiring but it seems that was one way it was done.
Click to expand...
Birken Vogt said:
Thing is, most generators these days are 20 kw or better. Even a 14 kw can run a 5 ton compressor no problem along with other things. So it would be a rare event to actually cut off the compressor for load shedding. Load shedding is mostly done to pass plan check and make the inspector go away. So the easiest and cheapest and least failure prone way to do it is usually best. Not getting all fancy and generate a callback or 3.
Click to expand...

In this application I know the generator is 26kW andlarge enough for the heat pump. I just want to tie it in to the load shedding out of caution in case the heat pump wants to start right when other appliances decide to start. I figure worst case scenario the HVAC system is down for 5 minutes to stabilize the frequency.

Rob G
Seattle
 
Birken Vogt said:
I did not see if the OP stated it has gas or oil for backup heat, or is electric.

If it is gas or oil, I might be tempted to use the load shedding output to *close* pink to pink inside the outdoor CU. Tricking it into thinking the coil is frosted. This should cut off the compressor and kick in the backup heat source. This would take an inverting relay.

Thing is, most generators these days are 20 kw or better. Even a 14 kw can run a 5 ton compressor no problem along with other things. So it would be a rare event to actually cut off the compressor for load shedding. Load shedding is mostly done to pass plan check and make the inspector go away. So the easiest and cheapest and least failure prone way to do it is usually best. Not getting all fancy and generate a callback or 3.
Click to expand...
I looked up the model number the op gave, it said gas.
 
marmathsen said:
In this application I know the generator is 26kW andlarge enough for the heat pump. I just want to tie it in to the load shedding out of caution in case the heat pump wants to start right when other appliances decide to start. I figure worst case scenario the HVAC system is down for 5 minutes to stabilize the frequency.

Rob G
Seattle
Click to expand...
I can understand that, but I would not bother.

Modern scroll compressors are not hard to start like (very) old reciprocating compressors, they don't need a 5 minute downtime.

The thermostat or controls themselves may also have a delay timer built in, so if you add another 5 minutes, you get 10 minutes total.
 
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