Shedding heat

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frofro19

Senior Member
Location
VA.
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Master Electrician
This is for the generator guys. Is there any way to shed back-up heat strips when using a back-up generator when running low voltage wire to the air handler isn't possible? I have used SMM's in the past but with that, it would shut down the back-up heat strips as well as the heat pump itself. I know low voltage wire would be best but if the air handler is on the 2nd floor, it would be nearly impossible to run that wire from the ATS to the second floor without cutting out some drywall possibly.
 
I'm not familiar with the internal wiring connection for the backup heat strip on the heat pump, but the SMM have direct inline capacity without needing control wiring to the ATS. Can you install the SMM directly inline of the backup heat strip only? Or is that wiring part of the listed product and not field connected?
 
Single circuit, no.
Two or more circuit yes as long as all heat strips are on separate branch circuit(s). Then a load shed can be installed. Or a manual load shed( turn off breaker(s).
Some times you can find extra lv wire in stat and use them as well.
 
Tulsa Electrician said:
Some times you can find extra lv wire in stat and use them as well.
Click to expand...
I was going to suggest ideas along this line. Often there are unused T-stat wires. Also, newer systems usually include a common wire that is grounded, and could be re-purposed by using the EGCs for the common.
 
Fred B said:
I'm not familiar with the internal wiring connection for the backup heat strip on the heat pump, but the SMM have direct inline capacity without needing control wiring to the ATS. Can you install the SMM directly inline of the backup heat strip only? Or is that wiring part of the listed product and not field connected?
Click to expand...
That's a good idea. Not sure, but I may check with an HVAC guy to see if that's possible.
 
Tulsa Electrician said:
Single circuit, no.
Two or more circuit yes as long as all heat strips are on separate branch circuit(s). Then a load shed can be installed. Or a manual load shed( turn off breaker(s).
Some times you can find extra lv wire in stat and use them as well.
Click to expand...
What do you mean by two or more circuits? The extra lv wire is also a good option but unfortunately the heat pumps are not close to the ATS either.
 
LarryFine said:
I was going to suggest ideas along this line. Often there are unused T-stat wires. Also, newer systems usually include a common wire that is grounded, and could be re-purposed by using the EGCs for the common.
Click to expand...
Do they make any kind of t-stat wires that could be run in conduit buried in the ground? That would probably be my only option other than installing a 100 amp SMM in the attic for the heat strips.
 
frofro19 said:
Do they make any kind of t-stat wires that could be run in conduit buried in the ground?
Click to expand...
Absolutely. Any wet-rated standard wire type, like THHN/THWN from 14ga as small as 18ga if you can find some. That's what I use for generator control wiring.
 
frofro19 said:
What do you mean by two or more circuits? The extra lv wire is also a good option but unfortunately the heat pumps are not close to the ATS either.
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There is maybe some chance the ATS is close to the loadcenter though. If so you can use contactors and interrupt some of the heat circuits at/near the origin of the branch circuits.
 
kwired said:
There is maybe some chance the ATS is close to the loadcenter though. If so you can use contactors and interrupt some of the heat circuits at/near the origin of the branch circuits.
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The panel is directly behind the meter and I plan on putting the ATS beside of the meter. In this case there are 2 double pole 60 amp breakers, I'm assuming for 10 kw heat in the unit in the attic and the unit in the basement. My concern is, if I cannot get a wire ran to the attic and there are no extra lv wires in the outdoor units, I would have to install the 100 amp SMM'S. There's no back-up heat, other than the heat strips, and if the power was off for a week in single digits, that would not be good. The problem with contractors is that a single 6/2 romex wire is probably feeding the unit in the attic and the contact would shut the whole unit down.
 
When there is more than one circuit.
You use a branch circuit for the air handler and a branch circuit (s) for the back up heat. This allowed load shedding with back up generation.
I do this when we install back up generation. They do a lot of geothermal which also has back up heat. So we take a small branch circuit for the air handler and compressor and than additional circuits for back up heat.
With a heat pump you can take a circuit just for the air handler and addition circuits for the back up heat. Then you can just turn off the back up heat according to what your generator will run.
With an air handler you can either us the outside condinsor or she'd it and use back up heat.
This way you have options. You can do it manually or automatic depending on client. Additional disconnecting means and a little rework at the unit and your done.
Using LV you can control the heat strips by interrupting the contactors in the unit. You can also set up a generator panel and put what loads you want in that along with shedding. Tons of options.
 
frofro19 said:
The panel is directly behind the meter and I plan on putting the ATS beside of the meter. In this case there are 2 double pole 60 amp breakers, I'm assuming for 10 kw heat in the unit in the attic and the unit in the basement. My concern is, if I cannot get a wire ran to the attic and there are no extra lv wires in the outdoor units, I would have to install the 100 amp SMM'S. There's no back-up heat, other than the heat strips, and if the power was off for a week in single digits, that would not be good. The problem with contractors is that a single 6/2 romex wire is probably feeding the unit in the attic and the contact would shut the whole unit down.
Click to expand...
If it is only 10 kW chances are you are shutting the entire thing down anyway , without making control modifications. Unless this is a heat pump and you just want to lock out the emergency heat then you still need air handler power and need to intervene with controls.
 
Tulsa Electrician said:
When there is more than one circuit.
You use a branch circuit for the air handler and a branch circuit (s) for the back up heat. This allowed load shedding with back up generation.
I do this when we install back up generation. They do a lot of geothermal which also has back up heat. So we take a small branch circuit for the air handler and compressor and than additional circuits for back up heat.
With a heat pump you can take a circuit just for the air handler and addition circuits for the back up heat. Then you can just turn off the back up heat according to what your generator will run.
With an air handler you can either us the outside condinsor or she'd it and use back up heat.
This way you have options. You can do it manually or automatic depending on client. Additional disconnecting means and a little rework at the unit and your done.
Using LV you can control the heat strips by interrupting the contactors in the unit. You can also set up a generator panel and put what loads you want in that along with shedding. Tons of options.
Click to expand...
Unfortunately this is an existing house and we are just quoting a price for the Genny install. Panel is in a finished garage so no good way to run a wire to the air handler in the attic. Not a good way to run a wire to the basement air handler as well. My only hope is to possibly run LV wires to the outdoor units and hope they have extra wires that are not being used and use those to control the heat strips. Otherwise I would have to use the 100 amp SMM and shut the whole system down. And I'm assuming it wouldn't shut the whole system down unless it's too much on at the time the heat is trying to come on?
 
I had another thought: How abut a simple latching relay at the air handler, for just the heat strip, that will drop out upon power loss, and have to be manually reset. You could run a low-voltage wire to a button by the attic door.
 
If you just quoting an gen install than make it clear no additional work. Exclude and make owner awear of why. Be fare about it.
This way your competive and may just win you the job.
 
Tulsa Electrician said:
If you just quoting an gen install than make it clear no additional work. Exclude and make owner awear of why. Be fare about it.
This way your competive and may just win you the job.
Click to expand...
If the load shedding is part of the scope of what is needed, anyone not bidding that is going to lose $$$ on that portion of the bid, possibly be challenged by owner if they try to claim they didn't bid what was specificied or just end up ripping off the owner (to some extent) if they just pay the extra without further questions, but that can also give the contractor bad reputation if too many complaining about such "rip offs".

If there is no detailed scope of work for bidding, you probably better write your own description of what will or will not be performed, or what may be options.
 
frofro19 said:
The panel is directly behind the meter and I plan on putting the ATS beside of the meter. In this case there are 2 double pole 60 amp breakers, I'm assuming for 10 kw heat in the unit in the attic and the unit in the basement. My concern is, if I cannot get a wire ran to the attic and there are no extra lv wires in the outdoor units, I would have to install the 100 amp SMM'S. There's no back-up heat, other than the heat strips, and if the power was off for a week in single digits, that would not be good. The problem with contractors is that a single 6/2 romex wire is probably feeding the unit in the attic and the contact would shut the whole unit down.
Click to expand...
When that is the case, I have the customer use a couple of portable heaters at marked locations (to balance load and not on the same circuits) that way they have some heat, but not overload the generator.
 
hillbilly1 said:
When that is the case, I have the customer use a couple of portable heaters at marked locations (to balance load and not on the same circuits) that way they have some heat, but not overload the generator.
Click to expand...
That's a great idea. I'll mention that to the owner if we get the job.
 
frofro19 said:
Unfortunately this is an existing house and we are just quoting a price for the Genny install. Panel is in a finished garage so no good way to run a wire to the air handler in the attic. Not a good way to run a wire to the basement air handler as well. My only hope is to possibly run LV wires to the outdoor units and hope they have extra wires that are not being used and use those to control the heat strips. Otherwise I would have to use the 100 amp SMM and shut the whole system down. And I'm assuming it wouldn't shut the whole system down unless it's too much on at the time the heat is trying to come on?
Click to expand...
Instead of a 100 amp SMM, use a low voltage one, and just break the white wire at the air handler, which would lockout the heat. Would be much cheaper. If the a/c is also too much, break the red wire.
 
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