Including load for Electric Duct Heaters being used for Reheat.

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Sparky2791

Sparky2791

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Northeast, PA
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Electrical Design
Working on a school where Air Conditioning is being added.

I did the calcs per 220 for a school and the service is large enough just have to let Power Company know of the additional load since their transformers are feeding the building.

AC is being done by a VRF (Variable Refrigerant Flow) heat pump system and some new RTU's. The total AC load with all condensing units and RTU's is about 800A. However there is also 80KW worth of electric duct heaters which will reheat the cooled air if a thermostat in a certain area is turned up so it will kick in the duct heater and heat up the already cooled air to help deliver air that is a bit warmer to the space. (used a lot commercially but not very efficient if you ask me)

As I am designing the panels required for the new AC loads I want to also use these panels to power up the new electric duct heaters. So typically you size the panels for the larger of the 2 loads Heat or AC. But in this case the duct heaters are designed to be used during the summer months. Is there a demand the NEC would allow in this situation. I know about the 65% for heat pump auxiliary heat in the optional calculations but that is for a dwelling unit, this is a school. I hate to have to add in the full load of the 80KW of electric heat knowing there is no way all 80KW will be in use the same time the AC is running. Its 'bad' enough I am required to use 100% of the AC load knowing there will be some demand factor and all 22 Condensing units and 4 RTU's will not run at the same time, I accept that as part of the NEC's requirement for AC load.
Can anyone offer anything in the NEC that tells me I do not have to include 100% of the duct heaters as well? I went through Article 220 twice.

I am in PA, we are NEC 2014

Thanks you!
 
How far along is this project?
Electric reheat isn't just "not very efficient"; it's insane. Yes, it used to be common, way back in the day, when nobody cared about energy consumption. But to install it today is borderline criminal. How did this ever get approved? Did nobody do an energy-cost estimate? Did nobody look at on-peak demand charges?
 
drcampbell said:
How far along is this project?
Electric reheat isn't just "not very efficient"; it's insane. Yes, it used to be common, way back in the day, when nobody cared about energy consumption. But to install it today is borderline criminal. How did this ever get approved? Did nobody do an energy-cost estimate? Did nobody look at on-peak demand charges?
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I was seeking an answer to my question not a barrage of questions about the mech design. This is an electrical forum. I thought that is what this Forum was for? Saying its criminal is a little bit drastic considering some of the horrible things that actually are criminal in this world. I am not a mech engineer so I could be explaining this incorrectly (my apologies if I am) I can ensure you that we are bound by very strict energy requirements in PA and we must complete several forms that shows it passes all PA energy codes before a permit will be issued and these are than verified by the code official at time of inspection. Perhaps you could offer some useful information to the thread regarding the actual question?
 
LibertyEngineering said:
I was seeking an answer to my question not a barrage of questions about the mech design. This is an electrical forum. I thought that is what this Forum was for? Saying its criminal is a little bit drastic considering some of the horrible things that actually are criminal in this world. I am not a mech engineer so I could be explaining this incorrectly (my apologies if I am) I can ensure you that we are bound by very strict energy requirements in PA and we must complete several forms that shows it passes all PA energy codes before a permit will be issued and these are than verified by the code official at time of inspection. Perhaps you could offer some useful information to the thread regarding the actual question?
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Well if you have poor inefficient design then your calculated load will be higher than if you have a good efficient design.

What you likely do not know is what is the design for worst case scenario, and what that worst case scenario is. A well designed system should rarely need to draw 100% of rating (including any accessory/backup loads) or at least for not very long duration, but this is going to be dependent on good heating and cooling load studies. If it turns out they did undersize the system for the needs, there is possibility more gets added sometime down the line as well.
 
But unfortunately the NEC does not make allowances for any design efficiency that is not reflected in the equipment nameplate.
There is not even any basis to justify getting a PE to do a new energy use computation for the as-built system.
 
GoldDigger said:
But unfortunately the NEC does not make allowances for any design efficiency that is not reflected in the equipment nameplate.
There is not even any basis to justify getting a PE to do a new energy use computation for the as-built system.
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Correct, NEC basically requires you to assume it can/will draw rated load, though you seldom ever see a new and still in good shape compressor/system that draws rated load current.

Cooling load is usually higher on heat pumps. Ground source draws somewhat same load all the time as long as the ground source doesn't change temperature on you, maybe a high and low stage current but both will remain fairly constant when called upon. Air source heat pump, draws less the colder they outdoor temp gets, so they draw the most current on mild days but heating is still demanded. They draw less on cold days but end up with longer run times to do the same amount of heating, there is less heat available for them to collect the colder the outdoor air temp is therefore it results in less heat being "pumped" = less work for the compressor.
 
kwired said:
Correct, NEC basically requires you to assume it can/will draw rated load, though you seldom ever see a new and still in good shape compressor/system that draws rated load current.

Cooling load is usually higher on heat pumps. Ground source draws somewhat same load all the time as long as the ground source doesn't change temperature on you, maybe a high and low stage current but both will remain fairly constant when called upon. Air source heat pump, draws less the colder they outdoor temp gets, so they draw the most current on mild days but heating is still demanded. They draw less on cold days but end up with longer run times to do the same amount of heating, there is less heat available for them to collect the colder the outdoor air temp is therefore it results in less heat being "pumped" = less work for the compressor.
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Thanks for the input gentleman. after further consideration I'll be checking with mech engineer to see if this system is set up with the duct heaters for use when the air source heat pumps can no longer keep up with the heating load on those days when temps drop below the heat pumps ability to maintain temperature. Here in PA that can happen in the dead of winter especially on those nights when temps can drop into the teens and lower, like last night it was 14 degrees. Basically its auxiliary heat in that case. The next question will be if the auxiliary heat takes over do the compressors still run. Now we are talking more the 65% rule for heat pumps with auxiliary heat. Including the total compressor load along with 65% of the auxiliary heat load. If the compressors shut down and we go strictly electric heat than its the standard heat or AC load whichever is larger. I am determined to get to the bottom of this!
 
LibertyEngineering said:
Thanks for the input gentleman. after further consideration I'll be checking with mech engineer to see if this system is set up with the duct heaters for use when the air source heat pumps can no longer keep up with the heating load on those days when temps drop below the heat pumps ability to maintain temperature. Here in PA that can happen in the dead of winter especially on those nights when temps can drop into the teens and lower, like last night it was 14 degrees. Basically its auxiliary heat in that case. The next question will be if the auxiliary heat takes over do the compressors still run. Now we are talking more the 65% rule for heat pumps with auxiliary heat. Including the total compressor load along with 65% of the auxiliary heat load. If the compressors shut down and we go strictly electric heat than its the standard heat or AC load whichever is larger. I am determined to get to the bottom of this!
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No definite answer on whether the aux heat can/will run at same time as heat pump. Sometimes they do, sometimes it is arranged so they don't.

Seen some where they have outdoor temp sensor and will lockout compessor so it won't run at all when below a certain temp, mostly because at some point they no longer have enough efficiency and possibly can't keep up with demand anyway.
 
Reheat operates at the same time as the compressor(s) during the summer, for humidity control.
It typically runs when the outdoor humidity is high and the outdoor temperature isn't.
The mechanical engineer might be able to provide a duty cycle or diversity factor for the maximum total load.
 
drcampbell said:
Reheat operates at the same time as the compressor(s) during the summer, for humidity control.
It typically runs when the outdoor humidity is high and the outdoor temperature isn't.
The mechanical engineer might be able to provide a duty cycle or diversity factor for the maximum total load.
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The mechanical designer should have a sequence of operations drawing for all the units, detailing under what conditions each piece of equipment will operate. He or she would be your best resource.
 
RE-Heat is typically used in a setting where the cooling is performing " Dehumidification " duty. Since it is necessary to keep the mechanical cooling ON in to do this the Reheat function is to keep the space from dropping below setpoint and getting the RH down. Computer Room systems such as Libert are set up this way to maintain space temp and RH in a certain range, all calculated by design.
 
StarCat said:
RE-Heat is typically used in a setting where the cooling is performing " Dehumidification " duty. Since it is necessary to keep the mechanical cooling ON in to do this the Reheat function is to keep the space from dropping below setpoint and getting the RH down. Computer Room systems such as Libert are set up this way to maintain space temp and RH in a certain range, all calculated by design.
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I found out its not for reheat it is auxiliary heat for when the VRF heat pump system can no longer maintain the temp in the building because it is too cold outside. Like Aux heat in a residential split system. I am familiar with the Liebert CRAC systems though. I've designed a few data centers
 
StarCat said:
RE-Heat is typically used ... Computer Room systems such as Libert are set up this way ...
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Um, no. Computer rooms almost never need dehumidification; they usually need humidification. Unlike people, computers produce only dry heat and don't exhale or exude any water vapor.
Back in the day, Liebert used heating elements to make pure water vapor out of impure tap water, but that's not what "reheat" means. And even when I was working there -- thirty-odd years ago now -- we were trying to phase it out to reduce energy consumption.
 
drcampbell said:
Um, no. Computer rooms almost never need dehumidification; they usually need humidification. Unlike people, computers produce only dry heat and don't exhale or exude any water vapor.
Back in the day, Liebert used heating elements to make pure water vapor out of impure tap water, but that's not what "reheat" means. And even when I was working there -- thirty-odd years ago now -- we were trying to phase it out to reduce energy consumption.
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Yes, years ago our department had a computer room with a Liebert, and we came in one moring and there was quite a bit of water on the floor outside the room. The humidifier had leaked or overflowed.
 
LibertyEngineering said:
I found out its not for reheat it is auxiliary heat for when the VRF heat pump system can no longer maintain the temp in the building because it is too cold outside. Like Aux heat in a residential split system. I am familiar with the Liebert CRAC systems though. I've designed a few data centers
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So does system design lockout the heat pump when calling for this aux heat or not?

NEC never mentions it but the compressor won't be drawing anywhere near rated load in that situation if it is running.
 
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