NEC 220.82(C): Is AHU supplemental heat a fixed heating system, continuous, and does Article 424 apply?

[PedroS]

We have received a comment from the building department in Florida on the load calculation for the apartment units in a new apartment building. We are using Part IV of NEC 220: Optional Feeder and Service Load Calculation.

The building department official is asserting that the supplemental electric heating in a split AC system is a continues load under 2017 NEC 220.82(C)(6) and should be taken at 100% instead of 65% per 2017 NEC 220.82(C)(4). They are using the Article 424.3(B) in the 2017 NEC to define the supplemental heat in the AHU as fixed space heating despite section 424.1 stating that Article 424 does not apply to air conditioning. The argument for ignoring section 424.1 being that when in heating mode, the AHU in the AC system would be central heating, not air conditioning, thus 424.3(B) would apply and treating the heating in the split system as fixed space heating.

The heating in the AHU dose not have it's own breaker, it is part of the AHU and thermostatically controlled. Using the building official's logic, I don't see where NEC 220.82(C)(4) or NEC 220.82(C)(5) would ever apply.

Are we interpreting NEC 220.82 correctly? Is the supplemental heat that is part of the AHU in a split AC system to be treated as a continuous load and if, when in “heating mode”, the AHU in one apartment in an apartment building is defined as central heating? Or is it part of the air conditioning and thus falls outside the scope of 424?

 

[mtnelect]

We have received a comment from the building department in Florida on the load calculation for the apartment units in a new apartment building. We are using Part IV of NEC 220: Optional Feeder and Service Load Calculation.

The building department official is asserting that the supplemental electric heating in a split AC system is a continues load under 2017 NEC 220.82(C)(6) and should be taken at 100% instead of 65% per 2017 NEC 220.82(C)(4). They are using the Article 424.3(B) in the 2017 NEC to define the supplemental heat in the AHU as fixed space heating despite section 424.1 stating that Article 424 does not apply to air conditioning. The argument for ignoring section 424.1 being that when in heating mode, the AHU in the AC system would be central heating, not air conditioning, thus 424.3(B) would apply and treating the heating in the split system as fixed space heating.

The heating in the AHU dose not have it's own breaker, it is part of the AHU and thermostatically controlled. Using the building official's logic, I don't see where NEC 220.82(C)(4) or NEC 220.82(C)(5) would ever apply.

Are we interpreting NEC 220.82 correctly? Is the supplemental heat that is part of the AHU in a split AC system to be treated as a continuous load and if, when in “heating mode”, the AHU in one apartment in an apartment building is defined as central heating? Or is it part of the air conditioning and thus falls outside the scope of 424?

The "Emergency Heat" is usually found in an Air Handler of a Heat Pump system and requires a dedicated circuit. It requires its own disconnect, within sight. Refer to the manufacturers Installation Manual for proper circuit size. Attached is an example.

 

[jaggedben]

It seems to me, especially since you used the word 'supplemental', that 220.82(C)(3) is exactly applicable to the equipment you describe. I don't see how anything in 424 is at all relevant, as 220.82(C) is rather clear by using the term 'heat pump.'

Also note that it isn't at all clear if you have to add any continuous use factors, or how you would do that, in order to implement 220.82. For an example of how gnarly that discussion can be, see here:

https://forums.mikeholt.com/threads/continuous-duty-residential-dwelling.2578707/post-2892502

It seems to me your question conflates the issue of continuous load and 100% vs. 65% in 220.82. They are separate issues, whatever one thinks about continuous load.

Again, in my opinion nothing in 424 changes that discussion.

 

[jaggedben]

Again, in my opinion nothing in 424 changes that discussion.

And 424.3 is titled 'Branch Circuits' and therefore does not pertain to service or feeder calculations.

 

[mtnelect]

You will have to do an "Engineering Heat Load Study" to determine the proper size unit. Attached is an example of a "Heat Load" study.

 

[mtnelect]

It seems to me, especially since you used the word 'supplemental', that 220.82(C)(3) is exactly applicable to the equipment you describe. I don't see how anything in 424 is at all relevant, as 220.82(C) is rather clear by using the term 'heat pump.'

Also note that it isn't at all clear if you have to add any continuous use factors, or how you would do that, in order to implement 220.82. For an example of how gnarly that discussion can be, see here:

https://forums.mikeholt.com/threads/continuous-duty-residential-dwelling.2578707/post-2892502

It seems to me your question conflates the issue of continuous load and 100% vs. 65% in 220.82. They are separate issues, whatever one thinks about continuous load.

Again, in my opinion nothing in 424 changes that discussio

The continuous non continuous question is not relevant, since the manufacturer has already taken that into consideration.

 

[suemarkp]

I think determining what is meant by "nameplate" rating for the heat strips helps to determine what to use in the service calculation. Typically, a heat strip will have a nameplate MCA value. This includes a 125% continuous factor. If you use that, times 240V, times the 65% heating demand factor i think that would be reasonable.

If the nameplate also has a watts rating, that is going to be 25% less than MCA times volts.

This sounds like a central heating heat pump, so the 65% factor seems correct. But do you use watts or MCA times volts?

 

[jaggedben]

I would use the nameplate amps times volts. 220 wants VA not watts.

 

[PedroS]

The "Emergency Heat" is usually found in an Air Handler of a Heat Pump system and requires a dedicated circuit. It requires its own disconnect, within sight. Refer to the manufacturers Installation Manual for proper circuit size. Attached is an example.

I believe I may have mistaken the terminology, it is not the emergency heat for a heat pump, it is not a heat pump. It is the heater (heat strips) in an AHU - part of a regular split system, not heat pump. Both the CU on the roof and the AHU in the unit have disconnects as well as their own circuits and breakers per code. The CU would not be running when the heater is functioning, it is all thermostatically controlled. The problem is that the bld. official/plan reviewer wants to treat it as fixed space heating instead of as part of the AC system. It's frustrating to say the least.

 

[PedroS]

It seems to me, especially since you used the word 'supplemental', that 220.82(C)(3) is exactly applicable to the equipment you describe. I don't see how anything in 424 is at all relevant, as 220.82(C) is rather clear by using the term 'heat pump.'

It's not a heat pump, just a regular split with factory installed heater (heat strips). Sorry, my mistake in terminology, it's not supplemental heat. Still 220.82(C)(4), or (C)(5) if more than 4 separately controlled, applies. As you said, 424 isn't at all relevant.

 

[mtnelect]

I believe I may have mistaken the terminology, it is not the emergency heat for a heat pump, it is not a heat pump. It is the heater (heat strips) in an AHU - part of a regular split system, not heat pump. Both the CU on the roof and the AHU in the unit have disconnects as well as their own circuits and breakers per code. The CU would not be running when the heater is functioning, it is all thermostatically controlled. The problem is that the bld. official/plan reviewer wants to treat it as fixed space heating instead of as part of the AC system. It's frustrating to say the least.

Technically, all Air Conditioning Units are "Heat Pumps". An Air Conditioning Unit only is one half of a HP, and a Heating Ventilation & Air Conditioning Unit HVAC has a reversing valve to do both.

I would like to know the manufacturer and model number of the unit you are talking about.

 

[jaggedben]

It's not a heat pump, just a regular split with factory installed heater (heat strips). Sorry, my mistake in terminology, it's not supplemental heat. Still 220.82(C)(4), or (C)(5) if more than 4 separately controlled, applies. As you said, 424 isn't at all relevant.

Okay. I guess I've never seen that, so like mtelect I'm a little surprised. (If the compressor could heat as well as cool that would be a lot more efficient.) But taking your word for it...

Then I agree with your interpretation. If it's the only heating in the dwelling then it would be counted at 65% under 220.82(C)(4). I don't think 424's branch circuit requirement informs which section of 220.82(C) is used. 220.82(C)(6) seems intended for things like liquid floor heating. That said, I don't know how to prove to them it isn't continuous without further documentation.

 

[mtnelect]

2011 NEC Section 220.82(C) Dwelling Unit (2011 NEC Handbook)

"Where the heat pump-compressor and supplemental heating operate at the same time, 100 percent of the compressor load plus 65 percent of the of the supplemental is treated as the central space heating load. If the equipment operates such that the compressor cannot operate concurrently with the supplemental heating, the central space heating load is required to be based on only 65 percent of the supplemental heating load".