I am trying to calculate the air unit portion of a heat pump for service size calculation. My local code guru is saying to add the KW rating of the heat package plus the motor. Per article 220.51 he is saying it should be taken at 100 percent. However, the engineer is saying that it is not a fixed Electrical Space Heating and therefore needs to be taken at 125 percent based on continuous duty. What do you guys think?
How to i calculate Heat Pump supplemental heat for service size.
- Thread starter Vmadden
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- Placerville, CA, USA
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It should be treated the same way as a simple forced air electric furnace in terms of circuit ampacity. For overall load calculation you may or may not have to add in both the heat package and the heat pump condenser, depending on the design of the unit.
Note that 220.51 only directly addresses the feeder circuit part of the load calculation.
I find it hard to understand what the engineer thinks the equipment is if it is not Fixed Electric Space Heating. Note also that the heat package will not be a continuous load but the fan motor might be considered to be by an inspector.
I am talking overall load calculation, not branch circuit. The heat package is a much greater load than the condenser, I believe for load calculations that cancels out the condenser since in this case the condenser does not run if the heat package is called for. I would agree for the largest motor should be increased by 125 percent. Not sure about the engineer, but his calculation is putting us past 400 amp and causing and issue with the service size.
The heat pumps I have seen, have the compressor and heat on at the same time. The compressor is providing heat until you reach
a set temperature. When that temperature is reached, the strip heat come on. Yours maybe different.
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- Placerville, CA, USA
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- Retired PV System Designer
That does not make a lot of sense in terms of my experience unless by "reach a set temperature" you mean drop low enough rather than get high enough.
The supplemental heat will usually come on under two distinct conditions:
1. The differential between the room temperature and the set point of the thermostat is more than a specified number of degrees. The assumption is that you need heat faster than the HP (heat pump) can provide it. This is why setting back the thermostat of a heat pump at night is usually not an energy saver!
2. The cold side (heat source) temperature of the system is so low that the HP is not contributing anything more than the energy used by the compressor motor.
Under condition 2, there is no reason to run the HP and the heat at the same time. It is a waste of system life and inefficient to boot.
Under condition 1, there may be a justification for running both in terms of heating fastest, but some units will lock the compressor out anyway in this case.
Under condition 1, as the room temperature gets close enough to the set point the heat pack is turned off (and the compressor is turned on if it is not on already.)
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- Connecticut
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- Engineer
220.51 has nothing to do with whether the heating load is continuous or non-continuous. It only says that there shall not be any demand factor applied for fixed electric space heating loads, unless permission is granted by the AHJ.
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- Placerville, CA, USA
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- Retired PV System Designer
Actually 220.82 does not say A + B +C.... It says to use only the larger of
100% cooling or
100% heat pump + 65% heat pack if not interlocked or 65% of heat pack only if interlocked or
100% nameplate rating of heat pump (normally including the fan even if it is not part of a separate unit.)
That should be even less than the 100% of heat pack plus fan that you have been trying to use!
Since the words "heat pump" appear explicitly in that section, I think the inspector may be persuaded to yield on that point.
Good find, Npstewart!
The section does not include any limitations such as having to calculate parts separately if they are on different circuits. The calculation applies to the system as a whole.
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- Placerville, CA, USA
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- Retired PV System Designer
424.3(B) refers to the branch circuit calculation of ampacity only. The service load calculation is entirely within Article 220 in the case of heating and A/C.
The cooling and the heating are the same thing and the same load on a heat pump. What I am saying is rather than examining the circuitry on the unit, just use the worse case scenario.
I design mechanical systems as well as electrical. Typically the first stage of a heat on a heat pump is the heat pump itself where the refrigerant system reverses. The second stage is electric heat and they call this supplemental heat because it is supplementing the heat pump. Usually I will put a small heat strip equal to about 5 kW to help the heating cycle during cold snaps. Usually the heat strip has LESS capacity then the heat pump itself. For instance, a 3 ton heat pump will have a capacity of 36.0 kbtu/h of heating capacity whereas a 5 kW heat strip has a capacity of 17.1 kbtu/h. In order to have a supplemental heat capacity of 36 kbtu/h you would need a 10 kW heat strip but why would someone put a 10 kW heat strip in the heat pump unless it ran at the same time as the heat pump?? In other words, the electric heat is supplemental and runs at the same time.
So your worse case would be:
100% of the fan (because it runs all the time)
100% of the cooling/heating (because they are the same thing)
65% of the supplemental heat (because it can run at the same time at the cooling/heating.
If you do this, then you are covered.
I design mechanical systems as well as electrical. Typically the first stage of a heat on a heat pump is the heat pump itself where the refrigerant system reverses. The second stage is electric heat and they call this supplemental heat because it is supplementing the heat pump. Usually I will put a small heat strip equal to about 5 kW to help the heating cycle during cold snaps. Usually the heat strip has LESS capacity then the heat pump itself. For instance, a 3 ton heat pump will have a capacity of 36.0 kbtu/h of heating capacity whereas a 5 kW heat strip has a capacity of 17.1 kbtu/h. In order to have a supplemental heat capacity of 36 kbtu/h you would need a 10 kW heat strip but why would someone put a 10 kW heat strip in the heat pump unless it ran at the same time as the heat pump?? In other words, the electric heat is supplemental and runs at the same time.
So your worse case would be:
100% of the fan (because it runs all the time)
100% of the cooling/heating (because they are the same thing)
65% of the supplemental heat (because it can run at the same time at the cooling/heating.
If you do this, then you are covered.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
In cold enough areas that the heat pump (particularly air source) may not work at all under really cold conditions, the standard design is for the heat pack to be larger than the HP. It is then called Auxiliary or Emergency heat rather than supplemental and is only used under the two conditions I mentioned in my post.
As long as the HP output is sufficient to heat the house, there is no reason to add "supplemental" heating unless you are willing to pay more per BTU to heat faster.
But sometime the HP will not be working at all or will not be getting the same multiplier of heat out to power in, and those are the exact conditions where you need the most heat output of the whole year. Also, when the outside air temp gets close to or below 32 degrees, you have to worry about the outside coils frosting up.
The heating efficiency of a heat pump decreases drastically as the temperature difference between the hot side and the cold side increases. For a given heat pump, you will finally reach the point where putting the same amount of electricity into a heat strip will get your more BTU inside than running the compressor would. (worst case: when the hot side coil is no warmer than the inside air.)
YMMV.
Also, what "nameplate" value are you using. If looking at the heat pump, the MCA will already include the 125% value required for the compressor. For the fan unit, the nameplate will have 125% of the heat strip (because this is a circuit sizing value) plus the controls and 125% of the fan (largest motor). So any continuous factor should have already been covered.
Unless you're digging through the thing looking for FLA values, heat strip KW, etc, I would think the intent is to use the MCA values. Some things aren't on the label (e.g. the furnace controls -- you can figure it out based on the fan FLA, heat pump RLA, using the 125% factor, and anything left is probably the controls).
Another reason to have a large heat strip is the "emergency heat" function. You use this when the compressor dies, which it will at some point (and on either the hottest day of the year or the coldest...). Do you really want only 20K BTU of heat available when the system probably requires 48K or more? If I could do it over again, I would have done my 4 ton heat pump with 15KW of aux heat instead of the 20KW that was installed -- make the strip and HP BTU values about the same.
Unless you're digging through the thing looking for FLA values, heat strip KW, etc, I would think the intent is to use the MCA values. Some things aren't on the label (e.g. the furnace controls -- you can figure it out based on the fan FLA, heat pump RLA, using the 125% factor, and anything left is probably the controls).
Another reason to have a large heat strip is the "emergency heat" function. You use this when the compressor dies, which it will at some point (and on either the hottest day of the year or the coldest...). Do you really want only 20K BTU of heat available when the system probably requires 48K or more? If I could do it over again, I would have done my 4 ton heat pump with 15KW of aux heat instead of the 20KW that was installed -- make the strip and HP BTU values about the same.
Gregg Harris
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- Virginia
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- Electrical,HVAC, Technical Trainer
Determine the load for the air handler and what ever heat package is installed using 220.51. Then determine the load for the heat pump/ condensing unit and then apply 22.60 and ignoring the smalest load.
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- Placerville, CA, USA
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OK: Let's look at some of the logic involved and see where we arrive:
1. 220.3 states that specific load related code sections listed in Table 220.3 can modify the general procedures in the rest of 220.
2. Fixed Electric Space Heating is listed in the Table and refers to 424.3 for Branch Circuit sizing only. We are working with a service calculation, so we will ignore that for now.
3. Part IV, section 220.80 describes an alternate method (not 100% of continuous and 125% of continuous) which can be applied. If you take this route you have to follow IV completely rather than picking and choosing which way to treat specific loads.
4. Specifically, you apply 220.82(B) to get the general load and then apply 220.82(C) to get the Heating and A/C load. No other rules apply if you take this alternate method.
5. The calculation for 220.82(C) comes out a lot smaller than the part III calculation for the heat pump case we are looking at. You do not go circuit-by-circuit, you do not go motor by motor inside the heating-A/C system, you just follow (C). Any other miscellaneous contributions are assumed to be covered by the number in (B).
6. The question the OP has to answer for himself is whether the calculated number in section (B), when added to the smaller number he gets to use in (C) for the heating, etc, is smaller or larger than the part III number. It is likely that it will be, but only he has the information to determine that.
7. Note that the list of loads in (B)(3) will NOT include the heat pump or any parts of the heating system if it consists of multiple units.
8. If the OP instead goes with the Part III, then the Heating load will have to be calculated according to the rules in part III. That may be as simple as 100% of continuous and 125% of non-continuous, or there may be more details related specifically to heating and heat pumps that have to be taken into account. That will be the subject of another post if I feel it is important. Or somebody else can do that. Or it can be an exercise for you.
This illustrates why the Alternate Method is just that, an alternate method to be used as a complete replacement for the part III method. This causes a good bit of confusion among those who are first doing service calculations.
If you like the number you got using part III, there is no obligation to calculate the part IV number, but if you are trying to save money on a bid or are up against a service size crunch, it is a good idea to do it.
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Perhaps I missed it, but I have yet to see the OP'er indicate the occupancy type. The alternate method discussed thus far only applies to a dwelling unit... emphasis on "unit". He did state service, so if a multifamily dwelling then 220.84 could apply, but states only the larger of A/C or heat, essentially same as Part III. The other possible alternate methods cover schools and new restaurants, both having a general demand factor table and do not cover environmental air treatments specifically.
I will not speculate further on the alternate method, lacking pertinent information, e.g. all parameters influencing 200.82(C) load determination.
*****
General thread comments:
A heat pump is used (typically) for both A/C and heating. 200.60 will not apply. And unless somehow made portable, fixed electric space heating requirements apply.
Mentioned previously, the heat pump likely has an MCA, so 125% for the compressor is already included.
125% continuous vs. 100% noncontinuous factoring is not a part of Article 220 load calculations. However, separation during calculation is necessary to properly apply conductor sizing and ocpd rating requirements of Articles 210, 215, and 230.
PS: 125% continuous vs. 100% noncontinuous factoring does not apply to motor loads.
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I'm very sorry if my post was misleading or not clear. This heat pump application is for an office building not a dwelling. That being said I will be more specific with the loads. there is a quantity of 2 of these units.
Heat pump strip heater KW is 37.42 and the voltage is 208 3 phase
The motor is 14 amps at 208 single phase.
The air unit with strip heater and motor has a 152 amp M.C.A.
The actual compressor has a M.C.A. of 102 amps
The HVAC contractor has told me the compressor does not run at the same time as the strip heater (emergency heat). Baltimore MD area, it does get pretty cold in the winter.
The engineer is using the M.C.A. for the indoor unit in his calculations. I have been told this is not right. Take 100 percent of the 2 strip heat KW ratings, then add 100 percent of the 2 motors except for 125 percent of the largest motor. This would lower the calc considerably.
Heat pump strip heater KW is 37.42 and the voltage is 208 3 phase
The motor is 14 amps at 208 single phase.
The air unit with strip heater and motor has a 152 amp M.C.A.
The actual compressor has a M.C.A. of 102 amps
The HVAC contractor has told me the compressor does not run at the same time as the strip heater (emergency heat). Baltimore MD area, it does get pretty cold in the winter.
The engineer is using the M.C.A. for the indoor unit in his calculations. I have been told this is not right. Take 100 percent of the 2 strip heat KW ratings, then add 100 percent of the 2 motors except for 125 percent of the largest motor. This would lower the calc considerably.
Still confused. Please clarify...
2 heat pumps and one or two AHU's?
Strip heaters in heat pumps are 3 @ 37.42kW each or 3 @ 37.42kW total? Are these heat strips included in heat pump MCA?
Does "motor is 14 amps" refer to name plate FLA of a heat pump compressor or an AHU blower motor? If the former, nameplate FLA of compressor is irrelevant if the unit has an MCA rating. If the latter, we need motor HP, not nameplate FLA.
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If there are two heat pumps and two AHU's with MCA ratings as noted in your post, and heat pumps do not run concurrently with AHU's (with the exception of blower motor), I'm inclined to agree with engineer that the AHUs' MCA rating is to be used in the calculation.
Also, IMO heat strips in a heat pump unit are a waste of energy when it is not the major heat source.
kwired
Electron manager
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- NE Nebraska
When the source gets colder there is less heat to absorb, meaning less heat is pumped through the system, this also means unit has less load and draws less current. NEC does not recognize this fact, but it is what happens. I have seen compressors rated around 20 amps RLA only drawing 3-4 amps when it is cold outside, the BTU output is likely lower than the energy input - it does take a certain amount of input just to run the compressor even if it is doing little work. The better HVAC guys around here will have an outdoor thermostat that locks out compressor operation when it is too cold for it to be very efficient. Also outside temp doesn't need to be below freezing for coils to freeze up, the coil only needs to be below freezing temp. I have not seen an air to air unit that doesn't go through defrost cycle of some sort. Simplest way to defrost is to reverse the unit - pump some heat from inside back to outside to defrost the outdoor coil. Some do run aux heat during defrost, and some people will insist this means compressor and aux heat run at same time increasing the load calc. I personally don't think the duration of this defrost cycle is that significant in most cases, but that is only my opinion.
On the hottest day of the year when the compressor dies, you really will not care how much electric heat is installed
You said in there that aux heater and compressor do not run at same time, so how can you justify using both for calculating max load on a feeder or service? Use only the larger of the two.
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