Load factor for HVAC with zoning

[Electriman]

I am working on a project that HVAC units are zoned with VAVs. So basically, this means one zone can have heat on and other ones can have cool on at the same time. Both heat and cooling is electric. Am I still allowed to use either cooling or heating to calculate load demand or should I add both loads to find out my demand load?

 

[david]

I am working on a project that HVAC units are zoned with VAVs. So basically, this means one zone can have heat on and other ones can have cool on at the same time. Both heat and cooling is electric. Am I still allowed to use either cooling or heating to calculate load demand or should I add both loads to find out my demand load?

it sounds like within each zone it will be heat or cool but not both

if that's true than apply the larger of the heating load or the cooling load for the building

 

[MAC702]

Is this some type of multi-head mini-split or something else that uses a common heat pump? Each zone is still only able to do one OR the other.

 

[Electriman]

Is this some type of multi-head mini-split or something else that uses a common heat pump? Each zone is still only able to do one OR the other.

This is a giant AHU 160 tons that cools a building with more than 20 zones. Each zone has the ability to heat independent of other zones.

 

[david]

This is a giant AHU 160 tons that cools a building with more than 20 zones. Each zone has the ability to heat independent of other zones.

it sounds like you are saying that both heating and cooling loads will be on at the same time.

if so than the larger load would not cancel out the smaller load and should be designed according to the actual load conditions

 

[MAC702]

This is a giant AHU 160 tons that cools a building with more than 20 zones. Each zone has the ability to heat independent of other zones.

Would the cooling power be a fixed load no matter how many zones are using it at the time?

The heating is provided by a different machine completely?

 

[__dan]

There are different ways of having a giant AHU and 20 VAVs. You could have one giant cooling compressor for the AHU, a cold deck in the giant AHU or, a more modern or efficient system may have a common warm water loop circulating between the 20 VAVs, and each VAV has its own heat pump compressor for local zoned (modulating demand response) heat or cooling.

You suggest electric heating as if it may be resistance type units. Could be set up as electric resistance backup or second stage heat, but if its primary electric heat, I would be looking to see it that's a problem. Modern system could have electric heat pumps at each VAV, with an air to water HX to the common shared warm water loop. With central plant supplemental heating and cooling equipment sitting on the warm water loop as necessary.

Supplemental primary central heat you would want gas (methane). Otherwise the VAV mounted heat pumps could cool and dump the heat where it is needed, or to a central chiller.

Your question pertains to the equipment topology and if the auto control system has a mode where everything runs (at full) at the same time. It's not clear what topology you could have.

 

[Electriman]

There are different ways of having a giant AHU and 20 VAVs. You could have one giant cooling compressor for the AHU, a cold deck in the giant AHU or, a more modern or efficient system may have a common warm water loop circulating between the 20 VAVs, and each VAV has its own heat pump compressor for local zoned (modulating demand response) heat or cooling.

You suggest electric heating as if it may be resistance type units. Could be set up as electric resistance backup or second stage heat, but if its primary electric heat, I would be looking to see it that's a problem. Modern system could have electric heat pumps at each VAV, with an air to water HX to the common shared warm water loop. With central plant supplemental heating and cooling equipment sitting on the warm water loop as necessary.

Supplemental primary central heat you would want gas (methane). Otherwise the VAV mounted heat pumps could cool and dump the heat where it is needed, or to a central chiller.

Your question pertains to the equipment topology and if the auto control system has a mode where everything runs (at full) at the same time. It's not clear what topology you could have.

Yes it is resistance heat.
There is a possibility that AHU with its chillers run all year around. I came to conclusion that I need to add the electric heat with load of HVAC load.

 

[MAC702]

...There is a possibility that AHU with its chillers run all year around.....

But does it run at a constant load or is its load dependent on the number of rooms that are demanding it? Those rooms would not be using heat.

 

[Electriman]

But does it run at a constant load or is its load dependent on the number of rooms that are demanding it? Those rooms would not be using heat.

No it does not. It adjust the speed to provide required supplied air.

 

[kwired]

But does it run at a constant load or is its load dependent on the number of rooms that are demanding it? Those rooms would not be using heat.

No it does not. It adjust the speed to provide required supplied air.

That is a reality that NEC doesn't really take into consideration.

During times of peak design cooling demand, it is likely there is little if any heating load, and same goes the other way.

If the system is designed for efficiency input will decrease as output needs decrease on both heating and cooling systems.

If at that sweet spot where you have 50-50 heating and cooling needs chances are input of each system is near 50-50 as well. If a system can't be varied, it will likely at least cycle as needed, and/or have fixed stages of different load levels.

Anything that would draw full rating of heating and cooling at same time would be very inefficient and unlikely - but NEC sort of doesn't recognize that.

 

[MAC702]

... but NEC sort of doesn't recognize that.

But does the AHJ?

 

[kwired]

But does the AHJ?

If going by NEC no, about only way NEC sees this situation as possible is to have history of demand data for existing loads.

I still think if you take larger of heating or cooling load you likely will be OK most of the time when it comes to whether or not you will actually overload anything.

 

[pjones]

If the unit has the option of heating with a heat pump and uses electric heat as backup heat then both the heat pump and electric heat will energize when the heat pump goes into its defrost cycle. If the majority of the building is calling for heat then this could create a full cooling and full electric heat scenario for the full duration of the defrost period.




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[kwired]

If the unit has the option of heating with a heat pump and uses electric heat as backup heat then both the heat pump and electric heat will energize when the heat pump goes into its defrost cycle. If the majority of the building is calling for heat then this could create a full cooling and full electric heat scenario for the full duration of the defrost period.




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I understand that, but personally feel that the time of the defrost cycle is probably short enough that it doesn't cause much potential overloading issue. For one thing you must have quite a bit of other loads that were factored into your load calculation running at same time, if those aren't running at the time you have extra capacity already when defrost occurs, plus the heat pump compressor likely is not pulling full RLA.

 

[david]

But does the AHJ?

It is unlikely the inspector (AHJ ) is going to ask , these issue need to be resolved by the designer during load calculations.

If the designer cannot apply Noncoincident loads than design accordingly

220.60 Noncoincident Loads.
Where it is unlikely that two or more noncoincident loads will be in use simultaneously, it shall be permissible to use only the largest load(s) that will be used at one time for calculating the total load of a feeder or service.

 

[Chamuit]

It is unlikely the inspector (AHJ ) is going to ask , these issue need to be resolved by the designer during load calculations.

If the designer cannot apply Noncoincident loads than design accordingly

220.60 Noncoincident Loads.
Where it is unlikely that two or more noncoincident loads will be in use simultaneously, it shall be permissible to use only the largest load(s) that will be used at one time for calculating the total load of a feeder or service.

I agree with David. That's where I'd go.

 

[pjones]

I understand that, but personally feel that the time of the defrost cycle is probably short enough that it doesn't cause much potential overloading issue. For one thing you must have quite a bit of other loads that were factored into your load calculation running at same time, if those aren't running at the time you have extra capacity already when defrost occurs, plus the heat pump compressor likely is not pulling full RLA.

The compressor is trying to melt a block of ice by using its hot gas. To do this the system usually compensates in some way to make a high discharge temperature. It should be expected that the compressors will operate at their normal amperage at that time.

How long do they need to run together before it is considered simultaneous operation?

What other options are you banking on being urned off? Dehumidification - depending on the complexity of the system this can often work by running air conditioning at the same time as electric heat (if not using hot gas reheat)....
humidifier- can be required due to long cooling cycle removing too much moisture or from heating cycle causing the relative humidity to drop and/or pulling in dry outside air....
ERV wheel - even large ones typically uses very little amperage

Another consideration, unless they are mechanically locked out, then the service or repair technician may use the controls to run all options at the same time. Although this might not be considered normal operation and be overlooked.


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[kwired]

The compressor is trying to melt a block of ice by using its hot gas. To do this the system usually compensates in some way to make a high discharge temperature. It should be expected that the compressors will operate at their normal amperage at that time.

How long do they need to run together before it is considered simultaneous operation?

What other options are you banking on being urned off? Dehumidification - depending on the complexity of the system this can often work by running air conditioning at the same time as electric heat (if not using hot gas reheat)....
humidifier- can be required due to long cooling cycle removing too much moisture or from heating cycle causing the relative humidity to drop and/or pulling in dry outside air....
ERV wheel - even large ones typically uses very little amperage

Another consideration, unless they are mechanically locked out, then the service or repair technician may use the controls to run all options at the same time. Although this might not be considered normal operation and be overlooked.


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Well it depends on the use of the facility to some extent, as well as design and settings of the HVAC system. My comments probably apply more to typical residential and light commercial systems than other systems.

Typical dwelling heat pump system, they simply switch the reversing valve and put it in cooling mode to defrost it, if they turn on resistance backup heat it may be more for inside comfort than for defrosting purposes. Most residential AHU's have the refrigerant coil ahead of the resistance heat in the air stream so that resistance heat is going into the indoor space instead of directly to the refrigerant coils.

At same time, especially in a dwelling, if your service was marginally sized, you would need a lot of load running that normally has a low diversity level before you need to be much concerned about tripping the main breaker. That means you need to be doing laundry, cooking - with higher than usual demand from cooking appliances, have about as many lights on as possible, and even then possibly find some more load to utilize that normally doesn't all run at one time.

Now for a retail store that has marginally sized service - there may not be as much room built in from things that have demand factors. If your service is marginally sized you have 100% of lighting, HVAC and maybe a few other fixed loads but not much more room for anything else.

Quite often we don't size services that close to actual calculated load though and leave some room for expansion.

If you overloaded a breaker by only 10-15% for maybe 10 minutes it likely doesn't trip.