Electrical Demand Factor for HVAC

[wsayle]

This may not be the place for this question, but I thought I would ask. 

Should electrical engineers separate out HVAC loads into heating or cooling? Trying to figure out how to get an accurate load demand for my projects. There are instances where a unit is a package unit where it has both. How do you tackle this?

[HVAC-Novice · Accepted solution]

Unless the equipment can simultaneously heat and cool, I would say the larger of the 2. The equipment will have a name plate rating that would tell you the electrical data.  If that isn't clear, contact manufacturers.  

for example a heat pump either cools or heats, but never at the same time. so the larger of the 2 loads is what you need for design (and it often is about actual motor-size, not the actual draw for that specific compressor). But a cooling unit with electric re-heat could do both at the same time and you would add them. 

But really, the manufacturer already gives you what you need and also account for the load due to controls, fans etc. 

[iainsavage]

Be very careful with refrigeration cycle machines in either cooling or heating mode because the power drawn is very dependent on the actual operating conditions rather than "standard", "typical" or "average" conditions.

Heat pumps are particularly susceptible if they are likely to operate in warmer weather (e.g. for domestic hot water generation) because the higher the source (outdoor air) temperature, the greater the heatpump output will be and if the compressor is not variable speed then the minimum output step will be one compressor running at enhanced output due to the higher source temperature.

Conversely, the lower the source temperature, the worse the CoP gets. In Europe heatpump data is usually stated at +7C outdoor air and 35C water temperature and CoP will be circa 3, but when it gets down to near freezing outside and water temperature is maybe 50C the CoP will be about 1 or less.

So its important to distinguish between likely maximum load for cable/breaker sizing and "typical" running load for analysis purposes.

[wsayle]

The main goal is to determine running load for analysis purposes. Sizing panels and service entrance for a building. We rely on manufacturer electrical requirements to size wire and breakers. Sometimes we don't get anything except HP or KW and we have to do some calcs to come up with a breaker size. But the main point of this thread was to think about how to determine overall load for a building. For example, a building may have a chiller and then boilers. One is for cooling and one is for heating. You likely wouldn't have them both running at the same time at 100%, but as of now we plan is if they were (no diversity is taken here). 

[HVAC-Novice]

Unless this is ancient existing equipment, you always should be able to get the appropriate data from the manufacturer. If the manufacturer doesn't provide the data, use a different manufacturer. Good manufacturers provide it all on the submittal. Maybe you have to specially request it, but it should be available. 

Modern equipment uses inverters and so on and include the fans etc. You can't just estimate the data yourself. That is the manufacturer's job. 

As mentioned by iansavage, the actual load when running depends on operating conditions and in any case should be less than the nominal rating used for sizing. But for system sizing, the actual nominal rating is needed. 

[orsgeorgerobinson844]

Yes, electrical engineers typically separate HVAC loads into heating and cooling when calculating load demand, especially for accurate panel schedules, demand factors, and equipment sizing. However, the approach depends on whether you're doing a peak demand load calculation or breaker/panel sizing. For systems like the dual split units available at MiniSplits4Less, if the heating and cooling systems cannot operate simultaneously (e.g., a single compressor reversing between heating and cooling in a heat pump), then only the larger of the two loads should be included.

[johnstewart01hvac]

For a multi-zone concealed duct HVAC system, the electrical demand factor reflects that not all zones operate at peak load at the same time. This allows designers to size electrical capacity more efficiently while still supporting simultaneous cooling or heating demands across multiple zones.

[HVAC-Novice]

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@johnstewart01hvac wrote:

For a multi-zone concealed duct HVAC system, the electrical demand factor reflects that not all zones operate at peak load at the same time. This allows designers to size electrical capacity more efficiently while still supporting simultaneous cooling or heating demands across multiple zones.

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For a VRV system, you have most power going to the outdoor unit with the compressors. There you use the electrical data that manufacturer gives you  (THEY worry about diversity, or if heating or cooling is larger, the demand is driven by compressor size). As electrical engineer you only need to know; " 208V and 72A", which is on their data sheet. In addition, the single zone units have line voltage to run their fans (and operate the controls, valves etc.). Each of those zone units also will have electrical data (i.e. "120V, 3A"). You don't worry about heating/cooling since the manufacturer will give you the design data. And even under partial load, all fans could be running (for ventilation etc.) at the same time. 

The data the manufacturer gives you will work for design and account for startup current etc. You don't need to apply your own logic or theories or ideas how the equipment operates all day long. Yes, in reality it will run at less load most the time. But you also need to size the system in case the system isn't set up properly, starts up, or has some other reason to run parts at 100%. We are not designing for the system to work well 99.99% of the time. We also cover that 0.01%, and that is what the manufacturer supplied design data do.  

Don't size the system smaller than the manufacturer tells you just because you think diversity or other conditions require less.