Just finished an audit on a 85,000 sq ft office building in Dallas served by Oncor under the GS-3 schedule. The building has four rooftop units that all start simultaneously at 6:30 AM. Last July we hit a 47kW demand spike that lasted exactly 15 minutes - cost the client an extra $1,240 that month just from HVAC startup. Has anyone dealt with Oncor on staggered startup schedules? The facility manager is pushing back on the cost of demand controllers but this is killing their summer bills.
HVAC Startup Surge - 47kW Demand Hit at Office Complex
Marcus, we see this constantly here in Connecticut with Eversource. The GS-2 rate hits hard on demand spikes. For a building that size, you're looking at maybe $8,000-12,000 for proper demand control but it'll pay for itself in 8-10 months with those kind of spikes. We've had good luck with Schneider Electric controllers - they can stage the units with 3-5 minute delays between startups.
That's brutal Marcus. Out here with NV Energy we see similar issues but our GS-2 demand charges aren't quite as punishing as Texas. What's the total connected load on those four units? If you're hitting 47kW just on startup, the inrush current must be massive. Have you considered soft starters in addition to the staging controllers?
I've got a similar situation in Atlanta with Georgia Power. Three 20-ton units on a warehouse, all controlled by one ancient Honeywell system. Every morning at 7 AM - WHAM! 35kW demand spike that sets our peak for the whole month. Georgia Power's Schedule GSD-8 is merciless on demand charges. Finally convinced the owner to install a simple time-delay relay system for $1,200. Cut the peak demand by 60% the first month.
Kim - total connected load is around 180kW across all four units. Each unit pulls about 45kW running, but startup inrush hits 60-65kW per unit for about 30 seconds. The problem is they're all on the same contactor system. Derek, that's exactly what I'm trying to sell them on. Even a basic staging system would drop that peak to maybe 15-20kW. The $1,240 hit was just one month - over the summer we're talking $6,000+ in avoidable demand charges.
You might also want to look at the power factor during startup. We had a similar issue here in Des Moines with MidAmerican Energy where the inrush wasn't just creating demand spikes but also power factor penalties. Installing capacitors helped with both issues. The building was seeing 0.78 power factor during HVAC startup which triggered additional charges under their General Service rate.
Marcus, definitely push for the demand control system. Here in Georgia I've seen paybacks as short as 6 months on similar setups. One thing to watch - make sure whatever system you install can handle the building automation integration. Some of the cheaper controllers don't play nice with existing BAS systems and you end up with comfort complaints. Schneider and Johnson Controls both make good units that integrate well.