Just had a client get slammed with an $18,400 demand charge from ComEd after their new 350-ton RTU had a startup issue. The unit was stuck in a restart loop for about 20 minutes, hitting 890kW every time it tried to come online. Normal demand is around 340kW. Anyone else seen this kind of cascade failure? The building engineer said it was a control board malfunction but ComEd doesn't care about the cause.
ComEd demand spike from RTU startup sequence - $18K hit!
Ouch, that hurts. We see this all the time in Texas with Oncor territory. The key is getting soft start controllers on anything over 100 tons. Also make sure the BMS has proper staging delays. What's the demand window on ComEd - 15 minutes? That restart loop would definitely nail you.
Had the exact same thing happen with Ameren Missouri last year. 250-ton unit, compressor lockout kept cycling. Client got hit for $12K on a normally $3K demand bill. The staging sequence was programmed wrong - all zones calling at once. Facilities guy thought he was saving money by doing the controls in-house.
TVA has that same 15-minute window. I always tell clients to install demand controllers with HVAC override capability. When you hit 85% of your target demand, it can shed non-critical loads or delay equipment starts. Costs about $8K installed but pays for itself the first time it prevents a spike like this.
The frustrating part is this was a brand new Trane unit with factory controls. You'd think they would have better startup protection built in. I'm pushing the client to add soft starts and a demand controller. Ed, what brand demand controller do you recommend? Looking at Honeywell vs Johnson Controls.
PSE&G just changed their demand tariff - now it's a 12-month ratchet at 75% instead of 50%. One bad month and you're stuck for a full year. We're seeing more clients invest in demand management after getting burned once. The payback is usually 18-24 months now instead of 4-5 years.
Yuri - I've had good luck with the Honeywell Spyder controllers. Easy to program and they integrate well with most BMS systems. The Johnson Controls are more robust but harder to configure. For a straightforward demand limiting application, Honeywell is usually the better choice.
IPL here in Indiana has a nasty demand structure too. I always recommend staging delays of at least 30 seconds between compressor starts on multi-zone systems. The inrush current on those scroll compressors can be brutal. Had one client save $45K last year just by reprogramming their start sequence.