We had a professional installation (by an HVAC company that is) of a cold-climate heat pump with 10kW heat elements as supplementary heat in the air handler (indoor) unit. This is for a house with forced air ducts, and the air handler replaced a broken methane gas furnace.
The installers tested everything - Thermostat Up = hot air. Thermostat Down = cold air.
What they didn’t test was whether this setup was making efficient use of the 10kW heaters. Most customers would not have noticed any problems with this setup, until the electric bill showed up. (I wonder how many other heat pump installs have this issue??)
IotaWatt confirmed: any call for heat caused both the 3.5kW heat pump (30A dual breaker) AND the 10kW supplementary heat (60A dual) to run. There was one clue - the thermostat contacts had no wire on the AUX terminal, in other words the only signal from the thermostat was “call for heat” and everything was responding to that signal.
Several youtube videos explaining thermostat wiring helped solve this. The installers were busy, I just fixed it myself, they eventually came by a week later and were satisfied, but strangely not too excited about this new failure mode.
Now I have Influx v2 configured and soon will have alerts or a dashboard for when things appear to misbehave.
The other serious issue for my installation was the Electric inspector’s requirement for a whole-house load calculation. It’s quite complicated with percentage loads depending if your kitchen stove is electric or not, plus square footage “base load” calculations. Managing certain loads could be done if (example) your total consumption is above or below 75% (eg allow AUX heat to run only if other loads are below 75%… otherwise something else is already heating the house). Managing consumption this way could avoid extremely costly panel/service upgrades in some cases.
I can provide more detail if someone needs it. 100A panel, Honeywell Home thermostat with HP/AUX lockout at different temperatures, and Adaptive recovery to reduce concurrent heat even without the lockout settings.
Specific to EV charging, a device called DCC-12 is being marketed to address the need to limit total electric consumption. By checking whole-house consumption (if you are thinking CTs on mains, you’re right!), this unit decides whether to allow the EV to charge or not. More than 80% max load → stop EV charging for at least 15 mins. I see some potential here for something like an IotaWatt (CTs on mains!) to perform part of this task, where multiple big discretionary loads are in play. In my case I’d like to insert some relays at the 10kW element heat signal (not necessarily the 120V/240V lines just the 24V ones). I’m still jumping through hoops to minimize total loads. Possible opportunities for IotaWatt here!
As is side note, we purposely tried to blow our breakers by having all big loads run at the same time. Nothing tripped. But that doesn’t satisfy the inspector unfortunately.