Unsolicited opinion: I can't help but comment that yanking the power from a running compressor isn't something I'd want to have happen. Doing that more than occasionally is bound to shorten its life, and doesn't allow your HVAC to work as designed.
Thks Hal.
I agree.
I don't believe the grid provider removes current while the compressor is running, altho, thats what a contactor does when it switches on & off during cooling cycling periods.
I'm sure they send it a signal, that prevents it from turning on, while in a low current state, but who knows.
I see it sense current, but there's no voltage sensing capability (unless I'm missing something).
This would tell me when it's drawing current, but not if the mains have been disabled, thus, no current. If my T-stat is not calling for cool, it will not draw current.
I'd have to come up w/ a way to differentiate if the grid provider is stopping current or my T-stat is stopping the cool air from the furnace.
In @user3420's setup, those RIB relays will not distinguish from when the utility has cut-off power to the A/C condenser from when the thermostat is not calling for cool.
Wouldn't a rule of some sort have to be made in any event?
How is this control actually done?
Is there some sort of relay in the house, outside?
Tapping into that for a signal might be frowned upon by the utility though.
Good point. I wasn't thinking of that, although if the HEM was accurate enough, I could be the case that the HVAC system draws a few watts when idle, but absolute zero if power disconnected. Not likely the best solution though.
The more I think about this, this would be really simple solution to implement. It would also be easy to put the power-detecting relay in a waterproof project box outside the condenser.
I must be misunderstanding the situation. If the power company control is at the HVAC system (like I would expect it to be, or hooked into the thermostat), why does this have to be on a separate power meter? My assumption was they cut power at the meter.
Correct. I can't touch the grid providers stuff. I can only get into the box AFTER their wires come out into the main disconnect/fused panel. At that point, I can pick off 1 leg to common. That will then need to goto a 15/20amp breaker of some sort.
The ones I've seen are directly at the outside unit. They're called "Power Saver Switches", which they control by some radio control to cut power at specific times to A/C condensers. Here's an example:
Yes, they open a contactor in their meter, cutting off supply current from getting to the compressor.
My furnace & blower is on a separate circuit entirely. there is a "control", low current relay contact, that comes from my T-stat, telling the compressor to turn on, IF it can.
Yes, that's what I would expect. The power saving control is integrated into the control of the HVAC. Alternatively, it is integrated with the thermostat. But neither of those approaches would require a separate power meter.
Having a separate power meter where power is turned on and off at the meter and not intelligently connected to the HVAC system control is what surprised me. This is what @user3420 described.
The reason for a separate meter is how the grid provider measured ONLY HVAC kWh usage.
They give a discounted kWh cost for air conditioning if they can level out the grid, based on demand.
Prior to SMART meters, they had a radio receiver that had a relay contact, in series w/ the T-stats cool control wiring going to the compressor.
Thank you ALL for your thoughts!
