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There is a room in an apartment that is heated by a packaged terminal (PTAC) unit. Based on the smart meter, during the night if the temperature is set to around 73 degrees Fahrenheit the PTAC unit consumes about 3kWh/hour of electricity, which seemed excessive for the amount of space it was heating. It is cold enough that I assume it is relying on resistive heating and not a heat pump.

This is almost exactly as much as two space heaters, so as an experiment I borrowed two space heaters and ran them overnight in the same room, on different circuits as to not trip the breakers. They kept the room significantly warmer, to the point where one had to be turned down.

Why would these units appear to be much more effective at heating the room than a PTAC unit?

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    Is your PTAC heating & recirculating the indoor air only, or is it bringing some air in from outside too?
    – brhans
    Mar 1, 2023 at 19:25
  • @brhans I actually do not know. I have found conflicting articles online if PTAC units recirculate or not in heat mode.
    – Chuu
    Mar 1, 2023 at 19:38
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    Is there any chance this is unit confusion? Your PTAC is consuming 3kWh, or about 300W average across ten hours. That's not very much at all. You then added 3kW of space heating to the room, I would naturally expect it to get much warmer! What was the total kWh consumed by your 3kW of space heaters?
    – KMJ
    Mar 1, 2023 at 21:10
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    @chuu kW is a rate of energy use. kwH is the amount of energy used over a period of time. If you run a 3kW load for 3 hours, you used 9 kWH. The H is (in fact) hours. The k is simply kilo, you could as easily say 3000 watts. Mar 1, 2023 at 21:42
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    @Chuu As KMJ and Harper suggest, it seems as if you are saying that your PTAC consumes 3kWh = 3000Wh over 10 hours, or a power draw of 300W. Your space heaters likely use 1500W each, or 3000W for the pair. That is 10 times as much as the power draw of the PTAC, which likely explains what you are observing.
    – Armand
    Mar 2, 2023 at 4:20

1 Answer 1

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There are a number of issues here, some of which have already been mentioned in comments but I will try to summarize and explain a bit more.

TL;DR Way too many unknowns, but it is likely that the PTAC is far more efficient and that measurement, usage and/or calculation problems are producing the strange results.

  • "Based on the smart meter"

Generally a smart meter refers to an entire building or dwelling unit, such as an apartment. So the usage will include multiple rooms. Even if there is nobody in the other rooms, there may be vampire loads (clocks, router, devices in standby mode, lights left on (possibly some always on for safety), etc. So the 3kWh reported may or may not have any relevance to the particular room, except to say that the usage includes the room.

  • kWh vs. kW and time

kW = kilowatt, which is a measure of the instantaneous amount of power being used at the moment.

kWh = kilowatt-hour, which is a measure of an amount of power that has been used, specifically one kW for one hour.

Usage is billed in kWh. (Well, there is peak demand charges in some places, but that is not of concern right now.) 3 kWh is a billable amount of energy used. But that 3 kWh could be 3 kW for one hour, 125 W (0.125 kW) for an entire day (0.125 x 24 = 3) or anything in between.

  • Space Heaters vs. PTAC Efficiency

A PTAC is a form factor, so that does not necessarily indicate whether it is using a heat pump or resistive heating. An electric space heater, by definition, is using resistive heating. (I don't care if it has a fan or not, ceramic, oil filled, whatever - in the end every electric space heater is a giant toaster.) A space heater runs at 100% efficiency - all the electricity becomes heat. A PTAC might use a heat pump and turn that same electricity into 2 or 3 times as much heat - 200% or 300% efficient! We really don't know what PTAC is in place here. Many heat pumps have resistance heat backup, but they vary dramatically in how cold it has to be outside to switch to resistance heat. A lot of unknowns!

  • Thermostats

A PTAC, like most fixed HVAC systems, will generally have a fairly effective thermostat. It might be "1 to 10" or it might have a calibrated display in F or C, but it will almost always have a fairly effective thermostat.

Most portable electric space heaters that I have found have very ineffective thermostats. That is primarily due to cost - on a $20 heater (yes, there are 1500W electric space heaters for $150 or more, but they are no more effective in heating a room than the $20 ones, so most people get the cheap stuff) which includes a heating element, power cord, usually a fan, plastic case, shipping from China, a box, profit for the manufacturer and the retailer, etc. there just isn't that much left for a good thermostat. And then the thermostat is placed within inches of the heating element, so it is prone to cycle too much (because it gets hotter than the room) or too little (because people turn it up to overcompensate for it turning off too soon). Notice that in a typical home with central HVAC the thermostat is in the middle of the house - far away from the HVAC unit itself, preferably several feet from any ductwork, on an inside wall so not affected by drafty windows, etc. Can't do that with a portable space heater.

The end result is that if you plug in a couple of space heaters to heat up the room quickly you are likely to overheat the room, and that indeed is what happened. Even if the PTAC produces an equivalent amount of heat (10,000 BTU) as 2 space heaters, the PTAC will do it in a much more controlled way, cycling to keep the room within a few degrees of the setpoint instead of wild swings between too hot and too cold.

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