The Money Mustache Community
Learning, Sharing, and Teaching => Ask a Mustachian => Topic started by: ChpBstrd on February 20, 2024, 12:13:27 PM
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Two years ago, we replaced our 13 year old heat pump with a new SEER 13 model, at a cost of about $10,000. If the new heat pump lasts 13 years like the old one did, we are experiencing $769.23 in depreciation per year.
Of course, our heat pump went out amid hot summer temperatures. While the contractor waited a couple of weeks for parts to arrive we installed an old, battered 5,000 BTU window AC unit into the dining room with cardboard and tape, and turned it on full blast 24 hours per day.
Amid 95 degree daily highs, the little window unit never let our 1940s 1300sf house get above 78 degrees. The best part was that it sucked the indoor humidity down to 50% at a time when it was 80-90% outside, so 78 degrees was more comfortable than it sounds! We learned a window unit running 24/7 will more effectively dehumidify a house than a central unit because it isn't kicking on and off - it's just steadily running and dripping.
Thus, as we waited for HVAC components to arrive amid pandemic supply chain shortages, we realized we could technically live most days with only a little comically undersized window unit. It was kinda loud, but it got the job done.
Our electric bill that month was much lower than the previous year, because we weren't running a worn-out central heat pump and because we were only cooling to about 78 degrees. However, the cost of our nice new central heat pump made me think about how we're paying more in annual depreciation than the thing is consuming in electricity.
In contrast, little junk window AC unit that kept our whole house comfortable only cost about $160 new. Thus, I wondered if we could double the life of our central heat pump by relying on a window AC unit in the summers, and only using the central unit for 100+ degree days. E.g. turn the thermostat up to 80 or so. Doing this might save a little less than half the annual depreciation of the central unit - worth maybe $300-350 per year.
The question is how much more or less I'll spend in electricity to run a small 10 SEER window unit constantly instead of a big two-speed 13 SEER central unit intermittently. A second question is whether the window units would wear out so quickly that most of the savings would... evaporate... when it's time to replace them. My window unit does not currently have another use and sits on the floor in a backyard shop, but if I'm thinking about depreciation I need to compare apples to apples and assume I'll buy a new one when this one quits. My electricity is very cheap, so that's an argument in favor of wearing out the cheap equipment to protect the expensive equipment.
Cost per kWH: $0.10341
Wattage of window unit: estimated 500W?
Size of central heat pump: 3 ton
Of course there are downsides:
1) The aforementioned noise
2) Lack of effective air filtration
3) Cooler and warmer areas of the house, though this wasn't as big a problem as we thought
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We use window AC in the summer. We've pondered installing central air, but haven't yet.
The downsides to window AC are pretty apparent. It's noisy. You effectively lose a window. You have to install it in the spring and remove it in the fall. Then you need to store it all winter.
But I suspect it might be more efficient and cheaper than central AC, especially central AC attached to the HVAC of an old house. We also have about 1300 sq ft, and we use three units. One large one in the lower story (kitchen, dining, living), and a small one in each of the upstairs bedrooms. We only have them running occasionally, though. That means that the main floor is on a timer for the afternoon when we get home from work, and the upstairs units are programed to kick on a couple hours before we go to bed. It's a small house, and the little window units can get it comfortable where we need it quickly even on a hot day.
Central AC seems easier and potentially more comfortable, but I doubt it's cheaper than running window units on an as-needed basis.
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A properly sized central AC will run most of the time in order to properly control both humidity and temperature. A larger and more expensive oversized unit, sold by a zealous HVAC contractor, will, as you note, run a lot less and fail to dehumidify.
In order to see how much each uses, you should probably do a trial running first one then the other during hot weeks of similar temperature. You can check the total usage online, if available, or via your meter. Just be careful not to introduce other variables. I expect the central unit to be more efficient than multiple window units but with just one unit, you are probably better off with just the window unit especially if you like the humidity control better.
Any effect on depreciation is secondary and hard to calculate.