Biting off more than I can chew, again

In a previous article I lauded the habit of tackling large, novel problems head on, so after my recent foray in HVAC, it seemed appropriate to demonize the same trait to ensure a well-rounded perspective.

To begin this story, let’s start at the beginning.

The Story

Ever since I realized that my AC unit wasn’t working any more, I’ve been researching options, fixes and replacements that would work for my house with it’s open floor plan, aged central air system, etc. I diagnosed the issue to be a blown transformer[1] in the blower unit, which was sort of good news. The part costs ~50 – 80 dollars based on availability; but being a solid state device with no moving parts, a blown transformer is often a symptom of another problem with the unit.

If I doubled down and replaced the transformer, I would also want a full maintenance call, and possibly a recharge, since the unit has not been maintained and been in disuse for a while (previous owners were hard on the house, can only imagine what they did to the AC unit). All toll that placed me doubled-down on a 8 year old, mismatched system[2] for $>500$ dollars which I imagined to be far from an ideal.

I did some digging to checkout what a new model would cost and what kind of changes have been made in the past few years. Cost was a difficult number to determine since the vast majority of sites list approximate quotes that vary by a factor of two depending on all sorts of factors–which makes sense if you’ve seen the range of construction/house layouts out there, but it made comparisons difficult. I had a local HVAC guy out to give a quote:

A new 2.5 ton, 30,000 BTU/h unit of both indoor and outdoor units, including removal of old unit and installation will cost 5,435 dollars with rebate. The system would have a rated SEER of 15 and a 10 year parts and labor warranty.

The SEER rating, or Seasonal Energy Efficiency Ratio[3], is the ratio of cooling power to the energy used over the course of an entire season. It’s almost a direct measure of how costly a system will be, and a great way to compare systems.

My current system, if it was working has an estimated SEER of ~8 and I would estimate a typical year’s of use to cost 2,000 dollars minimum[4], so now I can compare operating costs between the old system and the quoted system.

$$(\mbox{Annual cost of unit 1})\times(1-SEER_2/SEER_1)=\mbox{Savings by unit 2}$$

Using the number given above, $2,000\times (1-9/15)=2000\times 0.47 = 933$. So according to the formula, by switching systems I will save 933 dollars a year in electricity. That’s a huge payout and makes not switching units a difficult proposition.

tri-zone-ductless-mini-splitNevertheless, $5435$ dollars is still a lot of money and makes the years-to-payback higher than I would like: almost $6$ years. I started looking into DIY options and while a whole house AC unit replacement is not something I could do myself, I decided to do a little research into mini-split systems. A mini-split is basically a whole house unit but on a much smaller scale. They are designed to work for one room or one space only, and in that way they also offer the ability to control each one individually (zones). Over the course of my research I found that A. They can be installed without too much work (relatively speaking) and B. they have crazy high SEER values of $16-21$.

Over the course of my research I whittled my way down to two units, which together would offer 3 zones of cooling and a SEER of $~18$. Using the same formula as above, this would save me $1200$ dollars a year from the current system and about $200$ dollars a year more than the quote system. Not bad, but to top it off this new system would cost $~4,200$ dollars yielding a years-to-payback of $3.5$.

Without saying too much more, I obviously jumped head first and order the units and prepped my tools for a weeklong project.

Now that I am on the other side and have two beautiful systems installed, I am very happy to have taken the plunge. Since I had never worked with HVAC[5] systems before, everything was brand new: placing the units, installing the indoor blowers, running conduit, pressure testing the system with nitrogen, vacuuming down the lines, making flare connections, etc. I certainly learned a lot and am very happy with the results, but…

Am I insane? This entire week has been stressful beyond belief, and I have been to Lowes, Home Depot and Airgas more times that I would care to admit. My stomach has been tied in knots and very night I had dreams about AC units. Thank god it’s done. I can only imagine that this is the sort of feeling my dad has had numerous times when he undertakes a new project or does something crazy (like buy a farm).

I put this out there so that anyone else who is considering doing the same thing, let’s talk. I can give you a full list of materials I used, where to buy stuff and what to expect. I made a few mistakes but isn’t that half the reason for doing new stuff? I need a vacation.

Pictures and more to come.


  1. The transformer in a central air unit converse the 110/208/240V AC to 24V DC for use in the thermostat.
  2. As it turns out, the blower unit was also installed incorrectly and the outside unit doesn’t match the indoor unit for size, hence mismatched.
  3. See the wikipedia article on SEER.
  4. Down here in Florida the AC is used a ton, so I have no idea except that it will cost a lot.
  5. HVAC is one of those well known acronyms, which has a not so well known long form. It is one that you can guess, but now you don’t have to: Heating, Ventilation and Air Conditioning.

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