Obtaining quote for HVAC
Climate Zone 3, Atlanta
The evap coil on our 2011 R22 unit gave up the ghost and with R22 @ $150/lb along with $1800 for a new coil we’re going to look at just upgrading the A/C system.
The current setup is a 4 tonne evap coil w/ 3.5 tonne outdoor unit on a 3-zoned system. When asked, I told the technicians boss that I wanted someone to come out and confirm that our system wasn’t oversized because even though this was the size spec’d by the builder (1999) our system was short cycling. I thought maybe we could save some $$ by going a little smaller. The 2-story townhome w/finished attic (end unit) faces N/S with N/E/S facing windows (21 windows total of which 6 and rear windowed door in S/E corner “sun room”). E/S facing have a lot of shade. Total SQFT is approx 2k with 9′ , 8′, 8′ ft ceilings on 1st, 2nd, attic, floor respectively.
I’m interested to see what sort of estimate I get. In any case here’s my question.
Q: Is there any way to gauge how much wear/tear the compressor experienced from running on a low level of refrigerant? The technician had to add 2lbs yesterday just to get the system to cool below 77 degrees and I noticed the evap was freezing just 3 days ago.
My wife really doesn’t want to spend the estimated $3500 on a new A/C system, so she’s thinking about just doing the $1800 coil replacement. However, it’s not really worth it if the 7 yr old compressor goes out within the next year or two which puts us back to square one with regards to the availability and cost of R22. (The original builder grade system was replaced in 2011 with the current off shelf “dry unit”).
TIA.
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How do you know the evaporator coil is no good? Is it leaking? Sometimes technicians will add refrigerant when they read low suction and head pressures, assuming a refrigerant leak. Sometimes this is really just an indication of poor airflow (as is a frozen coil). You could just have a dirty filter, air handler, and coil. Unless they found a leak or an actual problem with your coil, you (or the technician) should check to ensure it's not an airflow problem. Does your zoned system have a bypass duct?
The technician checked the evaporator coil first and said he could see where an amount of lubricating oil had sprayed out onto the coil/inside of the plenum. We never had problems with regards freezing from insufficient air flow and I change the filters religiously (We use pleated filters). Current set is only 1 month old.
By-pass: Yes we have a by-pass damper/duct. The by-pass damper is a passive system (swing arm with a weight on the end) so force from the movement of air causes it to open/close. It vents into the attic space that is open to the atmosphere however the zone for the interior portion of the attic space is permanently open because it's so small so it effectively acts like an additional bypass.
Sounds like you had a good technician who was checking for the right things. I'm not an Hvac technician, what I know about troubleshooting is strictly from what I know about designing systems. If you had oil spraying out of your coil, then your compressor likely had liquid refrigerant flood the compressor. The only oil in the system should be in the compressor. (Stolen from ACHR because it's easier than writing it myself): "Flood back can ruin a compressor by diluting the compressor’s oil with liquid refrigerant. This has a tendency to ruin the lubricity of the oil and score bearing surfaces in the compressor. Flood back also causes oil foaming. This pressurizes the crankcase and causes the oil to be pumped out the discharge valve and into the system. Discharge valve damage can also occur from the oil-foam/refrigerant-rich mixture."
Depending on how long it was pumping oil around, you could have a damaged compressor. That's about all I can offer. If you trust your hvac technician to be honest, he would have the best idea of whether it has life left or not.
Thanks.
Estimate came in at $4100 for 14SEER 3.5 ton unit plus labor. Now to shop around.
Are you sure you really need 3.5 tons?
Replacement time is an opportunity moment for right-sizing things. A careful Manual-J, or measuring the compressor's duty cycle for several periods when the outdoor temps are near the 1% outside design temp would tell you if it can be down-sized. Most existing HVAC equipment is sub-optimally oversized for their loads, delivering lower efficiency & lower comfort than right-sized equipment.
See:
https://www.greenbuildingadvisor.com/article/how-to-tell-if-your-air-conditioner-is-oversized
and
https://www.energyvanguard.com/blog/air-conditioner-sizing-rules-of-thumb-must-die
Note that most homes under 4000 square feet have a square feet per ton ratio in 1400'/ton range or higher. So 3.5 tons could theoretically manage a middle-of-the road 5000' house, perhaps only 4000' after compensating for the parasitic load of ducts and air handlers being in the attic:
https://www.energyvanguard.com/sites/default/files/styles/panopoly_image_original/public/square-feet-per-ton-air-conditioner-sizing.png?itok=vsJxOobH
^^^this graphic was compiled from a few dozen Manual-Js performed by Energy Vanguard, in Decatur GA
Do you know that 3.5 tons is really the right size? See:
https://www.greenbuildingadvisor.com/article/how-to-tell-if-your-air-conditioner-is-oversized
Replacement time is an opportunity moment to get it right. Most homes under 4000' come in at about a ton per 1300-1400' (with outliers in both directions), so 3.5 tons would cool quite a big house. You have ~2000', and may have a total design load of less than 2 tons.
Oh I'm sure it is oversized. The rep said that for our neighborhood the builder spec'd the tonnage (3-3.5/unit) and w/out knowing how the ducts (covered in drywall, etc) are laid out he was hesitant to go smaller.
I'll have to find someone who will do a Man J on existing construction.
UPDATE:
Another company came in and performed a Man J. Interior space is approx 2k sqft spread over three floors (Ground, 2nd floor, attic). By his calculations (Windows, ceiling height, et al) he said between 2.9 - 3.3 tons. The rep said technically a 3 ton system would be fine however the builder was probably concerned about the amount of heat generated in the large finished attic space so they spec'd for 3.5. He also said that unfortunately due to the size of the blower motor on our 3 yr old furnace we are stuck with 3.5 ton system.
Interestingly the quoted price was the same but a different manufacturer (14 SEER Carrier vs 14 SEER Rheem).
You might consider getting a two-stage system if you felt like your single stage 2.5 ton was already short cycling...?
Can you match a two-stage A/C with a single stage blower?
Nope. Airflow needs to match the tonnage. Usually 350-400 cfm per ton, depending on latent (humidity) loads. Too little airflow, coil will freeze up and the compressor could see liquid refrigerant. Too much airflow and latent capacity is greatly diminished and the system will experience low sub-cooling.
Ya. Didn't think so. This is the problem with replacing heating and cooling individually rather than both simultaneously.
UPDATE:
New AC installed this past weekend. I had a discussion about the disconnected by-pass damper with the technician who was installing the evap coil.
For those who don't know, the by-pass damper on my zoned system was venting into the attic. When he asked I told him that I was told that with the damper hooked-up to the return plenum I was going to slug the compressor with liquid refrigerant when running the A/C. He took a look at the old evap coil and asked me where I bought it and he chuckled when I told him it was a warranty replacement from a home warranty company.
The technician said that whomever installed the evap coil was correct to vent by-pass air into the attic because the warranty company, for cost reasons, did not give him an evap coil which also had a TXV (aka Thermal Expansion Valve). He said that since the TXV meters refrigerant into the evap coil based upon the temperature of the air passing over the temp sensor it there will be no slugging back to the compressor when the by-pass damper vents back into the return plenum.
It feels nice to have the system back up and running properly. I look forward to a less drafty winter. Next repair in a couple of months will be the rebuilding of a return and main plenum with sheet metal.