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Our house is 21 years old; we've owned it for nine years. Last September we replaced the HVAC system, not because it was failing but because it was essentially at its end of life. The new system, now 10 months old, has begun to quit cooling in our current heat wave (temps > 95°). The service tech last week told us that Rheem error code 66 meant that the duct work was too restrictive. (That's one of two possibilities listed in Rheem troubleshooting.) He then dropped the air velocity from 2,000 CFM to 1,400 CFM. Unfortunately, that did not prevent the system from quitting cooling in the afternoon.

Is there any reason for duct work to become restrictive when the outside temperatures are high?

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The issue is that your system is losing capacity to the duct restrictions (TL;DR: fix your ducts!)

You are right to ask questions about this diagnosis: ductwork doesn't exactly change with the temperature! However, the service tech is probably correct, all the same. The reason this only shows up in hot weather is because the restrictive ductwork is restricting the ability of your air conditioner to deliver its rated cooling capacity to your house. How?

Basically, an air conditioner needs to be able to deliver a certain amount of BTU/h of cooling capacity in order to cool your house to the setpoint at a given outdoor temperature. However, it can only remove a set amount of BTUs from each parcel of air passing through the system. If the ducts are too restrictive, then fewer parcels of air pass through the system over a set span of time (the blower can't deliver it's design CFM rating due to excess static pressure/friction loss), resulting in fewer BTUs/hour delivered to the space because there were fewer cooled parcels of air delivered to the space in any given hour.

So, it's time to bring the HVAC crew out and have them fix your ducts using the ACCA Manual D rules for duct design. Ridding your house of HVAC-throttling flex duct is a good start, though, especially if you have a ductopus lurking in your house!

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  • I understand. Some follow-up questions: Does the HVAC system make the restrictive determination on BTU/hr or on static pressure? Why would the predecessor HVAC system cool the house in hot weather at a higher CFM than the new system? I'm just having a hard time with the concept of having to replace duct work just to make a new HVAC system work on a house that's not really old.
    – geoB
    Jul 31, 2022 at 17:16
  • @geoB -- you'd have to look at the docs for your Rheem unit -- it might be deciding based on blower current draw (likely, if your unit is variable speed, as that correlates to static pressure encountered). the answer to the second might have to do with the difference between fixed-speed and variable speed blowers, but I'd have to look at the specs for the old system to be sure. Jul 31, 2022 at 18:39
  • @geoB On the last point: it's very common for home HVAC systems to not receive the attention they deserve during house construction. This paper explains it far better than I can, although the TL;DR is that most builders don't want to spend the money needed to get a HVAC subcontractor in that knows what they're doing re: HVAC design, so they get the lowest bidder instead, and it all rolls downhill from there as good contractors struggle to get work while the cheapos proliferate Jul 31, 2022 at 18:44
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    Your comments are greatly appreciated. My wife found this that changes our thinking about the ducts here. The next step, it seems, is to get a professional opinion on our ducts. Don't envy the crawl needed to do that. We'll see what tomorrow's tech visit brings.
    – geoB
    Jul 31, 2022 at 19:27

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