Reply to HVAC replacement - looking for advice from those who have BTDT

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[quote=Anonymous][quote=Anonymous][quote=Anonymous]Here's a really good article (written by a PhD in building science!) comparing the results of a thorough Manual J with using rules of thumb:

https://www.greenbuildingadvisor.com/article/manual-j-load-calculations-vs-rules-of-thumb

[/quote]

This guy's firm does HVAC design for people who are building and remodeling their homes. If you are just talking about an HVAC replacement for an older home with old duct work, it is more nuanced than the results of a load calculation. Our contractor said he could do Manual J but at the end of the day he had to rely on how much air our ductwork could move. Basically it's be a fair amount of labor and then it wouldn't make a difference. So far we are super happy with our system in terms of both comfort and efficiency.[/quote]

Duct work is only a constraint if you want to increase the size. As he points out, most existing systems are oversized, often by a factor of 2 or 3. An oversized system will cycle on and off, whereas a properly sized system will run more continuously. This results in better comfort as it does a better job of dehumidifying, lower electrical bills and longer equipment life.

I get why most systems are oversized. From the installer's perspective an undersized system is a disaster, the customer will know right away that it's not keeping up on hot days. The problems with an oversized system are more subtle, the customer may not realize they're even happening. So if you're just guessing anyway the incentive is to guess too big rather than too small.

The Manual J doesn't eliminate guesswork, there are still some assumptions the installer has to make, but it constrains guesswork. [/quote]

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