Interpreting Manual J Results: 2-ton or 2.5 ton?

[Mr Bill]

Manual J cannot compensate for bad or old insulation in walls.

there is nothing that causes it to lose its insulating value. Show me anything that proves insulation loses any of its rating over time.........

Again, more words read into what I said, I never said “there was anything that causes it to lose its insulating value” some folks just need to take a deep breath, and read all the posts here on the forum a little more carefully.

[Edwin73]

Has ACCA Manual S become unnecessary? I am a homeowner and not qualified to tell another what the answer is. But I have purchased the Manual S book and read the way the AC is supposed to be de-rated for the design day conditions, and then selected so that it matches both latent and sensible loads separately. In other words I have always been told that if your house has 24,000 BTUH combined sensible and latent loads, you usually don't want your AC to have 24,000 capacity rating.

Hope this helps -- Pstu

Yep, yer right, sensible and latent need to be looked at separately. Then find a unit in the brand you want that fits the bill. This is how alot of companys get into trouble with sizing cooling loads.

[dash]

Manual J is accurate if the input is accurate,insulation in walls doesn't "get old" in a way that changes the R value.

No extra "input" or "guessing" is needed or wise.

[Robelander]

The reason for the increase use of these manuals is to qualify for the rebates that are offered in some states. I am in NJ and to get the rebate from the state run program they want to see a Man J and S calc within 15% variance. I am a builder and do most of my own HVAC work that only applies to A/C. I use boilers for heat. Not many hvac contractors (in my area)know how to do these calculations, they sub it out to an engineer. They size the units from experience. Sometime they hit it dead on other times.... well oversizing = musty problems. I may consider purchasing the manuals from ACCA.ORG.

[dash]

The reason for the increase use of these manuals is to qualify for the rebates that are offered in some states. I am in NJ and to get the rebate from the state run program they want to see a Man J and S calc within 15% variance. I am a builder and do most of my own HVAC work that only applies to A/C. I use boilers for heat. Not many hvac contractors (in my area)know how to do these calculations, they sub it out to an engineer. They size the units from experience. Sometime they hit it dead on other times.... well oversizing = musty problems. I may consider purchasing the manuals from ACCA.ORG.

Every contractor in the country should have and use ACCA manuals.

[arc8]

Every contractor in the country should have and use ACCA manuals.

As long as there isn't a must in there!
Let the local regulatory enforcement offices handle that!!!

[dash]

As long as there isn't a must in there!
Let the local regulatory enforcement offices handle that!!!

The local regulator broads couldn't make a better choice,what did they choose in your area,nothing at all???

[arc8]

The local regulator broads couldn't make a better choice,what did they choose in your area,nothing at all???

To each his own city!