Manual J sticking point with vendor: mean temperature value
I’ve had an issue with my builder and their HVAC vendor. They way oversized my system by doing the following items:
1. Wrong Window U-Values 0.58 instead of the correct 0.34
2. Not adding any window blinds
3. Not including the conditioned crawlspace
4. Using 0.4% instead of 1.0% mean location temp
5. Intentionally orientating the house to show the highest loads instead of using the correct orientation.
I hired ProCalcs and they exposed all these items over time. The last sticking point is that they are insisting on using the 0.4% Mean temp value and I need to understand if the industry standard is 1.0% and if so, under what conditions it is justified to change it to another value.
Right now my builder is wanting me to sign a $300 change request mainly to include the following verbiage:
Client is requesting an HVAC system smaller than the size recommended my the HVAC contractor. HVAC contractor is recommending a 2.5 ton system on main level (3 ton air handler, already installed pared with a 2.5 ton heat pump condensor) and a 1.5 ton system on the second (2 ton air handler is installed, to be paired with a 1.5 tone heat pump condensor). Client is requesting a 2 tone system on the main ( causing the current unit to me removed and replaced.) Due to this undersizine on the main level (based on the clients calculation of 91 degree outside tem0), SHC will not warrant system operating properly.
ProCalcs shows that using all the correct inputs and using 1.0% mean, the recommended tonnage is 1.5 on Main and 0.9 on second floor. The vendor started out at 3.0 on Main, 2.0 on second. After all the corrections and leaving the Mean temp at 0.4%, they ended up at 1.7 ton on main and 1.2 ton on second floor. With my forcing them to use the 1% mean, they come down to 1.5 on Main and 1.1 on Second.
So, I would like someone if possible to tell me under what conditions is it acceptable to use one temp mean setting over the other? I’d like to stand on firm ground when I push back on this..
Thanks for all who have been providing feed back on my questions the last few weeks.
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