Sizing HVAC Equipment
If we use ACCA Manual J as a reference, HVAC contractors consistently oversize equipment. For instance, on my house, ACCA Manual J calculated the heating load to be 46,000 BTUs….the Title 24 Environmental Engineers used a Micropas program to calculate a 52,000 BTU heating load….not a huge difference….then, they added a 30% safety factor, for extreme weather fluctuations (I assume, those days when storms hit and drop temperatures by 50 degrees over night)….bringing the load to 68,000 BTUs…..the contractor covered that estimate with a 90,000 BTU boiler, possibly because of size restrictions, etc…..twice the Manual J calculations! And Performance Contractors and others claim that Manual J is already oversized by 15-30%.
(1) Is Manual J an accurate calculation tool? Is it already oversized?
(2) Why does it appear that HVAC contractors pay no attentions to Manual J?
(3) Does the typical HVAC contractor even use accurate heating load or cooling load calculations?
(4) And, as someone recently told me, these guys (HVAC contractors) are not idiots….they install equipment that reduces the chance of getting callbacks.
I understand the inefficiencies related to oversizing (short cycling, etc) but I also understand HVAC contractors not wanting to get callbacks from super sensitive clients who are not willing to wear a sweater or add a blanket when storms hit. Where is the middle ground?