I'm doing a renovation on an unfinished area, size about 350 sqft. During my rough framing/electrical/mechanical phase inspection, my county inspector didn't mind the idea of tapping into the existing system for heating/cooling. However, when I called up a technician to schedule a time to take a look at setting that up, they mentioned that by law they can't do work on something where the unit isn't zoned for the square footage. Now, correct me if I'm wrong, but the sqft zoning is a rule of thumb and not a strict law, isn't it? Wouldn't the county inspector want to scrutinize the condenser tonnage/SEER if he thought it was in question? Is it possible the project would be halted because of this unless I supplemented with an air unit or changed out the condenser/HVAC system?
There are three pieces to this problem here.
- What will the permiting authority accept?
If the inspector says he will approve your renovation without upgrading the existing HVAC system or adding a supplemental unit, then make sure you get that in writing. In my county in Florida, anything renovation that adds roof or that changes the conditioned area by more than 100 square feet requires a new heat loss calculation.
- What can you get an HVAC contractor to do?
If they won't do what you want to do, then keep shopping. Considering the size of your job, you'll probably get a better response from a smaller company or an individual HVAC contractor. They are more likely to not dismiss a small job.
- Are you prepared to do something more if your current system doesn't work out?
I'm not particularly worried about the system handling the additional load
This could really go either way. If the current space is well insulated and properly ventilated, then that is the most efficient way to keep the room below it insulated. Adding a room above decreases the effectiveness of your HVAC system (more cubic feet to condition) and probably also increases the heat transfer in to your envelope (since you likely can't insulate as well and you're adding more exterior wall square footage to your envelope).
Be prepared to have to do something if your system can't keep up after your renovation. Since you'll have the walls open, etc., make a plan now for how an upgrade or supplemental HVAC might be installed. For example, instead of tying in to an existing vent nearby, maybe run the new vent all the way back to the current air handler. Or pre-plan a route for refrigerant lines and placement for an additional air handler.
It's an attic renovation, so I can't really knock the HVAC guys...it's not as simple or straightforward as it seems. You're affecting the whole home's comfort (more volume weakens all rooms' circulation), really just for cooling.
But, I have to agree it sounds like they're possibly concerned about money & should've gotten your input on cold or hot spots in the house that clearly weren't properly insulated or that could be bumped up to essentially shrink the new room's volume.
Many homes have "cooling lag" & rarely shut-off during the day, if you're one of those home's, you'll be compounding the problem & a new bigger Condenser, at a minimum, would be worth the money. If you're already cold-ing & shutting-off during the day, then this room would possibly weaken the entire system down to lag-level.