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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?

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    Have you contacted a second HVAC company or the inspector about this? – TFK Feb 22 '16 at 19:16
  • Yea, find another company that's not trying to up sell you. They make their money on selling equipment. – Mazura Feb 22 '16 at 19:27
  • Thanks. I figured it wasn't going to be a big deal probably. I'm not looking to buy a whole 'nother unit to heat/cool 350sqft more. I can insulate the rafters and walls of the remaining unfinished area to help make up for the extra load the unit may need. So I guess this is typical of some HVAC companies? Make up some regulation to suggest you need a new unit costing thousands more? – saigafreak Feb 22 '16 at 19:33
  • There's plenty of regulations on the manufacturing of and efficiency for systems, but that's neither here nor there if they're not installing any equipment. – Mazura Feb 22 '16 at 20:18
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    350 sqft is a lot. Where are you? How big is your current unit? How many sqft is currently conditioned? Do you have the original plans for your house? If so, it should have a heat loss calculation as part of the HVAC plan. Those numbers will be useful for determining if the AC you have now will be enough. – longneck Feb 22 '16 at 21:05
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There are three pieces to this problem here.

  1. 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.

  1. 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.

  1. 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.

  • Very informative, thanks. The proposed space is conveniently located near various supply lines and is even more conveniently speaking adjacent to the utility room with the HVAC system. There are also several metallic boxes in the upper attic space also adjacent to the room. Are those called air handlers? Or are they called distribution boxes? These boxes have flex duct directly connected to them but are not on HVAC unit themselves. – saigafreak Feb 23 '16 at 15:29
  • The air handler is the combination of the blower, AC and heating unit inside the house. That's the HVAC system in your utility room. – longneck Feb 23 '16 at 19:50
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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.

  • This is the unfinished "attic", a side room next to an existing bonus room. The upstairs stays very very cozy during the winter and very cool in the summer. Unless I'm missing something, I'm not particularly worried about the system handling the additional load, since also there will be less unconditioned space above the first floor since this new room will consume that space. – saigafreak Feb 23 '16 at 11:46
  • Oh, very good then, not starting from scratch is a huge plus! However, IF your cooling lags like most places & isn't actually "cold-ing" until the Sun goes down or outside temperature drops then you'll definitely be lacking comfort everywhere. IF your system's actually cold & shuts off frequently during the day, then I'd agree that you'll be fine & would only be stepping the system down to the lag-level. Balance of the air flow in the rest of the house is what this room will be effecting, output & return circulation will be weaker all over & you'll notice. – Iggy Feb 23 '16 at 13:14

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