HVAC Options for a large room

[Maigahane]

TL/DR: What the best option to get heating and airconditioning out to a 300sqft addition?

Our house has a 300sqft addition built on the back. For some reason the guy that did it didn't extend the HVAC system out there. So basically the room is unusable for 9 months out of the year because it has no heating or air conditioning. It's also not sealed well so the cold air is currently leaking into the rest of my house. We're looking into options to fix that. So far we have:

1. Extend the existing ductwork out there. I don't think they'd have to go very far since there's a vent about 10 feet away from the door but the room is 24 feet long. It would probably involve removing the drop ceiling in the basement which we would then need to replace. I don't know how old our system is but I found a service receipt from the 90's, I'm afraid they'd recommend upgrading and I'd have no idea if it was actually necessary or if they were just pushing a sale.

2. I don't know what it's called but there are these systems that hang down from the ceiling and are wired into the house. I kinda feel like this would be a ghetto fix though it seems like it would be easier than extending the ductwork. With this I'd be concerned that we'd have to redo the electrical in that room since it's not on the main breaker, it has it's own fuse box.

3. I've heard of mini-hvac systems but I have no idea what they actually are.

Any suggestions or thoughts on these options? Any other options I didn't mention? And I know it can vary greatly but any idea what these would cost? I'm planning on getting suggestions and quotes from a couple of companies but want some info beforehand so I have some idea if I'm getting bs'd

We're only planning on staying in this house another 5-7 years and it's already at a state where nicer improvements won't help the value much (one of the nicer houses on the street but only valued at $136k). So we want an option that works but is on the cheaper side.

[bogart]

I don't know where you live but I recently went through an unpleasant experience of firing a contractor building an addition on our home and hiring another.  TL/DR:  It emerged that the first guy was planning/hoping to do a lot of things outside of the zoning inspection process.  Basically, I learned (hindsight 20/20 all that) that he had permitted and built our addition as a 3-season (unheated) room to which he was planning to add HVAC outside permit (among other things).

Turns out that at least where I live (US SE, very humid area) you cannot do that (easily) and perhaps for good reason.  The insulation he told me he had used (some of which it turned out he hadn't, but that's another story), which was the standard paper-backed-fiberglass-stuff, and the way he had built the room (no soffits in roof, etc.), the inspector (and new contractor) tell me would have led to all kinds of moisture and mold problems.  Long story short (ha!) we've ended up paying a lot to deal with getting foam insulation blown in in order to be able to have the system built as permitted with HVAC.

Your region (and regulations) may be different.  But if you're looking at selling the house in 5-7 years I wouldn't think you want any unpermitted work done?

We had the addition tied in to our existing (gas furnace) ductwork, as we find the current system very cost effective; the tie-in cost about $1500, including various inspections and a separate zone/thermostat for the addition.  I think the other system you're talking about is a minisplit, and I don't know anything about those.  Based on what you're describing, though, your best options might be some space heaters and a window unit...

[Maigahane]

We live in the midwest. I would definitely want everything to be on the up and up with permits and code and whatnot.

We had a roommate that used a window heater/airconditioner but the windows are on one end of a 24 foot long room, the thing didn't come close to working for the entire room. I want something more permanent than a space heater. And less fire-prone. And more efficient. And I really don't want to have to explain to a potential buyer that they'll have to have a space heater to use that room.

$1500 to hook into the existing system would be very doable and well worth being able to comfortably use the room for the next 5-7 years

[bogart]

Gotcha.  To be clear, the $1500 was just the cost of the hookup.  Getting the room OK to hookup cost noticeably more -- existing sheetrock had to be removed, foam insulation had to be blown in, sheetrock had to be replaced ... now, this was "relatively" simple low-cost for us because the work wasn't done (the sheetrock had been installed but not finished), but it wasn't cheap by any stretch. 

But if you're interested, you should be able to get a contractor to look at it and give you a free estimate.

[Takk]

Solutions include:

1) Finding a window unit that has a decent "throw" of the air, this being the cheapest option available, it may not be as efficient, but the cost analysis really shows window units being the best price point. If you have it running all the time to maintain setpoint, it may behoove you to go for a more efficient method, but if you're only running it when someone is in there (and that isn't "all the time"), it turns out to be one of the most efficient methods on a cost basis, assuming of course it conditions the entire space. Cost: $300?

2) Extend ductwork, if your home system is oversized, this can be an option; to figure out if it is oversized though would require testing, or at least a calculation. This would require ductwork routing if it's capable to do so. back of hand test may help, how often does your AC Cycle (turns off and waits for temp change to turn back on) in the coldest/warmest (depending on climate)? If it often shuts down, it may be oversized enough to help. Old engineer method is ~300-400sqft for 12,000btu. (1 ton of cooling/heating)  This method may or may not help, as it may be region specific, I tend to do Load Calculations to verify. Message me and we can chat about other options to determine if your system is right sized, or if you could make it oversized by doing things such as getting new windows, which would create enough difference to add in the new area. I don't know cost, contractor would have to really dig into it for a full evaluation.

3) Mini-Split system, You can place it in a different location central to the room to cover the whole room, Misubishi, Daikin, and a few others make them. This does cost a bit more, as it requires wiring, piping, but no ductwork, so if the ductwork renovation costs a bit, this may be a decent solution. They are very efficient with scroll compressors and fans that are not one-speed. you can then separate that room from the rest much like a window unit. It hangs on the wall for the cheaper ones, and I enjoy the design of them but some view it as an eyesore. There are ducted units that are less efficient but allow you to make in essence a new system out of them. A contractor told me this may cost ~$3,000 installed, but of course region specific and who you have available. Equipment ~1k, install ~2k. Estimated of course.

source: HVAC Building Design Engineer.