Replacing an attic-ducted HVAC system on a small, chopped up house
I own a 900 square foot house in Climate Zone 2A (low SW desert) that’s now a rental. The roof mounted “gas pack” is about 25 years old. The duct work is in a low, poorly vented attic that is about 4 feet high at the peak. It’s a hip roof, and there’s a lot of carelessly installed fiberglass batt insulation on the attic floor that does nothing for the ductwork. The windows are new double-pane vinyl, the walls are six inch concrete block with no insulation. The tenant has had abnormally high cooling bills the past two summers, and while there could be a variety of reasons for that, I want to plan for the end of life of the current HVAC system and move the house closer to being a “pretty good house”.
My first thought was to abandon the ductwork and use mini-splits. If I did that I would remove the batts, seal the attic floor, blow in cellulose and try to improve the venting. The issue I see with mini-splits is that the house is chopped up in to small rooms (< 200 square feet) and it’s probably not reasonable to require the tenants to keep bedroom doors open. I don’t know if mini-splits make sense in this situation.
My second thought was to stay with a ducted system (perhaps a ducted mini-split?), seal the attic and make the attic a semi-conditioned space. The rafters are 3 1/2 inches so I’m not going to get a lot of spray foam in there. I’m more inclined to put a whole lot of rigid foam on the roof but I guess I could do both.
I’ve attached a floor plan which is not terribly accurate yet but shows the room configuration. The bedrooms are at the top, the kitchen is the lower right, the room on the far left is a living room, and the bathroom is the room in the lower middle.
I did a major interior remodel before I discovered GBA, so while I’m willing to spend money on HVAC, insulation and even a new roof, I’m hoping to avoid tearing into ceilings and moving walls. I would love to hear how folks here would approach this.
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Long DIY opinion:
Years ago, I had an 'energy audit' performed. I didn't get much of value out of it. Years later, I realized that what I wanted wasn't a boiler plate 'report' discussing my plug loads but rather a manual J and a recommendation on envelope improvements and/or system improvements.
You can pay someone to do this for you. Look for an energy rater who performs Manual Js (or more). If you pay for it, be sure to negotiate that you get the as built manual J but also 2 or 3 modified reports to reflect improvements you are interested in and can afford. (see below for quick ideas). It's no good to have a model of what's there if you don't get a model of what you might do instead.
But if you are savvy (you are on GBA), try out the free web load calculator coolcalc.com (or an older simpler one loadcalc.net, which works but I think doesn't do room by room). GBA has several articles to help you understand load calculations. Cool calc is relatively straightforward (there are a few sticky things, like whether you include the floor and ceiling area in each room and dealing with garage partition walls). After you have your as built model, you can tweak things to represent your planned improvement. Certain things would be very easy to try as tweak (increasing ceiling R value; increasing board insulation on your walls). Unfortunately, they don't yet have duplicate model or a model descendant type system to let you save the original or the tweak so make sure you know what you did to your as built (or build the model twice).
After you perform your load calculation carefully (or pay for one), perhaps you'll see some great opportunities to improve the envelopment or be more convinced that mini splits would help. Conventional wisdom (and probably your new coolcalc model) will indicate that roof heat gain and window heat gain rule your cooling load and your walls and slab matter less.
But load calculation aside, I want to mention some quick ideas:
Check your unit:
I'd do this now. If you can find a good refrigeration tech (not all companies have one), a quick do it right now for ~$200-$300 is to have the tech come out and perform a service on the system including checking refrigerant pressures. It's beyond my knowledge to tell you what they should look for but a system can sometimes provide years more live with some refrigerant and tuning. Yes, you want it cleaned but you need someone to check and adjust pressures and add or remove refrigerant. I got a lot of mileage out of a high quality tune recently. Don't call the 'ac service for $79' people; the job requires 1-2 hours.
Stucco over Foam:
If your exterior is concrete, consider hiring a siding/stucco company to install EPS or mineral wool board at least a couple inches thick. I would anticipate $12+/sf for this work but you might have cheaper stucco labor down there. You'll have to figure out how you want the windows trimmed as they will now be 2-3" back from the surface of the wall. Obviously this could cost a lot of money. Whether upgrading to stucco over foam increases the value of your building depends on your market. You'll need a load model to find out what % reduction in load you might experience; it's possible walls won't help much with cooling. It seems like something your building could use but it might not help and costs a lot.
Shade your windows:
If you have unshaded west or south facing glass, consider finding a way to shade it. European motorized exterior roller shutters, vegetation (juniper tree?), exterior plantation shades, awning, eyebrow roof, etc). Interior shading helps only a little for controlling heat gain. You could DIY shutters; whether the tenants will use them is up to them. This option could be your cheapest potentially but is usage dependent.
Roof space:
It sounds like your ducting and your insulation is a messy difficult system to maintain. Personally, I think your 'attic' sounds like enough reason by itself to look into a non-ducted system. So here are some ideas to consider:
- Just 'fill' the attic with cellulose. I'd just leave the fiberglass there unless you are really motivated. Advantage: less disturbance, reduce duct losses, reduce ceiling gain/loss. Risks: Condensation from lack of ventilation (probably minimal in your climate), breaking your drywall from all the weight in the ceiling, cellulose infiltration into the duct system, crushing flex duct.
- Abandon the system in the attic, ideally pulling the ducting. Install mini splits or another system with heads. Fill the attic with cellulose). You really need a load calc to figure out what's needed and what this gets you. I'm guessing pulling the ducting, insulating the ceiling to R-60+, and installing seer 16+ multi head mini splits is your best bang for the buck. The mini split install will be pricey of course but if you are looking at equipment retirement anyways, maybe it's not so horrible. Again, a load calculation will help you figure out if the individual room loads are high enough to justify individual heads. Mitsubishi and LG (maybe other companies too) makes heads as small as 6k that modulate down quite a bit (1500 btus??) and would run efficiently at 3000 btus. Yes more heads will cost more money. If you have an overhang on the building, you might think about putting those refrigerant lines outside the house tucked up under that overhang instead of in the attic. There are plastic cover kits you can buy online that would protect your refrigerant lines from direct sun and animals etc.
- If you have high ceilings (9'+) there are some options to abandon the existing ducting and install new tin on the inside of the envelope (again, remove abandon ducting and registers, 'fill' attic with cellulose, build drop soffit). In a more complex house that might be pretty expensive but your layout seems pretty simple. You would need a mechanical closet (perhaps outside the envelope, tightly sealed and very well insulated)
Hybrid system:
- I'm guessing your climate is too hot to bother and your building has too much uninsulated thermal mass, but you might think about a single duct insulated whole house fan. I'm not sure how you do that without a gable roof end. Perhaps a whole house fan manufacturer could recommend something. Perhaps it would help with your shoulder seasons.
Personally, if it was my structure, I'd:
- get a good refrigeration tuneup and replace all missing insulation on exposed refrigeration lines (the tech will say it doesn't matter, that's bs). (cheap)
- decommission everything in the attic, insulate the attic to R-60 and go with a 4 head mini split unit, putting the refrigerant lines somewhere besides the attic, assuming the loads remotely support individual heads. Your tenants will love the individual control. (expensive)
- Plants a few junipers to shade your worst exposure windows (as determined by the manual J). (relatively cheap)
Good luck.
Keith, thank you for such a thorough response. You intuited several things correctly. There is some south side shading needed as the eaves are too short to shade the windows. And I would love to make the attic something I deal with as little as possible.
I did have a service guy check the unit last summer and they didn't find anything out of spec. I did find that the return air flex duct (from the intake in the house) had come about 1/3 loose from its attachment to the main unit, so it was sucking in hot attic air. My attempt to repair that may have failed, so that's the first thing I'll be checking.
(Side note: over 2-3 days of 115 degree temperatures two summers ago, the two AC units I own that date to the late 90s came through fine. The upgraded (14 SEER) Rheem classic unit I installed new in 2013 failed and needed a $500 repair. Although it's a tiny sample, it's tempting to conclude they really don't make them like they used to.)
Sounds like the next step is do the Manual J and, with that in hand, come back for more help in determining if there's a mini-split setup that will work given the room sizes. If I'm willing to sacrifice some closet space perhaps I could serve the 2 bedrooms (and maybe even the kitchen) with a ducted mini-split system and serve the living room with a standalone unit. I know even less about ducted mini-splits but I'm thinking it might be a way to combine rooms to meet a minimum load requirement.
Hi Nick and Keith -
Keith--you nailed it!
Nick: I have not used Rheem equipment and I am a cold climate guy, but the performance and reliability of mini-split systems has really come a long way in the last several years.
The hardest part in almost any region of the country is finding solid experienced high performance HVAC folks. Might be your hardest nut to crack after getting such good direction from Nick.
Peter
HVAC, so we can assume you also use this unit for winter heating? Single story. Would it be possible to move the unit outside? Build its own little utility shed that runs up to the overhang and run the main trunk up and into the attic, if you don't want to reroute all the lines. (you could lengthen the over hang to match the roof line of the shed, or false chimney, if you have to for proper main duct area) You could easily install a return from inside the shed into the house. Gas piping and venting would be a piece of cake, as well as easy service if needed.
Then address the attic and venting insulation.
FWIW: To hit IRC code-min a CMU wall in zone 2 only takes 1" of exterior EPS (R4) under EIFS/stucco, which isn't usually a tough retrofit to pull off the way thicker foam might.
The exterior mounting and attic ductwork is probably doubling the system load. Installing 4" of reclaimed roofing polyiso above the roof deck under a CRRC listed "cool roof" membrane roof, and R15 rock wool batts between the 2x4 rafters, would bring the roof up to IRC code min on a U-factor basis, and probably bring the total cooling load of the house down to something a 1.5 ton mini-ducted Fujitsu 18RLFCD mini-split (or maybe even a 1-ton) could handle with the ducts- maybe even the original ducts running in the now conditioned attic. (It would clearly handle the heating loads as well.) Any attic floor insulation could just stay in place, but sealing off the venting would be necessary.
In my area 4" of reclaimed roofing polyiso runs about 75-80 cents per square foot, a buck a foot if done in two layers of 2". You'd be spending a similar amount for R15 batts. With a fairly simple hipped roof this is completely DIY-able for less than a couple grand in material for the roof insulation.
The roofing would be quite a bit more. A 5/8" OSB nailer deck through-screwed to the structural roof deck and a high SRI cool roof shingle layup might come in cheaper than membrane.
It's not worth spending a lot of money trying to optimize a 25 year old oversized gas-pack with ducts outside of conditioned space, but a simple service check to verify that it's working and charged properly could go a long way toward tenant goodwill in the near term.