[Decca]

Hi!
I am new to this forum but not to DIY. I am looking for feedback on a very atypical HVAC project I have. I couldn't find any similar project anywhere on the internet.
I would appreciate all constructive feedback! (but please read the whole post before answering):vs_cool:


Long story short, I need to replace my split heat pump HVAC system currently in an uninsulated attic. I plan to insulate the attic and to put 1 mini-split IN THE ATTIC, then add ventilation through the ceiling to cool down the whole house. :vs_bulb:
( my house: 1953 1300sqf Florida ranch style, 3bed/2bath 2living room 1 office 1 kitchen 1 laundry, not open floor plan. Gable roof with ventilated and not insulated attic, small crawlspace)


I did a LOT of research and I think this might actually be a surprisingly efficient and cost-conscious alternative to spending $20,000 on a full removal/ installation of a new system.

Why can't I just keep my current HVAC or upgrade to similar system?
- ductwork is in uninsulated attic (in Florida!). I cannot insulate the attic as the ductwork takes the whole space (attic is very small). I cannot even access the attic. In fact, I don't even have insulation on the ceiling, and I don't have access to put insulation because of the current HVAC system. On top of that, the general installation and design of this HVAC system is very poor, and the lack of space also means the ductwork is very poorly done and highly inefficient.(sharp bents, compressed ducts, etc.). I highly doubt that paying for new ductwork would result in a much better result due to lack of space and bad location of air handler / outdoor unit. And that still leaves me without insulation and access to my attic.


What are the other options?
- relocation : I don't have anywhere else I can put it (I have a small crawlspace, but it is too small and therefore not an option).
- High velocity : too much work for retrofit and way too expensive.
- Ducted mini-split. Other than being way overpriced, these unit also do not work for 5 or 6 zones (which I would need). It would mean several units inside and outside, a lot of complexity, a lot of cost, and without even the added benefit of zoning cooling (as one unit would be used for several rooms).
- Ductless mini-split. I keep going back and forth with this. I really like the idea of cooling by zones, but the problem is the number of units / the location. If I put it in a room, what about the other rooms? with 3bed, 2 baths, and 2 living areas + kitchen (and not an open floor plan) this would mean a lot of units, or/and a very uneven distribution of airflow. Having 4 or 5 units would mean way more BTU's than I need, and also the added complexity of 5 things to maintain, repair, etc. It is also quite pricey.
For a while I thought that my only solid option was to go ductless with 5 zones (one in each room, 1 in kitchen, one for living areas), which is a little ridiculous and overkill for a small house like mine.


And then I thought: why not remove the whole system, insulate my attic very well (making a hot roof and include the attic in the conditioned space) and then put a 30,000 BTU mini-split in the attic. The attic would then be cooled evenly, and I would add a small ceiling fan to each room to bring the air from the attic to each room. My attic then acts as a "plenum" for even distribution of the air between the rooms, and ceiling fans make the transfer of the air faster (10 months out of 12 I am cooling, so cool air would go down anyways). I could maybe add a ceiling fan for a return from the house to the attic right under the mini-split.
It seems to me that I would get a quite simple, solid and efficient system this way. It will make my house 1 big room as far as HVAC is concerned. I would get access to my attic. It should be cooled quite evenly throughout the house, and that can be improved / fine tuned with the use of fans or maybe even a ERV ventilation system if needed. It allows me to fill my attic with insulation, thus tremendously improving the efficiency of my house (now it has 0 insulation). I would go from 10 SEER to 25 SEER, improved insulation, and more space (in the attic and outside unit). And I won't see the mini-split on the walls in each room. So, win-win-win, or did I miss something?
What do you guys think? :vs_worry:

 

[user_12345a]

Insulating the roof deck itself is a good move, saves a lot of energy.

$20000 seems very high even for a totally new forced air system.

The mini-split in attic idea i don't like, it would be very difficult to properly regulate the temperature. You would have to put large return air style ceiling grills in all the rooms, attic would still be colder and air could recirculate into the head.

Space pack and ducted mini is worth looking into.

If you go mini-split put them into the rooms.

 

[beenthere]

Is there any romex/NM wires in the attic. If so, they are not allowed to be in a plenum. All wires in a plenum are required by code to be plenum rated.

You would need a min of 2 opening to the attic for each room in order to get a room cool. Air will need a way in and out.

If you cooled the attic to 70°F, then you would need to move about 180CFM into a 10x12 bedroom to cool it to 74°F when near design outdoor temps. Instead of 80 CFM.

 

[Bob Sanders]

You're essentially talking about using the entire attic as one large duct.

You will never get enough insulation into the attic to make that worthwhile. You'll just be wasting money with higher cooling bills. Not to mention you will have a tremendous increase in household dust levels.

 

[Decca]

thank you been there, that's very useful.
bob sanders: I hear you but I am having a hard time reconciliation your answer with what some pros have aledgely done. Many green houses are cooled with 1 mini split / level and are very comfortable in every room. How having a mini split in one room to cool others be any different than having it in the attic (by definition a room adjacent to all others)?

 

[Bob Sanders]

thank you been there, that's very useful.
bob sanders: I hear you but I am having a hard time reconciliation your answer with what some pros have aledgely done. Many green houses are cooled with 1 mini split / level and are very comfortable in every room. How having a mini split in one room to cool others be any different than having it in the attic (by definition a room adjacent to all others)?

Insulation in rooms is different than insulation in an attic. The air space in the attic itself is indeed a layer of insulation when using traditional duct work. The sun beats down on the roof and some of the heat gets through (regardless of insulation value). The attic airspace itself is vented so some of that heat is in turn lost. You then have the insulation on the duct work protecting the cool air from the some what 'treated' air in the attic. Long story short... by using the attic as a duct you have instantly lost one (important) layer of insulation.

 

[Decca]

if someone thinks it won't work, please explain why. Pros doing conditioned attics say that "the temp in a well insulated attic will approximate the temp in the house even without a return duct in the attic". Then, the opposite should be true, especially considering that I will be cooling (cold air falls down) and I will be installing ventilation to the rooms...

 

[Decca]

Thank you Bob for the explanation.

Regarding the air around the ducts serving as insulation: I do understand the theory very well, but my current reality is it is not just some heat going through the roof. In fact the inside of the attic is much, much hotter than the outside. My ducts are literally inside an oven.
Added to the fact that the duct layout is awful and very inefficient, I am having a hard time thinking that what I want to do wouldn't be better. However, I do see your point that I wouldn't have any air insulation. I guess that in "green" homes they make for it with an airtight, well insulated whole house envelop, which in my case would be really hard to do as a retrofit... just insulating the attic doesn't make up for the lack of wall/windows insulation.

 

[KnowItAll]

Is your roof flat? What is the height from the joist to the roof? Is the existing duct round pipe? Square duct?

 

[beenthere]

Insulation in rooms is different than insulation in an attic. The air space in the attic itself is indeed a layer of insulation when using traditional duct work. The sun beats down on the roof and some of the heat gets through (regardless of insulation value). The attic airspace itself is vented so some of that heat is in turn lost. You then have the insulation on the duct work protecting the cool air from the some what 'treated' air in the attic. Long story short... by using the attic as a duct you have instantly lost one (important) layer of insulation.

Spray foamed attics are very easy to heat and cool. The spray foam allows the attic to be conditioned for a small fraction of the cost of what the rest of the house requires.

When the attic is spray foamed, the insulation on the floor of the attic is removed. No need for it.

The problem the OP will run into. Is that of the air volume exchange require to cool the rest of the house from the cooled attic air.