mini split usage in 2-story home with small bedrooms on second floor

[aaron1]

Hi,

I have been exploring my options to add AC to my 2-story, 1600 sq ft 100-yr-old home. I have ducts but they are very old and would probably need to be redone, since I have no returns on the second floor. Seems like that will cost at least $10k alone, so I thought maybe a few mini splits could work.

Has anyone installed mini splits in a home similar to mine or have advice on sizing and airflow? My lower floor is very open, so I could put one large unit in the dining room. The upstairs has 4 smallish bedrooms, a small central landing/foyer and a small bathroom. I was thinking putting the smallest, highest turndown ratio unit possible into our main bedroom and then put a larger unit in the guest bedroom to do the rest of the rooms, as 95% of the time those doors are open.

Does this sound reasonable?

Anyone have input on the maintenance required with these mini splits? I'm a little concerned about wet, dirty messes created up on ladders in nicely finished rooms, but people do it somehow right?

 

[UpStateNY]

Warning Central Hudson increased their electric supply rates by 2.5 times on Feb 10, 2022 versus the previous 12 month average. I have never seen an increase in electric rates this significant before. I stopped using my mini-split ductless Fujitsu system for heat until Cental Hudson rates come down. I have been using my 2 year old Mini-Split ductless for heating house when outside temp is above 30F.

 

[aaron1]

Warning Central Hudson increased their electric supply rates by 2.5 times on Feb 10, 2022 versus the previous 12 month average. I have never seen an increase in electric rates this significant before. I stopped using my mini-split ductless Fujitsu system for heat until Cental Hudson rates come down. I have been using my 2 year old Mini-Split ductless for heating house when outside temp is above 30F.

wow, that is a big jump. It went up to 21.6c/kWh for the supply rate alone. I am not using CH for my supply. It will be interesting to see what happens to my supply rate (or my delivery rate.)

Have you thought about adding solar panels?

 

[aaron1]

Central Hudson Alerts Customers of Higher Winter Energy Costs

Central Hudson is advising customers of increases in supply prices for both electricity and natural gas as the region navigates a colder than average winter and as global energy factors come into play, prompting an increase in the demand and price for energy. Supply prices are market-based and...

www.cenhud.com

 

[woodgeek]

Not what you were asking, but I have a similar home, a side by side split level, with small bedrooms and bathrooms upstairs, and an open plan lower story. I went with a central system, and as I had hydronic heat and no ducts, put the air handler in the attic, dropped supply vents into the ceiling, and had one central return in the stairwell.

As for bedroom returns, I did cut about 0.5" off the bottoms of all the bedroom doors. Done.

While in new construction supplies and returns to each room are the standard, using the door bottom as a return works well in retrofits.

I am a fan of mini-splits, but when you have 5 little rooms upstairs, and they want to do split heads or several units, I don't get it. Just get a central system. They make central systems with inverter compressors (to match the eff of minis). And the duct losses are small if you mastic them properly and insulate them. Caveat: if you were in love with some super low temp mini-split, I can't say if the central inverter systems can match it.

The only 'problem' I have is balance upstairs/downstairs with one zone/compressor. So I have a (very efficient DC motor) ceiling fan in my stairwell to mix the two levels.

In a central system model, you could have two smaller units, one upstairs and one down, and two zones. This makes more sense if your square footage is larger, mine is 2200 sq ft.

 

[aaron1]

Not what you were asking, but I have a similar home, a side by side split level, with small bedrooms and bathrooms upstairs, and an open plan lower story. I went with a central system, and as I had hydronic heat and no ducts, put the air handler in the attic, dropped supply vents into the ceiling, and had one central return in the stairwell.

As for bedroom returns, I did cut about 0.5" off the bottoms of all the bedroom doors. Done.

While in new construction supplies and returns to each room are the standard, using the door bottom as a return works well in retrofits.

I am a fan of mini-splits, but when you have 5 little rooms upstairs, and they want to do split heads or several units, I don't get it. Just get a central system. They make central systems with inverter compressors (to match the eff of minis). And the duct losses are small if you mastic them properly and insulate them. Caveat: if you were in love with some super low temp mini-split, I can't say if the central inverter systems can match it.

The only 'problem' I have is balance upstairs/downstairs with one zone/compressor. So I have a (very efficient DC motor) ceiling fan in my stairwell to mix the two levels.

In a central system model, you could have two smaller units, one upstairs and one down, and two zones. This makes more sense if your square footage is larger, mine is 2200 sq ft.

Thanks for the input. I would love a ducted system, but I don't know what to do about the attic. You're saying it's ok to put ducts into the attic if they are insulated? Everyone on Green Building Advisor says you lose 20-30% efficiency having ducts in an attic unless they are not only insulated but super insulated, like flex duct within flex duct and then buried in a foot or two of loose cover fill. I could actually do that if I used the space behind my knee walls for most of it.

What about using an air handler in the basement to heat the entire house and then putting in a single mini split upstairs to cool the entire house from the landing location. Here is my house layout:

 

[aaron1]

Oh, btw, the under-door return idea does seem like a good way to go as well. I agree with that. these grills for the door bottoms seem pretty awesome too if you have a central return:

Perfect Balance In-Door Air Pressure Relief System

Economical Solution for resolving pressure imbalances in the home, removes hot and cold spots from rooms, baffle mitigates noise an light, saves money, buy now, made in the USA

www.tamtech.com

 

[woodgeek]

I love the GBA people (or at least I did before they paywalled), but they are referencing attic ducts as they were typically done. LEAKY.

You can compute the losses from the R-value of the duct insulation and the area of the ducts. The area of the ducts is much smaller than the area of your house envelope. If it has similar R-value (8+) then how can you lose 20% of your heat?

Ofc, installers in attics are uncomfortable and rushed and can be sloppy. There are many tales of improperly attached ducts popping off and blowing heated air into the attic for years. Also, my installer lied to me, told me 'taped' all the hard ducting in my attic, when what he meant was he taped the FG insulation on the outside of the ducts... with the wrong tape. The insulation was all inflated like a balloon, and that balloon popped after a month or two. I ended up tearing back all the insulation, taping all the metal seams (with the right tape, but should've used mastic), replacing the insulation, and then buried all the flex under new cellulose blown in. This was in 2008... installers now are better, and you just have to make sure your installer uses mastic on any metal ductwork.

Ofc, I use the 'hillbilly FLIR' method of looking at the snow melting on my roof. No melting spot over my air handler or trunk duct. My neighbor with the same house, and still using an oil boiler with hydronic radiators... his roof snow melts slightly faster than mine. My attic airsealing is probably a little better. And both of ours do better than most of our neighbors.

Bottom line, attic ducts in the past have been disasters. Mine were not great, as installed 13 years ago. But properly sealed and insulated (R-8 current codes or higher) are not a big loss. I estimate I lose 2% tops of my HVAC power, less than I lose through a set of French doors. The GBA folks have put attic ducts on their chit list after seeing horror stories for years, and fetishized duct free solutions as a result, IMHO.

 

[woodgeek]

Looked at your floorplan... I needed to drop three flex lines from my attic to heat my bottom (half) story (I have three levels side by side). I did one through the plumbing stack (to a utility/laundry room) and two larger ones through a top floor closet. I lost about 6" of width in the closet, and put up a new wall to cover the two flex. You could do the same in the bedroom closets, but getting flex to the other side of the house could require an annoying chase... if you really have no closets or anything over there...

Oh, I just recalled you have ducts. Those old ducts are probably metal, leaky, buried behind horsehair, and too small for the low delivery temps for a HP. Huh. If they are in the exterior walls, I would probably abandon them. In the interior I might try to keep them.

 

[gthomas785]

If it has similar R-value (8+) then how can you lose 20% of your heat?

Keep in mind that the duct temperature is usually much warmer (for heating) than room temperature and the air is subject to greater turbulence resulting in more heat transfer than your average wall or window. R-8 is not really that much for insulating the warmest and windiest part of your house.
Another way to get the duct loss would be to measure plenum and register temps to get ∆T along the duct and then multiply by each register's cfm*1.08 and add them up.

 

[aaron1]

The area directly to the left of the words "second floor landing" could be used to house a ducted mini split i suppose, mounted vertically. I wonder if that would be a good compromise of getting ducted but keeping the unit separate from my basement unit (which could maybe be just for heating)

 

[woodgeek]

Keep in mind that the duct temperature is usually much warmer (for heating) than room temperature and the air is subject to greater turbulence resulting in more heat transfer than your average wall or window. R-8 is not really that much for insulating the warmest and windiest part of your house.
Another way to get the duct loss would be to measure plenum and register temps to get ∆T along the duct and then multiply by each register's cfm*1.08 and add them up.

Exactly this. Since the plenum is hard to reach, I measured the register temps at multiple registers, in particular one close to air handler, and one at the end of the longest run. I was not able to measure a significant difference in those temperatures, which were 50 or more degrees warmer than the air in my attic... thus my estimate that I was losing less than 2% to conduction from the ducts.

 

[Ashful]

I've had both installed in old houses, one almost as "new" as yours (built 1877). I agree with woodgeek, the small rooms on the 2nd floor when combined with an available un-finished attic, make the minisplit less attractive for your case. A minisplit is a great alternative, when there's no space left for ductwork (eg. finished attic), or one large space to cool, minimizing number of indoor units. But in your case, it fails on both counts.

But to answer your other question about a wet mess up on a ladder, yes... minisplits can indeed create drainage and water problems. I've had one of my units in my carriage barn freeze up and start pouring water indoors, on several occasions. Of course, it's caused by the infiltration of too much fresh (humid) air, when I leave the barn doors open while working a project. In your house, that'd be less likely to happen, unless you have a habit of leaving windows open while running AC.

There have been many drainage issues tied to minisplits, but usually any experienced installer is aware of these sortcomings, and then tend to be cautious in their drain line slope and planning.