Does it make sense to replace old central air with multiple mini-splits?

[stickshift]

2500sq ft house with a very old central air system running on R22 (freon). It runs, but does so inefficiently. Replacing would involve replacing the evaporator and air handler in attic, which would require making a large hole and cutting joists to get the old unit out and new unit in, so it would be an expensive job.

Even with a new efficient central air system (that will probably not last as long as the old R22 systems), you lose efficiency because you're cooling the whole house rather than the areas that are currently occupied (e.g., no need to cool the bedrooms during the day; no need to cool the downstairs during the night), while mini-splits can be individually controlled. Another benefit of mini-split is the ability to operate as heat pumps, which could substitute for my boiler baseboard system on all but the very coldest days, and given price of nat gas and never ending push in the West to make usage of fossil fuels more burdensome and expensive, it might make sense to rely on mini-split heat pumps when temps allow their use.

So for these reasons, I'm wondering if going with multiple mini-splits makes sense. I'm not familiar with how much coverage you get from these units, but I guess you can cover the open ground floor (living room, dining room, kitchen, foyer) with one larger unit. But what about bedrooms, office, etc. - do people put small units in each of these rooms? Basement - small unit for that, or just rely on main floor cool air to migrate down plus fact that basement is naturally coolest part of house? Just adding up the rooms, seems like I could easily end up with 1 large unit and 5 small units, which seems like a lot compared to what I'm seeing on installations on youtube.

 

[American Locomotive]

Bedrooms would likely require their own units. Larger indoor units can cover larger spaces.

There are two ways you can do this:
- Small individual mini-splits sized for each room of the house. You have an indoor unit and outdoor unit pair for each cooling zone.
- Multi-Zone mini-split. You have one large outdoor unit paired with multiple smaller indoor units. One outdoor unit can usually do 2-5 indoor units with these systems.

The multi-zone units are all the rage right now because it looks cleaner on the outside. However, I just helped a buddy with this - we were looking at a 3 zone system, but determined that 3, single-zone systems would actually be cheaper and significantly more efficient than the multizone from the same manufacturer - 33 SEER vs ~20.

....however, it almost never pays to replace a functioning air conditioning system. Even an inefficient R22 one. By the time your mini-splits would pay for themselves (about 20 years in most areas), they will be needing replacement again anyways. This isn't just a mini-split thing either - it's any system. My advice is to just run your current system and just keep putting capacitors and condenser fan motors in it as necessary until it catastrophically fails (major leak, compressor burnout). Then replace it.

 

[Notgrownup]

I asked this to a local HVAC guy and he said a study would need to be done to be 100% sure but the practical side is the separate units are able to be set at different settings and that may over time be more comfortable to you. Don’t think of it as a cost savings because if your current system is working, even with less than efficient capacity, it will take a long time to recover the investment.

 

[jlv03]

Is this a 2 story house? Sounds like it, but hard to tell.

You also mention baseboard heat, it would be hard to give up that level of comfort.

If this is a 2 story house, and the attic unit is serving all floors - what about a new/smaller system for just the 1st floor? And limp along the old system for some time.

 

[Firebrick43]

I know a guy that replaced his with one mini split in a two story home. (at least 2500 sq feet) It was some what of an open floor plan but not fully. He placed the head unit in the top of the stair well. It worked surprisingly well. I was there on a humid 95 degree day and it was not uncomfortable at all. He didn't know the btu size of the unit

My duplex addition is 1450 sqaure feet and it does well with one 18000 btu unit but is superinsulated icf. It isn't insanely hot as long as you don't cook without ac.

That being said, I would expect 2 units necessary in a normal 2500 square foot house. But two units give you redundancy if one fails and and single unit minis are much more efficient.

 

[stickshift]

You also mention baseboard heat, it would be hard to give up that level of comfort.

Well I wouldn't get rid of the baseboard heat. The idea is it can be more efficient to use mini-splits to heat when it's not very cold. When it's cold enough that mini-split isn't running efficiently (frost building up on the outside unit), then use nat gas fired baseboard heat.

If this is a 2 story house, and the attic unit is serving all floors - what about a new/smaller system for just the 1st floor? And limp along the old system for some time.

You mean a mini-split for first floor? Yeah, I had thought about that for precisely the reason that the old system still works and works best at night when it's not being taxed very hard on the hottest days. So the idea is don't run central air during the day and rely on the mini-split(s) to cool first floor and basement. Keep the central air registers on first floor and basement closed, and run the central air only at night when bedrooms are occupied.

Benefit of going this route is less up front cost, and I can keep central air system going until it requires expensive repair, at which point it gets removed (and ducting abandoned). But when doing the initial mini-split, I'd still have to plan for eventually going mini-split for the whole house, so that probably means going with multizone outdoor unit from the beginning.

 

[Firebrick43]

Well I wouldn't get rid of the baseboard heat. The idea is it can be more efficient to use mini-splits to heat when it's not very cold. When it's cold enough that mini-split isn't running efficiently (frost building up on the outside unit), then use nat gas fired baseboard heat.


You mean a mini-split for first floor? Yeah, I had thought about that for precisely the reason that the old system still works and works best at night when it's not being taxed very hard on the hottest days. So the idea is don't run central air during the day and rely on the mini-split(s) to cool first floor and basement. Keep the central air registers on first floor and basement closed, and run the central air only at night when bedrooms are occupied.

Benefit of going this route is less up front cost, and I can keep central air system going until it requires expensive repair, at which point it gets removed (and ducting abandoned). But when doing the initial mini-split, I'd still have to plan for eventually going mini-split for the whole house, so that probably means going with multizone outdoor unit from the beginning.

I don't think there is any plus to a multizone mini split unless its a new build and there is no ducting there. They certainly are not as effecient as the single zone system and they are not any cheaper than two (or more) separate systems single zone system. And there is still one failure point.

 

[fitter30]

Using two mini split 3 head units for 3 bed 2 bath. Living, kitchen, dining and master bed one unit. Two bed and bath and laundry room the other unit. Most minis at least 2 wall units have to be turned on. Using two units because im very rural parts are a few days out. The wall units need to be cleaned every year or two look on utube to whats involved.

 

[stickshift]

I don't think there is any plus to a multizone mini split unless its a new build and there is no ducting there. They certainly are not as effecient as the single zone system and they are not any cheaper than two (or more) separate systems single zone system. And there is still one failure point.

I think the whole point of the multizone mini-splits is what @American Locomotive mentioned earlier - fewer outdoor units. With single zone mini-splits, I'd be looking at 5-6 outdoor units - that's a huge increase from the 1 outdoor unit of a central air system.

I didn't follow your point with regards to ducting, since as far as I know, there are no mini-splits that use existing central air ducting.

 

[stickshift]

Using two mini split 3 head units for 3 bed 2 bath. Living, kitchen, dining and master bed one unit. Two bed and bath and laundry room the other unit. Most minis at least 2 wall units have to be turned on. Using two units because im very rural parts are a few days out. The wall units need to be cleaned every year or two look on utube to whats involved.

So how many head units total? Are you running a head unit in every bedroom?

 

[Firebrick43]

I think the whole point of the multizone mini-splits is what @American Locomotive mentioned earlier - fewer outdoor units. With single zone mini-splits, I'd be looking at 5-6 outdoor units - that's a huge increase from the 1 outdoor unit of a central air system.

I didn't follow your point with regards to ducting, since as far as I know, there are no mini-splits that use existing central air ducting.

There are head units(that go under the floor/above the ceiling but that wasn't the point. The point was I don't think a multihead unit is a good replacement for an efficient central air if there is existing ducts. The seer ratings drop off for the multihead units and the better central air units are pretty close to the multihead ones. Single head minisplits have excellent seer ratings


I highly doubt that it would take 5 or 6 units. I would say two, three max and I doubt three. In asia where they use minisplits for the most part, they just stack the outdoor units.



Many of the multihead units are litterally stacked but there is not redundancy if one section fails.

 

[stickshift]

I highly doubt that it would take 5 or 6 units. I would say two, three max and I doubt three.

I think this is the disconnect between how I'm thinking about head units and what you and others are saying. Basically I'm thinking that each bedroom would need a head unit. So by this logic, a 3BR house is looking at at least 4 head units (1 for living room, dining room, and kitchen, 1 for each bedroom), possibly more if you want to cool basement, or an office far away from the ground floor head unit.

Where am I going wrong in my thinking? If you don't have a head unit in each bedroom, I guess you keep the bedroom doors open at night?

 

[Firebrick43]

My original section has central air. Even still, the doors are open most of the time. The Duplex type addition for my mother is the same square footage and the living room/kitchen/dining room are open, and the bedrooms are oppisite. It does fine. Yes the doors are typically open but when guest are visiting they are closed and no one complains. The doors are not tight fitting to the floor.

And as I said in post 5, the gentalman that I know has one mini in a 2500 sqaure foot house without issue. I did not ask if they sleep with the doors open or not, nor did I snoop upstairs but it was a 4 bedroom house, all upstairs. If you leave them open all day there is likely not an issue with them being hot at night.

If you are involved in extra curricular activities you may need to open the door afterwards to let the heat out?

 

[fitter30]

So how many head units total? Are you running a head unit in every bedroom?

Wall units one in each bed and two in living/ dining room/ kitchen and ducted in a bath and laundry room. No return in bath just laundry where unit is mounted. 6 total two condensers three zones each.

 

[dsimatt]

I think this is the disconnect between how I'm thinking about head units and what you and others are saying. Basically I'm thinking that each bedroom would need a head unit. So by this logic, a 3BR house is looking at at least 4 head units (1 for living room, dining room, and kitchen, 1 for each bedroom), possibly more if you want to cool basement, or an office far away from the ground floor head unit.

Where am I going wrong in my thinking? If you don't have a head unit in each bedroom, I guess you keep the bedroom doors open at night?

We cool our 1300sqft ranch with just 1 18k minisplit in our dining room, it takes awhile but will cool the bedrooms at the other side of the house. Ideally it would be nice have a head unit in every bedroom thou we only use 2 but price and budget dictated going with 1. One thing to remember is with multi head units they are all doing cooling or heating and can't mix, i much prefer our baseboard heat over the mini because it just feels warmer.

Iwould grt some quotes for replacing what uou currently have, like the HVAC guys told me that they recommend mini's when forced air isn't a option.

 

[Hobby_Man22]

Or just replace it with the same setup and be good for another 20 years instead of putting a bunch of holes in the side of the house.

 

[finn]

I have a three head Mitsubishi mini, plus a single head Hyperheat MITSUBISHI (for a basement tv/exercise room). The three head unit covers an upstairs bedroom, main floor bedroom and the living room.

In generally I am ok with the setup, although the first floor kitchen, which is an addition made after the three head unit was installed.

I also had another single story house that I cooled with a single unit. It was a very open floor plan though.

All in, though, I much prefer the central air systems we had (and have) in our other houses. No way would I abandon a functional central air unit.

The cons are poor air distribution with the mini unless you have a very open floor plan, or go to lower efficiency multi head units, and the unsightly clutter on the exterior of the house, which is compounded if you go to multiple single head units. Two story installations are particularly ugly.

You already know the positives of mini splits: shoulder season heating capability, and ease of effectively zone heating and cooling unused portions of the house.

My advice is to look at actual costs involved in replacing your current R22 system with a new high efficiency central air system. I probably wouldn’t actually pull the trigger until the current system has a major failure, though.

 

[jar944]

I stayed in a rental house this past weekend that was relatively new (built 2011) and had already ditched the central air in favor of many many minisplits. Seemed like a odd setup with the condensing coils growing like mushrooms outside the house.

I had previously considered moving to minisplits for the upper hvac system in my house but I would need a 5 head, or 2 ducted and a standalone just to get some level of flow to the bedrooms. Conceptually it seems good but in practical application I can't get it to make sense. Seeing all the coils outside the house confirmed my thoughts.

 

[rlitman]

...The multi-zone units are all the rage right now because it looks cleaner on the outside. However, I just helped a buddy with this - we were looking at a 3 zone system, but determined that 3, single-zone systems would actually be cheaper and significantly more efficient than the multizone from the same manufacturer - 33 SEER vs ~20...

That's too often the case. Still, I went with a two head unit for my basement, because I couldn't find space for two compressors outside, but one was easy.

 

[yeldogt]

It's a two story house with one unit and it's in the attic? Is the attic insulated or is the unit basically sitting up there in the unconditioned space?

The problem with mini-splits in bedrooms -- the heads are too big (BTU's). In most of the world the expectation for the operation of HVAC is not to the level we expect in the USA.

IMO -- a proper ducted system is always going to be a better fit for most in the USA -- that is not to say I don't like Mini's --- they are great problem solvers. I have them at the beach house and my new build has three of them (for odd places where I can't get ducts). I may stick one in my new place in SC

Multi heads are the answer for a system where all the heads will be running .... you don't want a 3 head system where only one head will be running most of the time. Multi heads all running will be very efficient as they run more in the sweet spot of the single compressor ...... remember ... the efficiency ratings for the units are all given when the unit is running at its most efficient ..... that's around 70% ... many singles are rarely there.

In SC I have a large bedroom bathroom all by itself ... the current system had ductwork that's not big enough. I'm trying to figure out what is best way to deal with the problem. I'm not sure I can get it to work even with zoning and a variable speed system. May be best to stick a small mini-split up there to add to the current system.

My beach house has a large addition that has a lot of glass ..... it was easier to tuck a mini up on a back wall to cool the room (we use it for some heat as well) -- the room is open to the kitchen with ducts. It's easier to just control that one room with the mini and in many cases when it is not real hot we have that running and the main system is off.

 

[Jackfre]

To answer I think you have to evaluate your existing duct work. Is it adequate and/or can it be modified to provide desired zoning. In my home when we moved in the ducted system was terrible. I pulled the whole system and went with mini-splits. I have a dual (16 seer) for the upstairs bedrooms and two singles (25 seer) for the downstair open floor plan. The upstairs units are really only run for cooling. The higher efficiency downstairs units are run year round and very economically. I have a Rinnai LP wall furnace on the N side of the house. I’ve been very pleased with the results. Couple the mini-splits with a good solar system and you can zero out H/C costs. Fitter makes a good point. Cleaning mini’s is a chore.

 

[American Locomotive]

It's a two story house with one unit and it's in the attic? Is the attic insulated or is the unit basically sitting up there in the unconditioned space?

The problem with mini-splits in bedrooms -- the heads are too big (BTU's). In most of the world the expectation for the operation of HVAC is not to the level we expect in the USA.

IMO -- a proper ducted system is always going to be a better fit for most in the USA -- that is not to say I don't like Mini's --- they are great problem solvers. I have them at the beach house and my new build has three of them (for odd places where I can't get ducts). I may stick one in my new place in SC

Multi heads are the answer for a system where all the heads will be running .... you don't want a 3 head system where only one head will be running most of the time. Multi heads all running will be very efficient as they run more in the sweet spot of the single compressor ...... remember ... the efficiency ratings for the units are all given when the unit is running at its most efficient ..... that's around 70% ... many singles are rarely there.

Sorry, but that is actually not correct. Air conditioning systems, refrigerators, heaters, whatever, are all tested at their rated capacity. That is the "EER" rating. The rated capacity is actually typically one of the lowest efficiency points of modern air conditioners. That's why so many mini splits have extremely high SEERs - their efficiency goes way up as they modulate down.

Most mini-splits achieve peak efficiency of around 20-30% of rated load. So for a bedroom, a 6000 btu mini-split would be running at ~2000-3000 btu/hr. Peak efficiency.

 

[mike93lx]

I would never put in a system that required leaving bedroom doors open at night. Doors closed can significantly slow the spread of a fire.

If your duct work is serviceable, I would stick with that. The mini splits will not be cheap and will require a lot of new piping

 

[reader2580]

I would never put in a system that required leaving bedroom doors open at night. Doors closed can significantly slow the spread of a fire.

If your duct work is serviceable, I would stick with that. The mini splits will not be cheap and will require a lot of new piping

One thing I notice in Youtube videos about HVAC is a lot of the systems on the East coast tend to have either one return period, or just one return per floor. Here in the Midwest houses generally have a return (or sometimes two) in every room except the bathrooms.

 

[yeldogt]

Sorry, but that is actually not correct. Air conditioning systems, refrigerators, heaters, whatever, are all tested at their rated capacity. That is the "EER" rating. The rated capacity is actually typically one of the lowest efficiency points of modern air conditioners. That's why so many mini splits have extremely high SEERs - their efficiency goes way up as they modulate down.

Most mini-splits achieve peak efficiency of around 20-30% of rated load. So for a bedroom, a 6000 btu mini-split would be running at ~2000-3000 btu/hr. Peak efficiency.

That 's not what the engineer at Mitsubishi told me ... I was trying to figure out in the ins and outs of the various systems. Singles, Multi-head all back to the one compressor and multi-head using a branch box back to one compressor. How different were hyper-heat got in the mix -- the early hypers were all those floor units with high COP numbers.

He told me the most efficient compressor output is in the 65 to 80 range. Now -- I would think when it's running at 25 percent it is using less power vs running at 70. The hypers still minimum out at 25%.

I really don't know what the published numbers correspond to.

The issue with the multi back to the compressor type is the lines to all the heads are active even with the head unit is off -- it's a low flow -- but it there. So if you have a far flung head someplace that you never use --- not a great spot for a multi as the refrigerant it flowing and will obviously lower the efficiency. This is why many complain about over cooling .... the unit fan on the wall units is always running and just the bleed of the refrigerant will cool.

The branch boxes are different and don't suffer the same.

I had an issue with my new build in that I had to use a mini in my kitchen .... the head on a multi does not have the same performance as when it's a single (range is less on a multi) ... it also would have been paired with either a very high up room seldom used or a lower level walkout that needs low output. So the best thing to do was keep the kitchen as a single .... I ended up doing three singles because the other two room worked best with the compressors on either side.

 

[mike93lx]

One thing I notice in Youtube videos about HVAC is a lot of the systems on the East coast tend to have either one return period, or just one return per floor. Here in the Midwest houses generally have a return (or sometimes two) in every room except the bathrooms.

I have one return on the second floor and it isn't ideal, but it works ok. I'd add something to balance or cut door bottoms before leaving doors open at night though.

When we have to replace the system, I will likely have returns added

 

[jar944]

One thing I notice in Youtube videos about HVAC is a lot of the systems on the East coast tend to have either one return period, or just one return per floor. Here in the Midwest houses generally have a return (or sometimes two) in every room except the bathrooms.

We just cut the doors short over here, fast cheap and inefficient

It's always one+ cold air return per floor, with usually one extra in the ceiling from my experience.

 

[Hobby_Man22]

We just cut the doors short over here, fast cheap and inefficient

It's always one+ cold air return per floor, with usually one extra in the ceiling from my experience.

I noticed yesterday my ac hasn't been taking out the humidity. Opened up a couple doors and the humidity dropped from 75 percent to 58 percent. You close off all the register vents and only have the main return vent I guess it was tricking the thermostat.

 

[Kaizen]

I just ordered a 3 head minisplit. LG. 18-18-12 and 32k outdoor unit. Once i get linesets and specialized tools i'll be around 5k.
fyi might want to see what is available. Not a lot are available. I wanted to get a setup last year at this time and NOTHING was available.
If you have the ductwork there are setups that use that. I'm not sure if the new setups like that would be able to be put in without all that destruction. I know mr cool had a good setup that uses ductwork. As i do not i did not investigate much.
I plan on running mine whenever possible for heat although i have a perfectly good oil boiler with baseboard. There is a transition time that it is cheaper to use fossil fuels vs minisplit. Probably dec-feb. I'm hoping to do solar at some point so able to offset some cost hopefully

 

[stickshift]

If you have the ductwork there are setups that use that. I'm not sure if the new setups like that would be able to be put in without all that destruction. I know mr cool had a good setup that uses ductwork. As i do not i did not investigate much.

Interesting. I'll have to look into this. Being able to use existing ductwork w/ mini-splits would solve some of the issues b/c then I'd need far fewer headunits, which would allow using the more efficient single zone mini-splits while not having lots of outdoor units as well as all the linesets running along exterior walls.

 

[American Locomotive]

That 's not what the engineer at Mitsubishi told me ... I was trying to figure out in the ins and out of the various systems. Singles, Multi-head all back to the one compressor and multi-head using a branch box. How or if hyper-heat got into the mix -- the early hypers were all those floor units with high COP numbers.

He told me the most efficient compressor output is in the 65 to 80 range. Now -- I would think when it's running at 25 percent it is using less power vs running at 70. The hypers still minimum out at 25%.

Perhaps on a multi-zone unit, but most single zone mini-splits are most efficient at very low load. You can look up minisplit COP/performance charts, and the DOE and EPA have done testing. Most are most efficient around the 25-40% range.

 

[Kaizen]

Interesting. I'll have to look into this. Being able to use existing ductwork w/ mini-splits would solve some of the issues b/c then I'd need far fewer headunits, which would allow using the more efficient single zone mini-splits while not having lots of outdoor units as well as all the linesets running along exterior walls.

To be clear these outdoor units take the place of your existing condenser outside and are run into something inside. i don't know what the indoor setup is as i said.
Also, depending on your floorplan, consider running your exisiting system on fan mode if you have outlets in all bedrooms. Not as efficient most likely but it would solve the issue of not getting 4 head units. To be honest a 10x10 bedroom with a small minisplit head i don't think would work right as they need minimum areas. call supply house.com. lots of info

 

[justinjoyal]

If your existing ducts can properly handle the cooling and heating load and provide adequate air to each room, I would not go with mini-splits. Nothing beats being perfectly comfortable in every room of your house. Mini-splits can't provide you with that comfort unless you put one in each and every room and that would involve a lot of work and money and would not make sense IMO.

There are "ultraheat" central heatpumps now, efficient down to -22F or so. I install a lot of them, they are great, really !

 

[Dagny]

No.

 

[428PI]

Got my multi zone mini split going last week. 9k unit in downstairs bedroom and 18k in dining room. Works fantastic. Very quiet. Outdoor unit is a triple so could add 3rd unit to upstairs. Square footage downstairs is around 1000. Have had 95 degree days and will easily cool to 72 but basement is still cool. Way less noise than heater through central air.

 

[dsimatt]

If your existing ducts can properly handle the cooling and heating load and provide adequate air to each room, I would not go with mini-splits. Nothing beats being perfectly comfortable in every room of your house. Mini-splits can't provide you with that comfort unless you put one in each and every room and that would involve a lot of work and money and would not make sense IMO.

There are "ultraheat" central heatpumps now, efficient down to -22F or so. I install a lot of them, they are great, really !

What are your thoughts or experiences with the horizontal ducted units, they look like a good option to put in my basement and run ducting to my bedrooms thru the floor.

I agree with you on using existing ducting, my house it would have been a major deal adding it into the house.

 

[bonneyman]

I used to think those minisplits systems were kinda funky, but with the prices of new central units lately I've rethought that opinion. If I had to replace my whole house unit I'd seriously look at a trio of minis instead. It wouldn't be cheap and they're not trouble-free but they might just fill the bill.

 

[theoldwizard1]

Replacing a central air system with multiple mini-splits will cost a lot more than replacing the central air. It will pay back in the long run especially if some units can be turned off and the area blocked off (bedroom) when no one is in the area. Hard to calculate the payback time.

Units that support multiple air handlers are less efficient than ones that support only one air handler.

 

[Syberia]

In my case, yes. But that's because our central air was builder-grade **** installed by flippers that ran through uninsulated ducts (1930s house) and didn't work very well at all. Using an Emporia Vue energy monitor, air conditioning the parts of the house that we actually use has cost us less than $20 since the beginning of the month, and that includes a room I use as an office 8 hours a day M-F.

Cost of equipment was around $6500 for three 9k single units and one 36k multi-split with three 12k heads. Labor was free because my time is worthless.

EDIT: We did not do this for "efficiency," as the person who said that they would never pay for themselves before replacement is likely correct. It is, however, an added bonus. We did it (along with insulation/window upgrades) because the existing system could not keep us comfortable during the summer.

Here is the 36k condenser, it's on the "ugly" side of the house with the swamp cooler and old central A/C there already so it doesn't bother me. Only two of the heads are currently installed (third room is just being used for storage right now and is a mess), but the lines for both are routed under the cover into the attic and then down to the kitchen/living room.


These are the single-heads we use for each bedroom. Only two are currently installed because the third room is also just being used for storage right now.

 

[jar944]

If your existing ducts can properly handle the cooling and heating load and provide adequate air to each room, I would not go with mini-splits. Nothing beats being perfectly comfortable in every room of your house. Mini-splits can't provide you with that comfort unless you put one in each and every room and that would involve a lot of work and money and would not make sense IMO.

There are "ultraheat" central heatpumps now, efficient down to -22F or so. I install a lot of them, they are great, really !

Unless your secondary goal is to get the hvac systems into the conditioned space, and the current system is in your attic. Splits are likely going to be cheaper / easier than trying to move the insulated layer to the roof deck.