Ductless vs HVAC - 2nd Floor

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Rafal

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I'm getting conflicting advice so I'm really hoping someone can help me in deciding (I'm a new home-owner with just the most basic understanding of this - not an AC specialist).

2 Options:
- Install Ductless (Mitsubish 14K BTU - GL15NA) on second floor
- Replace existing 80,000 MBTUH furnace (Keeprite N9MSB) installed in 2017 with 40,000 MBTUH one (Trane S9V2)

Situation:
- I'm in Toronto, Canada in a 100+ year old ~1,300 sq ft semi-detached house (2 floors plus basement and a closed off attic - only insulation there)
(so, according to this https://www.acdirect.com/gas-heat-learning-center-furnace-sizing-calculator , a properly sized furnace would be 1,300 * 50 (at least) = 65K .. the N9MSB is rated 92.1% so * 80,000 = 74K --> which to me doesn't seem all that far off from what we have)
- We do have ducts, HVAC and Gas furnace (basement)
- Issue is - 2nd floor (3 separate rooms plus bathroom) really heats up, both in Winter and Summer ..... for Winter, well, we just keep main floor cool at night so we can dress warmer and live with that; but in Summer - it's just too hot upstairs

I was hoping to get someone to diagnose the problem and offer different solutions (neighbours mentioned for example installing some fans in the ducts) with pros/cons and cost. Well, that did not happen.

Instead, I had HVAC guy come in and he said our furnace is too strong and so cools off main floor fairly quickly so not enough air gets in that time upstairs. His solution is to replace the furnace - i.e. air will circulate longer so getting more of it upstairs, plus removing more humidity. He said ductless wouldn't work well for us as it'd be prohibitively expensive to get one for each of the rooms (plus the rooms are small), whereas getting just one unit wouldn't be good either since we'd have to keep bedroom doors open and even then it still wouldn't cool them enough.

I had a ductless guy come too, it was the opposite - saying one unit could keep the entire floor/3 bedrooms cool, whereas even if we did install another furnace that wouldn't work since we have only one return (on main floor) and air would warm up in the big ducts anyways by the time it would get to the bedrooms.

Both solutions have a very similar price of about $5K after taxes.

How do I decide? Will one / both of those solutions really allow me to keep the bedrooms upstairs cool in the summer?
 

Fitter30

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2 store houses need a separate ac unit. Heat if your fine with what you have just leave it. Can get a mini with a ducted air handler or a regular air handler 1.5 ton.
 

WorthFlorida

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The problem with any old home built is the cold air returns for a second floor home. As you have none because of the extreme age of the home and even newer homes in the snow belts still placed the cold air returns near the floor. With one heating system the hot air has to be picked up at the ceiling, I'm in my 4th 2 story home and it's a balancing act. For heating season it is best to put the thermostat in "fan" mode. It will circulate the air 24/7 and the electric is not that much as you might think. The WiFi Honeywell thermostats have a "circulate" setting where a minimum of 35 minutes per hour the fan will turn on to circulate the air. I use it all the time in my 2 story Florida home with one HVAC air handler on the second floor. Most of the time it keeps both floors comfortable. With it not set to circulate, first floor cold, second floor very hot in cooling mode.

For cooling each room needs its own vent or cassette for best comfort. The AC guy that said one unit will cool the floor but that means the doors must be left open all the time.

On this forum is heating expert named DANA. Search on his name and he has hundreds of post on how to do heat load calc and many good suggestion on mini splits.
 

Drick

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What WorthFlorida said. If you have air returns on the second floor you can buy programmable thermostats that will run the fan for some number of minutes every hour. Doing this balances out the heat/air between the floors.

If you do not have return ducts on the second floor the HVAC guy isn't really solving your problem. You need to spend that $5k to get a couple of return ducts run from the second floor to the return in the basement. Then buy a good programmable thermostat with a timed fan cycle. No need to replace the furnace.

Alternatively you could go with a mini split cassette installed in the attic and ducted to the smaller bedrooms (you don't have to install a head in each bedroom) with a return in the hall. The mini split will be more efficient as well.
 

Rafal

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What WorthFlorida said. If you have air returns on the second floor you can buy programmable thermostats that will run the fan for some number of minutes every hour. Doing this balances out the heat/air between the floors.

If you do not have return ducts on the second floor the HVAC guy isn't really solving your problem. You need to spend that $5k to get a couple of return ducts run from the second floor to the return in the basement. Then buy a good programmable thermostat with a timed fan cycle. No need to replace the furnace.

Alternatively you could go with a mini split cassette installed in the attic and ducted to the smaller bedrooms (you don't have to install a head in each bedroom) with a return in the hall. The mini split will be more efficient as well.

Thanks!
I looked again and indeed I do have return in rooms and hallway, and was already thinking of programmable thermostat since I want to control remotely - good to know re programming it to turn on for period of time to circulate the air. I did have the 'on' option for fan turned on during summer - electricity went higher but not sure really helped :(

Interesting re cassette unit in the attic for main bedroom and ducting it to other rooms - wish local ac people would have mentioned that as an option - I'll have to ask.
 

Dana

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A 1.25 tonner seems CRAZY oversized for the upper half of a 1300' house. Even the minimum output of a GL15 is probably higher than the design cooling load(!).

That AC direct online BTUx square foot calculator is utter CRAP! I'd go as far to say that it is irresponsible disinformation! I live in a 1920s antique 2400' above-grade + 1600' of insulated semi-conditioned basement (stays above 15C in the basement all weather, but not directly heated) bungalow with antique windows + 1980s vintage clear glass storms (no low-E) that has a design load less than 40,000 BTU/hr @ -15C/+5F, which is pretty close to the -17C/+1F 99% outside design temp for Toronto. A 65K furnace would be nearly 2x oversized even for my house, and would end up delivering LOWER comfort levels due to the low duty cycle even during cold weather. ASHRAE recommends no more than a 1.4x oversize factor from the load at the 99% temperature bin, which is usually more than enough to cover the temperature extremes during a Polar Vortex disturbance cold snap. Anything bigger than that is just less comfortable.

Before talking to any more HVAC contractors, run a room by room Manual-J type calc using some freebie online tools to get a better handle on it- this is just nuts. Be aggressive about your input assumptions or the oversize factor will be similarly crazy. (When using those tools I always presume zero air leakage to the outdoors, and they still oversize relative to what a competently executed professional tool does.) Use both tools, and compare the output to see which one makes more sense. When in doubt, go with the smaller numbers.

A 3/4 ton cold climate ducted mini-split could almost certainly handle the doored off bedrooms zone with margin to spare, in both heating & cooling mode. Even a non-cold climate 3/4 ton version could probably handle it down to about -15C or so. But run the load numbers, we'll see.

Your existing furnace is probably a bit more like 3x oversized for your 99% load, but that can be determined by running a fuel-use based load calculation, which is pretty easy if you have some of last winter's gas bills. The fuel-use load calc would also be a reasonable sanity check on the online load tools too.

Are all the ducts inside the pressure and insulation boundary of the house?

Are the ducts well sealed?

Are there return registers for every doored off room with a supply register?

Is the basement cold in winter, with the second floor & attic warm? (If yes, that a symptom of significant air leaks at both the attic and the basement, both of which can be fixed fairly cheaply, improving net efficiency & comfort even with the existing beasty-furnace.)
 

Rafal

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A 1.25 tonner seems CRAZY oversized for the upper half of a 1300' house. Even the minimum output of a GL15 is probably higher than the design cooling load(!).

Dana, thank you for the comments. I am saving your post to do the calculations, re-view again your comments (and glad to read that my HVAC/Furnace guy aligns with yours, I started wondering previously). Sadly, I had several HVAC/ductless reps coming and not a single one of them would even entertain the idea of doing a J calc - so indeed I will have try the tools you sent myself. However, in the meantime I got some more urgent roof problems and an eye operation to worry about so AC is taking a back-burner until I can deal with those and will have to return to your post after. So, for now I just wanted to say the comments are much appreciated (especially the ductless potentially handling the 3 rooms) - so you don't think you wrote the above for nothing.
 

Dana

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I am saving your post to do the calculations, re-view again your comments (and glad to read that my HVAC/Furnace guy aligns with yours, I started wondering previously). Sadly, I had several HVAC/ductless reps coming and not a single one of them would even entertain the idea of doing a J calc - so indeed I will have try the tools you sent myself.

Most HVAC contractor Manual-Js have multiple errors pushing the sizing to the larger side. Someone who makes a living on the accuracy of the numbers does a better job than someone whose primary business is installing and maintaining equipment. Engineers, architects, RESNET raters etc are more likely to at least follow the Manual, which itself has a bit of padding even when done properly.

I reviewed an HVAC contractor's Manual-J a handful of years ago on a high-performance house that had a foot of cellulose insulation in all walls, imported high performance windows that met PassivHaus standards etc. The contractor had input only standard code minimums for the insulation & window performance except for air leakage which was HIGHER than code-min, and used heating/cooling design temperatures more than 5C cooler/hotter than appropriate for the location. The loads were later calculated by an engineer who included an appendix showing the math on how the U-factors were derived for the non-standard construction, and came in with numbers that were roughly 75% lower (which were also probably higher than reality.)

So even if they do the calculations using some Manual-J tool you would still need to see the entire report, not just the one page summary to see just how heavy the thumb had been pressed on the scale.

A consulting company in Atlanta GA area does it by the book, and put together this graph plotting square feet of space per ton of cooling load against house size using dozens of Manual-Js performed for clients:

square-feet-per-ton-air-conditioner-sizing.png


Bear in mind that most of these homes were in the hot & humid southern US, not far more temperate Toronto. If you look at the cluster between 1000-2000' of space the average is about 1 ton per 1300', and even the very WORST was about a ton per 600', probably a house with little to no insulation and single-pane windows, no low-E, no storm windows. You surely don't need a 1.25 ton mini-split for cooling the top floor of a 1300' house!

So, run the load numbers using the freebie online tools I linked to, and bear in mind that those tools often overshoot reality by 25-35%, sometimes more.
 
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