~1450 sq ft, 2 story cape with full unfinished basement + tiny crawl space. There are hot water radiators for heat. In contract on house with what inspector viewed as aging split mini duct system. They are labelled as mitsubishi mr slim, brown in color but look old or beat up, possibly both. There are 2 external compressors/ condensors and 2 indor handling units, both on the North side of the house serving only the main floor. For the main floor : The one handler is in the main living space, the other is tucked into the dining area; On the other side of the house are two bedrooms and a bathroom. For the 2nd floor there is currently a stand alone A/C shaped like a air handler but no external compressor; it is also on the north side of house. On the other side of house are a second bedroom and a bathroom. Basement : No HVAC. When I go to replace the mini split and the a/c, are the 3 units enough to cool the house or will i get hot and cold spots? Do I need an air handler in each room? Do I need more than 1 compressor/condensor? Can I just get more handler units and not the compressor? Also I want to tie in the crawl space (have to see what the foundation/waterproof guys says), if that matters. It's all boarded up right now so fingers crossed that won't be a nightmare when I open it up. The inspector said the house felt tight, just wondering if I should spring for heat/ a/c units if cost difference isn't huge for backup? Current heat source is oil but gas available on street. Thanks

Anyone?

I guess generally is it better to have an air handler in each room vs a few larger ones and (hopefully) equilization?
Just A/C or get the heater/A/C combo?

It's hard to say not knowing your climate or level of insulation. Saying the house 'feels tight' doesn't say a whole lot. Our house is 1400 sq ft, plus the same size basement. A single 12,000 BTU mini keeps the house comfortable year-round except for extreme (single digit) cold snaps, when we also run the 9K unit in the master suite. The basement ranges from 60 F. to 75 F. year-round with no HVAC at all. BUT- The house is ICF construction, R-60 cellulose in the attic, quality windows, and proper site orientation for energy efficiency. We are in a fairly mild climate in upstate SC. Our 99% design temps are 91 F in the summer and 20 F. in the winter. We've seen actual temps range from +3 F. to 107 F.

We keep all of the interior doors open in our very open house all the time, since we normally only use the one wall-mounted mini. our units are Mitsubishi HyperHeat units, and we are very happy with their performance, but of course, the house is a major player in their effectiveness.

A wall coil in every room is almost always EXTREME overkill, oversizing the equipment to the point that the mini-split operates well below it's rated efficiency.

Code requires that rooms be able to be heated to 68F at the 99% outside design temperature under an automatically controlled system (not a space heater), which is one reason to use ducted mini-splits, and also leads too many people to the "head in every room" oversizing mistake.

Some model numbers of the compressor units, the wall coil, and slim-duct cassette would be useful here.

Measuring up the equivalent direct radiation (EDR) of the radiators in each room (room by room, plus the whole-house total) would establish an upper bound on the total heat load, and the room-to- room load balance. The odds are the radiation is more than 1.5x oversized for the actual 99% design load, but it's a start. For a primer on estimating the EDR see:

http://www.columbiaheatingsupply.com/page_images/Sizing%20Cast%20Iron%20Radiator%20Heating%20Capacity%20Guide.pdf

jdebree - Hmmm I thought location was on there but I'm in Long Island NY. 0 - low 100s F I would say. There is only a crawl space type attic and there is in excess of a foot of loose insulation in there. I have to replace 3 windows probably but the rest of them are functional.

dana1 - very helpful. I'll come back and update the model numbers when I close. Will try to work out the EDR as well. Thanks.

The 99% outside design temperature for almost all LI locations is about +15F (yes, I know it can get colder than that, but for only about 87 hours in an average year.) Designing for a modest oversize factor on the 99% heat load is enough to cover the load during Polar Vortex events. The 1% outside design temps are in the high 80s (even though it'll hit 100F sometimes, it's only for a handful of hours per year.) See:

https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf

Most older capes have no foundation insulation, and only some even have fiberglass batts stuffed haphazardly under the subfloor. Air leakage is likely to be a double-digit fraction of the heating & cooling loads, as is heat loss out of the foundation. It's easier to get a reliable air tightness at the foundation rather than the subfloor- going with a sealed-insulated crawlspace and foundation wall insulation in the full basement has net efficiency benefits far greater than insulating and air sealing the floor would yield, usually with a 25% or more reduction in heating & cooling energy use.

A 1450' 2x4/R13 framed house that's been tightened up, with at least storm windows over single panes, and the IRC 2015 code-min R10 continuous insulation on the basement & crawlspace walls SHOULD come in under 20,000 BTU/hr @ +15F. A single 1.5 ton Fujitsu -18RLFCD slim-ducted mini-split has that much capacity, but the long duct runs and zoning to the second floor calls for at least two heads/cassettes, and maybe three. Almost any 2 ton 3 zone multi-split would have the capacity, but whether it's ducts vs. floor/wall coils or ceiling cassettes it's a matter of the floor plan.

A couple of years ago I was consulting on the HVAC on a house a bit more than twice that size in Martha's Vineyard (99% outside design temp of 13F) that ended up with a pair of 2-ton /3-zone Fujitsu multi-splits (one on each end of the house). The one serving three bedrooms was sub-optimally oversized for the room loads and the total load- they all cycle way too much rather than modulate. But the one serving the rest of the house does fine. (I tried to get them to go with a mini-ducted solution for the bedrooms, but they couldn't find a contractor who didn't insist on running the ducts in the attic, above the insulation, which would have been even worse.) I tried to convince them to get more serious about air sealing too- the older half of the house leaks like a sieve, and has no foundation insulation. But they stay warm, paying a lot less for heat than they would with propane, which was their other option, even at sky-high island electricity rates.

Dana1 Sorry been sorting out a lot of stuff before the closing. And I'm here on this forum because I want to get work done on the house prior to actually moving in.

You are correct in your assumption here that there is no foundation insulation but Just for clarification : aside from R10 on the foundation and crawlspace walls do you suggest lining / thermal break for the basement floor? It's just straight concrete at moment.

There are no HVAC ducts in the house and we want to keep it that way for allergy / asthma reasons. In terms of floor plans - On the first floor the 2 current wall units sit on the same (open floor plan) side of the house and then it converges into a very short hallway with the two bedroom doors, the bathroom door and door the basement stairs. On the upper floor the wall unit sits in the one bedroom exiting through a door to the bathroom door, other bedroom door, stairwell down combo... So a non ducted 2 ton 3 zone multi - split should work in theory...

Posted By honda99ex on 03 Feb 2018 05:40 PM
Dana1 Sorry been sorting out a lot of stuff before the closing. And I'm here on this forum because I want to get work done on the house prior to actually moving in.

You are correct in your assumption here that there is no foundation insulation but Just for clarification : aside from R10 on the foundation and crawlspace walls do you suggest lining / thermal break for the basement floor? It's just straight concrete at moment.

There are no HVAC ducts in the house and we want to keep it that way for allergy / asthma reasons. In terms of floor plans - On the first floor the 2 current wall units sit on the same (open floor plan) side of the house and then it converges into a very short hallway with the two bedroom doors, the bathroom door and door the basement stairs. On the upper floor the wall unit sits in the one bedroom exiting through a door to the bathroom door, other bedroom door, stairwell down combo... So a non ducted 2 ton 3 zone multi - split should work in theory...

Retrofit insulating a basement floor on top of the slab has a very long "payback" period, maybe "never" when viewed simply as the net-present-value of-future energy cost savings, but if you're planning to finish the basement or store moisture sensitive materials there, putting 1-1.5" of EPS under a vapor barrier + plywood subfloor before the finish flooring keeps the moisture susceptible materials above the dew point of the room air most of the summer, and avoids the "musty basement smell" without excessive mechanical dehumidification, and helps keep the overall mold counts down in the house.

In new construction it's financially rational on a lifecycle basis to put 2" of EPS under the slab, but when insulating over the top of the slab as a retrofit there are headroom issues, and the stairs have to accomodate the change in elevation, etc. R15 isn't insane for basement walls, even as a retrofit- even more so if using cheap reclaimed roofing foam at a steep discount. See the zone 4 row of Table 2, p.10 of this document as a rough guide:

https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

It's important to assess the moisture conditions and plan for it when making these decisions, since it affects the retrofit insulation implementation. Sometimes basements are damp enough that the foundation walls need a dimple-board for drainage between the foundation and wall-foam, draining into a perimeter drain that terminates in a sealed sump with a pump.