Replies

1. Expert Member
   Dana Dorsett | Feb 08, 2019 02:24pm | #1

   The system described is going to be insanely oversized for the loads, and won't hit it's HSPF or SEER numbers. You're probably looking at 1-1.5 tons of cooling load, and less than 25,000 BTU/hr of heating load, maybe less than 20,000. It sounds like the contractors have boat payments to make or something? :-)

   You could probably do the whole shebang with just ONE MVZ air handler using the existing ducts and a single half-ton head for the basement running an an MXZ-3C24NAHZ or 3C30NAHZ compressor.

   The typical load of a 115-160' bedroom is less than 2500 BTU/hr @ 0F with the windows closed. A GE06 or FH06 head on an MXZ compressor delivers about 8000 BTU/hr, which is so oversized that it won't be comfortable, and even the bypass refrigerant when other zones would running would overheat/overcool those rooms.

   Unfortunately insane proposals like those are the norm rather than the exception, which is why YOU have to do the legwork to figure out what's actually needed, then put it out to competitive bid.

   Since you have a heating history on the place you can start by getting a handle on the whole house heating load by analyzing wintertime oil use:

   https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new

   With a ZIP code and the K-factor off wintertime oil fill-up slip I could run the crude numbers for you here very quickly.

   But use freebie/cheapie Manual-J tools like coolcalc.com or loadcalc.net to do room-by-room load numbers as a sanity check. For the output numbers of those tools to approximate reality use the most aggressive assumptions about air tightness the tools will allow (eg zero infiltration and ventilation), and sanity check the whole house load (add up all the rooms) against the fuel use load numbers. Alternatively, start building a spreadsheet for running an I=B=R type load calculation, room by room:

   https://www.greenbuildingadvisor.com/article/how-to-perform-a-heat-loss-calculation-part-1

2. 
   FluxCapacitor | Feb 08, 2019 03:35pm | #2

   Dana,

   This post, and many of your past posts, have been super helpful to me. Thanks.

   Silly me! I thought it would be easy (but expensive) to simply throw a wall unit in each room and be done. Then I learned here on GBA that in a small house it could be very uncomfortable that way, with temperatures and humidity all over the place.

   Then I started exploring the ducted MVZ option, only to again find that’s maybe not great either because of the duct heat losses?

   “You could probably do the whole shebang with just ONE MVZ air handler using the existing ducts…”

   One contractor did suggest a single PEA ducted air handler (in attic) but I think I would like 2 zones so I can turn the bedrooms off during the day.

   I can’t use the existing basement ducting as it hangs too low in the basement (7 foot ceilings)

   So if I can’t use wall units but I want to remove the existing basement ducting would it be insane to consider the smaller 2 MVZ’s or single small PEA ducted in the attic?

   There’s lots of info here on ducts not belonging in attics but I can’t think of any good alternatives.

   Mark

3. Expert Member
   Dana Dorsett | Feb 08, 2019 04:32pm | #3

   Before this discussion can get very far we really need the whole house heat load, and the zone by zone heat loads. If not a K-factor, a ZIP code (for weather data and design temperature purposes) and exact fill-up dates & amounts is enough to at least accurately ballpark the whole-house load. But take a stab at the room by room numbers too- it's critically important to get it right, more so if going multi-split.

   The MVZ air handlers don't modulate at all, and the smallest one delivers a nominal 13,500 BTU/hr , which is probably more than half your design heat load. Multiple MVZs are overkill for air volumes and BTU output. If your whole house load really is as high as 27,000 BTU/hr (the output of a pair of MVZ-A12s ) there is likely to be some seriously low-hanging fruit on the insulation and air sealing front. Most 1950s vintage 1200' houses over a 600' basement with typical storm window / replacement window and insulation updates would be in the 18-22,000 BTU/hr range, or could be for comparatively short money.

   Putting ducts or air handler in the attic above the insulation punches holes in the pressure envelope of the building and requires exquisitely fine supply/return balancing to not add significantly to the load. That's truly a last resort.

   A 3/4 ton KD09NA4 mini-duct cassette delivers 10,900 BTU/hr in heating mode when married to an MXZ xxxxNAHZ multi-split compressor, and could all but certainly cover the heat load of the three bedrooms combined, assuming there was a reasonable way to route the ducts:

   http://meus1.mylinkdrive.com/files/SEZ-KD09NA4_For_MXZ_MULTI-ZONE_SYSTEMS_Submittal.pdf

   The similar P-series PEAD-A09AA7 is good for 10,000 BTU/hr:

   http://meus1.mylinkdrive.com/files/PEAD-A09AA7___SUZ-KA09NAR1_Submittal-en.pdf

   With a sketch of the floor plan (with the room loads indicated) we might be able to come up with some ideas. Mounted below ceiling level in a closet between the bedrooms is one approach that sometimes works.

   Fujitsu's 9RFCD mini-duct cassette can be mounted vertically taking up 6-8 square feet of floor area if built into a mini-closet/cabinet. These pictures are of a 1.5 tonner, which is bigger than the 9RLFCD but similar:

   https://uploads.disquscdn.com/images/2ffa6e108a7ded9f51130ff14126239b275b1244b7d53138beb63b4182d68f13.jpg?w=800&h=1097

   https://uploads.disquscdn.com/images/7843213f27734395e6ede8ea696552a8eafd3a2dd7f62c2b61241bb23189a293.jpg

   Note how the built a soffited duct run under the ceiling level for one of the 4 ducts coming out of that ultra-short plenum. That single unit heats and cools an entire house that is approximately your size. The house is in Berkeley, California and has a lower heating load than yours, but most likely a higher sensible cooling load (and lower latent cooling load.)

   Running two separate modulating compressors is usually cheaper, more efficient, and more comfortable than trying to do it with a multi-split. Cassettes/heads/wall coils don't modulate the individual zones when married to a multi-split, even when the same heads/cassettes can modulate when it has a dedicated single zone compressor.

   BTW: There is nothing to be gained by turning off the bedrooms zone during the day. It takes less total electricity to maintain them at temperature than it takes to recover from setback (especially if on a modulating dedicated compressor). Turning them off when you're gone for the WEEKEND can save some money, but not for just 6-12 hours. The same is true for the rest of the house- overnight setbacks end up using more power than keeping it at temperature. It's nice to have separate zones for the bedrooms to be able to sleep at a cooler temp than the rest of the house, but bumping the temperatures up/down multiple times per day will use more power than simply maintaing temperature. (This is the opposite of what is true for your oil burner.)

4. 
   FluxCapacitor | Feb 08, 2019 05:34pm | #4

   Thanks for all the info. I have some homework to do now.

   Here is my floor plan. Also, is there any advantage to the miniducts Vs one full ducts in my situation.

   Sadly this house has almost no closet space. I couldn’t possibly see putting any air handlers in the closets.

   “Putting ducts or air handler in the attic above the insulation punches holes in the pressure envelope of the building and requires exquisitely fine supply/return balancing to not add significantly to the load. That's truly a last resort….”

   What do you think would be a super rough ballpark estimate for how much difference the energy consumption would be in a house like mine between a couple of multisplit wall units (with all room doors open) VS one (or two ducted units)….10%, 20% 50%? It would be helpful if I could factor that in to my decision.

   Mark

   
5. Expert Member
   Dana Dorsett | Feb 08, 2019 06:33pm | #5

   A mini-duct cassette in a closet would live at the ceiling level, and only interferes with storage on closet shelves.

   Mini-duct cassettes have the advantage of modulation (if on their own compressor) , and more appropriately sized output capacity. MVZ air handlers are fixed output, and oversized for almost any zoning (other than by floor) that you would come up with.

   The efficiency differences come with the oversize factor more quickly when on a multi-split, since there is no modulation of the zones.

   A 350 square foot room with 42' on the long size is only 8' 4" wide. Is that really the dimensions of the living room?

   The corner bedrooms have more exterior wall area and more than 1.5x the heat loss of the one across from the bath. A really crude quick & dirty heat loss calc of the 160' bedroom goes something like this.

   Outside design temp: +10F, indoor design temp +68F, for a 58F temperature difference.

   Ceiling U-factor assuming 6" of fluff: U0.05

   160' of ceiling x U0.05 x 58F temperature difference = 454 BTU/hr of ceiling losses.

   Wall U-factor assuming R11 fiberglass and typical siding: U0.10

   Ceiling height 9', exterior walls about 26', for a gross wall area of 234 square feet. Assume 20 square feet of U0.50 window (a couple of double hungs with clear glass storms), for a net wall area of 214 square feet.

   214' x U0.10 x 58F= 1241 BTU/hr of wall losses.

   20' x U0.50 x 58F= 116BTU/hr of window losses.

   Assuming it's above a conditioned basement that's it- add it up and you're at 1811 BTU/hr, plus infiltration.

   Assuming 5 cfm infiltration per window that's 10cfm, or 600 cubic feet per hour x 58F x 0.018 BTU/F-ft^3= 626 BTU/hr

   Total heat loss is then 2437 BTU/hr. Still want to throw 7-8000 BTU/hr of ductless at that room? (Don't think so!)

   With the same window assumptions other corner bedroom will come in at about 2000 BTU/hr, the room with only one exterior wall about 1500 BTU/hr. For all three you're looking at something like 6000 BTU/hr, combined. That would be less than HALF the output of the SMALLEST MVZ air handler.

   If that's roughly where the numbers come out, Fujitsu makes an 7RLFC mini-duct cassette unit suitable for their multi-splits good for about 8000 BTU/hr that might be appropriate, if going multisplit:

   http://www.fujitsugeneral.com/us/resources/pdf/support/downloads/submittal-sheets/ARU7RLF.pdf

   But measure up the actual windows, walls, etc and it'll be a lot closer to reality than this quick & dirty example. With a better description of the wall stackup & siding we can fine tune that too.

   I suspect the floor joists are perpendicular to the hallway. A vertically mounted 9RLFCD in the basement wall below the partition wall between the two smaller bedrooms might work with essentially zero length ducts to those two rooms, with a duct in the joist bay feeding the larger bedroom. That takes only 6-8 square feet of floor area in the basement, and no head-banger ducts.

   If that assumption is wrong, fine, but think creatively- something along those lines is likely to work.

   The load in the bathroom is probably too low to matter. Leave the door open, or a small amount of electric heated towel rack or radiant floor could handle it. The living room and adjacent spaces can almost certainly be covered by a single 3/4 ton head unless it has huge west facing picture windows driving the cooling load through the stratosphere. A wall-coil head mounted where it can blow toward the kitchen and hall is usually enough to heat & cool the kitchen when the doors are open.

   The heating load of an insulated 600' basement is going to be tiny, and may not be worth a ductless head. But if you're already going with a 2-ton 3-zone multisplit instead of a 2-zone 1.5 tonner it wouldn't be insane to install a half-ton head there.

   FWIW: I have friends in MA living in a house more than twice that size who are heating/cooling with 6 zones comprised of two three-zone 2-ton Fujitsus and it's oversized for their actual loads. Their design temp is +12F, which is pretty close to northern NJ type design temperatures. Half the house is pretty crummy 1970s 2x4 construction originally built as a summer place, probably quite a bit leakier than your 1950s house. The other half of the house is middle-of-the-road 2015 code-min type construction. I kept on them to down-size and use min-duct cassettes, but they couldn't find a local contractor willing to deal with the ducted solutions other than running the ducts in the attic.

6. 
   FluxCapacitor | Feb 09, 2019 10:36am | #6

   Whoa! That’s a lot of good info. Thank you.

   I goofed on the house dimensions. As you suspeceted I switched the 42 foot and 27 foot house dimensions. Its 42’ long and 27’ wide. The living room is 14’ X 26’

   “….but they couldn't find a local contractor willing to deal with the ducted solutions other than running the ducts in the attic.”

   I think I will in a similar situation and/or it will be ultra expensive…but worth looking into. Also, my “biggest” closet is just 16” x 48”.

   I think you load assessment of my house is good. I came up with 22,000-27,000 Btu at zero degrees F. My home-brewed calculation is based on:
   -My beastly oil furnace is rated at 110,000 Btu output (142,000 Btu input)
   -Wife likes 70-72F indoor temp.
   -The furnace is single speed forced air (essentially on or off)
   -On the very coldest day this year (-5F low, 8F high) the furnace ran about 20-25% of the time.

   Mark