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Community and Q&A

Mini Split in 1905 All Brick Home

twoodson | Posted in Mechanicals on

I’m looking for a bit of help selecting a mini split setup for my historic district 1905 all brick 2 family home. This will be part of a major major gut of part of 2nd floor, so a number of energy improvements are on the docket. We will rent one of the units.

I would like to do ducted mini splits for each unit for aesthetic/historical reasons. I have heat pump water heaters already. This would be the only heat source, so heating loads dominate. Possibly 1 unit for front of each unit, 1 for back.

I have 4 quotes from contractors, but not many are familiar with mini splits in my area apparently (st louis). The most knowledgable contractor quoted me 4x 24,000 but ducted lennox units (MLA024S4S-1P outdoor, MMDA024S4-2P indoor). I think those are rebranded Gree. Quotes for Mitsubishi were 50% more. Other contractors quoted me 3 ton units with aux electric heat in the ducts.

Base on self-done load calc below (used manual J), it seems like these contacts are way over.

1) How far should units be oversized? Balance points for the Lennox units were -2 on the 1st floor and -5 on the 2nd.
2) Those on the Lennox medium static ducted units? They have a turndown to around 6k btu per unit, so not great. 12 year warranty.
3) Are there other system setups I should be look at?
4) Do my heat loss/cooling loads look way off? My 70k btu/h gas furnace has no trouble keeping up with our 1st floor unit with old windows and barn like air-sealing without the 2nd floor gas on (temps stay in the 50s on the 2nd floor).

Numbers below for climate Zone 4A @ 8F outdoor 70F indoor for heating. 93F outdoor 75F indoor for cooling.

1st floor (1250 sq feet over basement)
heating 30,000 btu/h
cooling 20,000 btu/h
1300 sq ft of uninsulated multi-wythe brick with plaster and drywall.
180 sq ft of windows
75 sq feet of doors

2nd floor (1400 sq feet over 1st floor)
heating 26,000 btu/h
cooling 18,000 btu/h
1550 sq feet of foam board covered multi-wythe brick.
200 sq ft of windows
75 sq ft of doors

Energy upgrades:
1) 3 inches of foam over the flat roof deck. Batts or spray foam in the roof joints. Aiming for R50 total.
2) Spray foam on the envelop side of parapet walls. Unvented “attic” (2 feet of space in the front, 0 feet in the back).
3) The walls are a tricky subject. We need to respect architectural details and make sure brick inside doesn’t get too cold. So R8 Foam board over brick on 2nd floor. 1st floor must be left as is.
4) 2 inches spray foam basement walls. eventually studs with batts and eventually replace slab and insulate under.
4) All window sash replaced with sash packs with low-e argon glass.

Based on all of this, it’s hard to know how tight we will get the house, but I intend to get a blower door to chase down leaks prior to sealing up the walls.

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  1. Expert Member
    Dana Dorsett | | #1

    The calculated heat load seems a bit high, but not outlandishly high given the lack of insulation on the first floor walls. Masonry walls tend to be pretty tight- so you may be overestimating infiltration. The accuracy of the calculation isn't easy to assess- let an certified engineer run the numbers and spec the equipment not an HVAC contractor.

    Why four units? At 6K per unit that's a minimum 24K of output @ +47F, well over your calculated heat load at that temp.

    If the existing ducts for the 70K furnace on the second floor are at all reasonable you can probably get there with a single 1.5 ton or 2 ton Mitsibushi PVA or MVZ air handler using the same or only slightly tweaked ducts. The 2-tonner can muster 28,000 BTU/hr @ +17F.

    If going mini-duct, the Fujitsu xxRLFCD units tend to have better specs than the competition. The 1.5 tonner is good for for over 25K @ +17F (rated for efficiency at 22K), and may be a better solution than a big-air handler Mitsubishi for the second floor.

    A pair of the 3/4 tonners would be good for over 30,000 BTU/hr @ +17F, (rated for efficiency at 12K per unit)

    The minimum modulated output for the PAIR is 6200 BTU/hr, which is about half what the Carrier/Gree units would put out.

  2. user-2310254 | | #2


    If you are wondering where to find an experienced HVAC engineer, see this article:

    The engineer will be able to run an aggressive Manual J. Right-sizing the system may cover whatever cost you incur by bringing in an expert.

  3. walta100 | | #3

    My guess is the guy that bid 8 tons of mini splits will have nice bid for a conventional system for half the price. This is the standard bait and switch tactic, bid the mini high and sell the conventional system.

    For your blower door test this guy covers St Louis my test with IR photography 3 hours on site 4 hours of drive time for the very low price of $150


  4. twoodson | | #4

    Thank you all for the input and suggestions. That definitely makes sense on the minimum output. I hadn’t looked into the Fujitsu slim ducted units; thanks for that lead, they look very promising. Their output is actually rated at -5f as opposed to the carrier/gree units that don’t even come close. And their COP at those temperatures is impressive.

    I’m not quite sure I understood how to go about finding a residential mech engineer to do a load calc here, but I enlisted my brother-in-law who spent a few years doing engineering design for commercial systems. He doesn’t have access to pro manual-j software anymore but helped me fill out a room-by-room on my building using CoolCalc. We even attempted to do an in-situ wall thermal conductivity measurement (that’s a crap shoot it seems- somewhere between R4 and R6 on my 1st floor). I fully intend to get a blower door done after the spray foam contractor finishes and the windows are done so thanks for the tip Walter.

    Finding a contractor who is willing to design to spec or doesn’t just want to sell me a conventional system is proving difficult. I have a few Fujitsu hvac contractors combing next week, but I have a few questions:

    1) Am I reading this right that the 9RLFCD combo puts out 14k @ -5f? and 16k @14f ? Incredible
    2) I don’t seem to see any mention of a basepan heater. My 98% design temp is 8f, so it definitely gets cold enough here and create ice. Any idea if that is recommended? If I went this route, I would put all 4 units on my roof.
    3) I can’t seem to find a clear answer on whether they will operate below -5f. Do you know if they have a temperature shutdown or a low suction shutdown (or both)?
    4) Thoughts on aux heat using ducted mini splits? Both the Gree and fujitsu untits can control electric resistance heating for backup.
    5) Is sizing at-load (no oversizing) a good practice with mini splits?
    For example.

    1st floor Zone 1 (2 bedrooms/2 bathrooms 640sqft) cooling 5.6k / heating 12.9k -> spec with 9RLFCD
    1st floor zone 2 (open living room/kitchen/dinning 570sqft) cooling 4.3k / heating 14.7k -> spec with 12RLFCD

    A few facts that would have probably been useful:

    1) My current ductwork is a rats nest of ducts patched into 115 year old gravity furnace ducts that all originate in the basement. Long story short, it needs to go, so I'm starting from scratch on that for both the 1st floor and 2nd floor.

    2) This may have been an important fact: I have 10 foot ceilings so that probably adds to the load.

    Sorry for the long post! I’ve become slightly obsessed with making this building green :) Next up is PV on my large flat roof.

  5. Expert Member
    Dana Dorsett | | #5

    Fujitsu's 3/4 tonners have more output than most vendors' 1-tonners, it's true. Labeling it as a 3/4 tonner gives it a marketing advantage, and also delivers higher efficiency at the lower-than-max modulation output level chosen for it's AHRI "rated" or "nominal" heat rating.

    I don't believe the xxRLFCD outdoor units have a pan heater even as an option, but they do have multi-zone units that do that should be able to utilize the mini-duct cassettes:

    Oversizing IS a problem, since the modulation range isn't infinite, and cycling the compressor on/off isn't very efficient. But you won't do much worse with a 4 zone 3 ton multisplit rather than four separate units, given the minimum modulation constraints:

    xxRFLCD: 3100 BTU/hr x 4 = 12,400 BTU/hr


    AOU36RLXFZH3 compressor: 12,000 BTU/hr

    Pretty similar minimum output.

    The -36RLXFZH3 is good for at least 42,000 BTU/hr @ +17F, but you'd have to delve into the manuals to figure out it's capacity at +8F with the collection of heads/cassettes you hope to run with it.

  6. user-2310254 | | #6

    Before you finalize your flat roof plan, you should read this article:

    If you are using EPS, four inches is typically the safe minimum (according to Martin). Maybe you are using a higher r-value foam to get to r-15.

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