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Mini-splits in a split-level home? (Zone 4)

Jerry2017 | Posted in General Questions on

Hi all,

Thanks to all GBA contributors for the info on this site, and to Dana Dorsett for the technical guidance on many related topics from another forum. I became a first-time homeowner last fall. Your contributions got me started in the right direction on my HVAC replacement project.

In summary, I will be replacing my decades-old HVAC in my split-level house in Long Island, NY. My goal is to air seal/insulate enough for two separate mini-splits to handle both heating and cooling – one upstairs, one downstairs-type simple/economical configuration. I estimated heating loads using fuel-use based approach (two years of mid-winter heating bills), and roughly checked with I=B=R calculations. Both were in general agreement. I hired a reputable local RESNET rater for room-by-room Manual J. To my disappointment, his pre-improvement results are roughly 50% higher than my estimates. He struggled to sanity-check his results.

I have attached floor plan and Manual J results for review.

My questions –
1) Based on details below, is the two-head configuration technically feasible, say two Fujitsu 12 or 15RLS3? Note that “Head 2A or 2B” as shown on attached floor plan will be mounted on interior wall – how tricky is installation assuming distance to outside unit <35′? 2) Please assess the attached Manual J results. Instead of wasting time going back and forth with this RESNET rater, how can we reliably reduce his results in a way that is useful for sizing, locate heads, while minimizing room-to-room temp balance issues? 3) Anyone has experience with mini-splits in split-level homes that do not have a head at each level? My house has three levels. I have no plan to install a head to cover the lower 20’x20’ den with open stairways, other than by convection from other units, or maybe a ceiling fan. Do you see major red flag in this? Note that the den has 75 sq ft of east-facing sliding glass door (0.6 U), slab-on-grade, exterior wall with fireplace to the south, and attached garage to the west. 4) By any chance, anyone has experience with NY HVAC contractor that does not fight sizing? All this exercise is moot if I cannot find a reliable installer to do the proposed work. I contacted many, no serious bid (one head per room, 4+ tons system, $15k+ install, etc). I found “diamond” dealers to be no better than others. The mini split option is only economically feasible for two heads. I may as well replace the beastly HVAC in-kind for cheap, and invest the difference in solar PV. I have included a lot of details below. I apologize for the lengthy post. ————————————————————————————————————————- Location: Heating and US Climate Zone 4, Nassau County, NY House: See attached floor plan. 1950s built, 2×4 framed, two-story high split-level structure. There are four levels in the house, each level occupies half of building footprint. Starting from bottom up – unconditioned finished basement, den/attached garage at street level, living/dining/kitchen 5’ above street level, and upstairs bedrooms. 1800+ sq ft excluding basement, 2400+ sq ft including basement. Master bedroom as bonus room above attached garage. Total less than 100 sq ft of west-facing windows. Existing Conditions: Heating – 1980s gas boiler, 165k btu/hr input, typical 70F indoor setting during winter, no major temp balance issue (even in bonus room, +/- 3F at worst on +15F nights). DWH – 2000s, indirect, 40 gal. Cooling – 2015 SEER 13 outdoor unit, 1970s air handler and ducts in unconditioned attic, 3-ton, 10 central ac registers throughout the house, no comfort issue on 85F days. Fuel-Use Based Heat Load: Ranging from 38k Btuh to 45k Btuh for temp range between +15F and +70F (+70F to match Manual J input, this is also my house’s temp setting) I=B=R Heat Load: Roughly 38k Btuh at pre-improvement conditions Pre-Improvement Conditions: Roughly 5” fiberglass batts on attic floor; fiberglass batts in walls; all double pane windows; unconditioned finished basement (rim joists and foundation walls not accessible without major demo); all attic floor openings are temporarily sealed with plastic film in winter (whole house fan and ac registers, etc); fiberglass batts between bonus room floor and garage ceiling. Pre-Improved Blower Door Results: 3800 CFM50. ACH50 about 13 using whole house volume (including basement) of about 17,000 cu ft. Extent of Ongoing/Planned Retrofit – Air seal as much as I can in attic and garage; remove air handler, ducts, house fan and seal/insulate all attic openings; 10” blown cellulose to attic floor; ceiling fan in each bedroom Manual J Results: See attached results using 3800 CFM50 infiltration, pre-improvement conditions at 99% design temp. – I specifically asked for aggressive analysis. He calculated 66k/38k Btuh heating/cooling loads, and struggled to sanity-check his results. His reported loads ignored unconditioned basement. – Saw some single-pane windows used – likely skylight windows. Can reduce u-value in half, or about 1500 Btu/h heat loss reduction. – 24k Btuh heat loss from “winter CFM” appears unrealistic. He could not explain how the software goes from measured blower door 3800 CFM50 as input, to 394 “winter CFM”. What is the correct conversion? (394 CFM x 55F ΔT x 1.08 ~ 24K Btuh of heat loss) Regardless, I understand blower door exaggerates leaks from neutral pressure plane. What would typically be a more realistic CFM to use for both winter and summer? – If I am removing ducts in attic, there should be no heating/cooling loss from ductwork? It reduces heating/cooling loads by 9k/5k Btuh. – Not clear if he added heat gain from four people and other equipment? This further reduces heating load. – His room-by-room cooling loads confirm there is no need for one head per bedroom, assuming I can trust his cooling load results. – Den peak loads are scary high. What’s the realistic range based on descriptions in Question 3? Please comment on my rationale blow. Without overthinking too much: – I am comfortable with my fuel-use based heat load on the order of low 40,000 Btuh at pre-improved conditions. If I understand correctly, at +17F, Fujitsu 12RLS3 covers 13,500 Btuh, and 15RLS3 covers about 18,000 Btuh. With some improvement, a combination of two 12 and/or 15RLS3 should cover my whole house head load at 99% design temp? No need to worry about minimum modulated output at +47F since my part loads are high? – If an existing 3-ton central ac with leaky ducts in attic can cool the whole house comfortably, with some improvement, a 2-ton to 2.5-ton sizing should be appropriate? – Only issues are upstairs head placement in a way that does not complicate installation, air distribution to bedrooms and den despite open stairways, and finding a competent installer to comply with intended design. Thanks to those who make it this far in my post. My family and I will appreciate any comment. Thanks, Jerry

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  1. GBA Editor
    Martin Holladay | | #1

    Jerry C,
    You may be in luck. It's possible that Dana Dorsett or another GBA reader will analyze your situation and provide advice.

    But -- you are pushing your luck. In general, the best use of our Q&A forum is to ask one or two specific questions related to green construction. This Q&A forum is not intended as a source for free heat loss calculation services, free engineering services, or free heating system design services.

    But -- hey, you might get lucky.

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