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Single outdoor condenser unit vs, a separate outdoor unit for each head

richmhv57
richmhv57 Member Posts: 3
I am looking to replace an oil heating system (hot water baseboard) with ductless mini splits. One contractor is proposing 4 heads each with its own outdoor unit, a second one is proposing to run 3 heads off one outdoor unit. The house is one floor 1200 sq ft located in the Mid Hudson region of New York. Both systems will total 36000 BTU. The first is 4 9000 BTU heads, 2 at each end of the house. The second is one 18000 BTU head at one end (living/dining) and a 9000 BTU head in each of 2 bedrooms. Naturally more wiring upgrades will be needed for the 4 condenser system since each outdoor unit needs its own 240V circuit. So which is better. Is the multiple outdoor condenser system truly better or is it overkill?

Comments

  • GGross
    GGross Member Posts: 395
    Maybe a bit overkill, honestly if I could afford to do it that way I would though....

    One thing to be considered is that with a single outdoor unit and multiple indoor units your total turndown ratio can be limited, as the ODU has a low end rating often times equal to or slightly higher than the max capacity of your smallest indoor unit. This can lead to some short cycling. In general this is really not a big issue, but if you use the 4 IDU with 4 ODU configurations you will get the full range of modulation in all of your units. It is probably a big cost difference, but yes if I could afford to do it that way I most definitely would. I would also keep my hydronic heating system intact if possible, even if you do not use it. If your old boiler still functions it may be a nice backup just in case, or in the future you may upgrade to an air-to-water heat pump when the tech gets a bit better and more affordable to have high temp units.
    SuperTechIn_New_England
  • pecmsg
    pecmsg Member Posts: 3,550
    I’ve gotten away from anything over duel zone. 

    WHEN something fails you at least have something operating. 
    GGrossSuperTechoffdutytech
  • jumper
    jumper Member Posts: 1,911
    Single machine has to do some acrobatics to ensure oil return.
    Nuts to variable refrigerant salesmen.
  • richmhv57
    richmhv57 Member Posts: 3
    The boiler is 50 years old and inefficient. The oil tank is also old and a bit rusted. It is going away. The baseboards will stay, just not be used. House also has a large wood stove to help with cold waves and power outages.
  • pecmsg
    pecmsg Member Posts: 3,550
    jumper said:
    Single machine has to do some acrobatics to ensure oil return. Nuts to variable refrigerant salesmen.
    I haven’t seen that issue!
    GGross
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,007
    No way to get ducting into the attic or basement? The one-to-one setup is better. 
    SuperTech
  • jumper
    jumper Member Posts: 1,911
    pecmsg said:


    jumper said:

    Single machine has to do some acrobatics to ensure oil return.
    Nuts to variable refrigerant salesmen.

    I haven’t seen that issue!

    Undocumented feature?
  • yesimon
    yesimon Member Posts: 38
    Usually people recommend two outdoor condensers for "typical American homes". One for the open layout common living spaces and one concealed duct or multi-split for the bedrooms. Multi-split is disfavored because the smallest wall cassettes are usually still oversized per room. However given that labor is expensive for ducting it's usually not financially preferred.

    4 condensers would be "better" in the sense that owning 4 cars is better than 1 car but you're getting taken for a bath and will never see any financial or environmental ROI over a more sensible arrangement.
  • clammy
    clammy Member Posts: 2,827
    Just wondering if your bagging your hot water boiler has your contractor done a heat lose gain calc and is he guaranteeing that your home will be heated properly on design day ? Personally I can’t see any mini split heat pump system being cheaper to operate then a hot water boiler unless piped and installed terribly and over sized to boot . The other issue I see w this set up is your gonna have no heat durning power outages unless you have a generator and one that produces clean power which most electronic preferr to operate properly . Make sure they install surge protectors in your outdoor units to protect the electronic a cheap investment . I think you should also look into the yearly maintaince costs including indoor evap cleanings which are usually a hassle and time consuming . I will double up on the multi head unit feeling that they have there issues over a single head unit just me . A possible better long term opt would be do a real ducted system and use a Bosch heat pump outdoor unit and keep the boiler and baseboard system as a back up or when the heat pump just don’t cut it . I personally would only install a heat pump mini split as a secondary not primary heat source and If primary I would have to be sure tha home was well insulated and that the heat pump would satisfy my heat lose on design day ,which in most cases is the case .
    In most cases I ve found that a properly installed ducted system cost a bit more then a mini split but has a longer life span and requires less intensive servicing and is not rated as a appliance as are most mini splits plus I really don’t think of any air based system to be superior to either steam or hot water in delivery the heat at the least cost of transmission . This is just my thoughts on it . At the best if they last 15 years you ve done great but when replacement time comes it’s everything is replaced and the costs will of course be higher being time has past and we all know that nothing gets cheaper ,so think deeply being you will be living w them a fairly a long time and as stated many times nothing deliveries comfort and ease of zoning then a hot water systems .
    Peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • clammy
    clammy Member Posts: 2,827
    Just a quick thought how you gonna heat your bathrooms and common areas w no heads ? It’s gonna be cool getting out of that shower come winter .
    Peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • richmhv57
    richmhv57 Member Posts: 3
    Two the heads will serve the common area, living room, dining room, and kitchen. Bathroom won't have one. The situation is this, The house has been mainly heated by a large wood stove in the living room with the oil as a backup and for mild 50 degree + weather. I am getting older and want to reduce reliance on wood as it ia a lot of work, The oil system did not work without power either. Without a generator, the woodstove is it in power outages and it will stay for that and help on extreme cold days. NYSERDA claims these systems have lower operating costs than oil or propane. I am in the country so no natural gas.
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,007
    edited October 2022
    Not hard for a heat pump to be cheaper to operate than oil in 2022 :smile:
  • TAG
    TAG Member Posts: 738
    Factoring the cost is just a matter of math .... go online and get a fuel calculator and punch in the numbers for electric and fuel oil.

    It's important to understand how the multi heads work. I have two houses with 3 head systems -- my new place will also have 3 mini-splits .... all single head. I have used mini-splits in many projects over the years starting back around 2000 and that's what does our house in South Africa (heat and AC)

    The multi head systems are never off -- even the heads off are not as some refrigerant must be flowing to all heads at all times. The metering devise is in the outside unit .... the inside units are basically convectors with a brain and fan. I have the most experience with Mitsubishi systems and you have to be very careful when you do your heat and cooling load and match the heads with the outside unit in the printed Mitsubishi tables. The head units are the same -- but when attached to the multi-head compressors have a much tighter operating range .. plus, you must factor the outside compressors lowest setting if installing low output heads. GW here seems to do many of them -- maybe he will chime in.

    My new project has a far flung loft that was impossible to get ductwork to and will be used infrequently .... Need a mini-split for the kitchen and the project also has a lower level that I could not get ductwork to. Having a far flung head that is never on is not a good idea with a multi head mini -- I could have possibly linked it with the lower one but the heads are on different sides of the house --- longer line sets and my guess is they will not be used together. When I looked at the operating range for the kitchen unit I really need that wide range the single gave me.

    I have a tri head at my beach house (3 x12k units on a 30k compressor) .... two ceiling units on the second floor of a cape and a third wall unit in a new single story room I built off the side of the house on the first. Here all the heads are on when we are there so there is no wasted conditioned refrigerant for the most part ..... but, if I had to do it again the twin ceiling units would have been one twin and a single for my sunroom. Factor in any snow for the compressors and I had one of my compressors wall mounted -- you can hear the larger ones ... they vibrate more.