A/C recommendation needed

Discussion in 'HVAC Heating & Cooling' started by woodguy00, Jun 8, 2012.

  1. woodguy00

    woodguy00 New Member

    Messages:
    22
    Location:
    Hull, MA
    We recently bought a 12 year old well built 2700 ft2 cape style home near the coast in MA. We have baseboard hot water heating from a oil fired boiler. No ducts currently. We are looking to add air conditioning. The attic space is wide open and there are well positioned 2nd floor closets that can provide locations for chases to the first floor.

    In this sort of home would it make sense to go with a multi-head mini split system or stick with a conventional A/C with the air handler in the attic? Any guesses on what the cost might run for either? Would it make sense and be cost effective to go with a heat pump for when the outside temps are above 30 degrees or so?

    Any suggestions are appreciated.

    tk
  2. Dana

    Dana In the trades

    Messages:
    2,837
    Location:
    01609
    Most cape houses have low ceilings and not a lot of wall area to work with, which might limit your mini-split/multi-split options, but maybe. (12 years ago they were probably building them with taller ceilings than those built as summer homes pre-1960 that were only later converted to year round use.)

    Ductless would be by far the more efficient solution than ducted, and with better models you'd be able to heat the place even at design temp most days, at a signficant discount from heating with oil. The 99% outdoor design temps in coastal MA are all roughly +10F (Boston is +12F, Weymouth is +11), but the closer you are to the water the higher it will be.

    Being pretty much surrounded by water it wouldn't surprise me if the true 99% number for Hull was +15F. A 3 ton multi-split can still put out more than 20KBTU/hr at +15F with a COP greater than 2 at that temp. If you set the ductless to 70F and the boiler's T-stat to 68F they would share the load when the load exceeds what the ductless can deliver, but it would be carrying the lion's share of the load under most conditions, and the average seasonal COP would be greater than 2.5 if you get one with a decent HSPF. When it's 30F outside the ductless has both more BTU output capacity and a COP of 3+. Well-built 2700' houses of Y2K would likely have a heat load between 28-32K @ 10F, but might be even lower if the basement/foundation is insulated and the band joist & foundation sill are insulated & sealed.

    Ducted heat pumps are unlikely to beat a seasonal COP of 2.0 in this climate, which would make it comparable in cost to heating with oil, but ductless you'd beat 2.5, and would even be nosing up on 3 in Hull.

    Taking the worst-case, even with 18 cent electricity and a seasonal COP of 2.5 (which would be a worst-case for you, you may even hit 3), you get (3412 x 2.5)= 8530 BTU for $0.18, or ($0.18 x 1,000,000/8530=) $21/MMBTU of heat into the house.

    With 87% efficiency oil at $3.50/gallon you get (0.87 x 138,000=) 120,060 BTU for $3.50, which is ($3.50 x 1,000,000/120,060=) $29/MMBTU

    Most people in MA are paying closer to 15 cents than 18 for electricity, and more than $3.50 for oil. Oil would have to drop below $2.50 to compete with ductless on heating costs a price not seen in MA in a decade. At an average COP of 3 and 15 cent electricity ductless would even be competitive with mid-efficiency natural gas heating (~$14-15/MMBTU- less than half the cost of heating with oil this past season.

    There are a very few high HSPF variable refrigerant volume variable speed ducted air source heat pumps out there, but I suspect a ductless solution would be cheaper if you have the wall/ceiling area to make it work. You don't need a head in every room, just high-loss/high-gain rooms, but the floor layout can make or break it too.

    Air handlers & ducts in the attic are always a bad idea, especially when (as is likely) you have a semi-conditioned basement to run them in. Putting equipment in the attic that punches holes in your ceilings, and makes it near impossible to air seal is a disaster. Any duct leakage would drive air-infiltration well beyond what natural stack effect & wind forces would too, raising the heating & cooling loads.
  3. woodguy00

    woodguy00 New Member

    Messages:
    22
    Location:
    Hull, MA
    Dana,

    Thanks for all the info.

    Our home is a pretty modern design cape. Full 8' walls, 2x6 construction, lots of anderson thermal windows, R30 in attic - which I plan to increase. With oil at $3.75 and electric at 14 cents in Hull I like the numbers of using heat pumps

    I spent last night online learning more about the ductless systems. Yesterday I received two quotes that recommended 4 ton traditional systems mounted in the attic. Both were over $17K - I about barfed. Seeing those prices, my wife got over her reluctance to have wall mounted indoor units real quickly.


    I see the equipment for a 4 ton quad head ductless at around $$4500-5000 online. Do you have any idea what it should cost for an installed system?
    Thanks
    Last edited: Jun 10, 2012
  4. nukeman

    nukeman Nuclear Engineer

    Messages:
    711
    Location:
    VA
    A 4 ton system sounds like it is probably 2x or more the size that you would need for modern construction (or even old construction) in MA. I have a 2t/3t ducted central heat pump with 3300 sqft in central VA and the system runs at the 2t speed in all but the hottest days. The lower speed in best as you want the A/C to run for long periods in order to remove the humidity from the air. If the system is too big, it will cycle quickly and not have the chance to remove humidity. This makes you have to turn down the temperature further to feel comfortable. Most of the ductless are more flexiable in what speeds they operate at, but you should still get the size right. At a minimum, it will save money on the purchase of the system and could save you money down the road.

    What you want is a Manual J calculation. Ideally, a room-by-room heat/cooling load (or at least a heat/cooling load for each area that each head will cover is what you want. If you can't find a contractor that will do this, you can do it yourself. There are a few free options that are okay. The one that I used is much more complete and runs $50 for a homeowner. You input things like wall area, which direction each wall is facing, insulation values, window/door info, etc. It takes some time to gather the info and do a real load calc, so many contractors don't bother with it. If you don't want to do it yourself and the contractor won't do it as part of the bid, it would still be worth paying someone say $200-$300 to do the calc. A side benefit of this will tell you if your boiler is the right size and may allow you to go to a smaller boiler when it comes time to replace the current one.
  5. Dana

    Dana In the trades

    Messages:
    2,837
    Location:
    01609
    There's no WAY your cooling load is anything like 4 tons unless you have a massive expanse of west-facing unshaded glass! For decently built newer construction 1-ton/1000' is a rule of thumb that even works in FL and TX. In cool-coastal (but still kinda-sticky) Hull even a 3 ton would be overkill from an AC-only point of view, but probably not from a heating-season point of view.

    The installed costs of single-head mini-splits runs about $2700/ton but it'll vary. With multi-splits figure on $2500/ton for the outdoor unit and ~$1200/head (so a 3-head 3-ton might run about 10 grand, but will often come in cheaper if it's an easy installation.) Since in your case heating is a primary consideration pay attention to the heating season performance factor rating (HSPF) more than the SEER/EER in cooling mode efficiency. Anything with an HSPF below 8.5 isn't likely to give you the heating season efficiency you want- try to hold out for 9+ if you can. (There are 1-head mini-splits scoring better than 10.5, but SFAIK no 2.5-3 ton units north of 10.)

    The three most-supported vendors in our area are Mitsubishi, Fujitsu, and Daikin, in roughly that order. For both heating at low temp and cooling efficiency stick to the Daikin Quaternity, Mitsubishi H2i "Hyper Heating", or Fujitsu Halcyon series. The Daikin Quaternity units have the advantage of being able to dehumidify in both a heating & cooling modes with independently settable relative humidity and temperature setpoints, which may be useful for those many spring<-> fall days when it's humid but not particularly hot, but I haven't looked very closely at their multi-split offerings- don't know if there are any models that fit perfectly. The others have "dehumidify" modes, but it's cooling-only (slow blower speed), and doesn't run under temperature or dehumidistat control when running in that mode.

    Unlike bang-bang on/off controlled AC units, there's little disadvantage to slight oversizing with ductless, since they have continuously variable compressor speeds (and variable speed blowers) as well as a (internally) controllable variable-refrigierant volume valving that give them phenomenal efficiency at part load (much better than when running full-out), which is what gives them such good SEER and HSPF numbers. Sizing it with Manual-J may be useful for determining which rooms/spaces get the heads, but it's unlikely that sizing it for the full heating load at 10F would make any practical sense. They all have output curves with temperature (and humidity), so while a 3-ton nominal unit might give you 3 tons of cooling at the 1% design temp, it'll give you maybe 15-18KBTU/hr at +10F. But that's still a large fraction of your heating load at design temp, and the average temp in Hull in January is in the mid-20s, a condition where your heating load is likely to be under 25KBTU/hr, and below the capacity of the heat pump, so seasonally it could easily be carrying 3/4 of the heating load or more.

    To get the best efficiency out of a ductless you "set and forget" the temp and let it modulate, which guarantees that it always runs at the lowest ( highest-efficiency speed.) To work in conjunction with the oil boiler you can use a setback thermostat for the boiler, but make sure that whether in set-back or warm mode it's setpoint is at least 2F below whatever you have the ductless set to, so that the ductless carries as much of the load as possible. Even if you have to set it to 72F in winter to do that comfortably, running the house at the warmer temp with the ductless is going to be far cheaper than maintaining even 65F with oil.
  6. woodguy00

    woodguy00 New Member

    Messages:
    22
    Location:
    Hull, MA
    Dana,

    Once again you have provided a wealth of information. Based on this as well as you PM, I'm sold on ductless - as much so for the heating as the cooling. I have a couple contractors providing quotes.

    Thanks for all your help on this board.

    Nukeman,

    Thanks also for your help here. It is greatly appreciated. I believe I found the free calculator online last night. It came up with 23,000 btu heat loss and just under 31,000 btu heat gain. That sounds in line with what both you and Dana are suggesting. Does a four head 2 1/2 ton system sounds about right?

    tk
    Last edited: Jun 12, 2012
  7. Dana

    Dana In the trades

    Messages:
    2,837
    Location:
    01609
    A 4- head 2.5 ton sounds right, but all else (HSPF & SEER) being equal you'd get a slight edge on efficiency with a 3-ton, since the average speed of the compressor is slightly lower. But if it's a choice between the smaller unit with and HSPF rating of 9.1 and an SEER of 14.5 and bigger one with with an HSPF of 8.5, and an SEER of 17.5 take the unit with the better heating (HSPF) efficiency.

    You have far more heating degree-days than cooling degree-days in Hull, so the cooling efficiency differences will hardly matter- optimize where it'll make the bigger operating cost difference.

    I'm not sure how many 4 head units are out there at 2.5 tons but there are several 3- head 2.5 ton units. The Mitsubishi MXZ3B30NA 3-header is pretty good in heating mode (HSPF= 9.5), if a bit tepid in on cooling efficiency (SEER 14.5). But that was Soooo last year- it's a moving target and a rapidly moving target at that. The same model numbers seem to test and perform better every year with tweaks to the control algorithms etc. according to some third-party test data.
    Last edited: Jun 12, 2012
  8. woodguy00

    woodguy00 New Member

    Messages:
    22
    Location:
    Hull, MA
    Dana

    Thanks for the advice here and in your email. I've received one quote that wanted to use two 2 1/2 two head units. Seems like overkill. I expect another quote in the next day or two.
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