Heatpump question - split system vs single

Discussion in 'HVAC Heating & Cooling' started by yds, Feb 21, 2010.

  1. yds

    yds New Member

    Aug 28, 2006
    Toronto, Ontario
    Hi. I'm in the process of building a new third floor on top of my two storey semi-detached downtown toronto home. It will contain a master bedroom, ensuire bathroom, reading area and small deck. Total floorspace of just under 500 sqft. The plans call for a supplemental HVAC system for this floor. I had originally figured a split system ductless would be the right choice. I've attached the floorplan below.

    I had a discussion with my contractor yesterday. The HVAC guy that he is dealing with has suggested that I don't need a split system, just a single outlet that will supply the heat and cooling to the entire floor. The contractor is saying that if the outlet is located over the sliding doors to the deck it will sufficient to get heat and cooling to all rooms, with a supplemental electric register for when it gets really cold occasionally. I'm concerned however that the bedroom would not get sufficient heat/cool, especially when we have the door to the hallway closed (we would typically leave it open during the day, but close it at night). Note that the floor in the ensuite bathroom will have radiant heat, so it would probably be ok heatwise in any case.

    My thought was that it would be a split system with one outlet over the sliding door, and one outlet in our bedroom. Still with an electric register in both rooms for when it gets really cold.

    Any professional advice out there?

  2. Lightwave

    Lightwave New Member

    Aug 20, 2007
    Vancouver, BC
    Can you define your terms more clearly?
    What do you mean by 'split system?' A ductless heat pump with two indoor units?
    What does your GC's HVAC guy mean by a 'single outlet?' A ductless heat pump with one indoor unit or something else entirely?

    A single head ductless unit will not handle three rooms under any circumstances. You need a register or ductless unit in every room. You can use a single specialized ducted minisplit to heat/cool two rooms--you could handle the bdrm and ensuite this way, but you'll need another unit for the reading room.

    You can drive two indoor minisplits from a single outdoor compressor.

    Heat pumps produce less heat as outdoor temperatures decline. They must sized to meet your heating requirements at the coldest temperature you expect the heat pump to handle without help from another heater. This switchover temperature is known as balance point. Sizing is a matter of doing a Manual J load calculation at balance point and then using heat pump manufacturer's published heat output ratings at the intended balance point to determine the correct unit size. Oversizing a minisplit for heating will not--in contrast to oversizing a full-sized heat pump--harm cooling performance.

    No minisplit will produce useful heat at outdoor temperatures below -4f/-20c. Many will shut down entirely at -14f/-10c. Given Toronto's climate, you must have a supplemental heat source capable of carrying the entire heating load for the addition in cold weather. Use Manual J to size the supplemental heat source.

    If you have natural gas, a gas fireplace (for the bdrm) and wall furnace (for the reading room) will work nicely for supplemental heat. If you don't have gas, then look at baseboard heaters, underfloor radiant, or radiant panels.

    For the heat pump, look closely at Daikin's multi-head MXS system. These have very good cold weather performance and support both ductless and ducted indoor units. Daikin 4MXS32GVJU is probably adequate for your needs but you need a load calc for a definitive answer.

    Sizing minisplits heat pumps for this application is a niche job that few HVAC contractors are able to handle. If your GC's HVAC guy doesn't understand all of the above like the back of his hand, find another HVAC contractor with a strong background working with minisplits.
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  4. gator37

    gator37 Retired prof. engr.

    Nov 10, 2009
    Retired prof. engr.
    You will not be happy with one outlet in either one of the rooms. If you can get an outlet in each room that would be good. You also need an exhaust in the toilet and undercut the door (1/2") to allow makeup air to the exhaust fan.
    It does not matter if you use a conventional split or a package system as long as it gets the job done. A conventional split would need a place for the inside unit plus ducting.
    If you are thinking about a split ductless system I would recommend a Mitsubitisi (or similar system) with two mini AHUs assuming you have room above the ceiling. This type of system also has more control advantage but is typically more expensive. You will also need a place for the condenser unit outside.
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