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Choosing the right indoor units is a critical step in heat pump design. In the first part of this two-part series, I described ductless options: high-wall units, low-wall units, and ceiling cassettes. Here, I’ll look at ducted indoor units.
Using ducts to distribute heated and cooled air has several advantages. One of the biggest is that a single air handler can serve multiple rooms. As long as the duct system is properly designed and installed, it will provide more even temperatures than a design in which some rooms have ductless heads and others don’t.
The pros
Ducted designs also allow for a better match between the capacity of the indoor units and the needs of the rooms being served. Take, for example, a cluster of three bedrooms, each with a design heating load of 2500 Btu/hr. The smallest ductless heads available have nominal capacities of 6000 Btu/hr. Putting a ductless head in each room would mean 18,000 Btu/hr total. This severe oversizing can lead to temperature swings and efficiency losses. A better solution would be to install a single, 9000 Btu/hr ducted air handler that serves all three bedrooms.
Reducing the number of indoor units also reduces equipment cost and complexity. Less refrigerant piping and fewer fittings mean smaller quantities of refrigerant and fewer potential points of failure. For this reason, choosing ducted systems should reduce climate impacts associated with leaks of high-GWP refrigerants.
Ducted systems also allow separation of the equipment from the occupied space, reducing noise and visual impact. They can incorporate better filtration than the coarse mesh on ductless units. And ducted systems can also be used to distribute ventilation air. In addition, centrally ducted systems (described below) allow easy integration of supplemental electric heat using off-the-shelf controls and resistance heaters.
The cons
Ducted systems…
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3 Comments
Question regarding what condensate line to use:
"Condensate from compact-ducted units can be drained by gravity if the location allows for suitable pitch. Most models also include a “condensate lift mechanism” that pumps the condensate up a short distance..."
Horizontal compact-ducted units (sometimes called "pizza boxes") are my go-to. Mitsubishi SEZ units have THREE condensate outlets, two lower ones that are gravity draining (conveniently one on each side) and an upper outlet that is pump-operated. I have always endeavored to make certain that the plumber figures out a sloped drain (when possible) so that I can use one of the lower gravity outlets. My thought is that one less pump/motor to worry about is better for maintenance and longevity. Pro Mitsubishi "diamond-rated" installers have told me that as a rule they have ALWAYS used the upper pump outlet without consideration for gravity and they are not sure why so but stated that perhaps using a pump helps to avoid soft clogs like particles or mildew. I hope that the truth is that the pumped outlet is always used so that the installer can use less brainpower and not because it's actually better to pump in lieu of using free gravity.
Does anyone have a preference? Pump or gravity? Also how to prep the machine for one vs the other? Mitsubishi was fairly unhelpful in install instructions to me (architect and builder but not a HVAC pro).
Further, should I unplug the pump from the board? I recently did so on my own SEZ units after two years of good operation, thinking that it's best if the unit STOPS working if it ever senses the condensate level rise past the pump trigger instead of defaulting to pumping (since it would just be shooting water into my soffit) if my lower condensate line ever clogs.
Hi Ric, These are great questions, and I don't have strong opinions. It's been my experience that both gravity drainage and condensate lift on compact-ducted units are pretty reliable, so long as as the pipes are pitched correctly and cleaned periodically. I tend to default to gravity drainage when I don't need the extra lift to get a good pitch (most attic applications) and to use the pump in dropped ceilings and in basements where I want to keep the piping up in the joists. We did have one customer complain that the pump caused a high-pitched whine. Fortunately the unit was gravity-drained, so the problem could be solved by unplugging the pump.
That's great stuff, thank you. I can't imagine the time it takes to write an article like this.
I just finished my own home and did PEADs for the first time. We are typically a "duct-less only company," but we wanted to try it. I am happy so far. We would normally use wall-mounted and electric in the bathrooms, but this is much cleaner and more efficient. I only wish we had added 2' or so between the return grille and plenum in the upstairs unit. We connected one after the other, with no sound deadening, and the sound is noticeable in the middle of the night when all was quiet. From now on, we will be more mindful of this fact.
My other comment is access. I am preparing to remove these someday, and provisions must be made in advance. I have seen several installations recently, and I pity the poor soul who must get at these for anything substantial.
Finally, with the refrigerant changeover, we use the City-Multi from Mitsubishi as a stop-gap until we get residential ones back. They have what is called a high-static unit. I am told it will ramp down to mid-static ranges, but I have no one to corroborate that story. Do you have any experience with these? The one we are looking at now is the PVFY-P30NAMU-E1.
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