HVAC retrofit options
Nick Van Kleeck| Posted inGeneral Questionson
I would like to replace the HVAC system on my zone 2B low desert house. The house has a 20 year old gas pack (a roof- mounted gas furnace with electric AC) and a 6-8 foot run of leaky return air duct on the roof. There is no attic, as the low-slope roof is on top of the ceiling joists/rafters. The interior of the main house is 900 square feet and has duct work in a 15 foot section of dropped ceiling in a central hallway. I plan to blow in insulation in the rafter space (currently vented to the outside) and put a couple of inches of foam on the roof. Ideally I’d like to get all the conditioned ductwork off the roof and inside the building envelope, and am looking for suggestions.
I’ve considered putting in a couple of ductless units, but the locations they’d need to go on exterior walls are problematic. I am thinking this situation might be a good fit for a ducted mini-split if I am willing to sacrifice a bit of space for the air handler, but here I could use a bit of basic HVAC education. I don’t understand the difference between a ducted mini-split and the more conventional heat pumps that are common out here. Seems like both need an exterior compressor, coolant lines and an air handler… so what’s the difference? Would one make an easier retrofit than the other?
In case more detail is useful I’ve attached a plan view. The 900 foot area served by ductwork is the rectangle more or less on on the lower right: the bedroom, kitchen, living room, bath and hallway. The existing ductwork is in the hallway, with vents in each of the rooms and the return air in the living room ceiling. (The rooms on the north side are part of an addition without ductwork that already has a mini-split, so doesn’t need to be considered).
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
"I don’t understand the difference between a ducted mini-split and the more conventional heat pumps that are common out here."
他们是相同的,户外单位just look different. Using the existing ductwork seems easiest to install, easiest to service and probably cheapest.
A quick, basic answer would be capacity differences, and indoor air handler sizes.
A Fujitsu slim Ducted Minisplit could be variable speed/output starting from 3k minimum to 7k btu max output (rated) unit, to a 3k minimum to 18k btu max rated unit, although both actually put out more than rated. A central type conventional ducted unit could be from 18k btu/1.5 ton up to 60k/5 ton.
The slim Ducted air handler can be 22”W,27”L, 8” tall, like a flat little box, vs a 17”x19”x30” long air handler.
Being in a dry climate, you want to aim for high system airflow for efficiency, around 600cfm per ton.
A dropped ceiling hallway is ideal for a slim Ducted air handler, with high flow/low static pressure ductwork in conditioned space, and adding a 20”x30”x2” return filter/grille assembly attached to the return plenum, right at the unit.
"I don’t understand the difference between a ducted mini-split and the more conventional heat pumps that are common out here."
Most all of the mini split units use variable speed compressors yes the top of the line conventional heat pumps will have variable speed compressors a small percentage of units sold.
The control system that are built into the mini split system allows the mini to operate almost continuously matching the units output to the buildings losses. Most conventional heat pumps cycle on and off overshooting the set point and coming back on under the set point.
The outdoor units of the mini splits are about ½-1/3 the size of a conventional heat pump in terms of cubic feet.
Generally the blower motors of the ducted mini splits are much less powerful than conventional heat pumps so the duct work must be carefully designed.
If you connect a smart or standard thermostat to a mini system you are sacrificing most of the variable speed abilities that allow the minis to provide more comfort for fewer dollars.
Walta
Thanks all. This is encouraging - the slim dimensions of the air handler mean it could fit in a lot of places. Houses built in the '50s like this one often had a gas wall heater in the main room, and a vertically mountable air handler in a small purpose-built vented cabinet could look right at home if for some reason the dropped ceiling didn't work out. It also sounds like I would do well to find someone experienced with these units to design the ductwork. Guessing that expertise might be a little hard to find, so any suggestions would be appreciated. Not sure if this is the territory of HVAC engineers or installers.