Question

Are ductless A/C units worth considering as a replacement to our aging central AC with poorly performing ductwork?

Asked by Andrew Cooper
Troy, NY

We have a 1920's 2 story dutch colonial with good attic (spray foam) and basement - partially finished- (spray foam and polyiso) insulation but nothing in the 1st and 2nd floor walls. Heat is forced air natural gas, with central AC. The AC was added after the fact. All 2nd floor supply runs are inbetween the studs without ducts. In servicing the AC this year a full replacement was recomended (r-22 system with probably leak) to include the furance. Though a wholy new system would perform better for sure, the poorly designed ducts will still limit upstairs cooling. We do not mind the tempature differential in heating season, but it's worse, and bad for sleeping, in the summer (~12 degrees summer ~ 8 in the winter). As the furnace is stil functioning, and 95% efficient, I am curious about changing to multiple ductless AC systems and removing the central air. Downstairs is relatively open, upstairs is not (4 bedrooms), 1650 squ ft total.

Answer

Andrew,

Thank you for your question concerning ductless mini split HVAC systems.

Someone has done a lot of good things to your home with regards to the spray foam insulation, but the HVAC and distribution sound horribly inefficient. The summer and winter temperature differences in the home are not unreasonable. Since heat rises it is very difficult to achieve minimal temperatures between levels without zoning the system. This can be impractical in an existing home with a poor quality distribution (duct) system.

Ductless A/C

The ductless (mini split) system is certainly an option. Many of these systems can handle up to 8 zones. The distribution panel is mounted on a wall in the zone you are trying to condition.

  • A single panel on the main or second floor is not going to provide satisfactory results.
  • Based on your description I suspect that you would need at least 5 panels (zones), one in each bedroom and one for the main floor. The more zones needed the higher the cost.

While this is worth considering you may also want to look at other options.

Duct sealing

First, I would get a second opinion and verify that the HVAC system distribution system to the second floor is actually stud cavities without any ductwork.

While this is common practice on the return side of the duct system it is highly unusual for the supply side to be lacking ducts. Leaky ducts (even stud cavities used as cold air returns) hidden in wall cavities can be improved by an aerosolized duct sealing system (http://www.aeroseal.com/) with minimal disruption to the finished home.

If the ductwork can be corrected it would be more cost effective to replace the existing HVAC equipment with properly sized system designed to utilize the ductwork system in place.

Flexible 3-inch ductwork

Another alternative would be to install a high pressure air conditioning system that utilizes 3‐inch flexible ductwork routed through attic spaces into individual rooms.

This is a common retrofit for homes that have hydronic heating with no existing ductwork. This system would allow you to abandon the existing AC unit and replace the furnace with a similar unit when replacement becomes necessary.

Whatever system you decide to install you will still need to address the leaky ductwork to ensure healthy, safe and comfortable air distribution when using the furnace.

Good luck in your retrofit.

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