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Community and Q&A

Ducted vs. Ductless Air-Source Heat Pump

anonymoususer | Posted in General Questions on

Hi, everyone. I hope someone can help….We are homeowners wishing to swap our decades-old oil furnace for ducted or ductless air source heat pumps. We presumed the best person to calculate heating loads for each room, and to help us figure out if ducted vs ductless vs combo made the most sense, would be a licensed mechanical engineer. We therefore looked up (on state-run website) every mechanical engineer with active license within 50 mi of our zip code. We reached out to as many as possible but dozens turned us down–for varying reasons, but most often because they simply did not have time or they had signed a “no moonlighting” agreement with their employer. Anyway, we finally found a mech eng willing to visit our home, consult with us, carry out room by room manual J, then help us figure out ducted vs ductless…or so we thought. He turned out to be incompetent and dishonest: Charged us thousands to produce a report based on a house that was not even shaped like ours, nor with the same dimensions, and with very simplified infiltration inputs. Pissed us off because we gave him precise drawings of each room with measurements, plus R value of windows, walls, doors, roof, blower door result, etc. Then he tried to blame it on us, claiming that when he first met us (at our house), we gave him inaccurate info. We are certain we did not but anyway…it’s water over the dam. Now we are desperate for some honest advice but, not knowing we to turn, are hoping somoene on this forum can steer us in the right direction…?

To begin, we purchased a software called Wrightsoft and used it to draw each level of the house and run heating loads for each room. Luckily, we had on hand precise measurements for each room prior to purchasing. Also, we know the u factor and shgc for most glazing (recently bought new windows and doors for whole house). Plus, we know the R value of the wall and roof insulation. Finally, we have infiltration results from a recent blower door test. So we were hoping that, with all these inputs, we could get accurate heating loads for each room, which it seems we have….? To our surprise, however, it seems that our loads are too small for a ductless design…?

I thought this only happened in tiny homes or homes with passivhaus levels of tightness….?

Our house is shaped like a T:

The easternmost extreme of the T (which has only 2 levels) has a bedroom on top level, then kitchen/dining/bath/laundry at grade.

The northernmost point of the T (3 levels) has a bed/bath on top level, bed/bath at grade, and bed/bath below grade.

The southernmost tip of the T (3 levels) has a bedroom on top level, living at grade, and office below grade.

Connecting the 3 points of the T are long hallways. Before doing the load calcs, we thought maybe we could put one condenser at each end of the T (north, south, east). Then connect the condenser at the east extreme to 2 indoor air handlers. Then connect the condenser on the north to 3 handlers. Then the condenser on the south to 3 handlers.

But the smallest cold-weather compatible condenser we can find in Mitsubishi catalog has a whopping 20K capacity ( ! ) if 2 indoor connections and a ginormous 24K capacity ( ! ) if 3 indoor connections. Our whole house load calc was ~ 48K for a 3500 sq ft house. I guess our loads per room are small due to low (yet not passivhaus-type low) air infiltration and relatively small size for all rooms.

Should we throw out the possibility of going ductless? We are open to a ducted or combination system but feel overwhelmed by the learning curve we would need to climb to compare ducted options (ie, how many indoor units, where place each one, will modern-day ducts fit in our existing chases, what configuration/material is ideal for ducts, etc)

In a ductless system, is it okay for the capacity of the condenser to exceed the sum of capacities of handlers that are connected to it…by “a lot”? For instance, if I have two air handlers, each with capacity of 6K, can I connect these (and nothing else) to a condenser with 20K capacity?

Regarding air handlers, how much can the machine’s capacity exceed the heating demands of a room? If I have a bedroom with, say, 1500 btu heating load, what dreadful things might happen if I put a 6K  air handler in that room?

I have read Allison Bailes articles on the dangers of oversizing but do not understand…?

We have not looked at Fujitsu, Daikon, Samsung, LG etc simply because around here it seems most hvac contractors are generally more familiar with Mitsubishi than other brands. For this reason, we have thus far been focusing on the Mitsubishi “hyper heat” product line.

Also, it is important to us that our system perform well without backup on the most bitter nights of February. We have heard Mitsubishi ashps are reknowned for their performance in extreme cold.

Any advice would be much appreciated. thank you.

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Replies

  1. paul_wiedefeld | | #1

    I’m sorry your experience has been so bad.

    In my opinion, you’re overthinking this: you have the ductwork in place, now add a ducted heat pump. It might need backup - that can be resistance strips or the oil furnace. Ductless is for homes where adding ductwork is cost prohibitive and performance isn’t important.

    I’d skip all manual Js, use this instead: https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler.

    1. Expert Member
      Michael Maines | | #3

      I disagree that ductless systems is for homes where adding ductwork is cost-prohibitive and performance isn't important. They are also for homes where operating efficiency is a priority, space is not available for ductwork, space is not available for a central air handling unit, sheet metal contractors are in short supply, mechanical engineers who can design a ducted system are in short supply, etc..

      But ductless systems are not ideal for homes without good levels of insulation and air-sealing, a lot of small rooms and no large, open spaces, occupants who want identical temperatures in every space, where concerns about refrigerant leaks are a priority, or where replacing line sets in 20 years will be problematic.

      1. paul_wiedefeld | | #4

        Right. In this context I mean when ductwork is already installed and the alternative is a multi-split, I should have been specific.

  2. Expert Member
    Michael Maines | | #2

    Simply: oversizing the system results in inefficient operation and will cost you more than necessary. Results vary but I have a mechanical engineer friend and neighbor who built a new house just before this fact became widely known, and he is finding that his system costs 50% more to operate than his careful calculations predicted.

    Did you know that Allison Bailles' company, Energy Vanguard, designs residential mechanical systems all over the US? They don't design hydronic system but a project like yours would be simple for them. For some things it's better to have local designers, but for mechanical systems it's less important unless you want them to also inspect and commission the system.

  3. anonymoususer | | #5

    Thank you, Paul and Michael. This is helpful. I have left messages with Vanguard in past and no call back but will try again

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