Hi, all 

There has been renewed interest in this conversation 


Let's keep it going 2018 / 2019 style! 

Tags: Ductless, HERS, Minisplits

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Hi Brett, since the archive discussion you linked is locked to new posts, I think the best way to continue the conversation would be cite something specific from that thread here, to stimulate additional discussion.

To answer the question posed in your headline, there's no inherent impact of ductless on HERS index beyond the equipment's efficiency ratings. Nor should there be.

In calculating the HERS Index, the software considers equipment SEER/HSPF ratings and duct losses, if any. So a 17 SEER / 9 HSPF conventional system with ducts inside conditioned space would be considered equivalent to a ductless mini with the same efficiency ratings.

Sounds fair but will anyone see it? Maybe this can get promoted? 

In our webinar about ducted air source heat pumps featured now on youtube here Mitsu claimed that ductless usually boost the HERS index rating over ducted systems. They went on to explain it further. 

So here is my question 

1) Has anyone done any studies to prove that?

2) As we try to get more people who have less financial means to zero energy, is going ductless one of the ways to do that by 1) Saving money in upfront costs and 2) Boost energy reductions to lessen the amount of solar needed to get to zero energy capable?

Also, reading your message David, I think there would be disagreement. 

Since the Home Performance Forum archives are hosted on a different server with different software, the only way to 'promote' an archive thread is by posting a link here, as you have done. Perhaps Diane (moderator) will promote this discussion in her next group email.

I didn't listen to your webinar but any claim that ductless boosts HERS ratings must be based on a higher SEER/HSPF rating. The absence of ducts has two affects on operating cost: less blower energy, which is reflected in the rating, and no losses to unconditioned space. However the latter is not a exclusive to ductless. You can eliminate duct losses by locating ducts in conditioned space. As for the lower upfront cost, HERS obviously doesn't deal with that.

Here's what I said: "there's no inherent impact of ductless on the HERS index beyond the equipment's efficiency ratings. Nor should there be."

What exactly do you disagree with or seek proof of?

I agree that 'whole house' ductless heat pumps may be most economical option for for low load homes both in terms of first-costs and operating costs (assuming there's not a significant cooling load and the home has an open layout). Efficiency ratings and HERS ratings already bear that out. So I'm not sure what you're looking for here.

Whole-house ductless is more problematic for homes with actionable cooling loads, something I addressed in the archived thread, but I think that's a different discussion.

When you say actionable cooling loads you mean higher than heating loads? 

I need to see evidence of some homes you ran in REMrate, apples to apples with only the ductless vs ducted swapped out. 

Dianne already did this. 


By 'actionable', I mean the load is such that the homeowner actually needs to use A/C, as opposed to what the software might predict. Most of the US has actionable cooling loads, except higher elevations.

I explain my logic for not using whole-house ductless in homes with significant cooling loads in my first reply in the archived thread you linked. A lot depends on the occupants' predilection for using A/C, especially in bedrooms. If you have to put a head in every room, the advantages of ductless (both in terms of efficiency and first-cost) are lost. Yet I digress...

I'll try to find time this weekend to run a couple of REM/Rate report comparisons and post them here.

Depending on what kind of home you have, your options for cooling systems may be limited. For example, installing ductwork in a home built without it is much more difficult than installing a mini-split system.

Though the two have a few energy-efficiency differences, if you’re concerned with the environment or want to save on energy bills, you have options. Remember that both new central air and mini-split systems are far more energy-efficient than window air-conditioning units.

Thanks Franco but how does that impact HERS? 

Assuming same efficiency - apples to apples - off the top of my head the only way a ducted system gets penalized is in the leakage to outside & where are the ducts / insulation. You have your ducts all inside the envelope there should be no difference 

As for actionable / real usage of some family, that is not the way HERS is setup. It is more akin to the baseline government testing that says your car should get X MPG or your appliance will use Y $ a year. Actual results will vary & as I like to point out to some it is good way to grade your conservation. With that said, HERS seems to be over on heating & under on cooling figures - at least it has been in the past

Hi Sean, my comment re: actionable cooling load was about the application of ductless, not related to Brett's question on impact on HERS. Sorry if I conflated the two.

Ductless mini split-system air-conditioners have many potential applications in residential, commercial and institutional buildings. The most common applications are found in multi-family houses or as an option for homes with "non-ducted" heating systems, such as water heating (hot water heating), radiant panels and heating elements (wood, petroleum, propane). They can also be a good choice for room extensions and small apartments where expansion or installation of distribution channels (for air conditioning or central heating systems) is not possible. 

Hello Brett,

Without looking at the previous discussion, the question can be answered in many ways.  Do ductless systems do better on the HERS?  Better than what on the HERS?

A ductless system will have a better HERS Index than a ducted system that is located in unconditioned spaces like an attic or crawlspace for sure.  A ductless system will have a similar HERS Index for an HVAC system and duct system in conditioned space.  There are factors that would adjust results such as total duct leakage, leakage to outside, duct location and duct insulation, duct surface area, number of registers, etc.  

In general, ductless systems will score better HERS Indices than ducted systems based on the design and leakage results.  


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