iQ drive vs mini-split?
My last house (2700sf)was made with SIPS and had a traditional 16 SEER heat pump. It worked fine and despite a convoluted layout only had minor airflow issues. Now I’m building a smaller house (2000 sf) with double stud walls and better windows so I expect the envelope to perform better than my last house with 4″ SIPS (climate zone 4 – east TN…go vols and all that stuff). I have been thinking of going with minisplits because they are all the rage…but with the inverter’s used in Nordyns iQ drive couldn’t it be close to as efficient? I’m sure I could use ducted mini-splits as well, but after reading a bit I think the more traditional unit might deal with airflow issues better, I can also hook up in-line dehumidification and air filtration…which I guess you could on a mini-split as well. I’m trying to think of what my actual question is and I think I might just be looking for a little validation on my thinking. Can the iQ drive deliver anything close to the efficiency of a mini-split? Would adding an in-line hrv be more cost effective with a more traditional system? Has anyone had air flow issues with ducted mini-splits? Thank you for your replies in advance.
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Andrew,
In general, I think that designers spend too much time worrying about heating equipment and cooling equipment. The key to a good design is to create an excellent thermal envelope: as airtight as you can make it, equipped with above-code levels of insulation and high-performance windows, and desgined to address solar heat gain -- that is, with roof overhangs that allow solar heat gain in winter but limit solar heat gain in summer.
Once you've done that, your heating and cooling bills should be low -- so your choice of HVAC equipment becomes less critical.
You can certainly design an excellent system with a Nordyne iQ drive air-source heat pump connected to conventional forced-air ductwork.
Q. "Can the iQ drive deliver anything close to the efficiency of a mini-split?"
A. Systems with conventional ductwork will never be as efficient as ductless minisplits, since ductless minisplits gain efficiency by eliminating ductwork. The static pressure of any duct system reduces efficiency, because you need a larger fan to overcome the static pressure.
Q. "Would adding an in-line HRV be more cost-effective with a more traditional system?"
A. It's always a bad idea to try to combine heating system ductwork with ventilation system ductwork. For more information on this issue, see Ducting HRVs and ERVs.
Q. "Has anyone had air flow issues with ducted mini-splits?"
A. Your question is unclear. What do you mean by "air flow issues"? If you want to install a ducted minisplit, you have to follow all the usual design steps needed for any system, starting with a room-by-room load calculation and a duct design calculation. Undersized ductwork or ductwork that has a high static pressure will cause a problem in any system -- whether the system has a conventional air-source heat pump or a minisplit heat pump. Since ducted minisplits have smaller fans than the usual U.S. air handler -- famous for huge honking fans -- you need to pay attention to your calculations when designing the system.
A lot of the reason that mini-splits are so popular is that they get great efficiency in mild conditions by using variable-speed compressors ... and they have that technology available at low cost because of economies of scale, not because of the number passive houses built in New England, but because of apartments in Asia. The iQ drive has the same capability and so should capture much of the same benefits as the mini-splits ... but not all.
As Martin says,
". Systems with conventional ductwork will never be as efficient as ductless minisplits, since ductless minisplits gain efficiency by eliminating ductwork. The static pressure of any duct system reduces efficiency, because you need a larger fan to overcome the static pressure."
To elaborate on that some:
A modern system like the iQ drive units with variable speed fans can drastically reduce the fan energy. You can also oversize the ducts if you like and have room for them.
On the other hand, if you run the ducts in unconditioned spaces, heat leakage and air leakage can lose energy and cause lots of other problems.
A quick web search only shows a best-case of 10 HSPF on any Nordyne IQ drive equipment. That compares dis-favorably with say, the Fujistu 9RLS3 at HSPF14.2. That's 42% more heat output for the same power use.
Some of the Carrier GreenSpeed air handler & compressor combinations can test as high as HSPF 13, which is comparable to say, Mitsubishi -FH09NA's 13.5.
But on a better than code house even the smallest GreenSpeed might be oversized for your actual load- you might be able to get there with a single 1.5 ton Fujitsu -18RLFCD mini-duct cassette (HSPF 11.3) or even the 1-ton -12RLFCD (HSPF 11.5) either of which would be less expensive than a 2 ton (smallest available) Nordyne iQ or GreenSpeed.
So, it really boils down to both the whole-house and room-by-room heating & cooling loads, all of which are under your control during the design phase. But unless you're designing in so much window area that your cooling loads soar, any 2-ton unit is almost assuredly going to be oversized for your loads in a high-R double-studwall house. Only when you know the loads will you have a shot at picking the right equipment, but standard split-systems are designed for more conventional code-min houses with much larger loads. With a low-load house mini-split solutions are a better fit.