What is an optimal HVAC strategy?
I continue to plan for my new home (Zone 3) and am now focusing on the HVAC system. Here is what I think I understand at this point. Let me know if I am off base.
– Local code usually requires Manual J, but individual inspectors may or may not ask for evidence of one to issue a COO.
– Most contractors do not perform Manual J calculations correctly, if at all.
– A third-party HVAC designer is more likely to provide an accurate Manual J (at a cost of .50 to .75 per square foot).
– An accurate Manual J may not matter. (I say this after reading that most systems are oversized and that there does not seem to be much downside in terms of energy or comfort when contractors follow their general “rule of thumb” for sizing.)
– Manual D is probably more important in determining comfort and performance.
– Most contractors don’t perform Manual D calculations or don’t perform them correctly.
– A third party HVAC designer is more likely to provide an accurate Manual D.
– All ducts must be within the conditioned space. Metal ducts are optimal and must be sealed.
– Mini-split systems from Mitsubishi or Fujitsu offer the best overall combination of control and conform.
– Mini-split system with variable refrigerant control are the best choice (budget allowing).
– Pricing on mini splits varies considerably depending on region and installer experience. Finding an experienced and qualified contractor is critical.
– Ground-source heat pumps rarely deliver the performance the manufacturers promise.
– There are several other Manual codes that generally aren’t discussed (H, S to name two).
– Fresh-air exchange is critical in a tight house. There are several ways to promote air quality, but a ERV (mixed climate) that delivers air to all rooms is optimal.
What have I missed or gotten completely wrong? Thanks in advance for your input.
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Replies
A couple of things...
Oversizing air conditioning DOES have a comfort downside at some point, since it leads to highly variable and sometimes uncomfortable indoor humidity.
All high efficiency mini-splits are variable-refrigerant volume- VRV is not a different (or more expensive) beast.
ERV may not always be ideal, even in mixed climates. The higher the summertime dew points, the more likely ERV is to have an edge over HRV, but it's not universal.
There are MANY players in the mini-split market other than Fujitsu & Mitsubishi that are appropriate for mixed climates. But those two are currently the only manufacturers with models specifically designed to operate at -25C/-13F or colder. While others may still be running at low temp, there many cold temperature operation issues that are not designed into other units (eg. When defrosting at very low temps, there has to be a method of removing the liquid moisture before it re-freezes at the bottom of the unit- a simple drin hole & sump doesn't cut it at sub-0F temps.)
Thanks, Dana.
I thought VRV was a fairly new feature and more likely to be found on multi-zone units like the Mit City Multi. My house will be two floors over a (eventually) finished basement, so I was considering at least three zones--although I think six zones would really work better for the layout.
Would you recommend any other brands besides Mit and Fujitsu? Online review suggest some of the other suppliers aren't quite as efficient and/or reliable.
With the ERV/HRV device, how do you decide? Atlanta is kind of split between heating and cooling, but cooling is likely the bigger need. And we have lots of humidity. Temperatures rarely get much below 30, so cold weather operation isn't a huge issue.
Steven,
Here is an article you might want to read: HRV or ERV?
Hi Martin:
Thanks for the article. It sounds like I could install an ERV (I was considering the FV-04VE1 WhisperComfort™ Spot ERV because its inexpensive and the house is relatively small and tight.) or could go with an exhaust only approach. I'm spec'ing Panasonic WhisperComfort bath fans in any case and could put one or more on a timer switch for air exhaust.
But I need a whole-house dehumidifier and humidifier setup to really maintain comfort, correct?
In-re mini-split manufacturers: Daikin tends to be a cut above the rest in terms of under-promising and over-delivering on performance, (but usually a bit pricier too) and their Quaternity series has the added benefit of being able to dehumidify to a relative humidity setpoint, in either heating or cooling mode. IIRC Daikin is the worlds largest vendor of bigger-scale VRV systems for commercial-buildings. If you narrowed it down to vendors with the best reliability track record, Daikin would probably top that list.
Sanyo and Panasonic both have a some under-appreciated models with good performance & reliability (HSPF 10+, SEER 17+ ) that work just fine at Atlanta's temperature extremes, but may be marginal for colder climates. (Though I've recently seen a couple of Sanyo installations in coastal Maine.)
(Sanyo & Daikin also have high efficiency R744-refrigerant heat pump technology that isn't yet sold in the US, which are far nicer to the planet in the event of refrigerant leaks, but I digress...)
LG has some high-efficiency numbers on some attractively priced units, but a checkered history on initial reliability (some of which is an installer training issue, the rest of which is usually factory quality control rather than fundamental design issues.) With sufficient local support from the installer to de-bug it if you get one of the lemons they might be worth considering, but otherwise move on.
For model comparison reasons sometimes useful to dig up "qualified models" lists for utilities who offer rebates, even if they tend to be a year or three out of date, eg:
http://mnpower.com/Content/Documents/EnergyConservation/OneHome/HVAC/ashp-qualified-mini-split-systems.pdf
It's a competitive marketplace, but local and distributor support is everything- you don't want to be told a 8AM on a freezing morning that the necessary repair parts are being air-freighted from Asia to California, and should be in Atlanta in only 3 or 4 more days. Mitsubishi & Fujitsu together dominate most US markets, but Daikin is well worth considering.
In-re VRV terminology: All mini-splits with variable speed compressors & multi- or variable- speed blowers (which is essentially all of them, these days) need adjust the refrigerant volumes in accordance with how fast the heat is being pumped into and extracted out of the refrigerant. This is adjusted via the same electronics/software in the mini-split controls responsible for tweaking the blower & compressor speeds to track the load. IIRC both the compressors and refrigerant volume control valves for several other brands are actually Fujitsu spare-parts- the "special sauce" in the mini-split designs related to the delivered efficiency are primarily in the coil design & control algorithms for a particular set of compressors/motors/valves.
In general, there is a slight efficiency penalty when going with multi-splits, since the compressor is slaved to multiple heads, and the speed & refrigerant volume can't always be fully optimized for any random set of load conditions on multiple heads. There is usually a first-cost advantage though, and a wider range of lower BTU head options than you get with mini-splits, so often it's easier to size the heads appropriately for smaller zones. A head that is oversized by more than 50% for the peak load on the zone will start cutting into efficiency too, so you can't just skip the Manual-J on the assumption that the modulating output will take care of it.
I just saw the "But I need a whole-house dehumidifier and humidifier setup to really maintain comfort, correct?" line.
NOOOOOOO! Humidifiers are the bane of buildings, designed to create rot and mold problems in climates that never should! In a tight house humidity builds up if you don't ventilate, but you can set the wintertime humidity levels setting ventilation rates via dehumidistat control. But keep it down to 35% when it's actually cold out, eh? With properly sized mini-splits summertime humidity is rarely an issue, since they run almost constantly (at very low speed.) Only if you OVER ventilate in the summer when the dew points are high would you likely run into humidity issues. (If you went with a Daikin Quaternitys you could just set the RH to 45% or 50% for the summer, 35% for the winter, and be done with it, since it can and will dehumidify with NO sensible heating or cooling when there's no sensible load.)
Hi Dana:
I will investigate the Daikin units.
FWIW. Both the Earthcraft townhouses I've rented while waiting to build have had Aprilaire systems with humidifiers as part of the HVAC (standard split systems). In the current place, I should turn the set-point control to zero and keep it off, and advise my neighbors to do the same. Is that your advice?
Turn the humidifiers off, even turn the WATER to the humidifier off!
If your house is too dry in winter in Atlanta you're overventilating the place, either parasitically via air infiltration (which should be zealously hunted down and sealed), or by excessive mechanical ventilation. There's no real science to support always ventilating to the ASHRAE 62.2 levels unless you have some serious indoor air pollution sources, such as a family that chain-smokes, or goes through a can of hair spray every day!
Monitor the indoor relative humidity with a $10 AcuRite (eg: http://www.homedepot.com/p/AcuRite-Digital-Humidity-and-Temperature-Comfort-Monitor-00619HDCSA2/202260980#.UaaDApzzt50 ) Only if it's chronically below 30% RH in winter would adding humidity (or lowering the ventilation rate) be necessary.
The wintertime outdoor dew points in my area are well below those in Atlanta, and my house is far from PassiveHouse tight, though it might meet code min on tightness after methodically air sealing the obvious (and some less-obvious) stuff. (The next round of air sealing will involve blower door testing, at which point I'd have a measure of where it really stands.) But the interior RH in this house never drops below 30%, even when it's in negative single-digits F outside.
Steve,
I agree with Dana on humidifiers. Humidifiers are never a good idea, except in a very limited way to protect valuable musical instruments or works of art made of wood.