View Full Version : Need HVAC Suggestions - Conventional/ductless/etc.?

04-05-2012, 11:34 AM
Looks like I'm in the market for a new central a/c and heating system. Currently have a heat pump with backup electric heat, no gas, etc. Typical stick built on slab house, single story (ranch I guess, not sure what that means, we have a lot of "ranches" here and all the houses are different), about 2000 square feet conditioned. 4" walls and 6" batts in the ceiling (will do more when I'm through remodeling).

I've always been intrigued by the mini split heat pump systems and I can see the obvious efficiency over ducted. Is this even an option? Our house has 3 beds, two baths, and one large kitchen/family room, etc. Is it crazy to think about a ductless system in a house that has existing ducts?

I briefly read about inverter driven conventional ducted units. Seems like this would be an increase in efficiency over standard compressors but still use my existing ducts, etc?


04-05-2012, 12:17 PM
If the dogs and kids won't be all into your stuff, a single-head mini-split with the head in the open kitchen/family area may be able to handle it all if you leave the doors open.

With that approach there may be some room-to-room temperature differences, some of which is easy to mitigate, others less so, but no matter what the mini-split would still dry the air and provide at least partial cooling in the other rooms.

If the bedrooms or other doored-off rooms are on the south or southwest side with a lot of afternoon solar gain, a 2 or 3- head multi-split might be called for.

Is there any exterior wall insulation (particularly in the doored-off areas)?

Window types, and orientation?

Are the ducts in the attic, above the insulation? (If yes, is there at least a radiant barrier between the ducts and roof deck, and are the ducts sealed & insulated?)

What is the cooling BTUs/tonnage of the existing system?

All of these will affect the decision of whether to go ductless and the relative sizing of the unit compared to the previous system. There may be combination of envelope upgrades (eg. blowing insulation into uninsulated wall cavities, adding more insulation in the attic, putting up low-E storm windows, etc.) that may both enhance comfort in the rooms that don't have a separate zone head, evening-out the room-to-room temperature differences, as well as saving on cooling & heating (and up-front unit) costs.

And yes, continuously-variable inverter-drive compressors send the part-load efficiency of these things SOARING over traditional ducted heat pump systems. While there are inverter drive ducted systems out there, they're pricey and still fall short on ductless for efficiency for several reasons, such as duct leakage, direct duct gains/losses when routed outside of conditioned space, higher power needed to move air via ducts- it all adds up.

For weather data & looking up outside design temps, what's your zip code?

04-05-2012, 04:19 PM
Dana, was hoping you'd respond. Let me try to answer your questions.

House is oriented lengthwise southwest to northeast, with the bedrooms on the northeast end. Open kitchen living space in the middle and unconditioned garage on the southwest end.

Whole house has I believe R13 in the 2x4 walls and then the batts flush with tops of 2x6 ceiling joists.

Windows are double pane vinyl windows. I would guess 15 years old but not sure. Seem to be good, tight windows. They are on all sides but remember the garage is on the southwest side. I do have large trees on the north side that shade afternoon/evening over two of the bedrooms. Rest of the house is in sun all day.

Ducts in the attic and above insulation. Rigid ducts and look to be decently insulated and sealed with mastic, although I found one tap pulled out of the plenum from somebody crawling over it. Metal roof with radiant barrier, but the radiant barrier is directly under and against the tin roof, no air gap at all, just metal roofing screwed down to rafters right on top of the foil. You may remember me asking about this before.

Current compressor is a Carrier Heat Pump 38BY048 - 48,000 Btuh. 12.00 EER cooling and 7.60 EER heating. Air handler/evap in attic was replaced several years ago with a Goodman unit (under new home purchase 1 yr warranty).

Doors to the bedrooms and baths stay open all the time but the hallway from the living area to the farthest bedrooms is 20'.

Zip code is 77904.

Hope this helps and thanks!

04-06-2012, 02:24 PM
The configuration is about as good as it gets for going ductless- most of the gain in the bedroom end is before the mid-day heat, and the place that's getting a mid-afternoon roasting is the open area where you'd most likely put the mini-split head. The question is whether you can keep the bedroom cool enough for sleeping via convection alone.

Even insulated ducts in the attic above the insulation are likely to be adding at least ~10,000BTU/hr to the peak load, plus infiltration air leakage from room-to-room pressure differences generated by the air-handler may be adding a bit more. Rule of thumb "typical" losses related to sealed-insulated ducts outside of conditioned space would be ~20% of total range, but it varies a lot. Whether the actual whole-house cooling load + duct losses actually adds up to 48K is another question. If your cooling load is bigger than that you probably would have noticed- is it running all day and the house is still climbing into the mid 80s? Odds are you'll be able to back off to 3 tons or even 2.5 tons going ductless, but it's important to run the numbers. I have less of a feel for the true average cooling loads in TX for a house that size, so it's important to run the numbers using a real heat loss tool lest I steer you wrong, but 4 tons would seem a bit high for a house with ducts INSIDE of conditioned space, higher still for ductless. (I'm usually having to pay more to their heating output at lower temperaturs than cooling capacity.)

The 99th percentile outside design temperature (http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf) for Victoria TX is 94F, but has a hefty 44 grains of water/lb latent load @ 50% RH. You might try running a room-by-room heating & cooling load calc using Taco's freebie download (http://www.taco-hvac.com/en/products/products.html?current_category=370). It's more of pro-type tool and delivers both heating and cooling loads, including latent loads, and tots up the whole-house loads. The default outside design temps in the tool for my location are substantially different from the ACCA Manual-J data (the first link) so change them to the ACCA values to keep from oversizing. If any of the bedrooms show up as substantially higher than the other separate rooms you might consider a 2 or 3 head multi-split, but barring substantial solar gain the temperature differences between rooms will usually be less than 5F at design temp, less than 3F at average mid-summer or mid-winter conditions if there's decent room for air to convect in hallways, etc.

If the heating load at the 28F design temp may or may not be higher than the 94F sensible cooling load or conversely it makes sense to upsize to a unit that covers 100% or even 125%+ of the load at design temp. Since these are modulating systems with a 3:1 turn-down ratio or more there's no efficiency or comfort penalty for oversizing up to about 150% oversizing in heating mode, at which point the wind-chill of the higher-cfm head for an oversized unit starts to take a toll. Unlike traditional heat pumps there's even an efficiency GAIN with modest oversizing, since their efficiency at part-load is higher than when running full-out (mostly due to the high efficiency of the ECM motors at any speed.) With oversized heat exchangers (for load) and slower compressor & blower drive, at 150% oversizing you gain about a full COP over what you'd get with a unit with output at design condition sized exactly for the load a that temp. So in heating mode instead of averaging ~4 in your location you'd be at 4.5+ if the unit is slightly oversized, with similar gains in cooling mode. This isn't something that necessarily shows up in an HSPF or SEER number (but units with higher numbers perform generally better than those with lower numbers.)

04-11-2012, 01:56 PM
I would add a large temp. controlled attic fan for summer along with enough insulation to bury the ducts - after making sure they are connected right. Then I would just replace the existing unit with a better one.

You will never recover the cost of tearing the houe apart to install a scattering of white boxes on your walls just itching for one of its hundreds of componenets to fail.

04-11-2012, 03:23 PM
Attic fans depressurize the attic and usually result in a net INCREASE in total cooling energy use due to air-infiltration driven by the fan. The compressor power saved by use of the fan is universally less than the power used by the fan. (This was studied to death by the folks at Texas A & M and many others back in the 1970s, '80s & '90s.) If you're not using the ducts in the attic it would be a fairly STRONG net loss.

The exceptional case is small (not large) solar powered attic ventilation fans in attics with ducts that are in use, which have been shown to peel off mid-single-digit percentages on grid-power use. Source (http://www.fsec.ucf.edu/en/publications/html/FSEC-GP-171-00/).

Note that the average system COP on the AC unit modeled in that FSEC study is a modest 2.5. With a ductless split system you're likely to hit 3 even at 110F, and about 4 at 90-95F outdoor temps. See figure 14, (p.27, in the .pdf pagination) of this document (http://www.nrel.gov/docs/fy11osti/52175.pdf).

SFAIK nobody makes one, but a balanced-flow attic fan that didn't generate pressure differences could potentially lower the total power use, even if running off grid power. But without ducts in the attic, any potential savings would diminish rapidly with higher-R in the ceiling, crossing quickly over to a net-loss.

Bottom line: You're probably better off spending the money on more insulation than any type of attic fan, ESPECIALLY if you go ductless.

Runs with bison
04-11-2012, 08:42 PM
We have a useless attic/whole house fan installed in this house. It actually would pressurize the attic and depressurize the house since it pulls interior error and pushes it into the attic space. Arrangement of the house is half attic, half cathedral so that the attic fan is at about 60% of the max interior elevation: worthless. Worse than that actually as it provides a heat loss/air leakage path from the living space into the attic. After experimenting with it and finding it useless, I fitted pink foamular across it behind the shutters and sealed it up.

The only time an attic fan makes sense that I can see is in rapidly pulling in cool or hot air over a few hours, then shutting off the fan and closing up the house for the day or night. I do this manually already in Spring and Fall without using our poorly situated fan. We've got enough elevation difference that I can get a good draw by opening windows. I use two ceiling fans to stir up static layers in the cathedrals when I want to ventilate.

04-12-2012, 11:20 AM
An attic fans & whole house fans are unrelated species:

Attic fans are simply forced attic ventilation designed to ventilate the attic only, whereas whole house fans are designed to ventilate the conditioned space (sometimes using a ventilated attic as the exhaust, other times not.)

Most whole-house fans are poorly implemented and lose more energy than they save. But in areas with low summertime dew points and reliably cool nights there are insulated-shutter versions that can be used for a night-time ventilation strategy to some advantage. But with the advent of very-high SEER ductless AC the whole-house fan approach may still use more power to achieve the same end. In most of TX summertime humidity makes whole house fans a less attractive solution under any circumstance.

04-23-2012, 07:52 PM
Been busy at work so sorry for not getting back here. Started a new job last Monday.

To make a long story short, cap, relay, etc. checked out fine on the old unit but it would not start so I figured the compressor was stuck. Put a hard start kit on it and it fired up. Been working the few times it came on this last week. I don't have high hopes for it lasting though. We'll see...

As far as an attic fan, I know mine would pull conditioned air out without major sealing being done. My ceiling/walls are not very airtight (wire perforations, etc.).

After doing a little reading and pricing I'm thinking I would just probably go back with the same setup I have now (ducted heat pump). The few guys I talked to about the ductless said I would need an evap coil in every room to eliminate hot spots. That was my big concern as well, whole house has 8' ceilings too. I just can't see enough air circulation by convection alone.

04-24-2012, 07:53 AM
There are now variable speed inverter drive compressors & air handlers for ducted systems too. It won't address the inherent system-inefficiency of attic-ducted systems, but it's more comfortable, and drives less infiltration due to the lower pressure differentials at low air handler speed.

04-24-2012, 04:59 PM
I'm thinking that is the way I would go in this current house if I need to do a replacement.

But I am very intrigued by the ductless systems and would heavily consider one when I do new construction someday. I read a little on the internet about some home builders using them in new houses.

I believe this whole house remodel I'm finishing up is the best thing I could have done at my age (28) before I would build a house for my family. I have so many ideas for improvements in both design and efficiency and just flat out preferences that I would not have had if I had not done so much of this remodeling myself. I can't imagine someone making decisions on home building not knowing what I have learned. But then I guess they wouldn't know any different anyway...