Jennifer: We do that. Just click the link for “HVAC Design” at the top of the page, which I’m also putting here:

https://www.energyvanguard.com/hvac-design/

My husband and I are building our retirement home in Athens, TN (that’s east TN btwn Chattanooga and Knoxville). We are working with a GC who fills in the disciplines we haven’t bought out ourselves. We are at the point of getting down to business with regard to the hvac. There are plenty of installers around and even more opinions as to what would be the best option for our heating/cooling needs. Most are based on the rule of thumb method which just doesn’t satisfy our curious nature’s!
Each of us are engineers: he’s electrical, I’m civil — we picked the wrong disciplines for this question! We would like to get legitimate load analysis report (manual J) to properly size equipment, and recommendations as to best options to heat/cool this building efficiently. We have cheap electricity (TVA) and natural gas available. We want to use natural gas for heat. Water will be by rainwater harvest and underground cisterns for primary water source (well and public water options are cost prohibitive).
We feel it would be worthwhile to get an independent hvac analysis, system recommendations, sizing and specifications and then request quotes for implementation. Where would you recommend we look for a reputable technical group for this service? Following are a few details regarding the build:
It is a 2 story log cabin (~2100 sft)
-Full basement: unconditioned space, 12 ft ceiling including 24-in trusses between basement and 1st floor.
-First floor: 9 ft ceilings throughout the first floor with 16 inch truss space between first and second floor (no great room feature opening to second floor). The exterior wall logs are 6-in deep white pine. North and South facing covered porches the length of the house.
-Second floor: vaulted ceiling follows roof pitch: primary 10:12 with 3:12 shed dormer the length of the back, south facing wall. Exterior walls are stick frame 2 x 6-in (4-in are adequate though), with cedar board and batt exterior, log siding interior walls (1/2 log applied to 2 x 6 ext. framing).
Thanks for any thoughts/recommendations on the matter,
Jennifer

Also, what qualifies as “newer” or what insulation specs does this imply? My home was built in 1999 but about all I know about it is that it has dual pane windows.

Thanks for any advice, and thanks for your education efforts!

@Bob, there are some tricks for discovering the info needed to model an existing home, old or not. For example, a lighter or match will reveal if the glass has a low-e coating (google ‘flame’ and ‘low-e’). You may be able to tell what’s in the walls by removing a few outlet or wall switch plates until you find one with a gap between the drywall and electrical box. A thermal (IR) camera will reveal how well the insulation was installed so you can estimate the effective R-value for the assembly. If ducts are outside the envelope, you can usually inspect most if not all of the insulation. One advantage of modeling an existing home: you can do blower door and duct leakage tests to measure actual leakage rates.

Having said all that, the most accurate way to size a replacement A/C, heat pump or furnace is through cycle timing. This assumes the existing system is in good working order and you have the luxury of time to wait for a hot day (or cold, if sizing a furnace). Also, if the system is multi-stage or variable capacity, it must be locked into high stage. And unless it’s your own home, a data logger (indoor & outdoor temp) is usually the best way to get the data.

@Charles, that’s an excellent question. I believe you’re conflating ventilation, filtration and internal air changes. Let’s start with ventilation…

As an example, the residential standard for ventilation (ASHRAE 62.2) stipulates a 2,000 ft2 home with 3 bedrooms needs a ventilation rate of 90 CFM (minus the verified infiltration credit). That works out to 0.3 ACH, assuming a volume of 18k ft3. If the ventilation air is distributed by the main air handler, it can be set to run on low speed 24/7, or x minutes per hour. Likewise, one can improve air filtration by running the air handler off-cycle (potentially leading to condensate re-evaporation in humid climates). But these are separate considerations from internal air changes, which is what you appear to be asking about.

There’s no code requirement for internal air changes, at least not for residential (nor for commercial, AFAIK). ACCA’s Manual LLH (Low Load Homes) refers to air turnovers per hour (ATH) and “air loading factor” (ALF = system airflow / conditioned floor area). These are somewhat controversial concepts driven mostly by diffuser considerations, i.e., register throws. It’s interesting to see how Manual LLH dances around these old-school concepts that might still apply to homes built 20 or 30 years ago.

I’ve been designing mechanical systems specifically for low load homes for more than 15 years. Some of my projects (including my own home) have ALF’s as low as 0.15 cfm/ft2 and air turnover rates less than 1 ATH, and here’s what I’ve learned:

As we ratchet down the cooling load, register throws (and air mixing in general) become less and less important. For low-load homes with normal sized rooms, you only need to get the supply air to the room. I would argue that most homes built (and verified) to meet today’s energy code would qualify as low-load homes. In that context, there are more important things to worry about than register throws and internal air turnover.

More important when designing an A/C system is that your airflow achieves the desired sensible-latent split based on the loads. For example, 350 cfm/ton increases latent capacity, whereas 450 cfm/ton increases sensible capacity (and efficiency).

When we landed a large church hvac renovation, 28 mini splits were going to be ordered and the added reduction in price was the inspiration needed to add two more that would be for our home. Only one problem: my bride was also our office manager and bean counter. She immediately understands my intentions and stated – very emphatically- they better not be for “her” house! Turned out she didn’t like the look of wall or floor mounted inverter mini splits! Yikes!!! No matter, the two extra were sold within days.

Concealed mini split air handlers were just coming out, but had ultra low abilities to overcome resistance to airflow. I had designed for .03 & .05 (S/R). Solution? Convert all returns to supply ducts and cut in return air filter grills.

The manufacturer told me if I did that, there would be no warranty. Their national trainer said if anyone could pull this off, it was me. More than a decade later my inverter mini splits with the concealed air handlers are performing perfectly. At the time, I elected to also replace the still working second system and micro zoned the home to four instead of two zones.

You asked about run times. Given that they are inverter systems, they seldom are off. Our home is 100% hydronic radiant heating with a high eff NG boiler that is also modulating/condensing. When I first installed the inverter mini splits, I heated with them alone to see specially how our comfort level would be in our living room with its vaulted ceiling and was pleasantly surprised to find the comfort level a close second to radiant. Long story not so short, the inverter mini splits serve as backup for heating.