New house CZ3, ducted HVAC & air handling design questions
2500sf single story 3 bedroom north bay, California house with conditioned rat slab crawlspace. 3 bathrooms, family rooms with a gas fireplace, “great room” kitchen/DR/LR is 30×42.
Plans call for 2×6 walls with 1″ of well sealed foam (exact type still tbd), 500sf of casement windows and french doors with sub-.30 ufactor glazing, unvented 6/12 exterior pitch scissor trusses.
Part of the crawlspace will be effectively a basement (7′ tall) and that’s where the utilities – heat pump water heater, ducted heat and AC – will go. Client (my wife) will not tolerate wall units so will be using ducts.
I have read extensively on this site, and the conclusion I’ve come to is that I still haven’t learned everything I’d need to know to design the mechanicals for this house – and I should be skeptical about an HVAC equipment vendor design. So, here are my questions:
– does anyone have a recommendation for a good HVAC design engineer in northern California?
– assuming that all 3 bedroom doors stay closed at night, how many zones would this house need?
– will good air circulation design significantly mitigate the need for multiple zones? 80-90% of the time, only one bedroom will be used.
– Is there any “comfort” reason for locating air vents and/or returns high on the walls, versus floor registers?
– I have floor plans and a door and window schedule. What else do I need to have before signing someone up to do the Manual J/S/D calculations/plans?
Thanks for helping out.
Rob
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Rob,
This article should help. The HVAC designer does not need to be local. He or she just needs the correct inputs to run the necessarily calculations.
https://www.greenbuildingadvisor.com/articles/dept/musings/who-can-perform-my-load-calculations
Getting to the HVAC solution starts with well executed heating and cooling load calculations at the 99% & 1% outdoor design temperatures. ACCA Manual-J methods are enshrined in CA Title 24 code requirements for specifying new systems. Hiring a professional engineer or RESNET rater to make those calculations based on your full house design, that includes specifications for the exact windows you'll be using, and the insulation & wall stackups etc are critical to getting this right.
That said, climate zone 3 locations in CA have a 99% outside design temps ranging from the mid-40s to the low 20s, and 1% outside design temps from the low 80s to the low 100s- it' not possible to just take a WAG and get it right. Find the design temps in your area list in the California microclimates here (just about every municipality in CA is in there):
https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf
That said, a typical 2500' tight 2x6 (no foam sheathing) house with U0.30 windows, an insulated basement/crawlspace and R38 in the attic will have a heat load at 30F of less than 25,000 BTU/hr, and a cooling load at 90F of less than 30,000 BTU/hr. A lot of this is under your control during the design phase, and it's possible to design-out the load on both to get it into the capacity range of a 1.5 to 2 ton heat pump. If you can do something else other than the gas fireplace for ambience you can skip the cost of hooking up to the gas grid & gas plumbing and go all-electric, spending the gas infrastructure money PV solar to offset the energy use.
In a tight better-than-code house in zone 3B or 3C microzoning isn't usually necessary for comfort, nor does it save much energy, but until there's a room by room load calc it's hard to say if your house would be the exception.
Location is Santa Rosa; heating is 29, cooling is 95 (although if I had data for the specific site microclimate, it might be a couple degrees warmer in winter and cooler in summer. I'm meeting with structural engineers next week to go over roof design details, and interviewing Title 24/CalGreen consultants too.
My goal is to make sure I know enough about my options, and preferences, to participate actively in decision-making. This is going to be a "good enough" house: not necessarily 'Passive' or 'Pretty Good' but optimized based on sound building science and economically optimal energy efficiency. In CZ3, that sounds like conditioned crawl space, ventless R40+ roof, R5 taped foam sidewalls with good air sealing, did I mention excellent air sealing yet?, big overhangs to moderate solar gain... add 6KW of solar capacity and careful attention to details and it should be good enough. Assuming I use the right energy appliances and air distribution approach - hence this thread :)
Also, I'm still puzzled about the "how many zones" question: do I need a zone for each BR, or should I just use a good set of fans to move conditioned air around the house?
Rob,
I'm going to provide a link to an article that discusses ductless minisplits, while noting that much of the advice in the article applies to ducted units as well: Rules of Thumb for Ductless Minisplits.
If your house avoids large unshaded east-facing or west-facing windows (that is, windows that affect the cooling load), there's every reason to believe that the house can be treated as a single zone, and that you don't need a register in every room. That said, a lot depends on whether your bedroom doors will be open during the day, and whether your family members can tolerate room-to-room temperature imbalances of about 5 F° or so.
As Steve Knapp noted, you should also read this article: Who Can Perform My Load Calculations?
I re-read the article Steve called out. It reminded me that this should be easier and more affordable in states like California that mandate both energy efficient construction and pre-construction planning . It is neither easy nor affordable: my quotes for running Title 24 calcs - required to get a building permit - are running about $4K and that doesn't include Manual S or D.
I'm going to refactor my zone question and ask it separately, I realize now I haven't posed it properly.
In Santa Rosa's outside design temps you're probably looking at a total load of 1.5 tons of heat pump, if all of the ducts (if ducted) ar inside conditioned space (such as an insulated sealed crawl space/basement.) If you're not careful about window glazing & orientation and overhangs it might end up at 2 tons or even 2.5 tons, which is why it's worth downloading a copy of BeOpt and fine tuning it before handing it over to a qualified party to run the Manual-J.
If you analyze and tune the window glass size, type & orientation for the bedrooms it's unlikely that it would make sense to micro-zone the bedrooms as individual micro zones. The individual room loads are (or could be made to be by design) too small to really matter.
$4K is a ridiculously high number for a Manual-J on a standard 2x6/ + insulating sheathing type construction. In my area it can be done by a licensed & certified P.E. for under a grand, maybe $1500 in the most gold-plated playgrounds for the rich & famous like Nantucket or Martha's Vineyard.
As you're aware, the sun in California is intense. Overhangs aren't perfect on the south, given that you're into October before it really cools down and are useless on the west.
Consider solar screening for your south and west windows where it makes sense along with your overhangs, window area and glass SHGC. I used these on both a northern and southern CA house with great success. I had no natural shade, no overhangs and clear glass. I had the curse of the typical 60' lot with a drive alongside the house on the west side. The screens were ~$50 each for maybe 2x4' double-hungs and dramatically cut the heat gain, from feeling like I'm under the "french-fry lights" to no discernible warmth when standing in the sunlight. Daytime privacy and easy take-down in winter for solar gain were a plus.
I know I sound like a salesman, but they just made a huge difference. They might make sense for some of your window locations.
Maybe Mike may know of a new construction designer for you or perhaps he would do it . In either case , you'll not find someone more well equipped for this type task .
http://energydocs.net/about.php
Richard, I will reach out to mike. Redding is a fair distance and I'm sure he/his team are busy - California has no shortage of construction work after fires and floods - but he may well know some good people up my way.
Dana, thanks for the suggestion to look up BeOpt. I really hadn't thought about it as a user-friendly tool but having downloaded it this morning, and now running through the Help system, it looks like a powerful modeling tool that I will try to implement. [Learning new software was easier for me 30 years ago!]. It may answer most of the questions I was planning to post here!
Bennett, I have minimal east and no west glazing on this house (other than fully covered patio areas) and the south facing windows have 2-4' overhangs. That being said, there is one really big window in this project - a 20'x8' lift-and-slide pocket door - that would be a big solar gain issue if it didn't have a 12' deep trellis providing cover. The views to the south are the reason this house is being built, and I'm going to great lengths to ensure that they are accessible.
I've reached out to a couple of the folks on this site that do Manual J/S/D calculations and consulting, and will get at least J&D done before we start construction of my "Good Enough" house. I have a small basement area (conditioned space) that will house mechanical equipment. Question: where should the heat/.AC compressors go: inside or outside? Would much prefer to put them in the basement (noise notwithstanding, I think it will be tolerable). Assuming a 3 ton HVAC unit and a 100K BTU pool heat pump, does that require external venting of these compressors/air handlers? Sorry to ask such an ignorant question, but HVAC is the one part of building that I have always avoided..
Rob,
I assume you are asking about an air-source heat pump. This short video describes how this type of unit works to heat and cool a home.
https://www.youtube.com/watch?v=4V-Ou-TiybU
The compressors all go outdoors, so they can have access to their source of heat, which is the thermal mass of all the air in "the great outdoors".. Putting it indoors would heat the compressor room to 140F (or whatever it's failure temperature would be) while trying unsuccessfully to cool the rest of the house in summer, and turn it into a freezer during the winter when trying unsuccessfully to heat the rest of the house.
The air handlers and ducts (if any) all stay indoors.
Thanks to both Steve Knapp and Dana Dorsett for clarifying. I will make the "pool equipment room" portion of the crawlspace an unconditioned, well vented space and and use it for both house and pool heat pumps, along with the pool circulation pump. Now all I have to do is make sure it's acoustically separated from my master bedroom well enough to keep down the equipment hum!