View Full Version : HVAC System Recommendation?
Dee in SC
03-06-2011, 09:02 PM
I'm in charge of building a new house while my husband is in Afganistan. I'm making all the decisions on appliances, including doing the necessary research for reliability and quality. I don't even know where to begin with HVAC system. I know nothing, zero, zilch. Since you all are best informed of what works best and what breaks down often, could you suggest your preferred HVAC for the following situation -- and point out some issues I may want to consider? I can then use that as the basis for my research.
Mid-state, rural, SC home near GA border, 2,900 sq. ft., 1 level, 3 full bathrooms, looong rectangular floorplan with master bed/bath (his and her bathrooms) and washer/dryer at one end of the house. The other end of the house has the kitchen, husband's man-room and guest bed/bathroom. We will probably need a two-zone system, since the house is very long. We're on SCE&G electric and will have propane tank for cooking and whatever else we select to run off propane gas. We will have an emergency generator for backup power.
We're using well water, if that matters to anyone.
I've spoken to a lot of people who say they now regret not installing a "high quality" system, such as Trane. I'll take this as good advice and am willing to invest a little extra in our HVAC and postpone our landscaping or something. Budget is definitely an issue, but so is low-maintenance HVAC.
I appreciate your time and input.
03-07-2011, 03:30 AM
Well Trane is good stuff indeed but.......whatever you decide on make sure that the equipment can be purchased locally as well as any repair parts for it.
Dee in SC
03-07-2011, 06:44 AM
Excellent point, Mr. Sawyer. Thank you for weighing in and giving your time.
03-07-2011, 08:06 AM
I agree with Tom. You may consider calling your local Trane Rep and ask them what contractor sells Trane in your area. I would then check with the BBB in your area on the contractors you select. With the BBB you can usually find out if and how many complaints a business has on them. You would like to know if the contractor that you will be dealing with is going to be around for a little while since he is typically going to carry the warranty at least for the first year. Also ask about extended warranty. Try and get more then one bid and also have them do a heating/cooling load calculation and not this rule of thumb stuff.
Your electric has to be included by somebody either your electrical contractor or the HVAC contractor. Other things to consider are do the grills/diffusers/registers have adjustable dampers (opposed blade or multishuter). Best duct is going to be round metal or retangular metal insulated although many contractors will want to sell you insulated flex duct. (metal duct is more durable and less air friction loss) You may also want to consider two smaller HVAC units instead of one big one since your house is 2900 SF. and have them serve different areas of the house based on the estimated peak time of you load or use. (bedrooms opposed to day living areas). (I see you have already considered that.)
You need to determine what type of system either gas heating or electric heating w/ straight cool unit or heat pump with electric aux heat strips.
Your only alternatives are propane and if that is the case you my want to go w/ a heat pump.
Looks like you already have a good handle on it.
Every good HVAC system design starts with a room by room heat loss/gain calculation (per ACCA Manual-J or similar.) Oversizing of systems for loads is common, less comfortable, and more expensive to buy & operate than a "right sized" system. Any contractor you use should provide you with those room-by-room numbers, and those that offer it up without needing to be asked move to the head of the line! (And, "Lessee, 2900 feet times a ton per thou comes out to..." is not a cooling load calc any more than "...2900' times 15BTU/foot ..." is a heat load calc. A real calculation will involve measuring wall & window areas on each room, assigning the appropriate R & U values, and estimating solar gain & infiltration rates, etc. )
In new construction there are many cost-effective (even cost neutral or better yet, cost-negative) ways of reducing both peak and average heating and cooloing loads:
Optimizing glazing type/size and overhangs for solar gain/rejection based on the shade factors and oreintation can make a huge difference.
Going with blown or sprayed insulation over batts (even if there some cost premium) gets you more reliable performance.
Designing and building it to be air tight, and VERIFYING/rectifying the air-tightness a the point the athe sheathing, windows/doors and ceiling are up to get it under 2 air-changes per hour at 50 pascals pressure (a standard blower-door test pressure) is also worth it. (Caulking the stud plates & sheathing works best if done as you go, but going after all of the holes once the shell is up will find & fix a world of errors & omissions much better than any retrofit approach.)
Building with 2x6" 24" on-center instead of 2x4" 16" on center costs less to frame (same board-feet of lumber, fewer boards to cut), and allows you to add another R7 or so to the insulation package at relatively low cost, with reduced thermal bridging from framing.
Using insulating concrete forms for the foundation wall/stem wall is relatively cost neutral, but if done such that the outer foam of the ICF is co-planar with the exterior sheathing reduces the thermal bridging between the inteior & exterior.
Building the trusses to allow anouther 6" of blown fiber insulation in the attic can take a half-ton off your cooling requirements, and make it more comfortable year round.
Designing it without ducts or air-handlers in the attic, but rather in a conditioned basement/crawlspace or other conditioned space can take another half-ton off the AC and make air-sealing the house easier.
Use CRRC rated "cool roof" materials (http://www.coolroofs.org/) appropriate for your roof pitch. It's usually cost-neutral, and results in double-digit percentage reductions in peak cooling loads.
Investing some money in a higher-performance building envelope you go 30-50% over code-min on all R values, minimize summer time solar gains and can squeak in some summertime gains you can easily cut back 25% or more on the HVAC sizing (and reduce energy use by an even larger fraction.) But better yet, you end up with a more comfortable house. High efficicency HVAC may still be an option, but there may be better "payback" in spending the money on higher-efficiency envelope.
With higher R values and well-managed solar gains your latent cooling load (humidity) will likely be several times your sensible (temperature only) cooling load , so cooling equipment that employs a dehumidification mode can be more important for comfort and opperating costs than a simply opting for high SEER equipment.
In your climate & utility situation it's almost certain that it would be cheaper to run with heat pump in winter than any propane-fired appliance. Ducts should be both sealed & insulated to at least R6 if in conditioned space (higher-R if in the attic- but don't put them there!). The duct design should conform to ACCA Manual-D, and all seams should be sealed with mastic or FSK tape. (And forget about flex-duct.) Depending on the overall layout and how far you've taken air-tightness, R-values, and heat gain/loss factors you may be able to get by with a couple of mini-splits.
03-07-2011, 01:25 PM
Insulated Concrete Forms (ICF) construction is what I'd probably do if I was going to build a new house. My sister went to a continuing education class on them and part of the discussion was a high school out in the cold northern midwest where they added on a new gymnasium. It ended up costing them about $100 to heat the thing during the winter! Termites and carpenter ants won't eat the stuff, although they can burrow in if there's some other food source (but that can be handled easily). The house is VERY quiet, no drafts (assuming you do your windows and doors properly), and has a huge amount of thermal mass so you don't notice the effects of the daily temperature swings. I'd go all the way to the eaves with the stuff, then use engineered foam filled panels for the roof. You can specify them with drywall already installed on the interior so you only need to do the seams and paint. Doing it this way, once the footings are in, you could be closed in in maybe less than a week if everything was staged properly. It doesn't take all that long to stack the blocks (sort of like big Lego blocks, but hollow for the concrete and rebar) and insert the rebar. Then, less than a day to pump the concrete in and the walls are up. Some prep work, and a crane, and the roof is on. You'd probably want to put the floor in before the roof to help tie things together, but with the reinforced concrete walls, probably not a big deal.
For the roof shingles, it'd consider a metal roof. Most are guaranteed for life, good to over 100mph winds, decreases your fire insurance, and acts like a decent radiant barrier. You can cover the exterior walls of the ICF with anything you want: shingles, vinal, brick, stucco...I'd go with something that didn't need any regular maintenance. There have been some problems with fiber cement products, but most of it has been in poor installation. I'd look into James Hardie stuff, by a certified installer. Again, no termites, or carpenter ants can eat the stuff, it's fire retardent, and you should not have to paint it for the first 15-years unless you want to change the color. Proper installation is critical on this type of stuff (well, on many things!).
Trane used to run an add about the benefits of their (and other companies offer it too) variable speed air handler (or furnace). In the same design houses in the same area, the one with the variable speed fans drew enough extra moisture out of the air to fill a backyard swimming pool over the identially sized house with standard single speed fans. The variable speed fans allow it to start out slowly, which causes the humid air to linger a bit longer over the cooling coils, and draws much more moisture out of it. It then ramps up faster, if needed to cool the house. This, in my opinion, is a huge advantage, both in comfort and economy for a small upgrade charge.
ICF gets to be a bit unwieldy at higher lifts, and the thermal mass aspects have been highly over-rated (but still relevant in high-dry areas with large diurnal temperature swings like the Rockies. Not so much in the humid SE.) The steady-state whole-wall R value is still primary, and there are far cheaper methods of air sealing. ICFs make air sealing a lot easier, to be sure, but it is by no means guaranteed.
The R-values have to be pretty high to get the $100/year thermal performance out of it, but a minimal R16 ICF will outperform 2x6 stick built even with closed-cell (R25+) foam cavity fill if done right, due to the very low thermal bridging. But that's probably a bigger change-order to do without spousal input, eh? :-) It's definitely a cost-adder over 2x6 24" stick built with spray cellulose cavity fill, even if you added R5 in rigid foam over the stick built to bring the whole-wall thermal performance in line with ICF.
Unless you get versions with borate-loaded foam, ants and termites can still tunnel through ICF- I'd definitly look into borate-loaded forms if doing ICF in SC/GA.
Not all metal roofing would meet CRRC cool-roof specs, but some do. That too is a big bump in the budget though.
Variable speed blowers and multi-stage compressors add a lot to comfort and are quite efficient, but getting the loads down, and the HVAC sized to the actual loads is still primary.
Dee in SC
03-07-2011, 10:47 PM
Dana, Jim, and Dave,
I hate to lose the momentum, but have the packers and movers coming in a few hours and I'll relocate to a temporary apartment for the upcoming home construction. I'm blown away by the great information here. As soon as the guys are gone, I'll reply and provide the information about our plans for insulation --I'm very interested to know your opinion of this! Also, will talk about our 2x6 and 2x4 use and our trusses. I've got questions about the foundation and ICF. I'm going to do some checking on the duct shape, Dave, so I can ask questions. Dana, I want to ask you about my plans for mixed use of spot spray foam and batt on the studs . I most definitely want to talk more about the ants and termites, both as it pertains to the walls and the block crawl-space foundation. Please check back with me in the next 24 hours. Husband has put his complete faith in my choices, as I've been researching home construction for the past year --as well as investigating consumer report ratings on appliances, have been choosing tile, wood, etc. But it's a lot for one person, especially on this technical and critical part of home construction. We're looking for a good "above average" option to all our choices, but I also have to make sure we can afford the house in the end.
Dee in SC
03-18-2011, 10:20 AM
Well, lost the internet much longer than expected. I'm back online, but a little deflated. We can't afford the house as it is currently designed. Though we specified our budget to the architect in the beginning. We're back to the drawing board, in the middle of switching to all 2x4s, dropping ceiling height to 9' (plenty, if you ask me), changing to a truss system that is both load bearing and decorative (exposed) vs the system designed for us that had load bearing + separative decorative beams. sigh. We'll have a crawlspace. Our foundation walls will be block rather than poured concrete that the architect wanted. Block is standard here anyway, so I don't have a problem with it, but want to make sure we can address all the insect/moisture/insulation/vented-not-vented, drainage issues. I've read over your suggestions for insulation a couple of times and have read everything ever posted on the internet about insulation possibilities. I spent yesterday on 2x4 vs 2x6 and the today on how to best insulate 2x4s. I can't read any more. But since I can't sleep anymore, I have more time to read. The internet info is completely conflicting, mind boggling, and doesn't help me find the optimum balance of insulation for our area and a tight budget. Yes, the budget is tight, but insulation is really, really important to me, as I want to do it one time only and we're never moving. But then there's the budget. I'm willing to postpone a lot of interior stuff...just need the occupancy permit and the bank's blessing...but I have to get the house built. Unless I want to start chopping off the back deck and the enclosed garage from the construction plans, I need to switch to 2x4, etc.
I have read and mostly digested the guidance from http://www.energycodes.gov/training/consumer_ed/insulation.stm But still am left wondering what might be the most economical and yet most effective combination of insulation and insulating techniques for my situation. Blown? Batt? Spot spray foam? Thin foam layer plus batt or blow in? Rigid foam board? SIPS? for my ceiling,crawl space, foundation walls, exterior walls, vents, ducts, etc. Fill the block or not? With what? Asphaltic paper (roofing paper) for exterior walls seems one thing I need to do. Crossing over the paper on the insulation and stapling to the interior side of the stud walls is another. But beyond that, what to do? I don't see a category in Terry's forum for new construction insulation...just remodel. Shall I take myself away to another construction forum and then come back here with just hvac? Your instructions, please. Dee
P.S. Vapor barriers? Moisture control? Venting? AYEEEEEE!
03-18-2011, 10:53 AM
Many of the questions are things your architect should be addressing for you. Sprayed in foam, in the shallow 2x4 studwalls may be your best bet for insulation...it will minimize the air infiltration (which limits how much 'extra' air you have to condition) and maximizes the insulation capability. Dense packed cellulose would be the next less efficient choice (but somewhat less expensive). Both of these, and really any, insulation that is installed, relies on the skill and craftmanship of the installer. Insulation is one of those things that you either pay now, or pay more forever. With the costs of energy having probably nowhere to go but up, now is the more prudent. A good housewrap, done properly, will help immensly in minimizing air infiltration, too. Depending on the type of siding you plan, a drainage layer underneath, can lengthen the life of the siding by allowing a good path to allow the backside to dry. Orienting the windows and making the roof overhang sufficient to shadow the windows during mid-day sun can go a long ways towards minimizing heat buildup. Unfortunately, many architects never actually built a house and got their hands dirty. It might look good, but they may not have enough or any field experience in how things actually go together to maximize efficiency in the build, only in how it looks after it is done. Hopefully, yours has some real, practical experience.
Dee in SC
03-18-2011, 02:49 PM
Jim, you have missed my point entirely.
03-18-2011, 03:38 PM
The only way to get the recommended minimum insulation in a 2x4 wall is either foam or blown dense packed cellulose, so no, I don't think I missed the point. Not sure what your local codes have, if anything, on minimum insulation levels, but a 2x4 wall is pretty limited with batts and is a far cry from the minimum recommended. Air infiltration control is paramount to energy efficiency. It's harder to do with the fairly narrow tar paper, and much better achieved with a housewrap. the labor to use foam board, and then seal the joints would likely make the foam in place choice about even. Doing blown in cellulose is simpler if the wallboard is on, but then, it's harder to ensure you fill the channel up all the way. It can be installed prior, but can get messy. Foaming makes adding a wire or circuit later harder, as does the cellulose.
03-18-2011, 04:05 PM
Knowing I was going to live there many years, it would be more cost effective to build and insulate to maximum efficiency, even if it means reducing the overall square footage of the house. It would be very easy to plan and build the house in a manner that would allow for later expansion.
ONe thing to consider when you do decide on a unit, is that MANY of those units are the same one with different brand lables. Water heaters are the same way, there are dozens of "brands" but only a few manufacturers, and the only difference is the price you pay.
03-20-2011, 07:20 AM
Seems like a heck of a time to be making decisions like stud size, etc. 2x4 only gets you R-13 I think, in the walls. You will pay for that every year in heating costs.
03-21-2011, 04:20 PM
Your architect should be familiar with the code minimum requirements for insulation thickness (or max U values) and should be able to give you some indication on the cost for construction using different composit materials for walls, ceiling, etc. (Not sure what codes SC uses.)
The only way I know that you can get an approximation of effective combinations for materials (energy savings) is going to be an iteration method using an HVAC energy analysis program that will provide you with an approximation of energy consumption based on materials of construction and HVAC types and efficiencies. Trane has such a program if you know somebody that will let you use it. At minimum it will give you some insight but is usually done on large commercial projects or government housing (people that can afford it, HAHA). I am sure there are many other programs but I am not familiar with them.
There is another web page at Oak Ridge National Lab in Tn dealing with energy saving vs. construction that may make it easier if you have not already found it.
03-23-2011, 07:53 AM
Seems like a heck of a time to be making decisions like stud size, etc. 2x4 only gets you R-13 I think, in the walls. You will pay for that every year in heating costs.
You can get R15 in 2x4 walls.
The difference in whole-wall R value (the average value after the framing shorts are factored in) between R15 cavity fill (any type) and R13 cavity fill (any type) of a 16" o.c. 2x4 wall is the difference between ~R10 and R11. The R3-4 of framing dominates the heat loss/gain in a typical 25% "framing factor". The framing only adds up to 25% of the surface area, but since it transfers 4x the heat per unit area the framing is transfering well over half the heat. The real performance difference of going with R15 batts is that the density of the batt makes it a better air-retarder than a low density batts, but that will only show up if the assembly hasn't been detailed to be air-tight (or nearly so.) Cellulose (at any density) is at least as air-retardent than R15 batts, and either wet-sprayed or dense-packed has far fewer gaps, and no compressions to allow thermal bypass convection to occur within the cavity. Sprayed/blown-in-bag high-density fiberglass such as Spider or Optima at 1.8lbs density has similar air-retardency to low-density cellulose and a slightly higher R-value, but is usually more expensive.
The difference in whole-wall R-values between real-world installations of batts vs. blown R13 (rock wool, fiberglass, or cellulose) is the difference between ~R8 and ~R10. Blown/sprayed insulation is almost ALWAYS going to be nearly-perfect, where batts are only perfect in test-sample wall structures, never in the real world.
Spray foam vs. blown/sprayed cellulose or fiberglass is only an issue if the sheathing (or other layer) isn't detailed as an air barrier. Foam won't allow convection within the cavities the way fiber insulation does (but even low-density cellulose allows only ~10% the convection of low or mid-density fiberglass batts), but isn't guaranteed to be air-tight on it's own without detailing other layers in the assembly. While foam makes air-tightness easier, it is by no means assured.
Note, the amount of lumber in 2x4 16" o.c. construction is nearly the indentical amount of board-feet of 2x6" 24" o.c. With the 2x6 option you have a somewhat lower framing factor of R5-6 rather than R3-4, and with cheap ~R19 fiber fill you end up with a whole-wall R-value of about R14 as opposed to R10 for about the same amount of money. The insulation costs a bit more, the framing material cost is about the same, but the framing labor should drop slightly with fewer boards to cut.
With either the 2x4 or 2x6 framing options, putting 1" of rigid XPS or iso on the exterior, and detailing both the structural sheathing and the rigid foam as air barriers (and lapping their seams) is well worthwhile from comfort, HVAC sizing, and longer term energy savings points of view. If 2x4 you end up at about R15-R16 whole-wall (as opposed to R10), in the 2x6 case you end up around R19-R20 (as opposed to R14.)
The difference in comfort between an R10 whole-wall and R20 whole-wall value is something you can FEEL. Yes, the inch of foam is a cost adder, but it's a HUGE performance boost.
For the CMU foundation and crawl, insulating the interior with 1.5-2" of closed cell spray foam (up over the foundation sill and band joist too) and putting a vapor retarder on the floor to protect against ground moisture/gases has huge advantages in places as humid as SC. CMU is inherently air-leaky (there are always cracks), and moisture-leaky. Deep subsoil temps are in the ~65-66F range, and with an air-conditioned space above, the summertime temperature in the crawlspace will spend MANY MANY hours below the dew point of the exterior air. If you ventilate with outdoor air, you're begging for a mold & rot problem. Closed cell foam on interior air-seals the CMU, foundation sill and rim joist, and at 1.5-2" will have an R value of R10-R12, effectively earth-coupling the house to fairly comfortable soil temps, making it unnecessary to insulate between the floor joists. (It's probably worth putting an inch of rigid XPS or EPS on the crawlspace floor if you're going to pour a slab- even a rat-slab down there.) With the joists and subfloor fully within the insulation boundary it's moisture content stays well-moderated by the HVAC, whereas if you instead insulated between the joists the thermal short of the joist from the air conditioned space into a ventilated crawl makes it the coldest spot in the crawlspace, concentrating the condensation on the wood.
With the ducts in an insulated conditioned crawlspace the duct-insulation requirements fall to ~ R6 too. You'll still want the ducts to be mastic-sealed though. A very modest amount of HVAC air can be circulated in the crawl (required by code in some areas) to eliminate any air stagnation or gas buildup issues, but it doesn't need to be anywhere near the volume as in living space.
06-23-2011, 07:04 AM
Trane from our experience is a good choice. I would echo Tom. Make sure that you can get equipment and parts locally. That will be important.