Keeping HVAC inside conditioned space
We have just started construction on my new house. I want to keep the HVAC inside the conditioned space for energy efficiency. The crawlspace will be conditioned and we are using open web floor trusses on the second floor to accommodate the ducts for the second floor.
From my initial talk with the HVAC contractor, it sounds like they don’t want to put the actual unit in the crawlspace. I realize it’s harder to work on and install. The suggested I use a packaged unit so it could be installed outside where there is room. Either way the ducts stay in the conditioned crawlspace. How much of a difference will there be by having the unit located outside? The third option would be use a regular split system and put it in the garage, but I like my garage space and the garage is not conditioned.
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Michael,
I think that you forgot to tell us your location or climate zone.
Once again, it sounds like we're talking about a construction project that was started before the design was finalized. That's not a good idea.
The answer to your question is that the HVAC system belongs in your conditioned crawl space. It's possible, of course, that your excavation contractor and your foundation contractor made mistakes, and you ended up with a 3-foot high crawl space when you really need a 5-foot crawl space. I hope that's not true, but it's possible.
If you have a crawl space that is at least 4 feet high, and it is designed to be dry, conditioned, and well lit -- ideally with a slab floor -- you should be all set, and your HVAC contractor should go ahead with Plan A. Of course, this type of crawl space needs good access if you want to put equipment down there. You've planned for good access -- right?
If your HVAC contractor doesn't want to work with your plan, look around for a different HVAC contractor.
To answer your questions: you definitely don't want your HVAC equipment to be outdoors or in the garage.
Sorry, climate zone 4, the coastal region of south east VA.
You are correct, for a couple of reasons the HVAC system was not designed. I have never seen a house with a 5 foot tall crawl space in this area. My builder is adding an extra course of concrete blocks, but at the moment I'm not sure of the exact height. The crawlspace will however be dry, conditioned and well lit (no slab though, 15 mil vapor barrier on top of sand). I will discuss access door requirements with the HVAC company when I meet with them Wednesday.
Thank you for continuing to answer my questions.
-Mike
Ducted mini-split cassettes don't need a lot of headroom, and could be installed in almost any crawl space, but having at least 2-3' sure makes working in there a lot easier and less claustrophobic than a bare minimum 15". At 4-5' it's a piece o' cake if you have good set of knee pads.
Michael,
If you plan to install a horizontal furnace, it's best to pour a concrete slab to accommodate the furnace, even if the rest of the crawl space has a sand floor.
I like the idea Dana, but we will also be using gas for heating.
Is gas really cheaper than a mini-split running an HSPF of ~11.5? (= 11,500 BTU/kwh) or better?
eg: Wintertime gas in VA has been running $0.95-1.10/ccf, call it a buck a therm. In a 95% efficiency gas furnace that's about $1/95,000 BTU or about $10.53/MMBTU + the cost of running the air handler.
http://www.eia.gov/dnav/ng/hist/n3010va3m.htm
Electricity in VA has been running about $0.115 /kwh. So with an HSPF of 11.5 that's $0.115/11,500 BTU or $10.00 / MMBTU.
http://www.eia.gov/electricity/monthly/epm_table_grapher.cfm?t=epmt_5_6_a
Seems like it's a dead heat on operational cost.
The Fujistu -xxRLFCD ducted mini-splits have HSPFs in the mid-11s, and the 1.5 tonner is good for 20,000 BTU/hr or more, which is a large fraction of the heat load (or maybe all of it) of a 2000' code-min house @ 20-25F (a typical 99% outside design temperature for coastal VA.
http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf
http://www.fujitsugeneral.com/PDF_06/Submittals/18RLFCD%20Submittal.pdf
http://www.fujitsugeneral.com/PDF_06/Submittals/12RLFCD%20Submittal.pdf
http://www.fujitsugeneral.com/PDF_06/Submittals/9RLFCD%20Submittal.pdf
Most gas furnaces are oversized for the heating loads of code-min or better houses too,and having both the furnace HX and the cooling coil in the air handler uses more air handler power.
What are your calculated heating & cooling loads ( by floor, since they'll probably want to be separate zones)? I suspect a pair of mini-duct cassettes (whether on a 2-zone multi-split compressor or separate units) could be correctly sized to cover the loads of most medium size houses on the VA coast, and would deliver very quiet even-temperature heating & cooling, unlike the typical gas-furnace + cooling units.
The first floor is 1902 sq ft. The second floor is 1536 plus a 364 sq ft room over the garage for a total of 1900 sq ft on the second floor. The HVAC contractor submitted an estimate that included a 2.5 ton AC unit for each floor, and a 53,000 BTU furnace for each floor. I am going to get a copy of their load calculation when I meet with them, but in the mean time I tried to do a load calc myself at loadcalc.net. I wasn't able to figure out what to enter for a conditioned crawl space, so I just entered R19 floor insulation (I didn't add the crawlspace volume to my volume calculations either). My load calculation came out to less than 4 tons for the whole house. If I enter the full ASHRAE 62.2 ventilation rate, which was something like 150 or 160 CFM, I was able to get it to go over 4 tons.
If your HVAC contractor is still installing package units in new construction goodness knows what their duct sealing practices look like - sounds like they're twenty years behind the times. If there's nobody better in the area you should consider hiring an energy consultant to advise and supervise. Dana certainly knows his stuff but my hunch is you'd be going way out on a limb with ducted mini splits in this context. Better to stick with conventional tech, but make sure the installation meets current best practices. My guess is you're going to have to keep close tabs on insulation & air sealing too. In this area too an energy consultant will be a good investment.
BTW there are plenty of ducted air systems with air handlers installed in 3' crawl spaces, though more is always better from an installer's point of view. I doubt you'll need a slab, the unit can be hung from the FF joists, and most units are capable of being introduced through a 3' x 2' door. Again, bigger is better if the space is available. You're certainly on the right track with your HVAC distribution strategies. Good luck!
Thanks James. I believe the city here requires a duct leakage test, so they must do at least some duct sealing.
A cooling load of 2.5 tons for a ~2000' first-floor with a conditioned floor above it seems very high, and 53,000 BTU/hr is almost certainly higher than the heat load for the whole house, let alone a single floor.
Rules of thumb aren't very precise but 53,000 BTU/hr over the whole 3800' would be a ratio of about 14 BTU/square-foot, which is quite a bit higher than typical new code min homes in my area come in at with at a 99% outset design temp of 0-5F. (That's about the heating/area ratio of my 2x4 framed sub-code antique at a 99% design temp of +5F.) TWO of those furnaces would be a ratio of 28 BTU/ft^2-hr, which is about where homes with no wall insulation and R13-R19 in the attic come in at at 0F. There are new homes in central Alaska with design load ratios in that range (!), at outside design temps in the -40s or -50s. Your 99% outside design temp is quite a bit warmer than 0F (and WAY warmer than -40F :-) ) , and I'd expect a well built reasonably air tight IRC 2012 code-min house in coastal VA to come in at about 8-9 BTU/ft^2-hr (or slightly lower) @ +20F at least on the first floor, and possibly all of it. I see new houses in my area coming in at 10-12 BTU/ft^2-hr @ 0-5F often.
Cooling rules of thumb are even less precise, due to the high impact of glazing orientation & site shading factors, but tight code-min houses in Florida are coming in at about 1-ton per 1000' of floor area these days. With some shading and minimal west-facing window area it can be in the 1-ton /1500' range. It's possible that yours has a whole house load of 4 tons, but it could also easily be 3 tons. It's unlikely to be as high as 5, but the upper floor with the attic load and less shade would usually have a higher load per square foot than the first floor with the conditioned space above, and likely better shading factors.
Before picking the HVAC mechanicals, have a qualified engineer or energy nerd do an aggressive room by room Manual-J load calculation with better load calc tools, using the most aggressive assumptions about air tightness. ASHRAE 62.2 is a ridiculously high ventilation rate for most homes, but if you used heat recovery ventilation the additional load even at that rate would not be very much (and mostly latent load.) If it's not too late, you may want to modify window sizes/types & orientation too. Really great "sunset view" floor to ceiling windows are a recipe for high peak cooling loads and can usually be scaled back considerably without killing the aesthetics.
The bonus room over the garage usually needs to be it's own zone, and is almost never going to balance with the other rooms, since both it's heating & cooling characteristics are very different from a first-floor over a conditioned crawlspace with a conditioned floor above, and very different from a second floor with a fully conditioned floor below. In your climate a through-the wall PTHP may be the "right" solution for heating & cooling that room, unless the loads are high enough to warrant a 3/4 mini-split (unlikely, but not impossible.)
Without careful load calculations you can't really spec any of it, but I'd be surprised if a 1.5 ton mini-duct Fujitsu per floor and a half-ton PTHP in the bonus room wouldn't cover your loads with margin, as long as all ducts & mini-duct cassettes were fully inside of conditioned space. (And being modulating systems, the comfort level would be higher, noise lower.)
Simulating the house with BeOpt or HEED would give you a better sense of what makes a difference than monkeying around with Manual-J load tools, but there's a bit more data-entry involved. It's better to do this at the design phase rather than after construction begins, but at the very least it would sort out the heating & cooling load issues:
https://beopt.nrel.gov/
http://www.energy-design-tools.aud.ucla.edu/
"Your 99% outside design temp is quite a bit warmer than 0F (and WAY warmer than -40F :-) ) , and I'd expect a well built reasonably air tight IRC 2012 code-min house in coastal VA to come in at about 8-9 BTU/ft^2-hr (or slightly lower) @ +20F at least on the first floor, and possibly all of it. I see new houses in my area coming in at 10-12 BTU/ft^2-hr @ 0-5F often."
The design temps for the closest city to where I'm building is 91 degrees F and 24 degrees F. Just FYI for local code minimums here. Wall is now R15 insulation (or R13 + 1), Attic is at least R38, not sure if it's increased to R49 here or not.
"Before picking the HVAC mechanicals, have a qualified engineer or energy nerd do an aggressive room by room Manual-J load calculation with better load calc tools, using the most aggressive assumptions about air tightness."
I may end up doing just that. I am going to have my friend who is a mechanical engineer who designs commercial systems to take a look at this contractors load calc.
"ASHRAE 62.2 is a ridiculously high ventilation rate for most homes, but if you used heat recovery ventilation the additional load even at that rate would not be very much (and mostly latent load.) "
I agree, especially for a large house with only 3 occupants (well, 6 if you count dogs and a cats). I plan on using an ultra-aire whole house dehumidifier which will provide supply only ventilation.
"If it's not too late, you may want to modify window sizes/types & orientation too. Really great "sunset view" floor to ceiling windows are a recipe for high peak cooling loads and can usually be scaled back considerably without killing the aesthetics."
It's not too late, but for the most part it's not going to happen either. I'm all for less and smaller windows. My wife, not so much. It's a losing battle. She loves lots of sunlight. I calculated the total window area for each side and floor of the house, I'll have to post that later when it's in front of me.
"The bonus room over the garage usually needs to be it's own zone, and is almost never going to balance with the other rooms, since both it's heating & cooling characteristics are very different from a first-floor over a conditioned crawlspace with a conditioned floor above, and very different from a second floor with a fully conditioned floor below. In your climate a through-the wall PTHP may be the "right" solution for heating & cooling that room, unless the loads are high enough to warrant a 3/4 mini-split (unlikely, but not impossible.) "
I had the same thought, but the only wall to the exterior is on the front of the house, so that would mess up the aesthetics. Regardless, it may not even fit because of a pop out (kind of hard to explain without seeing it).
Thank you again for your awesome suggestions and information!
So, 2x4 / R15 or R13+ 1 meets local code- that's a wall U-factor of about U0.09 (counting interior & exterior air-films) but what are you actually building? At U0.09 the walls are probably going to be about 1/3 of the total conducted heat loss or a bit more.
With the 2x4/R15 wall, adding 3/8" fan-fold XPS under the siding would drop the U-factor to about 0.078 without changing any of the design details much. Adding 1.5" of polyiso (admittedly a significant detail change in how it might get built) would drop the wall U-factor to about 0.05 (R20 whole-wall.) That's approaching what might be the financially rational limit at the warm edge of zone 4A, but certainly not way out in super-performance territory.
The R38 attic vs R49 is probably not going to impact the total load for the mechanicals much, given that it's a 2- story. It would be a bigger number if it were a 3800' one-story rancher, with ~2x the attic floor area of what you're looking at.
The whole-house dehumidifier is probably overkill- definitely overkill if you had a modulating AC system given the long duty cycles it would be running in a code-min building envelope. An HRV or ERV for the ventilation, and a room dehumidifier located somewhere that gets good air exchange withe rest of the house would do it. (IIRC the Building Science Corp guys did a high-R house in TX that was dehumidified with a room dehumidifier in a closet that had all of the ventilation air passing through it, or something similar.)
Specifying low SGHC glazing for all of the west facing windows should whack a big chunk out of the cooling load, if indeed that's what's driving it to 4 tons.
Sounds like you may be stuck marrying the bonus room to the second floor zone. With the ducts sized correctly it might do OK during the cooling season, but may run cold overnight during the heating season and may need supplemental heat to stay above 65F even when the rest of the zone is coasting along at 70F or something. (TBD.)
Great thread! Dana’s comments are always so thorough and insightful. I hope more and more people read this stuff to apply pressures in the building industry and all the contractors out there who just don’t give a flip and race to code minimum.
Dana, instead of a room dehumidifier, could it be a good idea to implement a smaller Ultra-Aire unit? Perhaps one nominally rated for lower square footage? I ask because I have musical instruments and am considering a whole house Ultra Aire unit tied into ductwork to keep humidity consistent throughout seasons in my new construction build. Better for the instruments’ wood and better for the tuning.
I’m also leaning heavily to a ducted VRF system in conjunction with Ultra-Aire. Are we talking overkill here too? I’m in a very humid CZ4.
I’d rather avoid taking up floor space and emptying water reservoirs on those portable room-sized dehumidifiers.
In a well-sealed home, with a ducted VRF, is wise to A) pass on the Whole-house dehumidification system or B) just downsize the Ultra-Aire unit?
You pay a real premium up front for an Ultra Aire (even the Ultra-Aire 70H , rated
70 pints/day @ 60% RH) but they work, and will likely outlast a room dehumidifier.
With a right sized ducted VRF cooling solution you may already have sufficient latent load control, depending on what controls & adjustments come with it.
The Daikin Quaternity mini-splits are advertised to be able to dehumidify without sensible cooling, with independently settable RH and temperature setpoints, which might be a better place to spend the extra money than a whole house dehumidifier if the instruments are usually stored in one area that can be served by the mini-split rather than multiple remote rooms in the house:
http://www.daikinac.com/content/residential/single-zone/quaternity/control-humidity/
http://www.daikinac.com/content/assets/DOC/SiUS04-921%20Quaternity%20Systems%20Service%20Manual.pdf
The Daikin can be ducted?
I don’t need independent room controls. One setting for the whole house is fine.
I’ve been looking at the Mitsubishi units, mainly because the HVAC company I’ll likely be dealing with is an authorized installer.
Quaternity series mini-splits can't be ducted, but Daikin is the world's largest manufacturer of VRF equipment, primarily for the commercial building world. I don't know if they have independently settable temperature & RH controls on any of their their bigger VRF air handlers.
The full-sized air handler Mitsubishi units that are fully VRF are the PVA series air handlers, and then only when married to the compatible PUY 0r PUZ compressor units. The max to min turn down ratios aren't really great either, especially when compared to most ductless mini-splits, though comparable to the 2.3:1 turn down of the smallest Quaternity.
The PVA air handlers have a "DRY" mode for better latent load handling, but does not operate to an RH setpoint, and requires manually changing between DRY and the normal COOL modes with the remote. It's not automatic. Getting the sizing right with a room-by-room Manual-J load calculation will improve how well it manages the humidity without resorting to DRY mode.
With the Mitsubishi MVZ air handlers compatible with MXZ multi-split compressors you don't really get the fully modulating VRF benefit. As with the PVA series, there is also a DRY mode with MVZ air handlers, but they operate on a duty cycle rather than modulating with load.
It may still make sense to run a Quaternity for the zone/room with most of the instruments for tighter humidity control (since it's automatic) even if the rest of the house is being cooled/heated with a PVA or MVZ air handler.
Dana,
Can you provide some insight on the need for supplemental dehumidification for a 2300 SF house I am building in Austin, TX (climate zone 2A)?
Here are the specs I am thinking:
Walls:
* 1” Zip- R sheathing (R-3) with Blown in cellulose insulation on interior studs to achieve R-19
Roof:
* Unconditioned attic. Detail the perimeter to allow room for deep cellulose (raised-heel trusses or rafters on stacked plates on top of ceiling joists) and blow the cellulose deep.
* NO HVAC equipment or ductwork in unconditioned attic, all equipment and ducts will be within the conditioned spaces
* Air barrier at the ceiling plane/attic floor
* seal all can lights/penetrations
* roofs will be insulated to R-38 min.
* Radiant-barrier roof sheathing (LP techshield or equal) unless we use a metal roof which meets Austin’s radiant barrier requirements.
* highly reflective roofing — ideally, white metal (galvalume) roofing or white concrete tile roofing
Mechanical System
* Plan is to get this professionally designed even though it is not required. This will allow for
* Properly sized system
* Coordinated duct runs
* NO HVAC equipment or ductwork in unconditioned attic
* Ensure the ducts are sealed
* properly size the AC system for the home
* Install a conventional heat pump system or ductless mini split system 15 to 16 seer
* Install a ventilation duct from the exterior into the Return Air plenum so that it would be filtered and go through the evaporating coil before running through the house. The fresh air intake is set on a timer/programmer to open when the air handler is running.
* Ensure there is positive pressure
Thanks!
The ventilation rate and indoor humidity sources, the particulars of the cooling load, and the SHR of AC system will dictate whether supplemental dehumidification is necessary.
If there is little to no sensible load you can almost count on needing a dehumidifier, but without so much as running the Manual-J there's no way to even take a WAG at it.