Advice Sought for a High Performance New Construction in Houston
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  1. #1

    Advice Sought for a High Performance New Construction in Houston

    My wife and I are in the early phases of designing a new house in Houston. We have started working with an Architect, and we hope to engage a builder here soon. I've spent about three years reading and getting educated on building practices and energy efficiency (e.g., I have read a lot of what Joe Lstirubek has written), and I would appreciate getting some advice from the Pros here before we get too much farther along with the design and to help educate me about how to know when I've got a great HVAC contractor vs. when I've got someone who knows enough to put the right words on the bid, but whose team does the same-old-thing when it comes down to installation.

    Some quick project "specs" (goals, really):
    1. Tight construction - hopefully ~1ACH50, my limit is 2.0 ACH50 -- think "quality" and "attention to detail" here, not just "energy savings".
    2. Lots of glass, and, unfortunately, a lot of it West-facing (SHGC will be <0.25 for these, hopefully <0.20) due to lot orientation and building setbacks.
    3. ~3,000sf on 2 levels - public spaces downstairs, private spaces upstairs. We will most likely want at least 4 zones.
    4. We will probably have a high reflectivity and high emissivity roof, but that is probably for a different discussion (e.g., energy savings vs. damage resistance/durability vs. lifecycle costs)
    5. DHW will most likely be gas - either tankless or something like an AO Smith Vertex 100
    6. Man J and Man D will be a must.


    We will have a big challenge with heat gain from the western afternoon summer sun (overhangs only go so far and no trees on that side yet), but the rest of the day should be fairly moderate. As I understand it, we will have two basic paths to consider for HVAC, each with advantages and disadvantages:
    • Air Conditioner and Gas Furnace
    • Heat Pump (little need for 'emergency' heat given the market and our climate)


    The Air Conditioner + Furnace option will be standard construction for a lot of Houston homes, so it should be relatively cheap. However, the gas furnace isn't free, so the heat pump might have all-in costs that are lower, plus a heat pump saves me a roof/wall penetration.

    Further complicating the picture is the need for dehumidification and fresh air intake. We can go with an ERV or a damper for the air exchange, but the more interesting question (to me) is whether a VRF or other continuously variable system will eliminate the need for a dedicated dehumidifier, or whether we would still want a dehumidifier even with a system like a ducted/split Mitsu VRF, Carrier Greenspeed or Lennox XP25.

    For t-stat, I'd like to be able to change the temperature setpoints from a phone/tablet while we are in the house, rather than have to get out of bed, but I also hate the idea of having to pay a subscription charge to do this.

    I'm willing to pay for performance (comfort, control), but I don't want to overpay, and I am especially concerned about paying for anything oversized. Since some of these options are manufacturer-specific, that will need to inform which contractors we ultimately look at when we get to that point. However, for now, I want to be sure that we are asking the right questions in the right ways.

    Thanks for any thoughts!

  2. #2
    Join Date
    Nov 2004
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    SW FL
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    SHGC < 0.35 could have a penalty with resultant VT < 0.4.
    Q west windows = radiation level * SHGC * A might be ~ 2,800 BTU/Hr between 2 pm - 6 pm.

    You'll be at 1.5 ton for each level if you use spray foam and all ducts in conditioned space.
    ___________ _ _ _ _ _________________
    Q sensible makeupair = CFM* Dt * 1.08
    CFM = ~ 100
    dT = 98- 76 = 22
    Q sensible MakeUp Air=~ 2,400 BTU/Hr without a ERV.
    ERV might be 60% efficient
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  3. #3
    Join Date
    Aug 2003
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    Fort Worth, TX
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    11,274
    My initial thoughts, and I may have more to say in detail later, is work with your architect right now to do something about those west facing windows. You can't help what direction they face given your lot and setback constraints, but you can do something about the quantity and quality of each opening.

    I would highly recommend some form of external shading for each window. There are even electrically operable models that can be raised and lowered without needing to go outside to do it. And opaque enough to not kill all daylighting benefits. Drawbacks are first cost and potential multiple points of failure over the entire west facing facade.

    If you design the house to incorporate ducts within the conditioned space, cool roof choices are still beneficial, but not pertaining to duct heat gain. That said, if you plan to foam the roof deck inside the attic, DEFINITELY go with a cool roof. You will get the best of both worlds in one fell swoop regarding incredibly reduced heat transfer into the home from the attic.

    I have a cool (reflective) roof on my 53 year old house with a conventionally ventilated attic. I can't say enough how much it alone has improved our indoor comfort levels and HVAC performance since it was installed last year. Our thermostat sets the temp up to 78 at 8 AM and drops it to 75 around 5 PM. Recent monitoring of our smart meter's historic data shows that the a/c is hardly running at all during that time, yet the house never feels warm or stuffy during that time. And when the a/c does run, it's a two stage unit that uses much less juice to pull down and keep the house at 75 in the evening than our old system did, even with the cool roof.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  4. #4
    Join Date
    Jan 2009
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    Keokuk, IA
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    5,520
    You could go dual fuel and use the Vertex for hot water heat with a hydronic coil. Heating requirements are minimal where your at. Almsot any furnace will be oversized on a hig hperformance home. Seal and insualte it well and you could hear 3000sqft with <20k BTU's. Why both with a furnace. Air handler have higher effciency ratings with an air handler instead of a furnace.

    Greenspeed is also a nice option and would be pretty slick with a hot water coil so you can get the most out of it.

    Another solution could be using radiant floor heat downstairs, and have 2 sytems and upstairs a heat pump either electric heat or again hot water.

    Just some options to toss out there.

    We just installed a medium sized commerical 200k BTU 100 gallon vertex at my work. Pretty nice unit overall. Great info for troubleshooting that shows you all the interlocks, unit status and tank temperatures.

  5. #5
    Quote Originally Posted by Shophound View Post
    My initial thoughts, and I may have more to say in detail later, is work with your architect right now to do something about those west facing windows. You can't help what direction they face given your lot and setback constraints, but you can do something about the quantity and quality of each opening.
    Quality will be as high as I can afford, but the quantity is largely fixed and is coming from us (my wife wants as much natural daylight as we can get).

    I would highly recommend some form of external shading for each window. There are even electrically operable models that can be raised and lowered without needing to go outside to do it. And opaque enough to not kill all daylighting benefits. Drawbacks are first cost and potential multiple points of failure over the entire west facing facade.
    We are working on the shading aspects. There is still going to be a big heat gain (comparatively) between about 4pm and 7pm during the summer, which will end up as a light load during the day followed by a fast ramp up and then a fast fall-off once the sun goes down. The average load will be minimal. The peak load will be high. This is what is driving the interest in the more advanced compressors.

    If you design the house to incorporate ducts within the conditioned space, cool roof choices are still beneficial, but not pertaining to duct heat gain. That said, if you plan to foam the roof deck inside the attic, DEFINITELY go with a cool roof. You will get the best of both worlds in one fell swoop regarding incredibly reduced heat transfer into the home from the attic.

    I have a cool (reflective) roof on my 53 year old house with a conventionally ventilated attic. I can't say enough how much it alone has improved our indoor comfort levels and HVAC performance since it was installed last year. Our thermostat sets the temp up to 78 at 8 AM and drops it to 75 around 5 PM. Recent monitoring of our smart meter's historic data shows that the a/c is hardly running at all during that time, yet the house never feels warm or stuffy during that time. And when the a/c does run, it's a two stage unit that uses much less juice to pull down and keep the house at 75 in the evening than our old system did, even with the cool roof.
    All good stuff. We will minimize spray foam, but the house is going to be largely as air tight as we can get it. Ducts in the conditioned space are a must, as is using as much sheet metal for the ductwork as practical.

    Given the high expected peak load, are we going to kill a conventional A/C unit with short-cycling?

  6. #6
    Quote Originally Posted by motoguy128 View Post
    You could go dual fuel and use the Vertex for hot water heat with a hydronic coil. Heating requirements are minimal where your at. Almsot any furnace will be oversized on a hig hperformance home. Seal and insualte it well and you could hear 3000sqft with <20k BTU's. Why both with a furnace. Air handler have higher effciency ratings with an air handler instead of a furnace.

    Greenspeed is also a nice option and would be pretty slick with a hot water coil so you can get the most out of it.

    Another solution could be using radiant floor heat downstairs, and have 2 sytems and upstairs a heat pump either electric heat or again hot water.

    Just some options to toss out there.

    We just installed a medium sized commerical 200k BTU 100 gallon vertex at my work. Pretty nice unit overall. Great info for troubleshooting that shows you all the interlocks, unit status and tank temperatures.
    Hydronic radiant is out. No one in Houston seems to do it, and the heat transfer is too slow (probably why no one does it).

    The idea of using the water heater tied to the air handler is a good one. I assume that I'd need a device of some sort that would activate the hot water loop? Also, would the air handler be a standard one or some add-on module?

  7. #7
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    Thumbs up FIRST , APPROXIMATE MANUAL J

    Quote Originally Posted by Bear_in_HOU View Post
    .... my wife wants as much natural daylight as we can get.

    We are working on the shading aspects.

    There is still going to be a big heat gain (comparatively) between about 4pm and 7pm during the summer,
    which will end up as a light load during the day followed by a fast ramp up and then a fast fall-off once the sun goes down.

    We will minimize spray foam, but the house is going to be largely as air tight as we can get it.

    Given the high expected peak load, are we going to kill a conventional A/C unit with short-cycling?
    High Peak HEAT GAIN may not even be a significant issue

    MAYTAG or Carrier Greenspeed obviously deserve consideration
    ( although that may lead you to one unit with Several zones).

    Houston ~3,000 SF on 2 levels
    House dimensions : 52 x 30 ?

    T-stat set Winter 72'F ___ Summer 76' F __ ?

    Wall _ R-15 __ ?
    Ceiling R-30 ___ ?

    U-value _ 0.40 ?
    S.G.H.C. ___ 0.37 ?

    _______ Window Areas
    ____ First __ ... _____ Second
    ____ Floor __________ . Floor
    Total 260 ? _________ 170 Sq Feet ?

    N _______ ,,,,,,,, ______ Sq Feet
    S _______ ......... ______
    E _______ ------ ______
    West ____ __ __ - ______

    A.C.H. 0.2 (air change per hour)

    Minimal duct heat gain even at R-4 or so.
    Supply Air 54'F
    ___Attic 86'F
    ____dT 32'F
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  8. #8
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,274
    There is a point of diminishing return via increasing glazing surface area to increase natural daylighting. I would advise working with your architect to model interior light levels as window sizes are tweaked. Computer models exist that can do this; your architect may have them or have access to it.

    i have lived north of you in DFW most of my life. Cooling season in Texas coincides with glary, hot sunshine. I do not like being inside a house awash in glary sunlight on a hot Texas afternoon. Might be nice in winter, as it's not so glary then due to the lower sun angle, but forget it in summer. Give me light from a shaded window any day over that.

    Cooling and dehumidification is your primary design criteria for Houston. It should figure not only highly in HVAC design choices, but also the building itself. You are already way ahead of most people in Houston who contemplate building a home from scratch. The devil is in the details. As for heating domestic water with the heat pump, I would weigh the first cost of the equipment required to do that vs. a natural gas water heater. First cost and operating cost of the latter may be lower. Any water tank heated by a heat pump will require auxiliary heat when the heat pump does not run much, unless it can run just for heating water.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  9. #9
    Join Date
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    Keokuk, IA
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    I have tons of daylight. You can offset it by adding mass. Get tile, stone and as much concrete as you can into the design. Stucco on exterior. Insulating a concrete floor slab. I always thought of using concrete up to the bottom of the widow sills at least... Then you don't have to form openings.

    Direct sun is you enemy. Add natural shading with deep overhangs. That adds to roof cost, but a huge payback.

    Built right, your target should be 3 tons of cooling.


    Sent from my SGPT12 using Tapatalk 2

  10. #10
    Quote Originally Posted by dan sw fl View Post
    High Peak HEAT GAIN may not even be a significant issue
    AS near as I can tell from the modeling that I've done, it should be my only major design issue. The rest will be a simple matter of execution...

    MAYTAG or Carrier Greenspeed obviously deserve consideration
    ( although that may lead you to one unit with Several zones).

    Houston ~3,000 SF on 2 levels
    House dimensions : 52 x 30 ?
    Closer to 20x80, at least on the ground floor. The upper floor will be a bit wider (cantilevered) to create both a capillary break and to create additional overhang for the ground floor where a wall of windows faces west.

    T-stat set Winter 72'F ___ Summer 76' F __ ?
    We are typically 72-ish, but we live in a 1930 bungalow currently. I have no clue what it will be like when we have actual humidity control.

    Wall _ R-15 __ ?
    Ceiling R-30 ___ ?
    Clear-wall should be closer to R-20, roof will be R-40+

    U-value _ 0.40 ?
    S.G.H.C. ___ 0.37 ?

    _______ Window Areas
    ____ First __ ... _____ Second
    ____ Floor __________ . Floor
    Total 260 ? _________ 170 Sq Feet ?

    N _______ ,,,,,,,, ______ Sq Feet
    S _______ ......... ______
    E _______ ------ ______
    West ____ __ __ - ______
    There won't be a single consistent U-factor or SHGC. Budget will rule what we do for the north, south and east-facing glass.

    A.C.H. 0.2 (air change per hour)
    I'm targeting 1.0ACH50, so unpressurized it should be <<0.1 if I remember the conversion correctly. Fresh air will be part of the design.

    Minimal duct heat gain even at R-4 or so.
    Supply Air 54'F
    ___Attic 86'F
    ____dT 32'F

  11. #11
    Quote Originally Posted by Shophound View Post
    There is a point of diminishing return via increasing glazing surface area to increase natural daylighting. I would advise working with your architect to model interior light levels as window sizes are tweaked. Computer models exist that can do this; your architect may have them or have access to it.
    SWMBO disagrees. I'm mostly down to the mechanicals in trying to understand what the extent of the trade-offs are.

    i have lived north of you in DFW most of my life. Cooling season in Texas coincides with glary, hot sunshine. I do not like being inside a house awash in glary sunlight on a hot Texas afternoon. Might be nice in winter, as it's not so glary then due to the lower sun angle, but forget it in summer. Give me light from a shaded window any day over that.
    I'll have 3 - 4 hours a day during three months of the year that will be problematic until the shade trees get big enough.

    Cooling and dehumidification is your primary design criteria for Houston. It should figure not only highly in HVAC design choices, but also the building itself.
    Agree highly, and it is. I am mostly down into the weeds of comparing specific parts so that I can ensure that we ask the right questions for the HVAC contractor. I don't want to get locked-in to a Trane guy when what we really wanted was a Mitsu VRF (or a Lennox, or a Carrier). I'd also like to know going into the point where we begin bidding what the relative tradeoffs are so that I'm not spending a ton of extra money for a bad solution.

    You are already way ahead of most people in Houston who contemplate building a home from scratch. The devil is in the details. As for heating domestic water with the heat pump, I would weigh the first cost of the equipment required to do that vs. a natural gas water heater. First cost and operating cost of the latter may be lower. Any water tank heated by a heat pump will require auxiliary heat when the heat pump does not run much, unless it can run just for heating water.
    Thanks! I think the solution proposed was to use a gas-fired water heater as a supplementary heat source attached to the air handler, rather than using a heat pump water heater.

  12. #12
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    Quote Originally Posted by Bear_in_HOU View Post

    AS near as I can tell from the modeling that I've done, it should be my only major design issue.
    The rest will be a simple matter of execution...

    Closer to 20x80, at least on the ground floor.

    There won't be a single consistent U-factor or SHGC.
    Budget will rule what we do for the north, south and east-facing glass.

    Fresh air will be part of the design.
    Without the Estimated glass area and orientation, one doesn't know (cannot calculate) the Heat Gain.

    Without a proper ratio of East and West facing windows for adequate daylighting,
    the glazing imbalance Creates the Heat Gain issue that has to be 'fixed'.
    Address the root cause.

    What you describe (" sun house", nor just a sunroom) likely raises the window heat gain to >> 2/3 of the overall heat gain.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  13. #13
    Join Date
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    Here's how the west window matter appears to me, and perhaps Designer Dan...you are putting forth huge effort to make the house energy efficient and comfortable, perhaps even "green" to some extent, but then shooting a hole in your foot by what amounts to a giant solar collector disguised as low e/high SHGC glazing, all because someone is worried there won't be enough light in the house.

    I know, I know...the SWMBO factor. If you can't find mitigation there, I would not then wait for shade trees to mature. Do something structurally to shade the west facade, such as a pergola or trellis. Get it incorporated into the design so maybe it can get financed with the construction, maybe?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

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