Advice Sought for a High Performance New Construction in Houston

[Bear_in_HOU]

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!

[dan sw fl]

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

[Shophound]

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.

[motoguy128]

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.

[Bear_in_HOU]

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?

[Bear_in_HOU]

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?

[dan sw fl]

.... 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

[Shophound]

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.

[motoguy128]

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

[Bear_in_HOU]

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

[Bear_in_HOU]

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.

[dan sw fl]

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.

[Shophound]

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?

[teddy bear]

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.
.

Thanks for any thoughts!

Did you find Joe's Houston study on ventilation and humidity control in Houston? He measured several styles of ventilation and humidity control methods. I will see if I can find it.
Regards TB

[dan sw fl]

http://apps1.eere.energy.gov/buildin..._hot-humid.pdf

[Bear_in_HOU]

Without the Estimated glass area and orientation, one doesn't know (cannot calculate) the Heat Gain.

120sf - 150sf (still running the numbers), looks like that got left out of one of my earlier posts due to a Firefox crash. The heat gain is relatively straight-forward to model to a level that is "good enough" for where we are at in the process, as far as I can tell.

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.

Strictly read, this is not correct. If you add in the nuance of reflected vs. direct light, then we are getting closer. If the target were simply to generate an ambient luminous flux, then I suspect that I have better tools to measure that than most anyone on this board (I have access to a $20k spectrophotometer at our lab, and I routinely have a few desktop spectros and other light meters in my home office). I'm not trying to be rude, but I was more looking for equipment-specific advice, not general design advice.

What you describe (" sun house", nor just a sunroom) likely raises the window heat gain to >> 2/3 of the overall heat gain.

Given that, and given that I am willing to pay for higher-spec items if they bring better performance to the table, I am back to my original questions...

[Bear_in_HOU]

Did you find Joe's Houston study on ventilation and humidity control in Houston? He measured several styles of ventilation and humidity control methods. I will see if I can find it.
Regards TB

Yep. There have been several "Houston Studies". Joe's work with a local production builder is what caused me to investigate them. However, for my neighborhood, that builder's surcharges basically made them uncompetitive with local custom builders. There have been other studies done on Houston homes, esp. with respect to energy savings for EPA Energy Star homes, and the findings regarding insulation are informing the design.

[Mr Bill]

Howdy neighbor. Mr. Bear, you will find tons of good information here, and lots of great Pro's with all the science, but "bear" in mind "no pun intended" we live in the "Bermuda Triangle" of the a/c world, they haven't developed any a/c science really works 100% here "yet" but I hear NASA is on it.

[Bear_in_HOU]

Let me perhaps get more clear about the various options under consideration:

1) Single stage AC + (small) Gas furnace - standard Houston practice, we have this in our current home. Probably two units (one up and one down). If I don't ask for better, this is likely what every contractor my builder polls will spec.
2) Single-stage Heat Pump - as above, but without the gas furnace. Electricity runs $0.11/kWhr (more or less) and gas is $0.80/Ccf (more or less). Higher acquisition cost vs. the AC unit, but indeterminate vs. the package.
3) High-efficiency, two-stage iterations of both of the above. Somewhat likely to get "upsold" into a higher tonnage unit(s) to handle peak loads, while the lower speed stage may or may not still be overkill for our average needs. Ductwork and other items also having to be sized to the peak airflow means more added expense.
4) VRF Heat Pump (Japanese) - Mitsubishi can operate more than one air handler internally (great!), but there are few contractors locally who work with these units (not great), and the ones that seem to work in a ducted application (we need high performance filtration) seem to only have mid-market efficiency ratings. I've read a studies done by FSRC, among others, on these types of units to see what the practical efficiency is. This is still an open question.
5) Variable-Speed Heat Pumps (Team America) - Lennox or Carrier both offer variable speed heat pumps, but zoning is done via dampers (as far as I can tell). This complicates the ductwork, and the Carrier units seem to have had some teething pain. The Lennox units are even newer.

One of the issues I am especially curious about getting expert advice on: would we need dedicated dehumidification (e.g., Ultra Aire XT105H or XT155H) if we went the VRF/Greenspeed route?

I like the idea of using the DHW source as the back-up or even primary heater, though finding someone locally who can do it may be a challenge. There are also people whose opinion I respect who say that I would want to keep the water that I use for heating separated from my potable hot water (this was in the context of in-floor hydronic systems, but the general principle probably applies...).

Thanks!

[Mr Bill]

Let me perhaps get more clear about the various options under consideration:
2) Single-stage Heat Pump - as above, but without the gas furnace. Electricity runs $0.11/kWhr (more or less) and gas is $0.80/Ccf (more or less). Higher acquisition cost vs. the AC unit, but indeterminate vs. the package.

You "might" find one heat pump per 100,000 conventional a/c's in Houston, that ratio should bring you up to date and how popular and energy efficient the HP is here, gas is just way to cheap here to even entertain the thought of a HP. I would sure like to know what company in Houston even gave you the option of a HP, but he must be a big share holder of Reliant shares.