Designing HVAC for super insulated home
Hi all,
I am building a super insulated house in central KY. There aren't too many (actually...none at all) super insulated houses in our area, so I am trying to do as much of the HVAC research as I can so that I can help the HVAC subs in our area understand the old "rules of thumb" won't work on this house.
I have done the room to room manual J calculations, and I have come up with a total of 15k/9.6k BTUs heating/cooling for the 1st floor and 9.5k/6.8k BTUs heat/cool for the 2nd. This is well below what most conventional heat pumps can efficiently produce, so I am considering a mini split system. However, my floor plan is not ideal for a mini split system, so I have come here to ask if my ideas below will work.
See the attached floor plans below. For the first floor, I have included 2 mini split ductless heads (marked with the red boxes). The concern I have with this idea is 1.) Whether you can mount a mini split head over a window right on top of a header, 2.) Whether the ductless heads will be able to extend into the ends of the floor plan, especially the guest bedroom and bath (right side), and 3.) The fact that 2 mini split heads will be overkill for the heat/cool load. The smallest mini splits out there are going to produce 7k/6k BTUs, which means that one head would almost handle the entire floor, meaning that 2 heads would almost be double the amount of supply I need. I have a full basement under the first floor, so I could use a ducted mini split and run ductwork. This would allow a more properly sized unit to be used. The question, then, is whether ducts would be too long to be effective with the small static pressures of most ducted mini splits.
The 2nd floor I assume is too cut up to allow for ductless units. I guess I could put a ductless unit in the central hall to serve the front bedrooms and another ductless unit in the master closet to serve the back rooms, but I'd have the same oversizing issue that I have on the first floor, but even more so. So I am thinking about using a slim ducted unit (marked with the red box), mounted vertically in the master closet wall that allows me to run the supply ducts (marked with the blue boxes) overhead to the rooms and then has a return vent at the base of the wall in the hall.
Does anyone have any comments or critiques about these ideas?
Comments (11)
mike_home
7 years agohomepro01
7 years agoRelated Discussions
HVAC contractors do not know how to design for this home....
Comments (21)I would be curious in seeing if you have a good grasp on the numbers I posted originally and the effects they truly have on a home, along with a program you use that actually takes into account some of these things, such as thermal bridging and extremely low infiltration rates...cause ours does. Some of the hvac guys I talked to their programs maxed out at r29 insulation in the walls! Not to mention their "infiltration numbers were, well not numbers at all, but rater "loose, medium, and tight". (tight being 2 ach@50 pac, still way too loose). Its not a fight between who is smarter. Its a matter of new ways of doing things. Tried and true and gut feelings are all fine and dandy on typical code homes, but not here. It gets people out of their comfort zones and actually size things correct, heaven forbid. My HERS rater and ES rater came back with similar numbers now. I guess we are all crazy and the home will really, really suck. (just like the PH homes in VT, IL, MI, and other places that use a single point cooling such as a mini split with great results....)...See MoreHVAC advice: spray foam insulated house with BAD air quality
Comments (3)hrv for cold climates erv for hot humid climates. it matters what your location is. adding fresh air or make up air is easy. how much to add, is the question. was a blower door test performed on the house? what were the air changes per hour? using the blower door numbers, & volume of the house, ach is calculated. once amount of air required is determined then you make choices of how to do it. La. for instance is hot humid climate. an energy recovery ventilator will cost you approx $1200 installed. but a better choice is a whole house dehumidifier with fresh air intake. for a few hundred more. there are other methods of adding fresh air like barometric dampers etc. some work very well. you'd do best to find out how tight the house is. and go from there. best of luck...See MoreHelp designing HVAC for 4 level home in Seattle
Comments (16)The hydronic solution for the basement is an excellent idea if this hydronic air-source HP system were to be installed. First of all, the HP is essentially a geothermal HP without the ground loop; commercial quality that even includes a desuperheater for domestic hot water and an electronic expansion valve for greater control and efficiency! I would suggest properly insulating the slab in the basement with 2 to 4-inches of blue Styrofoam sheets, installing closely spaced in-floor radiant throughout the house for a lower supply water temperature and hydronic fan coil units in the basement and elsewhere where you want quick control of temperature, dehumidification or air-conditioning. Hydronic fan coil units are designed with drip trays and in some cases with condensate pumps for the removal of condensate (water) in air-conditioning mode. This system configuration does not require a gas line, chimney, side vent or much annual maintenance. I would install buffer tanks for both domestic hot water and for the radiant heating and cooling for energy efficiency and to prevent short cycling when only 1-zone is calling for heat or cooling. This is a big house. Even with gas forced air they will still require large electrically powered fan coils to distribute the air. With in-floor radiant they only have to heat from the floor till about 6-feet off the floor. Who cares what happens above that height - remember, this is radiant! Forced air requires that you heat the entire volume of air within the envelope – a completely different animal! Haven’t even discussed the possibility of using chilled beams for air-conditioning that’s possible with this system. Heating without ducts, A/C without a fan coil – imagine that! Think Water! http://www.multiaqua.com/index.php/water-fan-coils/mhww-3-hi-wall-120v#mhww-3-installation-operation-manual http://www.multiaqua.com/index.php/water-fan-coils/cffwa-universal-2p IMPO SR...See MoreNew construction design input requested; all elec HVAC
Comments (4)If the HP will be your only source of heating I would look at the Carrier Greenspeed over the Lennox. Avoid contractors that only sell one brand of equipment, maybe ask your builder for the contractor he/she uses. HVAC contractors that do builder work are typically much cheaper than the big hvac dealers who sell directly to customers. The prices for the highest efficiency lennox/trane/carrier equipment is likely within a few hundred dollars of each other...if you want a particular brand and the quote is so vastly different find another contractor. I would definitely spend the 650 for the additional zone. The ability to zone off that first floor bedroom will save you the $650 in a relatively short period of time and will help with comfort in the bedroom. HRV is probably a good idea in a new construction house that is very tightly insulated. An alternate would be to use bathroom fans that automatically turn on for a set period of time each day but you would lose efficiency as you would be exhausting conditioned air from the home...I have no idea what the payback time would be on the HRV....See Morecvw_ky
7 years agolast modified: 7 years ago- PRO
Air and Energy Soutions
7 years agoCongrats on doing your homework well! A couple of things to ponder.
- The heat load numbers you list may be correct, but are extremely low at around 2600 sq. ft. per ton. Again, your numbers may be correct. I've never used coolcalc and it may be right, but before the plunge, it might be a good idea to have someone with a very comprehensive Manual J software to just double check, just for fun. That said, a full basement is NICE and makes a conventional system really tempting. A package unit would make a nice install and offer cost savings, noise reduction, and reliability all in one box. Just a thought.
- I think you're right about the first floor and the two ductless split systems. This should give you good coverage. Remember though, the supply air will exit through the bottom of the front of the air handler. Make sure the airflow doesn't interfere with curtains or window treatments and the drain is properly run so you don't end up with condensate in a window header. Fujitsu makes an excellent product and is cost efficient.
- For the second floor, I would recommend a 16 SEER conventional split system. I agree, the second floor is too cut up for a ductless system. The reason for a 16 SEER conventional system is cost per energy savings and reliability.
- I'm not sure about your area but technical talent in West Tennessee can sometimes be a challenge. Make sure that whatever system you choose, you will be able to find qualified service contractors to help when needed. The last thing you want is Bubba's second cousin learning on your new $10,000 AC. Just sayin...
Once again, you have done an excellent job of researching your project and have come up with great ideas. I have written several articles on heat load calculations and energy efficient HVAC on my blog. If you would like to check it out, the link is HERE. One more thing. If you have a certified infrared thermographer, please consider taking advantage of this valuable technology. This is great for validating what you believe about your home's energy efficiency and for checking behind contractors. The picture below is from a construction job I was called in to verify. You can clearly see four bays of insulation missing. The insulators just didn't think these bays were important enough to insulate. The sheetrock guys just walled over the empty bays. Thermal imaging will save money. Please feel free to check out this information also HERE.
Once again, GREAT JOB! Please let me know if I can help.
homepro01
7 years agolast modified: 7 years agocvw_ky
7 years agoionized_gw
7 years agocvw_ky
7 years ago
Richard Badger
7 years ago- PRO
Dynamic Restorations, Ltd. Co.
2 years ago
energy_rater_la