Need advice on unit for new build
We are building a new house in central SC. We foamed the roofdeck, upgraded the wall insulation from R-13 to R-15 and the garage ceiling (room over) from R-19 to R-30. We have low-e windows and the house faces west. It is a 1.5 story with approximately 4,400 sqft (2570 down and 1800 up). I will attach the Manual J calculation.
I'm looking for advice on the unit proposed. We need to be at no less than 14 seer for the reduced rate from our local electric company but I'm not sure that going a lot over 14 seer is cost efficient for us. The company is quoting approximatly $800-1000 per increased seer level for EACH unit. Here is what is proposed:
3.5 ton split gas system DOWNSTAIRS
14 seer with variable speed blower
Indoor Furnace: 59TP5A100E21-20
2.5 ton split heat pump UPSTAIRS
14 seer (ARI# 3701069)
HEAT KIT: 9 KW
A couple of questions:
1. I asked about the size of the furnace proposed for downstairs as it seems oversized compared to what our Manual J report says we need. He suggested the following: "We can go with a 59TN6A080V21-20 furnace with a cnphp4821 coil and get the seer rating to 14.5 with a smaller furnace." Is this a better choice? Any advice on the proposed system is greatly appreciated as well as recommendations for other options. We do have to stay with Carrier as that is what my builder has chosen and they've started the ductwork installation.
2. We asked for natural gas heat since that is what we were familiar with but now I'm wondering if a electric heat wouldn't be just fine for us considering the increased insulation we've done (foam roof deck, upgraded walls and garage ceiling). I read an article called "Just say no to furnaces in high performance homes". Any thoughts on gas vs heat pump? Our electric and rates are as follows:
NG - All Therms @ $ 1.00525 per therm
First 800 kWh 0.12301
Over 800 kWh 0.13544
First 800 kWh 0.12301
Over 800 kWh 0.11802
I'm curious what he used for desing temperatures and conditions. Off hand, I'd say he upsized by 0.5 tons on each for his own fudge factor. That's not how you do it. The safety factors are already built it. 3 ton and 2 ton should be plenty and will manage humidity better. With all that sealing and insulation, 5 tons should be plenty.
Cople notes: Bigger is not better here. Especially in a large home, that will have a lot of mass. BTU/sqft shouldn't even be listed because it's irrlevant. IUt's wall surface area and solar heat gain that drive cooling load, not square footage. Therefore the smalelr the home, the greater BTU/sqft is will have ALWAYS when doth are designed the same.
This is clearly a luxury home. 2 things you should consider. Higher end equipment are 2 stage which will increase comfort and make the system quieter most of the time. I'd go with a 2 stage unit upstairs. Then downstiars I'd put in a Carrier Infinity zoned system... and consider hybrid heat that wil use a heat pump in mild weather. Furnaces should be as small as possible. IF you go with 3 ton upstairs, you can use a small 45k BTU furnace. That will be plenty of heat. Downstairs, again, heat pump, then a 2 stage 60k BTU Infinty furance. I'd add zones for the kitchen, Dining Room and Living room areas since thsoe have the most diversity based on occupancy. Or seperate the East and West facing sides of the home depending on floor plan.
Yes the price will go up... but with have a big luxury home with lots of nice fixtures and finishes, flooring, why not put in premium equipment. Not nessesarily all the bells and whistles and the highest SEER, but you may find that with rebates right not, it's worthwhile to go a little higher. That's at least my opinion. I do give you a big +++++ for using higher insulation. That's the best place to put your money. Again, I think you could go a little cheaper upstairs, and spend a little more downstiars.
One other thought. The upstairs heating and cooling load is small enough, that you might be just as well installing a heat pump only and not using a furnace. Any furnace you put up there will be a little oversized, even on low stage in your climate. Realistically, your real world heat loss upstairs is probably <20k BTU's when occupied.
Also, since you home will be tight, consider an integrated Infinity ERV for ventilation and winter dehumidification.
Thanks motoguy128. I will need to re-read in order to take in all of the information and recommendations as I really don't know a lot about hvac but one thing jumps out at me. You suggested a zoned system. We gave that a lot of thought and while we agree it probably the best way to design the system for our house, we have had a previous bad experience with zoning. I understand that it was most likely due to improper instillation but we don't have a lot of confidence in this company. The bottom line is that our builder hired them, they have started ductwork and he believes they are the best in the area. The reason that we've lost confidence and I am researching the equipment myself rather than just taking what they advise is that they initially proposed a 5 ton system for the downstairs and 3 ton up. When I questioned the need for a 5 ton system, they performed a manual j calculation but just used minimum code and "standard" options for the inputs with the exception of the foam roof deck that he did know about. It still came back at 5 tons and I questioned again leading to a revised calculation. This is why I'm trying to educate myself about the equipment rather than just taking what he proposes. Again, we can't change hvac contractors at this point as my builder doesn't want to have one company install the ductwork (which is pretty much already installed) and have another company order equipment for fear that if there is a problem, each will point to the other. So based on no zone, what would you recommend?
I would encourage fresh air ventilation for a well built air tight home. Fresh make-up air is a must to purge indoor pollutants and renew oxygen. A simple method is to use a whole house humidifier with the make-up fresh air option. Check out a web site co-sponsor- Ultra-Aire. I can't believer all of the new that do not include fresh air ventilation.
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"
About 5 years ago when the roof foam became popular, we had several builders use this after we were done with the job. Just to get that better "seal" on the home. I really dislike that stuff. Teddy Bear is right, you need fresh air make up and still need some attic ventilation. I have been in 3 of the homes we did with foam everywhere and all the duct work sweats because the attic cannot breath. I personally dont recommend the foam. Eventually the sweaty duct work will start dripping on your ceilings. I work in the Tampa Bay area where it gets humid just like S.C.
Foam on a roof deck needs a vapor barrier placed above the roof deck. I have a similar issue in my home. Wood and foam when heated act like a dessicant and drive moisture into the attic until enthalpy between hte wood and the air in hte attic are equalized. SO if hte wood is 115F, and maybe 5% relative moisture content, how humid does a 80F attic have to be to reach equilibrium. The answer... about 60%RH.
My solution was to add a 6" duct with a powered damper on a dehumidistat. It works pretty well. I'd still using a lot less energy than having the entire attic 120F most of the summer. My attic was already sealed, but unvented BTW.
But anyone putting up a new roof with a foamed deck, need ot add a vapro barrier under hte shingles. I'd also recommend going with metal roof or metal shingles too to reduce roof temperatures in the first place.