Homeowner
I am just starting construction on my new home here in Northern Utah zone 7a. The house is 4,000 square feet (2,000 above and 2,000 below grade) and being built with ICF (below and above grade). I am having a hard time deciding on an HVAC system based on cost/efficiency. I have a heating load of 55k Btuhs. 6.7kw solar array. My options are 1) Mini Splits 2)ASHP/Natural Gas forced air @ $15k. 3)Geothermal forced air @ $40k before 30% tax credit.
I am trying to go all electric and power most of it with solar/battery bank which is why I am leaning towards Geothermal or Mini Splits but money talks.
My issue with a Mini split system is air distribution. Will I be stuck with cold/hot rooms? I am also concerned with ever increasing natural gas prices which leaves me with geothermal which is quite expensive.
Any recommendations as to which system will serve me best? How long would it take to recuperate the cost of Geothermal?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
In terms of comfort a gas furnace is much better than any heat pump because it supplies much warmer air. If you have city gas available it will generally be the lowest cost to operate. Over the past few years the price is gas is down, what the future holds is just a guess. Some will not consider it a green choice but think it will save greenbacks.
I assume your geothermal bid had standard duct work. Make sure no duct work will be in the attic. It is hard to look at geothermal operating costs as most of the published numbers fail to include the pump that moves the water thru the loop as it is supplied by a different vendor and power usage would vary from sight to sight. Most of the sales lecture talk about 55 ground temp but most loops are at or below 32 by February.
If you are willing to have duct work consider a variable speed heat pump SEER 17+. Without city gas this could be the low cost option. Yes it will have backup electric heat but you will not use it unless the temp falls below 10. Where I am that is not many days a year.
If you want to answer your question in dollars and cents model your home with BEopt it is a very powerful tool but plan on investing 30 hours. Adjusting for inflation in fuel and install costs.
https://beopt.nrel.gov/
Walta
Btavery,
First of all, can you tell us your name? (I'm Martin.)
The cost of a ground-source heat pump (sometimes called a "geothermal" system) is so high that it almost never makes sense for a single-family home. For more information, see "Are Affordable Ground-Source Heat Pumps On the Horizon?"
It doesn't sound as if you have gotten a quote yet for an installation of ductless minisplits or ducted minisplits. Why not get a quote?
If you want a fully ducted system, you might want to read this article: "Ducted Air-Source Heat Pumps from American Manufacturers."
I'm having a hard time with the notion that a 2000' ICF over 2000' of mostly below grade would have a heat load of 55,000 BTU/hr. How was that heating load number derived?
My antique 2x4 framed house is much crummier than than an ICF, and a less efficient shape with about 2400' above grade, 1600' below grade, and it would have to be about -25F before my heat load hits 55K.
Before pulling the trigger on any solution, get verification on the load numbers!
Only when fairly accurate room by room load numbers are known is it possible to get heat pump solutions to work at optimal efficiency, or hot air furnaces to work at optimal comfort. 55K seems unlikely. Without running the numbers I'd hazard reality is something around 30K @ -11F (Ogden's 99% outside design temp.) Only an unusually large amount of window area would take the peak load into the 50K+ range. A continuous ~R22 is better than IRC code even in zone 7, roughly twice the whole-wall R of my house and the mass effect of ICF construction shaves another measurable chunk off the peak load.
There is no county in UT that is in zone 7A. Much of northern Utah at commonly populated elevations are zone 6B. At higher elevation at very specific locations it might be arguably 7B (as in the Wind River range across the border in Wyoming) , but not 7A.
Got a ZIP code? What is the elevation?
https://beta.greenbuildingadvisor.com/app/uploads/sites/default/files/images/DOE_climate_zone_map.preview.jpg
Thank you all for your responses, I have been back and forth for too long on HVAC systems and this is helping more than you know.
Walter: Thanks for helping me see the flaws/biases in geothermal efficiency ratings. Your image on natural gas prices helped me realize that prices could easily skyrocket again at any time over the next 20 years again pushing me towards all electric/PV system.
Martin: My name is Brandon Avery and thanks for the articles, they are great reads and further reinforce the unjustified high cost of a GSHP. I will be getting a couple quotes back for a mini spit system but based on the numbers I might be going with a DIY install and use the equipment purchased to also install mini splits at a couple relatives houses.
Dana: My geo guy also told me that my my J load calcs were much higher than they needed to be. I attached they calculations I was given, can you see where he could have messed up? or any other idea where an error could have been made? I think I mistook hardiness chart for climate, 6B is more like it here at zip code 84037 at 4,300 ft above sea level.
Didn't see the attachment.
In the SLC area use +10F or +11F as the 99% outside design temperature, but I've seen people using 0F or +5F, which is some even less frequent temperature bin. For a high-R high-mass house like ICF it's reasonable to even back off to the 97th or 95th percentile temperature bin if the mass effects aren't being taken into account by the software.
Here let's try that again. There should be a manual J report as well as a diagram of where I plan to place the Mini Splits. I will also place two 9k mini splits in the basement when we decide to finish that down the road. Does this look logical? I am trying to get my ducks in a row because honestly I trust your opinions here more than the installers I'm waiting to hear back from. Any comments/suggestions?
Thanks again!
I find it strange on your Manual J that your windows have a glass spacing of 1/4". Most double pane windows run 1/2"-3/4". Also, your window U-factors seem low, with an R-value of around 3. I would think you would be shooting for closer to R-4 (U .25) or R-5 (U .20)
Also, on the Manual J, it shows no floor insulation. Is this really the case in your CZ?