View Full Version : New House
01-26-2012, 02:13 PM
We are getting ready to finalize the plans for a new home. Now that the structure is done I can look at HVAC and HWH. It is just my wife and me. Our kids live far away and only visit once a year. The house will be a ranch with a walkout basement and an attached garage. Main living area on first floor is 3100sf, garage is 750sf, basement (part of the thermal envelope) is 3100 (with about half of that being a workshop). Workshop Heat should be independent of the main house. Insulation will be R27 or better in walls and R49 or better in attic. House will be air sealed to the highest degree possible, I'm planning 2 blower tests (one before drywall). Windows are in the UF .28 & SGHC .24 range. I realize that we need to have a formal heat/AC load calculation done but I'm interested in general ideas on how to heat and cool this place. Located in Central PA in a heavily forested area. My wife and I just retired, so this is the house they will carry us out of. I want to do it right!
Will have propane tank installed for the standby generator, it is available ($$$$). My initial thoughts are for a high end air source heat pump with a propane furnace as a backup for those cold days (also as primary heat in power outages).
Have plenty of land to work with so geothermal is a possibility, but not sure it is worth the money (even with the tax credit)
HWH - was looking at tankless at first, but based on some of the negative (longevity/cold water sandwich/complicated) things I hear, not sure they are the way to go. Also considering heat pump HWH like AO Smith. Since it is just the 2 of us, the HW demand will be lowe most of the time, but when the kids visit, need to be prepared for loooong showers. This xmas the routinely drained the 40 NG HWH I have now.
Would appreciate any ideas/concerns/thoughts/comments/questions
01-26-2012, 04:13 PM
ALthough heat pumps are better these days, I still think in your very cold climate it may not be the best idea. You will spend most of the winter on the backup heat anyway, and will spend a lot of electricty on the crankcase heaters.
Gas forced air, or a boiler and hot water radiators , may be a better idea. Best idea is to talk to a heating contractor who works your area.
01-26-2012, 07:22 PM
Geothermal (earth source heat pump) works, regardless of the air temp. Depending on the land, the soil cover, availablility of water, the possibility of an easy well(s), all could cause that to be viable. Now, if you sit on granite with little ground cover, that may be out.
Central PA is US climate zone 5, which is cool but not "very cold" ('ceptin' by San Diego standards ;-) ), and continously variable speed air-source compressors can run at geo-like efficiency on ductless or air-to-hydronic systems.
It sounds like you're willing to spend the money on the building envelope before spending in on the mechanical systems, and that's generally a good idea. Some issues to ponder:
Rather than R27 center-cavity, shoot for R27 as a "whole-wall" R. As a sample stackup, that's do-able in a timber-framed house with a 20% framing fraction (=16" o.c. framing, but simple surfaces, not a lot of bump outs and extra corners) by going with 2x6 with dense-packed or sprayed cellulose, and 2" of foil-faced rigid polyiso on the exterior, edges foam-sealed, seams FSK taped. With 1x furring through-screwed to the studs 24" o.c. on which to mount the siding, that comes out to a true R27, but performs closer to R30 (on average) due to the low-emissivity and high IR reflectivity of the foil facer that's facing the "rainscreen" cavity between the foam & siding.
It's pretty easy to utilize either (or both) the structural sheathing and (or) the foil facers as part of a continuous defined primary air barrier to the house, and identifying the primary air barriers in the construction plans is important to achieve your air-tightness goals. Test & rectify is also important, but you'll have a lot less to rectify if the know where to apply the foam/caulk/gasketing ahead of time.
Everything is in the building design, orientation and site factors. Modeling the house with DOE (http://doe2.com/)2, and optimizing it with BeOpt (http://beopt.nrel.gov/) is worth it, before committing to a final design.
For a rough idea as to where the break-even point is on whole-assembly R values for each major surface of the house see table 2, p10 of this document (http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones). (Use the values for zone 5, maybe zone 6 if your site is above 1000' of altitude.)
Take special note of the foundation recommendations, which are the most-overlooked (and most difficult to rectify after the fact) aspects of home construction. You really DO need to thermally break all thermal bridging of framing and structural elements, including that between the concrete an the earth in your climate.
If you can get the whole house heat load under 36KBTU/hr (likely) there are MANY ductless air-source heat pumps that can handle it, but if you're willing to spend a bit extra for higher comfort, with radiant floors set up for low water temperature operation an air-to-hydronic heat pump will exceed even that of ductless mini-splits/multi-splits on efficiency. SFAIK the only version currently available is the Daikin Altherma, but the offices of this company in Maine (http://www.nrtradiant.com/) are heated with one, in a climate with design temperatures probably more than 10F colder than yours. It costs more than a multi-split or (lower efficiency) ducted air source heat pump, but far less than geothermal. If you go with radiant, add R5-R7 to the sub-slab recommendations in the table though.
On any new house designing the roof pitches & orientation for future (or even now) photovoltaic (PV) solar arrays is worthwhile. At the rate with which the installed cost of PV is falling, if not now, sometime in the next decade the economics will be compelling. And if you're considering heating & cooling with air source heat pumps vs. geothermal, do the math on how much PV the difference in cost would deliver, and even a ducted system that averages a COP of only 2.0 (likely in that climate, vs. 2.5-3 for a ductless, or 3-3.5 for the Altherma, 3.5-4 if you're lucky with geo, after all pumping and air handler power is factored in). Net-metered the PV + air source often wins. Local & federal subsidies for either geo or PV can change the balance though- it's a moving target. But the system design risk is higher with geo, since presumptions of the soil conductivity etc vs. actual has big error bars around it. The thermal properties of air is the same pretty much everywhere, and the local temperature averages & extremes are known quantities, making the system performance of air source heat pumps far more predictable.
02-12-2012, 06:18 AM
I second most of Dana's comments (and learned some things from them myself about construction techniques). Ductless systems have many advantages and deliver amazing performance even with outdoor temps down around 20F. They're widely used nearly everywhere else in the world where energy rates are much higher. I'm not sure why they're relatively rare for residential use here in the USA. I think part of it is many HVAC contractors quote rather obscene prices for ductless splits as they're not that familiar with them, or want to put their union sheet metal guys to work building ducts. Some homeowners probably also don't like the look.
Propane can run $2.50/gallon and fluctuates rather wildly. And at that price it's often more expensive per BTU/H with the efficiency losses up the flu then using even electric resistance heat. A heatpump running at anything over a COP of 1.5 can be a worthwhile improvement over propane (at least at our rates here in the Northwest). The trick is getting enough BTU/H with cold temps and the house will be very slow to heat unless you add supplemental heat.
If you plan to have one or more propane fireplaces I would upgrade those to something that's at least AFUE rated for efficiency and they can help bring the house up to temp when the stat has been set low and/or when it's really cold outside.
Ductless depends partly on your floorplan. It works best with open floorplans to minimize the number of air handlers needed. When you don't need a lot of heating or cooling the better ones are surprisingly quiet and they continuously vary their fan speed rather than cycling on/off. Smaller rooms that don't need cooling can just use resistance electric heat rather than another ductless zone.