Replace AC with a heat pump?
I’ve recently inherited a house in Virginia mountains, climate zone 6, that’s been used as a three-season retreat. I want to keep it open in winter. Heating is via coal-guzzling electric baseboards. Conventional AC w/ exchanger in unconditioned 3rd floor attic and flexiducts.
1) Can i replace the AC w/ newest generation heat pump like those used in mini-splits? Local HVAC folks are not familiar w/ the technology, since climate is too cold for conventional heat pumps
2) Attic holding exchanger has some ceiling insulation, none in roof. It has only a ridge vent and gets VERY hot under blazing sun on black shingles. Is there a better way? I know attic fan would be counter productive, but its crazy for system to operate in heat in summer and cold in winter.
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Replies
Yes; a mini split will probably work, but will work much better if you upgrade the envelope. Have an energy audit done and air seal and add insulation to your new home.
Jerry,
Yes, you can abandon (or remove) the equipment in your attic, and install one or more ductless minisplit units to heat and cool your home.
Once this is done, you should seal the air leaks in your ceiling, and make sure that you have a deep layer of insulation on your attic floor.
For more information, see:
Air Sealing an Attic
Borrowing a Cellulose Blower From a Big Box Store
Unless this is some unique location on a high ridge above 5000' of altitude, even the cool-valley elevation parts of VA are still only US climate zone 5 or cool edge of zone 4, definitely not zone 6. (Do you have the ZIP code?) There is no single county in VA that's colder than zone 4A.
http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/ba_climateguide_7_1.pdf
Venting the attic is for moisture control, not temperature control. All vented attics run hot, but unless there is also soffit vents the ridge vent it even properly purge moisture effectively. Ideally a vented attic with a ridge vent would also have soffit vents, with 30-50% more free area at the soffit than at the ridge, to reduce stack-effect infiltration drives for the rest of the house.
The key to controlling the energy use is to not install the ducts & air handlers up there above the insulation, or to insulate at the roof deck. How deep are the rafters?
If you rip out the existing ducts & air handler and air-seal at he upper floor ceiling plane you can insulate to R50 at the attic floor pretty cheaply- far more cheaply than a roof deck insulation solution.
Unless the individual room-by-room loads are pretty low you won't be able to effectively heat or cool adjacent rooms with a wall-coil type mini-split, but there are also modulating mini-duct cassette versions from several vendors, some of which are nearly as efficient as the wall-coil types (notably, the Fujitsu xxRLFCD series "slim-duct" mini-splits: http://www.fujitsugeneral.com/duct_specs.htm ). These are mini air-handlers whose output can be split between 2-4 rooms with short duct runs. On a typical multi-floor 2 x 4 framed house with R11-R13 cavity fill & clear glass double-panes or wood-sash single-panes + clear storm windows it would likely need one of those per floor, which is a natural way to zone multi-floor houses anyway.
First order of business is to determine what you're going to do for upgrading the building envelope, but keeping the HVAC in mind still matters, since that affects whether you're going to bite the bullet on the more expensive attic insulation options. Then, run the heating & cooling load numbers on the "after" picture.
If the house has a full basement or a crawlspace, don't neglect air sealing & insulating the foundation too. Stack effect infiltration drives are determined by the elevation change between the lowest & highest air leaks in the house, and it's far easier to effectively air seal a foundation wall & band-joist than it is to air seal under the floor.
Jerry you must be looking at USDA climate zones which are entirely different from the Department of Energy classification. It's a common confusion. The DoE Zone Map is shown at the top of the Q & A section of GBA. All of Virginia is in Climate Zone 4A.
Thanks all for getting me started on this problem.
You're right, I was looking at horticultural zones.
The house is 20 years old, built for an elderly client by an equally elderly builder and not up to even the standards of the era. Example: the cedar siding is stapled on. Double pane window exposure in main floor living area is about 50 % of wall area to maximize view. Ceiling insulation is 8 inches.
Roof venting is not up to the standards cited, because of dormers. Appears to be less then 50% of
ridge length. No doubt a blower door test would blow my mind.
Needless to say, I'm not aiming for passive house standards. But I'm an archibuff and FHB reader since the days it was in a larger format and want to do what I can to make the place at least livable.
The house does self-ventilate nicely on all but the hottest days, so AC is less a concern than heat.
So where do I start? The building envelope? Rip off the interior trim and foam around the windows? Add roof insulation? Insulate ceiling of finished walk-out basement?
Jerry,
Deciding what type of energy retrofit work to do on an older house depends on your budget and your goals. If you have no idea how to proceed, the first step would be to hire a home energy rater to perform an energy audit (using tools like a blower door and an infrared camera). Most audit reports include a prioritized list of energy retrofit measures that is specific to your house.
If you don't want to pay for an energy audit, the first place to start is usually air sealing. For more information, see these articles:
Air Sealing an Attic
Air Sealing a Basement
2x4 or 2x6? Insulated, or empty cavities?
Foundation type?
Clear windows, or low-E coating? (Sometimes there is still a label or printing indicating U-factor to be found on the window if you're not sure.)
Are those big windows facing west, east, some other direction?
Are the ceiling joists 2x8s, with R25 batts, or are they deeper with only 8" of blown or batt insulation?
If you can run a standard spreadsheet tool it's pretty straightforward to run room-by-room heat load calculations using I=B=R methods and add it all up by zone or floor to come up with a reasonable estimate of the heat load. With a ZIP code we can come up with reasonable approximation of the 99th percentile outside temperature bin (a common heating design parameter.)
That won't show you where the air leaks are, but it'll tell you pretty much where the rest of the heat is going, and where it might be upgraded. The heat IBR heat load after any upgrades can be totted up, which would tell you which mini-splits or other heating equipment might be appropriate.
The way those calculations work is you measure up all of the exterior surface areas of a room by type (window, wall, attic floor, door, exposed foundation, etc). Based on it's characteristics of the surface types we can estimate their "U-factors", which is is in units BTUs per hour per square foot per degree of temperature difference. Then it's a matter of arithmetic. The difference between the 99% outside design temp and your preferred indoor temp is the relevant temperture delta. Then it's:
U-factor x area x delta-T = BTU/hour.
You add it up for each surface type for each room to get the room loads, then add all of the room loads that you think would be on the same zone (probably by floor, in a 2 story), then add the zone totals together to determine the whole-house load.
Eg:
A 2x4 R13 wall with cedar siding comes in at about U0.08
Clear-glass double panes are about U0.5
8" of fiberous fluff between attic joists 24" o.c. comes in at about U0.04.
Say you have 9' ceilings, and you want to know the heat load of an upper floor corner room that is 13 x 17', that has a two 5' x 7' picture windows on one wall, and a 3' x 4' casement window on the adjacent wall. Assume the 99% outside design temp is +15F, and an inside design temp of +70F, for a difference of 55F.
Total ceiling area is 13' x 17'= 221 square feet of U0.05 ceiling. The ceiling losses are then:
U0.04 x 221' x 55F= 486 BTU/hr
The total window area is (5' x 7') + (5' x 7') + (3' x 4')= 82 square feet of U0.5 window. The window losses are then:
U0.5 x 82' x 55F= 2255 BTU/hr
The gross wall area is 9' x (13'+17')= 270 square feet, less the 82' of window leaves 188' of U0.08 wall, for a heat loss of:
U0.08 x 188' x 55F= 827 BTU/hr.
Assuming the floor below is conditioned space (it's a 2-story, right?) there are no floor losses, so add it all up and the heat loss of that room is
486 + 2255 + 827 = 3568 BTU/hr.
But you'll notice that the window losses dominate the load number. Adding a low-E storm window over a U0.5 window improves the performance to about U0.3, which would reduce the window losses to 0.3/0.5 x 2255= 1353 a reduction of about 900 BTU/hr, and worth considering, even though it's not cheap.
Doubling the depth of the attic fluff to R50 would cut the attic losses in half, a reduction of 243 BTU/hr.
Unless you're stripping the siding or gutting the interior, you're probably not going to do much to the walls if they have R13 batts in them. So, after improvements you're looking at a heat load of:
243 + 1353 + 827= 2423 BTU/hr
In a 270' room that's a heat load ratio of about 9 BTU/hr per square foot, which isn't terrible, if far from PassiveHouse. With air infiltration it will be a bit higher, but reducing air-leakage is job-1 on any envelope efficiency upgrades, so we'll ignore that for now.
But tell us the real wall & foundation construction, and we'll come up with reasonable U-factor estimates for what you have, and with a ZIP code we can estimate the 99% outside design temp from weather data.
Measure it all up, room by room. On the walk-out basement only use the wall area that is above grade, but keep track of how much is below grade. The foundation type and foundation insulation (if any) are all necessary factors for figuring this out.
As far as insulating the ceiling of the finished basement is concerned, that's pretty much a wasted effort, since the basement is conditioned space. It's far more important to concentrate efforts on the above grade exterior surfaces, since that's where the big temperature differences driving heat loss occur, and where you need to limit air leakage to the extent possible.
When you say you want to keep it open in winter, are you living there full time or using it as a weekend retreat? If part of the time you are keeping it at 50 F rather than heating to 70, the advantages of a heat pump may be greater than they would be for keeping it at 70.
Another thought: Dana is right that it's best to improve the envelope first and buy a small HVAC system, rather than buying an HVAC system sized for the poor envelope you have. But another path would be to buy a mini-split right away, and use it for as much of the heating load as you can, while still using the electric baseboards for the rest. Then gradually work on the upgrades to decrease the percentage of the load that relies on the baseboards.
Yes, you always size the HVAC for the "after" picture, even if you are months/years away from making those changes, unless those envelope upgrades are in the "Deep Energy Retrofit" realm. If you're not re-siding the place or swapping in high-performance triple-panes in all the windows, sizing it for the "after" picture today will have you completely covered for the average wintertime load, if not the peak wintertime load.