New house! Eternal heater w/ solar preheat?

Discussion in 'Tankless Water Heater Forum' started by jjwest29, Aug 24, 2011.

  1. jjwest29

    jjwest29 New Member

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    Hello everyone, I'm helping out with some research on what types of setups would work well with the new house we are building. It will be built in Sedona, AZ and will be about 3800 sq. ft. Also, only 2 people will be living there, so there won't be a large amount of water usage.
    We're looking at going with an eternal water heater, and need to know if that's the way to go with a recirculation system instead of a conventional water heater. Additionally I'd like to know if it could be effectively used with solar preheating.
    Thanks so much!
  2. Dana

    Dana In the trades

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    You'd be potentially blowing a lot of the efficiency gained in the heater by wasting it in the distribution lines if you don't insulate the recirc loop to R4 or better and use a demand /flow-driven rather than temp-at-tap driven control scheme on the recirculator.

    Solar pre-heat works fine with any HW heater. The combustion efficiency of the condensing version of the burner won't be 98% with incoming water warmer than ~75F, but 95% with 92-95F pre-heated water is still pretty good, eh? (and half of the heating was already done by the solar under those conditions, assuming 75F water at the street, 120F at the output of the Eternal.)
  3. jjwest29

    jjwest29 New Member

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    Thanks for the input Dana. I also read one of your blurbs saying how the efficiency of an eternal wouldn't really pay for itself unless it was placed in a higher water usage situation, so we might be leaning more towards a conventional system now with that in mind.
  4. Dana

    Dana In the trades

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    You pay a lot for solar + highest-efficiency hot water heating, but that's not necessarily the best bang/buck for reducing energy use for 2 people in a 3800' house in Sedona.

    Unless you're filling a large spa or soaking tub every day total HW use for two people will likely be on the order of 50 gallons/day or less, and heating that water with natural gas in a pretty-good 0.62EF tank would cost $250/year or less. If the solar pre-heat system costs $7500 and the up-charge on the Eternal is $2500 and it's reducing your HW fuel use by 80% you've spent ten grand to save maybe $200/year. Even if subsidies & incentives knock that in half, that's still $5000 to avoid $200 in costs- a 4% after-tax return on investment (and that's assuming ZERO maintenance costs on the systems.)

    If your volume draws are fairly conventional, you can reasonably heat your HW with a tank-top heat pump hybrid and spend the other $9K on grid-tied photovoltaics. Given the location and the generous AZ & federal incentives, (now that AZ has net-metering) on a new home if you designed the roof angles & orientation reasonably optimally, spending the money on a grid-tied photovoltaic system would likely give you a much higher return, with lower maintenance costs, and lower hot water heating costs. Even if you don't install it today, desiging the roof pitches, even pre-wiring for photovoltaics is a good move for new construction, since even if the numbers don't work for you today, at some point in the next 25 years they likely will.

    Ten grand out of pocket can buy you lot in the way of building-envelope efficiency, limiting your total cooling & heating loads & energy use. Spending some money during the design phase of a new house can reduce as-sited energy use without huge cost-adders to construction. Being careful about window orientation/size/type as well as insulation types & better-than-code-min R-values, air sealing (and blower-door testing/rectifying well before the paint goes on) can buy a lot more than $200/year return on $10K for the entire lifecycle of the building. Reducing the peak loads and designing the floor plan such that it can be cooled & heated with ductless split air-source systems means you can then get geothermal-heat pump type efficiencies at 1/3 the upfront cost too. Many things are cost effective in a new home that would be cost-prohibitive as a retrofit.

    If timber-framed, designing for exterior rigid foam insulation is a lot cheaper than trying (and failing) to make up for it by filling the stud bays with closed-cell foam. (With thermal bridging of the studs a 2x6 studwall with spray cellulose (or R21 batts) comes in at around R14-15. Add an inch of foil-faced iso on the exterior and it's R20+, whereas 5" of closed cell foam in a 2x6 wall incurs 3x the insulation cost, and delivers only ~R16, despite center-cavity value of R30, etc. Adding 2" of exterior foam for a total of R25+ is still economic in that climate over the longer term.

    Using only casement & awning windows (or fixed, non-opening windows) rather than sliders & double/single hungs reduces window air leakage by half or better. Using swinging patio doors rather than sliders, same story. Swinging doors offer 2x the opening area per square foot of glass too. Think U0.34 windows & doors as an R3 hole in an R20+ wall, and you'll find that they soon dominate the heat gain/loss figures. Minimizing glazed area where possible can make a huge difference. (A push-out casement offers 2x the egress cross section of a slider or double-hung in a bedroom application, for instance.) Rationalize every square foot, cut back where you can. (Views are great, but views of your neighbors back yard aren't necessarily THAT great.)

    Designing to avoid penetrations of the upper floor ceilings and air-sealing that ceiling can be huge too. Designing for 1.5-2x code min using a higher density fiber (cellulose is usually a good value) may require a few more trusses to handle the static load, but it'll still be cheaper than spray foam solutions, and more effective than low-density fiberglass. Designing to keep all ducts and air handlers etc inside the thermal envelope can cut the peak loads in half (ducts in hot attics are TERRIBLE), and worth it. Even if you have to increase the height of the top floor a foot to accommodate the ducts it usually saves money, even in the short term. (Ductless split systems are still a cheaper & more efficient solution, if they can be accommodated.)
  5. Steve29

    Steve29 New Member

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    Steve

    Dana, thank you, you are a wealth of information! I am jjwest29’s dad, and it’s our house he’s helping with. I really appreciate your willingness to put down such great advice. Even though the size of our Sedona house has gotten out of hand, we ARE low-energy people who did things right with our current house, built 28 years ago, and we want to do things smart this time, too, and stay on budget. Info: Our upcoming Sedona house is already designed and in the construction-drawing phase (conventional timber-framed), but the HVAC and other energy stuff is still up in the air. We have natural gas to the site. Electricity is fairly expensive, with an estimate of $200 for a month that uses 2000 kWh (the actual cost per kwh varies based on time of day and year, but it reaches 25 cents/kwh during peak summer usage). We will be at the house all day long, normally. Sedona’s annual heating degree days are about the same as cooling degree days. We’ve read quite a bit and are open to anything. A few comments on your notes:

    We already planned on pre-wiring for PV. Our roof is flat, southwestern style I suppose, with full solar exposure. We will have a pool, if we can afford it, and have considered seasonal solar heating for it, hopefully to be combined with domestic water heating (and maybe for in-floor radiant, too, if it makes sense), so that the solar system can work for us all year long. Are you in favor of this approach?

    Re radiant, we’re considering doing Gyp Crete upstairs (slab on grade down), laying our own tubing, and hooking them up to a couple of gas water heaters. We hear each heater can have 7 zones, and that this is a DIY (sort-of) way to get radiant without spending the tens of thousands when having it done conventionally. Comments? We know radiant is a real luxury in a temperate climate like Sedona’s, but we’re used to having radiant, and love it. And are you OK with Gyp Crete, and if so, how thick?

    Was your reason for using a tank-top heat pump hybrid for HW, instead of natural gas, because its electricity usage could be balanced with PV, or because it would be smarter than gas anyway? (and I’m confused about what a tank-top heat pump hybrid is.) Right now, natural gas is a pretty cheap heat source.

    Your window comments are spot on, but unfortunately our house will have lots of glass, as views are what it’s all about. We’ll do what we can to minimize our loads, including continued rationalization of every SF, and we’ll try to get some mass in the house (Gyp, for one thing) and cool it down at night with whole-house ventilation (Sedona’s summer nighttime lows are usually in the 60’s). Still, our loads might be pretty high. I am very interested in your ductless split air source system comment – do you think that approach should be considered, knowing what you know now about our house? Your last comment said ductless splits are the way to go “if they can be accommodatedâ€. The whole idea sounds terrific to me if it can work for us. Not sure if they’re noisy if you’re sitting outside, though.

    Do you have a couple of favorite window manufacturers, for the money? We love our Pella’s, with hidden roll screens (a huge plus), but they’re awfully expensive.

    Great input on insulation! To minimize bridging, I want to consider advanced framing using 24†stud spacing, and you didn’t mention open-cell foam, which is cheaper than closed-cell: should we still stick with your rec of spray cellulose and exterior foam for the walls? The house will be stucco’d, so I’m not sure high-R iso can be used (we had to use low-R EPS when we stucco’d 28 years ago). For the roof, you mention air-sealing – how is that done? Somebody else recommended open-cell there, but you say cellulose still makes more sense. Re your duct comment, we plan the ducts (if any) to be in the upstairs floor, with none in the upstairs ceiling.

    Thanks again!
  6. Dana

    Dana In the trades

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    If you're doing radiant with a condensing gas boiler, an indirect-fired HW heater is the "right" solution 99% of the time, from a total economic point of view. If you have a "real" heat load due to all of that glass, doing it with a real boiler is a far better solution than with any standard hot water heater. For loads under 25KBTU/hr you might consider a condensing tank type hot water heating solution. But if you're doing a full on solar-radiant, the solar designer would be your best guide on that. (The HTP Versa series has versions set up for solar, but it's a hunk o' change.)

    If your heat load per square foot it sufficiently low that you can do it all with 100F water or cooler, the Daikin Altherma heat pump would do well in a Sedona climate. Since the latent cooling loads are low in that climate you can probably use the slabs for sensible cooling as well.

    But that's all a LOT of money compared to a mini-split. If your heating load is less than 30KBTU/hr, you might skip the radiant in favor of high-efficeincy/low-cost air-source solutions. In a Sedona climate a decent mini-split will have a heating seasonal average coefficient of performance higher than 3.0. (At 47F and higher outdoor temps they run about 4.0) At a COP of 3 each kilowatt hour buys you 3 x 3412 BTUs of heating, call it 10,000, which is about 1/9 of the heat you'd get per ccf or therm of gas in a condensing boiler, or 1/8 that burning it in a HW heater. If your electricity costs 20 cents/kwh that's equivalent to $1.60-1.80 gas (pretty expensive), but if your electricity is 15 cents it's like $1.20-1.35 gas (New England prices). If your electricity is 12 cents it's like a buck or so (maybe you're that cheap?). During the milder shoulders seasons with COP of 4 or better it's a major discount over heating with NG, but it's less fuel (or power) during those seasons.

    But since it's also an air-conditioner, the systems-costs have to be factored in. I you took the 10 grand you were going to spend on pumps & plumbing & gypcrete and put it into PV, net metered the output of the PV can offset the bulk of the operating cost of a mini-split. It takes a sharp pencil and good energy-use modeling of the house in-situ to determine where all of the crossovers are. And ductless systems also need open floor plans or R25+ walls to really work well, or you'll have some rooms that run too warm/cool at some part of the year. They're VERY quiet compared to their reciprocation-compressor cousins though, utilizing scroll compressors (a rolling action, not a piston action), and continously variable speed drive (on both the blower and compressor for some units). They're about as loud as a refrigerator at full speed, but most of the time they'd be cruising a long at part-load, and whisper-quiet.

    With a big investment in PV and heat-pumps for heating & cooling, the heat-pump water heater might make more sense than gas-fired.

    A 16" on-center 2x6 with either cellulose or open cell foam comes in at around ~R14 after thermal bridging, bumping it to 20" o.c. with advanced framing only increases that to ~R15. Fill it in with 5" of clo$ed cell instead and you're still at only~ R16 & change, a performance boost that COULD have been had with 1/2" of EPS outside the cheaper cavity fill solutions. Adding R5 of exterior foam the R14-R15 wall brings it up to ~R20, adding R10 brings it to R25. You're correct that iso isn't the best choice under stucco, but 2" of EPS (R8) or 1.5" of XPS (R7.5) would be the ticket, using cellulose in the wall cavities for a low-cost R20+ (whole-wall) stackup. Alternatively, 1.5-2" of closed cell foam (R10-R13) on the exterior makes for a continuous and nearly assured air seal, albeit at a boost in price. (About 17 cents/r/square-foot compared to 10cents/R/foot for EPS.)

    To air seal a studwall without spray foam you can either use the Huber ZIP approach (painted sheathing + tape), or caulk/glue the sheathing to the studs as you go, and lay a bead of caulk under the studwall plate etc. Spraying foam in the stud bays only treats 60-75% of the potential leakage, and there's no such thing as "too tight" (active ventilation is appropriate if you can get the place to test under 3 air changes per hour at 50 pascals in a blower-door test, which takes attention to detail to come anywhere near.)

    For more on wall stackups & whole wall R, see:

    http://www.buildingscience.com/documents/reports/rr-0903-building-america-special-research-project-high-r-walls

    and

    http://www.ornl.gov/sci/roofs+walls/AWT/InteractiveCalculators/NS/Calc.htm

    With a flat roof/no-attic, consider putting 4-6" of EPS above the rafters, below the membrane to make an R15-R23 thermal break over the framing, and filling the rafters with cellulose or new-school superfine fiberglass (Spider or Optima) between the rafters, and make the rafters 2x12s. That way you'll get a true R50 roof out of it, which will be important for both comfort & peak cooling loads, since you have no convective cooling of the roof going on and the peak temps on the surface will be high. New-school blown fiberglass might be a better choice here, since without an attic a roof leak might go undetected too long if you used cellulose. Open cell is an option, but it'll be more expensive, with very little if any performance gain, since a membrane-roof is already air-tight. Spot sealing the ends of the rafter bays with foam is still a good idea though.

    Also use a CRRC rated "cool roof" material for the finish roof, which will take a significant edge off the peak & average loads.
  7. Steve29

    Steve29 New Member

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    OUTSTANDING info, Dana, thanks so much! I'll re-read tonight, and absorb, and try to figure out some of those things, and terminology, that are confusing. I wonder if you also have knowledge of proper installation/design for radiant barriers. Walls and roof, with roof being the most important, I imagine. I understand they need an air space to work properly, i.e. they can't be up against anything.
  8. Dana

    Dana In the trades

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    Regarding radiant barrier: Heat rejection is better-accomplished above the roof deck than under it (which is why high solar reflectance high infra-red emissivity finish-roof is so important on flat or low slope roofs.) Don't buy any product that isn't listed on the CRRC site- there's a lot of magic-mouse-milk & BS in the energy-efficient coatings & paint biz. Avoiding those who advertized using pictures of the space shuttle is one way to filter, but not necessarily the best & only. :) Products listed as suitable for low-slope roofs on the CRRC or California Title 24 low-slope compliance works, and it's not always more expensive than non-rated roofing material (it's often just a matter of the color.)

    Radiant barrier needs an air space on at least one side to have any effect, and needs it on both sides to maximize the effect, but the economic value of that effect is pretty much nullified by a high-R roof and cool-roof materials. Even if you were to insulate only to code-min using low-density fiberglass the economics of adding RB aren't necessarily cost effective:


    http://info.ornl.gov/sites/publications/Files/Pub24434.pdf



    http://www.ornl.gov/sci/ees/etsd/btric/RadiantBarrier/RBFactSheet2010.pdf


    ^^See the "no ducts" portion of the graphic on p.5 of that document.^^ Unless you have foolishly designed it with ducts in an attic, even in an R19 batt-insulated roof (sub-code-min) radiant-barrier not buying you much performance, and the money is probably better spent elsewhere.

    Using high-reflectance high emissivity cool-roof type exterior finishes/colors on the sun-exposed side walls can also be a factor, but if you're going big on the glazing the heat-rejection coatings on the windows will make a much bigger difference in the cooling load & overall comfort- even on the north facing windows. If the views out those windows are medium to light-colored the heat gain through windows can be nearly half what it is on the sun-exposed sides (!).

    Designing-in and installing roll-down exterior window shades can cut down unwanted solar gains from windows substantially as well:

    http://www.builditsolar.com/Projects/Cooling/RollupSunShades.htm

    Using table 2 on p 10 of this document as a rough guide on "whole wall" R values (with the thermal bridging of the framing factored in, not just the insulation itself) Sedona is on the zone 4/5 boundary. Assuming this is slab-on-grade, there's a long term economic argument for putting 1.5" of XPS or 2" of EPS under the slab, (even in the middle). If you go ahead with radiant heat in that slab, add at least R5 to those recommended R values.
  9. Steve29

    Steve29 New Member

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    Thanks again, Dana - great stuff, and very clear! I've tried to absorb your 8/29 reply. Based on your comments re setups that would work best with certain heating and cooling loads, we're now trying to figure out those BTU/hr numbers. Do you have a recommended program? (preferably free!) I am aware of the heating calculator on builditsolar. It sounds imperative I get these answers now.

    I'm very interested in your mini-split comments and wonder if that approach would be practical for us. Our 2200 sq ft upstairs has a large great room, large master suite, and laundry rm and powder bath. The 1600 SF downstairs has a large open area, two bedrooms and 400 SF studio. I wonder if we'd need so many units that it wouldn't be practical?

    If we splurged and did radiant heating, for cooling what do you think of doing an evap cooler? You've never mentioned this as an option. Larger than normal ducts would be needed. Basically Sedona is a dry climate, but a month or so of the annual monsoon would be problematic. Perhaps mini splits in the bedrooms could get us over that hump? We will have outdoor living spaces around most of the house, so quiet machines will be hugely important, and an evap sounds like it could be noisy (never had one before). It cannot go on our roof. And one person told us that "upper end" homes should not be swamp-cooled.

    You suggested 4-6" of EPS over the rafters - why not half that thickness of high-R iso? Iso has (or used to have, anyway) and half-life, which is a negative. (We have some height restrictions, and we're actually down to worrying about inches now.) Also, a fair portion of the roof will be roof-deck, so will be walked on, sat on, etc.

    Thanks again for sharing your knowledge, Dana!
  10. Dana

    Dana In the trades

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    The Taco heat load freebie is a much more sophisticated & easier to use tool that the BIS heat load calculator. Knowing your loads as-designed is a good starting point for figuring out the crossover to determine whether its cheaper to go with thicker foam on the studs & rafters and maybe even triple-pane glass and low-cost mechanicals like a mini-split, than more substantial for heating & cooling mechanicals and a lower-performance building envelope, etc. It's an iterative process.

    EPS is ~R4/inch, iso is ~R6/inch, so yes, you could achieve the same R of 6" of EPS with 4" of iso. To make it a walkable-duty surface you may have to go with a higher density EPS, but with sufficient rigidity in the decking you can use almost anything. In the event of a minor leak iso will absorb water and lose-R, whereas EPS will not, but in bulk they're priced about the same ~10cent/R/square-foot. To be IRC 2009 compliant with an unvented roof you'll need at least R15, and maybe R20 (is Sedona in US climate zone 4, or is it 5?), so 3.25-3.30" iso (a standard thickness) would surely get you there. See: http://www.hpanels.com/images/stories/pdfs/lit_prod_color/english/H-Shield.pd

    The "half life" issue with iso is primarily a labeling issue- it's about R6.8-R7/inch initially, but it never goes to zero. The aged-value is around R6/inch for moderate temps, but in places like Minnesota it's aged-value has to be derated to R5.6/inch since it's R-value drops somewhat when it's average temp is below 25F. EPS on the other hand runs ~ R3.8/inch at moderate temps, but goes well into the 4s at temps below 25F, but at 100-120F it's closer to 3/inch (another reason to go with a cool-roof building material.)

    Building a deck over a foam-insulated roof takes some thinking it may be easier to design it with some structural elements penetrating through to the rafters and some small clearance between the foam and the decking rather than relying solely on the compressive stregth of the foam. With an OSB subflooring type panel you could do the lattter, but engineering the slip-surface for the roofing may be an issue if you do. Hunter & Atlas and a handful of others make iso panels bonded to 7/16" OSB nailer decking for composition roofs, but even that might not cut it for an active deck.

    Whether or not a ductless system really works for you depends somewhat on the room-by-room design temp heating & cooling loads, and your tolerance for the temperature differences that might occur in the rooms doored-off from the open spaces, as well as the overall heating & cooling loads. If it's under 3 tons (36000 BTU/hr) for both heating & cooling there are many 2-4-head options that might work. In high-R buildings with better low-U windows & doors the temperature differences can be pretty minor. I've been advising recently a high-R retrofit project where the design condition heating load was under 20,000BTU, and the cooling load was even lower, and they went with a ~1.5-ton single-head Mitsubishi (under $5K, installed). The estimated temperature difference between the coldest room and the main area with the interior head was ~3F on the coldest hours of the year, and bumping the temp a couple degrees in the open area was considered a more cost-effective comfort option than going with a 3-head 2 ton multi-split. In my market 3 or 4 head multi-split would likely come in at around $8-10K installed, similar in cost to three 9000 BTU 1-head minis. (~4.5-5K for the out unit, ~$1200-1500 per head for the interior units.)

    The Coolerado evaporative cooler is probably a win on operating costs in Sedona, and unlike other swamp coolers it doesn't add a moisture burden to the interior space as a function of cooling. But it doesn't solve the heating portion or the dehumidification issue, which is the attraction of mini-splits & other heat pumps compared to heating-only or sensible-cooling-only equipment. The smallest version (M30) is still 3 tons of cooling, and may be more than you need if you manage the solar gain well, and have high-R walls. It's essentially a 450 watt air-handler, and it'll almost surely be louder than a mini-split running at mid-speed, but they don't provide acoustic data. (They're kinda bulky compared to a mini-split compressor unit too.) In very-dry weather it's effective COP/EER is considerably better than a best-in-class mini-split, but not 2x better, and during the monsoon season the mini-split would eat it's lunch.

    The tighter you make the home from an air-infiltration point of view, the less you'll be wanting to add moisture, and the interior relative humidity can often be managed by the varying the ventilation rate- operating a heat recovery ventilator (HRV) under dehumidistat control.
  11. Steve29

    Steve29 New Member

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    Thanks for the great info, Dana! I've downloaded Taco and will use it instead. I'd say Sedona is more of a zone 4 than 5, since Nebraska, Denver and Chicago are 5's. Sedona is 5-10 degrees warmer than Denver. So maybe a 4.5 at most?

    Fascinating info on the R value vs. temps, time, etc. And thank you for the thorough mini-split and evap discussion. As always, I'll spend some time over the long weekend absorbing and learning about what you've said.
  12. Dana

    Dana In the trades

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    I figured Sedona was zone 4, but altitude & radiational cooling of clear dry air can fool you. (Design heating temp for Sedona is +16F, which feels like zone-4 to me too.)

    IIRC Mitsubishi has a 3 ton ductless that can take up to 8 heads, but methinks that would be a bit ridiculous in a residential app. Daikin, Fujitsu & Mitsubishi (and probably a few others- it's a moving target) have some VERY efficient 2-2.5 ton 3-head versions, if you can bring your heating & cooling loads down that low. (The bottom section of this list has a few current 2 & 2.5 ton 3-fer options. Something with an heating efficiency HSPF of 9+ is preferred, but 8.5 still qualifies for subsidies in many places.) Part load performance of ductless split systems can pretty remarkably good compared to ducted air source systems, and will likely beat ground-source (geothermal) heat pumps in your climate.
  13. Steve29

    Steve29 New Member

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    Thanks for the above info, and the really helpful links! BTW, we’ve been working very hard on the Taco load analysis, and still have more to do before we have some numbers that are (hopefully) believable. (It’s all your fault, Dana, because you had different recs depending on our heating and cooling loads!)

    Also, I finally have some accurate unit costs for gas and electricity based on a neighbor's previous 12 months’ costs. GAS PRICE varies during the year, with a max (January bill) of about $1.17/therm and a min (July bill) of $1.10/therm. So we could say $1.17/therm for seasonal space heating and $1.14/therm for year round water heating. ELECTRICITY PRICE is 12.75cents/kwh in the winter and 15.1 cents/kwh in summer. (These gas and electric prices include usage-based taxes and fees. The electric price is approximate, as it's based on the neighbor's usage on a time-advantage plan.) So using these costs with your earlier comments, heating with a mini-split with a COP of 3 would be slightly cheaper than heating with gas, and even less during the swing seasons.

    I've looked over the list you linked for me with your most recent reply - the multi-head units are less efficient than the singles, and you've said they are about three times the installed cost. Why not go with as many singles as needed instead of the multi's? Too many ugly outside units, too many exterior wall penetrations, or ?? The interior units can be put well inside the house? I have a poor concept of how it all works, like can the ~3" ducts be put through an upstairs interior wall instead of going through the ceiling, etc. (I'm ashamed to admit that in the upstairs great room we have entire walls of glass, so a mini unit would have to be in a ceiling or interior wall and somehow get its duct outside....)

    Thanks again, Dana!
  14. Dana

    Dana In the trades

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    The difference between an HSPF of 9.0 and 11.0 isn't as much as you might think, and actual performance will vary- it largely depends on running the compressor in it's more-efficient low-to mid-speeds as much of the time as possible.

    Since the spaces heated/cooled by each head will likely not have the same heating & cooling loads, they multi's be better matched to the entire home's load than a fistful of one-head minis. A 1 ton mini in a big open space with a 1.5 ton load won't cut it, and would likely be oversized for bedrooms, so if even if two or three of them added up to the whole-house load (or more) it's less than optimal- there will be times when one is going full-bore (at lower efficiency) while the other is idling along. With a multi-split you can size the interior head capacity to the zone's load, and the compressor speed will modulate with the combined whole-house load. Oversizing the compressor (within reason) can often be beneficial for it's as-operated efficiency, but 2x+ oversizing will lead to the units cycling rather than modulating, for lower comfort and more rapid wear on the equipment.

    There are ceiling-mount mini-split heads availble, but they have a kinda commercial/office look to them. They don't have ducts- installations typically have a 3-4" conduit that contains refrigerant, power and control signals between the interior & outdoor units. This isn't the most beautiful installation I've seen, but it's clear that those aren't ducts:

    [​IMG]
  15. Steve29

    Steve29 New Member

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    Excellent summary of singles vs a multi - thanks! I didn't realize the 3" lines were carrying refrigerant. I'll look into wall vs ceiling mounts and continue on the Taco expedition (am a little nervous of what it'll tell us....)
  16. Dana

    Dana In the trades

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    01609
    The Taco heat loss tool will just tell you what you already know, but it'll put a number on it. Having a lot of glass is pretty lossy (gain-ey, if in direct sun). Finding the right balance between the aesthetics, thermal performance, and costs (capital & operating) is never simple- it's an iterative process. Cutting the glazed area 20% is usually far cheaper than increasing it's performance 20% with extra coatings, exotic gases, & extra layers. It's sometimes possible to cut the losses elsewhere with high-R wall & roof assemblies, but costing that out is also not simple.

    The last thing you'd want to do is keep the field of view large, but end up cutting out so much of the light with the high-performance windows that the view looks dull and it's so dark you need to turn on the lights to read your watch, as sometimes happens with some of the more serious heat-rejecting goods. The view out of a 15' x 8' wall of glass isn't much compromised by reducing it to 14' x 7', replacing that ~18% of U0.33 glass with R20+ (sub U0.05) wall is a 17%+ cut in total heat loss out of that wall. Then bumping the windows from U0.33-0.34 down to U0.29-0.030 buys you another ~10%, usually without breaking the bank or ruining the daylighting or view. Adjusting the size & shape of the windows to optimize the view using the minimum of total area counts for a lot- make the case for every square foot.

    The conduits housing the insulated refrigerant lines DO look a lot like high-velocity mini-ducts, but "ductless" is a key factor in the high efficiency of ductless systems. Without ducts there's no duct leakage losses or backpressure increasing loads to the air handler. If you literally never clean the filters on the interior units of split system it'll cut into efficiency measurably, but not as much as going to a ducted system. Using indoor units with continuously-variable speed blowers rather than some of the simpler 2-speed also improves average efficiency.

    The ceiling mount units tend to look like this:

    [​IMG]

    But there are flush-mounted versions too:

    [​IMG]

    Most of the ceiling mounted units are well north of 10,000BTU- finding one appropriately sized for a bedroom might take a few discussions with distributors or manufacturer's reps, since they're usually targeted at larger commercial installations that have far more ceiling than wall as opposed to single-office/room type installations.
    Last edited: Sep 9, 2011
  17. Steve29

    Steve29 New Member

    Messages:
    15
    Location:
    Sedona, AZ
    Dana, our Taco load results are intimidating compared to the numbers you've tossed around: 115KBTU/hr cooling, 109KBTU/hr heating. Despite the ton of detail we put into it, I'm not sure how correct they are, as I get no change to the cooling load when I add a huge overhang (to simulate exterior shading) over our expansive east side glass. We will try to get these numbers lower using your recommendations (less glass, more insulation, etc), but they'll still be high. (We input .5 ACH/hr in Taco, which seems pretty agressive.) I have cost estimates for four different HVAC approaches (but this guy hasn't done a load analysis, he has just reviewed our plans):
    1) conventional GFA/AC = $24.9K (two Carrier 16 SEER AC units, two 93%-eff. furnaces)
    2) central (ducted) heat pump = $34K (highest eff. Carrier, 21 SEER)
    3) geothermal = $47K before incentives, about $21.5K after incentives (30% fed credit plus an $11.4K after-tax estimate of the incentive from APS, our utility company, to be given to us by check)
    4) Evap and radiant = $30K, or $36K with gypcrete upstairs (2 evap units needed)
    5) ductless (minis) - I had to ask him about this as an option, and he said he does like them and will get me an estimate.

    Our energy unit prices are currently $1.17/therm, and 12.75 cents/kwh in winter, 15.1 cents in summer.

    Your thoughts at this point? (I've looked into possible tax credits for the high efficiency heat pumps but believe they are available only for existing homes.)
  18. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    Very interesting read especially without a fee!

    And a few bottles of wine and finally some guesswork.

    I cannot imagine a house without radiant heat. You dont need those absurd systems with outdoor sensors, numerous automatic valve zones, and a mass of wiring that insures a lifetime repair profit to the installer.

    http://www.radiantec.com/systems-sources/open-system.php

    Thats the system a lot of guys in the business hate because billy bob can install and adjust it. I find the cost estimate on the site terribly high.

    I feel the mini splits add a huge level of complexity in a large house also, not to mention the lack of a warm floor - the KEY to comfort in the winter. I see the splits as nice in motels and small rentals.

    I've been doing radiant for 30 years starting with polybutylene [no leaks yet] and always plumbed in with the potable hot water. 1 pump [or as you wish] and 4 to 10 valves, and a GOOD checkvalve, and you are ready to heat. The day-laborers sitting on the safeway parking lot can run the pex zones in a day. And another day for this cheap labor for mortar mixing to fill in betweeen the 3/4" wood sleepers on the second floor where the pex is tacked to the floor. Lay wood or carpet over it. The gyp crete guys are way out of line in cost. Call in a pro to make the manifold connections and pressure test it.

    I have laid out systems for many owner builders, and they have done it all with laborers and good instructions. And they still call back to thank me for a house they cannnot imagine living in without the radiant. I think the only callback was for a mouse in a Polaris vent.

    My vote is water cooler and radiant, hands down. Then you guys can duke it out about what water heater to use. I wouldnt trust the Korean infernal-eternal.

    Every unvented flat roof I have cut into gives me a blast of hot wet air and the smell of mould... How are you insuring against this?
    I am just now adding flat roof vents to my home. Hard to do a perfect vapor barrier. And ANTS! be sure your insulation has some borax or D-earth in it. Around here it snows styrofoam in the spring in houses with open beams and the old styrofoam above. And my ants love the yellow "toxic" foam too.

    You seem quite quick with numbers - can you calc 8 cent per KW winter and 12 cent summer KW against $2.50 per gallon Propane on a say, 85% efficient water heater?
    Last edited: Sep 24, 2011
  19. Steve29

    Steve29 New Member

    Messages:
    15
    Location:
    Sedona, AZ
    Thanks for the info, and for your vote! Shoot, just when I was getting radiant out of my mind.... (And here I thought Californians didn't care about heating.) You've probably gathered we have a slab downstairs (not a basement, it's just the first level), so what do you suppose our cost could be to do that slab and upstairs? The slab is ~1650 SF, but maybe we'd have to heat only 60% of it or so. Upstairs is 2179 SF, but we would need to radiate maybe only 75% of it or less.

    I hope/think I understand what you're suggesting - you mortar in the pex inside channels that are created with 3/4" wood strips? Not sure why this done - better radiant heat distribution? And I still wonder how radiant can work well when it has to heat through a wood floor. That's why I thought about gyp ($6200 for 1 1/2" thick upstairs).

    I'm not sure of the flat roof construction method. The roof will be a combination of trusses and TJI's. Perhaps some blown fiberglass on the bottom, and on top of the roof sheathing a couple inches of rigid foam (to combat thermal bridging). So we could also spray a thin layer of open-cell foam at the top of the trusses/TJI's, to fight infiltration. Suggestions? If you used a vapor barrier, where would you put it? I assume under the blown fiberglass. There's a lot of dispute about using vapor barriers. And it doesn't have to be near-perfect to be very effective.

    What type of insulation are you suggesting putting Borax or D-earth in? Googling it shows that cellulose has 10-15% Borax and Boric acid, both non-toxic fire retardants. Dana suggested staying away from cellulose in the roof in case there is an undetected leak in the "flat" roof.

    I calculate that your propane, in an 85% efficient water heater, equates to 10.9 cents/kwh if using an electric heater (100% efficient).
  20. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    I can't figure out vapor barriers - not enough time or wine.

    The boric acid is more toward bug killing than fire. By the time a fire gets to the blown insulation, its time to start planning the rebuild.

    You need foam with boric acid in the board. Not carried usually by the lumber yards. The main ingredient of Terro ant killer is boric acid.

    Sedona likely has my climate, hotter summers I might guess. I have about 1/4 of the house with thick pine floors over the hand set cement or mortar, but I rarely even turn this zone on, as the heat from an adjacent 800 foot slab keeps that area warm enough. Heat transfers through wood quite well. put a piece on your electric hot plate. Or use a thin engineered flooring. I use 1/2" pex on 300 to 400' zones, and with sleepers, mortar, and pex, and all the unemployed workers, you should be able to do the upstairs for $3000 MAX. Were you going to leave the gyp exposed? Your gyp cost did not include the pipe, I suppose.

    On my third floor, I have carpet over the pipe set in cement. Almost all windows. No problem with heating.

    These idea might not work in Northern wisconsin, however.

    On one job, I got the exterior stuccoers to don knee pads and mortar and trowel a great finish right over the pipe without sleepers. Stained and varnished the floor. Even got them to thinset plaster the walls for less than the cost of garbage sheetrock mud.

    The most important issue of radiant heat is the entire mass of the house becomes warm, and the comfort feel is irreplaceable. Not like warm air blowing out of a chinese or Korean box on the wall.
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