Looking to put in central air this year

Discussion in 'HVAC Heating & Cooling' started by LLigetfa, Apr 29, 2012.

  1. LLigetfa

    LLigetfa DIYer, not in the trades

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    4,050
    Location:
    NW Ontario, Canada
    I have a 1 1/2 storey home in the shape of an L, 1200 sq ft on the main floor, 800 sq ft upstairs due to cathedral ceilings. 150 lineal feet of wall with 8 foot ceiling downstairs. 2x6 construction so R22 walls, 2x12 rafters so R40 roof. Low-e argon double pane windows. No basement. The FA gas furnace is a lowboy so it needs a horizontal coil. It was installed in with future A/C in mind so has an extended plenum.

    The wife is working on two estimates, both 2 ton 13 SEER. First one from the guy that installed my furnace came in high. Second guy's ballpark estimate came in a lot less but then he started waffling about the added cost of a horizontal coil and saying 2 ton is too light. Waiting for him to firm up the price. Meanwhile the wife went back to the first guy, asking him to sharpen his pencil.

    Do you guys think 2 ton is enough? What is a ballpark price installed? I don't know what sort of price disparity there is between the US and Canada.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    No way to know without more info.
  3. jimbo

    jimbo Plumber

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    A horizontal coil costs no more than a vertical, except he may not have one in his stock so has to order.\

    Is everyone just pulling cooling capacity numbers out their rear end? There is a forma way to calculate it, and very easy today with software which any HVAC contractor should have.

    WHen you say the furnace is a lowboy..does that mean it is horizontal in the attic? The ease of access to the work area will affect the labor cost.

    No idea about Canada..... you could be looking at 5K to 10K for a two-ish ton unit, installed.
  4. LLigetfa

    LLigetfa DIYer, not in the trades

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    4,050
    Location:
    NW Ontario, Canada
    They are saying that the horizontal coil is more than double the cost of a vertical one. I think it might be a bait-and-switch tactic.

    The lowboy furnace is in my crawlspace which has almost 4 feet of headroom and easy access. The crawlspace has a concrete floor. It is an easy straight 14 foot run to the exterior wall. Easy access to the electrical panel too on the same wall.
  5. Dana

    Dana In the trades

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    Has anybody done a room by room Manual-J type heat loss/gain calc that included the site orientation & shading factors, etc? A 2 ton might be enough, too much or not quite enough, depending on the true summertime solar gain factors. (I suspect 2 tons is overkill for most decently insulated homes with all low-E glazing in NW Ontario, but certainly not all.)

    At anything above ~ R5 or so wall-R is of almost no consequence from a cooling point of view, since even through low-E windows the solar gains through the windows will dominate gains through the walls. Roof gains & window gains are going to be 90% of the cooling load.

    South facing windows with low solar gain factors have comparatively low gain due to solar geometry: The bulk of the heat & light are mostly reflected at high incident angles of mid-day sun, and roof overhangs may partially or fully shade those windows. There is still significant gain, but southwest & west side glazing has a bigger effect on peak loads, since the sun is lower in the sky when shining on them, and the air-temp is higher in the afternoons. The solar gain factors of even low-E windows varies quite a bit, and modeling the solar gain factors, particularly of the SW to W side windows correctly can change the peak load numbers by quite a bit.

    Roof insulation types & finish roof materials can also affect the peak loads. Rock wool & cellulose are opaque to infra red, whereas fiberglass is somewhat translucent, and the peak temp in the insulation layer can be a few cm in from the top under a hot roof. Darker composition shingles have higher solar gain, but about the same emissivity as light colored shingles, and run significantly hotter. Metal roof finishes also vary by quite a bit from a gain/emissivity point of view.

    Are the ducts in the crawlspace insulated? Are they air sealed at every seam & joint? If not you may run into some condensation issues, and duct losses cooling off the crawlspace may be of modest cooling benefit to the first floor, but not a lot, and it would add to the total tonnage requirement. Are the crawlspace walls/floor insulated & air sealed?

    Don't expect anyone to do the more rigorous analysis for free, and I suspect only 1 in 10 are even up to the task. Being an energy nerd, I'd run a Hot2000 simulation on the place to come up with both the heating & cooling load. It's time consuming to do the data entry, but may point you to ways of cutting the load that are cheaper (and more comfortable) than upsizing the compressor.

    Is the floor plan open enough to consider going ductless (mini/multi-split)? Split systems are inherently more efficient (no duct gains or losses), and inverter drive ductless are more efficient & more comfortable due to fully modulating compressor speeds and automatically adjusting to the load. (And if your furnace is propane rather than natural gas, a heating/cooling heat pump would cut your heating costs, even if sized to only keep up with the heating load down to -5C or so.) A 2 ton high SEER high HSPF single-head heating/cooling mini-split would likely come in under $5500, maybe $4000 if cooling only. (Add ~$1000-1500 for a 2-head.) Many 1.5 story houses that have only moderate gains on the first floor can be cooled adequately with a single head mini-split on the upper floor if air is able freely convect up/down the stairwell.

    At 2.5 tons or lower the choices of central-air compressors are relatively few, but 1-3 tons is the sweet spot for ductless, with many options to choose from. Oversizing an inverter-drive ductless by even 50% on cooling tends to yield higher average efficiency, since they run more efficiently at part load than when running flat-out. Whether there's "pay back" on the efficiency bump from upsizing just for cooling depends on the length of the cooling season, which isn't super-deep or long in your location. But if it's being used for both heating & cooling, in many locations it's cheaper to heat with a ductless at 0C than it is to run condensing natural gas. It's unlikely that even a 2.5 ton would be oversized for your design condition heat load (but it might handle your full load at -5C or even -10C.) Efficiency drops off pretty dramatically below -10C or so, but most are still more efficient than electric baseboards even at -20C. At -25C or lower, forget about it.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    Probably the bigger desire for a/c in your location is to remove some humidity rather than significant cooling. that all comes into play when determining the size of the unit you want, and the system design. With the existing furnace, you probably couldn't take advantage of the benefits of a variable speed blower as you could with a new unit or by going ductless with a minisplit. But, if the goal is humidity reduction, running the fan at lower speeds at least initially really makes a big difference in the amount of moisture it can extract. Trane described two identical houses, identical a/c units, difference was one had a variable speed air handler, the other the traditional single speed. Over the course of a summer, the variable speed unit claimed it pulled enough extra moisture out of the air to fill a typical backyard above ground pool compared to the same unit with a single speed blower.

    Ideally, you want the a/c to run nearly all the time to provide max dehumidification and constant temps. Too big, and it ends up cold and clammy, running short cycles since it doesn't need to run long enough to extract the humidity.
  7. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,050
    Location:
    NW Ontario, Canada
    Not that I know of. The guy that put in my furnace did the ductwork design off the blueprints and supposedly factored central air. That said however, I don't have any high returns upstairs so not sure how well it will work. It does work well for heating but I understand moving cold upstairs can be challenging.

    Window size, placement, eaves, and shade trees were factored when I designed the house and works well to keep solar gain to a minimum. That said, some of the mature shade trees didn't take well to having their roots trampled and am losing a few.

    Ducts are not insulated and not mastic sealed. Foil tape over some of the joints. I've considered mastic but so far the losses have not been enough to even close off the three takeoffs that currently heat the crawlspace. I have R22 batt in the stud cavity and 2" foam on the exterior side of the crawlspace walls. It is air sealed using R2000 techniques. The crawlspace is conditioned space.

    I thought of a mini-split in the upstairs master bedroom but would not want the noise for sleeping. Furnace blower is single speed and the noise is tolerable. Downstairs is open plan but not the bedrooms upstairs.

    I heat mostly with wood so heat pump not of any value. Natural gas is cheaper than electricity.
  8. LLigetfa

    LLigetfa DIYer, not in the trades

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    Location:
    NW Ontario, Canada
    I got a bit more info on the two models proposed.

    Payne 2 ton PA13NA

    York 2.5 ton TCGD30S41S3
  9. Dana

    Dana In the trades

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    If you're on the gas grid it rarely pays to heat with heat pumps. AC-only takes a grand or so off the price of a ductless.

    Inverter drive mini-splits with continuously variable speed blowers & compressors are pretty quiet, especially at low speed (which it would be at night), and quieter even at mid or high speed than most ducted air conditioning. The compressor/condenser units are also ghostly quiet, using scroll compressors rather than reciprocating compressors. A mini-split head at the top of the stairs, or in the bedroom (with the door left open) might be a viable whole house solution- it just depends.

    With a well insulated crawlspace duct losses to the crawlspace aren't huge, and keeps the floor warmer during the heating season, but is a net drain on the air conditioning. During the cooling season a cool brow counts for a lot more than frosty feet, quite the converse of the heating season, where toast-toes count for a lot. If you go with a ducted solution it pays in both comfort and power use to seal and insulate the ducts.

    With your shading factors I suspect your design cooling load is well under 2 tons. But a heat load calc that takes those factors into account is in order.

    A ~$2000 (f.o.b. the distributor) 1.5-2 ton Mr. Slim or similar with an SEER north of 15 would likely more than cover your peak loads, but being continuously variable, modulating with the actual load would be more comfortable & quiet than any bang/bang on/off ducted system. Rather than running with a temperature hysteresis, it runs continuously or nearly so, mostly at very low speed, and would have no duct losses to the crawlspace. Most have a turn-down ratio of greater than 3:1 so the only time the noise factor would be at all an issue would be a 2PM on the hottest day of the summer.
  10. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
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    Location:
    NW Ontario, Canada
    With the wife, it's all about aesthetics first, and silence a very close second. She would not go for a Mr. Slim hanging from the bedroom wall even if it were quiet. If it were not such a long and difficult run to hook up a mini-split, I'd be tempted to convince the wife to go that route.

    Yes, I was thinking the same. It's one thing to leak out some heat into the crawlspace as there is more than enough to go around plus, warm air is easier to get to go up. Heavy cold air will take more effort and more will leak out. I will seal with mastic first and wrap the ducts with insulation if need be.

    We decided to go with the Payne 2 ton PA13NA that my current heating contractor offered despite his bid not being the lowest. I have a couple of questions WRT to the installation. Knowing that the contractor would likely not want to fish the wire through the wall from my panel, I plan to fish it myself. I assume a 20A 240V feed with 12/2 Romex®? With the placement of the concrete pad, how close can the outside unit be to the house? He will supply a 36"x36" pad and I think he said the unit is 30"x30". I have a small area with pavers where my hose reel, garbage cans, gas and electric meters, HRV, furnace and HW intake/exhaust etc. all culminate and it's a bit tight to fit it in there.
  11. Dana

    Dana In the trades

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    There's no fightin' "the boss" over visual aesthetics, I get it.

    Putting insulation on the supply ducts may be necessary to avoid visible condensation on the ducts, especially if the thing is oversized and running only intermittently, not drying out the air very much.
  12. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,050
    Location:
    NW Ontario, Canada
    We got the unit installed today. It fit on a 24"x24" pad so wasn't as crowded as I feared. Plenty of room for the hose reel to still pay out the hose.

    I still need to order a tub of mastic and start sealing up all the ducts. I won't know how well the ducts will perform to push the cold up and pull the heat down until we get some hot weather. Bring it on!

    While we had the plenum off the furnace, we discovered that the heat exchanger is in need of replacement. The gas furnace is only 13 years old and we don't use it very much since we heat mostly with wood.
  13. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,050
    Location:
    NW Ontario, Canada
    As I was about to fill out the warranty registration online, I went out and got the details off the side of the unit. It turns out to be a 2-1/2 ton pa13na030, not 2 ton as originally discussed. The odd thing is that the spec plate lists less current draw for the 2-1/2 ton than what I found online for the 2 ton pa13na024.
    pa13na.jpg
    I picked up a half gallon of Red Devil Duct Sealant at Manards for $12.
    http://www.reddevil.com/index.php?l=product_detail&p=1376

    Do you think half of a gallon might be enough? The guy that sold me the A/C only has gallon sizes of Ductmate brand PROseal for $40.
    http://www.ductmate.com/product.aspx?id=23
  14. jimbo

    jimbo Plumber

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    8,997
    Location:
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    A very telling comment. When Willis Carrier invented A/C in 1902, his task was to control the HUMIDITY in a printing plant because it affected the colors.
    Turned out the humidity control also COOLED the air.....and the rest is history!!
  15. Dana

    Dana In the trades

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    It totally depends on how much ductwork there is to seal- how many joints & seams. IIRC I went through a 3-4 of the half-gallon tubs sealing both my (way oversized and 90- year old) supply & return ducts in the basement of my house, but you'll probably be able to manage with fewer. (How far is it to *******, if you run short? For me it was under 2km driving each way.)

    I'd almost be willing to put money on your cooling load being well-under the 2.5 tons, but it's probably not so ridiculously oversized that it'll short cycle on you. The old-skool geezer rule of thumb was 500 square feet per ton, which was almost always 2x oversized even for barely insulated leaky houses with single-pane windows. Their scion is now drifting toward a 1000 square feet per ton rule of thumb for new code-min houses, but even that oversizes by a good factor for most (but maybe not in Florida with uninsulated ducts in a 55C attic, which is still legal to build there). In a tight well insulated Canadian house with high performance windows (much better than lower-48 US code-min) that 1k'/ton rule of thumb might be more than 2x oversized, but it won't take a huge hit in efficiency. At 2.5 tons for 2000' of space you're at 800'/ton in a space where a 1.25ton would probably have had margin.

    While it probably won't short-cycle, it won't have the steady temperature & constant dehumidifying comfort of a mini-split. Modulating systems that track load are pretty nice to live with, even if some are pretty ugly to look at. I'm sure it'll be comfortable enough.
  16. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,050
    Location:
    NW Ontario, Canada
    I spent a few hours in the crawlspace and got most of the ducts sealed with the half gallon. I worked my way outward from the furnace so if I ran out of material the higher pressure zones would be done. Less than a quart should finish it off. I'll probably pick up another half gallon later but I'll wait to see how well it performs first.

    According to Google Maps, it's 17.6 km to Manards.
  17. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,050
    Location:
    NW Ontario, Canada
    For 13 years I justified not sealing the ducts, reasoning that the leakage was warming the crawlspace. After all, I have three take-offs open down there as well to heat the space. I figured that if I did not heat the crawlspace adequately, that I would have heat loss via transference through the uninsulated metal ducts with the longest runs losing the most.

    Now I wish I had done it years ago. I cannot get over how much quieter it is since doing it. There is much less turbulence and vibration. I'm enjoying how quiet it has become.

    It would have been a lot easier to do while the ducting was being installed and had I known the benefit then, I would have spec'd that the contractor do it. I did all my own wiring, plumbing, insulation, sheetrock, carpentry, etc, but when it came to the ductwork, I sub'd it out. Sheet metal and me just don't get along. Anytime I work with it, I end up losing a lot of blood.
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