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Old 07-15-2013, 12:05 AM   #21
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Well, I guess the heat will soon be on, so......time for a song!

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Old 07-15-2013, 09:21 AM   #22
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+1 on additional quotes and more education.

The AFUE figure refers to the efficiency of the furnace, not the A/C unit. I wouldn't think furnace efficiency would be an important consideration in Los Angeles.

The A/C unit's efficiency is reflected in it's SEER rating.

Thats right! Also, when going to a higher efficiency furnace, they will most likely have to change your flue stack, as the older stacks will not work with these newer more efficient units.
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Old 07-15-2013, 11:25 AM   #23
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True, but I tried getting a manual J calculation from any of the 4 bidders and was unsuccessful, even with me providing all the necessary input data. I finally did my own with an on-line calculator.
Same here. Most wanted to use the data from the unit being replaced (despite R-30 of additional insulation in the attic and replacement of all single-pane metal-framed windows with double-paned ones) or their rule-of-thumb based on sq ft. Like you, I had to do the heat-load calcs myself, and it resulted in a much smaller furnace that has worked very well.
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Old 07-15-2013, 11:41 AM   #24
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....... Like you, I had to do the heat-load calcs myself, and it resulted in a much smaller furnace that has worked very well.
Exactly what I found out. I ended up with a two stage furnace (35,000 / 70,000 btu) that rarely ever goes into the second stage. They wanted to put iin a 100,000 or 125,000 btu furnace.
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Old 07-15-2013, 11:43 AM   #25
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Oh, I'm sure I could have gotten a better price and maybe even I overpaid for it, but from what I understand, Costco does keep a lid on the prices re: what their independent contractors can charge. So, I know that I'm not getting the best price, but a fairly decent price. And from what both my son and daughter have told me (they both at one time worked for companies that contracted with Costco, Costco makes sure their customers get fair deals. And, if there is a problem, the contractor answers to Costco (and sort of to me).
....

The situation here is, I don't have an understanding or a feel for this kind of stuff. So, I'm willing to spend a few extra hundred/thousand dollars and play it safe. It's the right way for me. Actually, I do wish I could enjoy and get energized from these sorts of projects, but I don't.
I think that's reasonable. W can't all take the time or even have the ability to learn everything about everything we purchase. It's good to know that you have some inside info that Costco is watching over the contractors. Sometimes you just need to go with some reasonable level of trust.

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Same here. Most wanted to use the data from the unit being replaced (despite R-30 of additional insulation in the attic and replacement of all single-pane metal-framed windows with double-paned ones) or their rule-of-thumb based on sq ft. Like you, I had to do the heat-load calcs myself, and it resulted in a much smaller furnace that has worked very well.
But is it so bad to base the needs on how the current system is working? I would likely replace my furnace & AC with a similar output (accounting for eff%) units. My present furnace runs at about 50% duty cycle with thermostat at 68F during very cold weather. I think it would keep up if we hit a really cold and windy night.

AC does OK, I could maybe go a bit bigger. It has been marginal when we had a crowd here on a really hot day. DW didn't understand why I was trying to get the house temp down to 68F before company arrived - we needed that buffer. But being maybe marginally undersized, it does a fantastic job of pulling the humidity out,which is actually the most important thing for us most of the time.

Since I actually observe and monitor this stuff, I'd feel good basing my decision on that, rather than go through calculations.

-ERD50
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Old 07-15-2013, 11:54 AM   #26
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But is it so bad to base the needs on how the current system is working? . . .
Since I actually observe and monitor this stuff, I'd feel good basing my decision on that, rather than go through calculations.

-ERD50
Direct observation is probably better than a calculation, and will work well for you. But I'd bet few folks know/notice the duty cycle of their AC/furnace, and it's harder yet these days to know how hard the equipment is really working given today's variable speed fans/multilevel burners.
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Old 07-15-2013, 02:43 PM   #27
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Direct observation is probably better than a calculation, and will work well for you. But I'd bet few folks know/notice the duty cycle of their AC/furnace, and it's harder yet these days to know how hard the equipment is really working given today's variable speed fans/multilevel burners.
Plus too large a furnace is less efficient and provides a less stable temperature. Too large an AC unit does not dehumidify properly.

Incidentally, I double checked my Manual J calculations by looking at my old gas bills and calculating the gas usage per degree day to verify my actual needs. You need to know the efficiency of your current furnace to do this calculation.
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Old 07-15-2013, 06:19 PM   #28
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Just so you know, included in the price is a new duct system. Or, is that just to be expected when putting in a new HVAC system and everybody but me was aware of that? Ah, maybe Meadbh wasn't aware if it, either. The house was built in 1962 and the HVAC system came with the house.
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Old 07-15-2013, 07:08 PM   #29
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Just so you know, included in the price is a new duct system. Or, is that just to be expected when putting in a new HVAC system and everybody but me was aware of that? Ah, maybe Meadbh wasn't aware if it, either. The house was built in 1962 and the HVAC system came with the house.
A new duct system adds a considerable amount of labor and done correctly, is a bit of science in itself. There are computer programs that design these properly and installers that just use a rule of thumb. It is worth your time to read up on this a little so you can tell a good design from a bad one.
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Old 07-16-2013, 12:12 AM   #30
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Just so you know, included in the price is a new duct system.
Are they just building the collars/adapters to hook up your new furnace to your existing ducts, or are they really building an entire new duct system? If the latter--why?
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Old 07-16-2013, 12:39 AM   #31
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Are they just building the collars/adapters to hook up your new furnace to your existing ducts, or are they really building an entire new duct system? If the latter--why?
I don't know. Do ducts from 1962 still work well? Whatever they are doing, it's all included in the original price. I do know that the heat and the air conditioning are really weak in some parts of the house. I guess there might be several reasons for that.
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Old 07-16-2013, 01:32 AM   #32
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Older houses have metal ducts that used to be built or assembled on site. They have a rectangular section, and most often are of smaller sizes than what's put into new houses built in the last 30 years. Modern ducts are universally big gooseneck-like plastic hoses that make installation very fast. They all have a circular cross-section, and are double-walled with fiberglass insulation in between.

I was surprised to find out that while the metal duct's life may be limited by corrosion, the modern plastic hoses also deteriorate due to heat. At least that's true in the SW. A friend of mine needed to have all his ducts replaced after 25 years. I do not know if it is a problem with a particular brand.
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Old 07-16-2013, 08:22 AM   #33
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I don't know. Do ducts from 1962 still work well? Whatever they are doing, it's all included in the original price. I do know that the heat and the air conditioning are really weak in some parts of the house. I guess there might be several reasons for that.
redduck, you seem to be taking a lot of this on faith. A single quote for a big job--and you're not fully cognizant of what they are doing. According to your post, you don't really know what you've bought.
There's no way I'd pay to have good, solid, well-sealed metal ductwork (of any vintage) replaced with quick-and-dirty plastic slinky duct until someone made a good case for it.
Cosco does get paid based on what this job costs, right? No matter what we may think, they are not your agent in this transaction.
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Old 07-16-2013, 12:34 PM   #34
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I do not know the details about redduck's situation, and am also not an HVAC expert. But I will offer some observations made in my parents' past homes and my own homes. These homes range in age from 1940s, 1950s, 1980s, and my newest home built in 2007.

The earlier homes had all metal ducts. The branch lines going into each room tended to be small, of 8" diameter, I think. I think I have seen 6" also, but am not sure. These older homes were also built before the popularity of the AC.

Newer homes use slinky hoses, but they are bigger, of 10" or even 12" for longer runs. But even the newest home uses a big square rigid metal duct as the main trunk. The AC/heat pump blows into this large duct, from which the flexible hoses fan out to run to each room.

If the airflow to the distant rooms is inadequate, it is reasonable to replace the corresponding duct with one of a larger size. In an older home, that would mean replacing the old smooth round metal duct with a slinky. I do not see that as a big deal. I would prefer to have a larger duct to ensure a good airflow. If too much flow, I can always close down the louvers at the registers.

Perhaps redduck's contractor was talking about upsizing the branch lines to the remote rooms. That would make a lot of sense, else all that extra capacity of the new AC/furnace would be for naught.
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Old 07-16-2013, 02:56 PM   #35
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Perhaps redduck's contractor was talking about upsizing the branch lines to the remote rooms. That would make a lot of sense, else all that extra capacity of the new AC/furnace would be for naught.
In homes originally built without central AC, the ductwork can sometimes be too small to accommodate the needed flow rates for AC. This problem is even more apparent when going from a combustion heat to a heat pump. But if the branch ducts are inadequate then the registers are likely inadequate, too. So, they'd need to add more or make the existing ones bigger. I'm wondering if that's included in the estimate.
The only advantage of the slinky ducts is their speed of installation. The rough sides slow air velocity, the junctions are prone to leakage, they get brittle over time, and they trap dust that is very hard to ever clean out.
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Old 07-16-2013, 03:24 PM   #36
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Come to think of it, of the two homes that I still own, only the newer one built in 2007 has the branch lines being of round plastic ducts. The duct work is laid out like a central spine, with the big square metal one in the basement, then with the branches connected like ribs to the former.

My main home built in 1986 has all square metal ducts, and yet it is a more complicated set up because it's a two-story. The slinky parts were only added because it was the only way to bring air to the new 2-room addition. And then, when the new 5-ton unit was installed, its huge 18" inlet and outlet were connected to the metal trunk with giant slinkies and round-to-square collars. It was at the slinky connections that they messed up, causing the massive leakage I described earlier.

It is very true that metal ducts are superior. The insulation they put on also appears to be thicker than what is in the slinky. Nowadays, I suspect that the metal work may cost quite a bit more though, what with workers who do not even take care to duct tape slinkies correctly. Very very bad workmanship nowadays.
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