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.
the heat is on (or, will be on)
- Thread starter redduck
- Start date
travelover
Give me a museum and I'll fill it. (Picasso) Give me a forum ...
- Joined
- Mar 31, 2007
- Messages
- 14,328
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.
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.
travelover
Give me a museum and I'll fill it. (Picasso) Give me a forum ...
- Joined
- Mar 31, 2007
- Messages
- 14,328
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.
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.
NW-Bound
Give me a museum and I'll fill it. (Picasso) Give me a forum ...
- Joined
- Jul 3, 2008
- Messages
- 35,712
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.
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.
Last edited:
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.
Last edited:
NW-Bound
Give me a museum and I'll fill it. (Picasso) Give me a forum ...
- Joined
- Jul 3, 2008
- Messages
- 35,712
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.
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.
Last edited:
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.
NW-Bound
Give me a museum and I'll fill it. (Picasso) Give me a forum ...
- Joined
- Jul 3, 2008
- Messages
- 35,712
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.
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.
Last edited:
