I'm glad you get it. I'm a bit mystified as to why your contractor went on with that downsizing nonsense as though it were the root of your problem. Something doesn't compute. Either he doesn't get it or some material facts have been left out. Or it could be that I'm the guy using the library computer.
Using the existing coil is fine. Add a TXV and you'll be good to go whether you use a 4 or 5 ton AC. It's unlikely that you'll be using a 3.5 ton. A house that big isn't likely to get by on anything less than 4. The only thing you lose by not replacing the coil is an ARI rating. But an ARI rating is a construct used to steal from the lower middle class and give to all of those above them. A buying decision based on government manipulation may or may not be the best decision.
If you get a new coil anyway, the use of an oversized coil is a VERY good thing to do in dry climates. But again it may or may not come with an ARI rating. And those ratings are so very important in enabling you to get your wealth redistribution check... err, I mean rebate check. See that? I let politics seep in there again. I'm bad that way.
Irascible your one liners here have me rolling.
Originally posted by Irascible
But again it may or may not come with an ARI rating. And those ratings are so very important in enabling you to get your wealth redistribution check... err, I mean rebate check. See that? I let politics seep in there again. I'm bad that way.
A Carrier guy came by today. He is not the sharpest tack in the box but seemed like a good guy with 20 years of practical experience who would take the time go beyond a wham-bam replacement install to work with me on my ducts. I like that. He offered the Carrier line but then really pushed a low priced Payne 12 Seer Condensor/ADP coil combo that qualifies for the small local ARI 13 seer $200 rebate. The Payne looks cheap but then it is cheap. Not sold yet on the Payne but he seems like he would stay committed to do the replacement correctly and would make a good partner to help me fix some duct issues. One more bid to go..
If your up for another round of mentoring I have a couple of basic fundmentals questions which will go toward helping head me in the right direction to resolve my issues, well at least my cooling issues.
I understand how in areas of high humidity a key element of provide comfort is to get the humidity out and how lower than 400 cfm might help with that.
1. To successfully the nether regions of my house is my primary objective to push more cold air in the hot rooms farthest from the blower or is the priamary objective to get the hot air from those distant rooms into the return to cycle a nice cold coil.
2. In drier climates why does 500 cfm provide better cooling than 400 cfm?
If the Carrier guy has a well developed gut feel for things then I suppose that's a good thing. But I don't know how one can reliably "work with you on your ducts". In other words, how does one know what the heat load in a particular room is? How does one know what size ducts to use to address that heat load as well as compensate for distance? The only way I know to do those things properly is with calcs.
Maybe it's that my design experience is limited to the last three years or so. Maybe after I've had 20 years in the trade (I’ve got 11 so far) I'll be able to feel my way through those questions more comfortably. Or maybe it's that the Carrier guy is affable, reliable and "rule-of-thumbable". That's better than a lot of the alternatives. A lot of "rule-of-thumbers" are also unreliable and crooked. So if your guy is honest and reliable then that may be your best bet. Trying to meet my standards may leave you with no one to work with. But still, I'm skeptical as to the wisdom of doing anything other than proper calcs.
You’re system cools the house by pushing, not sucking. I’ve been told anecdotally by those in the know that there were experiments where they tried reversing things a bit by focusing on sucking rather than blowing. They put returns near the heat sources (windows or whatever) in an attempt to suck hot air out of the house preemptively. It was a miserable failure. The position of a return is mostly irrelevant. The ONLY thing return positioning is good for is to eliminate dead spots. If you have proper unobstructed velocity through a supply diffuser (500 FPM or greater) then dead spots won’t be an issue in a typical bedroom.
Incidentally, to get 500 FPM through a diffuser quietly means getting high quality diffusers.
And to even have a clue that you’ll get 500 FPM you have to engineer it. Rules of thumb won’t cut it.
If you haven’t gone through my site you really need to. It’s tailor made for pedantic geeks that like to suffocate themselves on too much information. Just between you and me, I choke all the time. ;^)
When you first posted that link I did not realize that this is your web site. After you posted the first link I read through most of the content in one sitting. Congrats, That your site is awesome! In web sites 'content is king' and you win. Your content is the best organized, best writen HVAC content I have found on the web.
My sister lives in Corcord and does not have HVAC issues right now but I emailed her a link to your site and told her to bookmark it so she will know who to call when she does.
If I could have found a contractor of the same cailber down here I would just floated some cash and not spent time learning this space.
Ok. So the Carrier guy. "if the Carrier guy is affable, reliable and "rule-of-thumbable" He is all of those things to a degree that surprised me. I wasn't kidding when I said he would be a good guy to work "with me." When I asked him the duct system was not well designed he said "Thats not possible. If the house was not designed well it never would have been approved by the building dept." When I asked about calcs he said "oh, an engineer would have to give me that data." That Engineer would be me armed with HVAC Calc and knowledge from living in the house for 10 years. What he brings is vision, willing skill and labor on "how" to cleanly solve the problems that have been identified. I built my first house from the ground up when I was nineteen, in another state I used to be a licensed general and am now an engineering manager. The controls we are talking about here are strangely not that far from some of the controls we use when we design prototype product production lines (our ovens have 36 independently timed tempature and humdity zones.) So, I am ok with this relationship if it is the best that I can get. And so far it is... although getting this involved was far from my first choice. The weather turned too cold to spend the day boating tomarrow so I will measure the house and do a calc in hvac calc instead.
Now the for good news. The house came with high quality diffusers on every vent. Even the return register is high grade too bad it only gives me 28" X 17" of grill space and sings...
More mostly good news. I crawled around in the attic today armed with all the info I have learned from the great people who share here and I discovered some good things about the original design. The trunk line going in the attic is a large 20" flex duct. The 8" flex duct with weak flow to the room where I need the most cooling has a hard 90 degree turn and is pinched three times within the bracing structure trusses. Yea.. poor installation is much easier to fix than poor design. I will do something about that this weekend and see how much difference it makes.
The news in the attic was not all good. The attic is very tight and its gunna be a bear to route return ducts for each of the rooms upstairs. The return route down to the furnace wont be easy. There are three possible routes but they require busting out drywall and firestops to confirm. I did not see quick fixes to all the upstairs cooling imbalance issues. There are no balance dampers. The attic plenum is a hacked together small duct board cube. Its so poorly put together that I feel like it will crumble and I will have to replace it if try to make any changes .. like add a duct or a few balance dampers.
All in all with your help and the help of other people generous with their time I am over the hump on having plans to solve a lot of the problems identified.
The upstairs return is half solved.
The Zoning is half solved.
The upstairs flow imbalance is half solved.
I know how to quiet down the furnace and airflow that is outragously loud in the living room.
I know what I need to know to pick replacement equip and a contractor.
I learned more about Manual J and Manual D than I wanted to.
I can do the calcs to know what bid proposals will and wont work and know what I really need.
I demystified two speed and variable speed furnaces/blowers and thier relationship to AC.
I know a lot more about what I need for heating and cooling and what I don't.
Now I have a lot of real work to do bathing in the fiberglass in the attic, need to close on a contractor and equip.
I do have two questions about the return vent locations in the bedrooms. Where I can squeeze through the attic I will try to locate them as far as I can across from the intake or to deal with a known dead zone. Any reason I should do the extra work to cut through the top plate to be able to mount them in the wall or should I just mount them in ceiling where I just need to cut drywall? The attic will only let me get to the middle of one room as I try and reach a hotspot. Apart from looking wierd any reason I would mount a ceiling return vent in the middle of the room?
Thanks much for your help.
[Edited by billyray on 09-10-2005 at 02:14 AM]
I'm glad you like the site. It's taken a few work weeks worth of time over the past two years to get it to that point. And I have a big list of things I'd still like to add. The problem is... I HATE writing! It's tedious as hell!!! Ahem. (You might have figured out the connection sooner by clicking the big yellow button. It's not there just to spice up the forum you know. )
"Thats not possible. If the house was not designed well it never would have been approved by the building dept."
Affable, reliable, "rule-of-thumbale" and a friggin comedian! That's hilarious.
Duct board boxes not only like to fall apart but they're horribly turbulent. If there's room you should replace it with splitter wyes.
In the explanation on diffusers on my site there's a link to Titus-HVAC.com. They have a bunch of engineering data published there. At least one of those documents mentions return location. It is irrelevant except for overcoming dead zones. You can put it [/i]anywhere[/i], including right next to a supply diffuser. That's not to say I recommend doing that. I'm merely emphasizing the flexibility of where you can put it. No top plate cutting is required. It could be in the middle of the room, inside wall, outside wall, whatever. Just make sure that the supply diffuser doesn’t blow directly at it. And of course you want have at least 500 FPM of unobstructed velocity coming out of the supply diffuser so that you get proper mixing. Without proper mixing the placement of the return becomes more of an issue.
If you want a wider selection of opinion (which is always a good thing) then start a new thread with a different question than this one. The other guys are happy to help. But once the replies get as long as they did here you lose them. Few people have the patience to read through all of my bluster.
The engineering docs on Titus-hvac.com were a great reference. Those will keep me busy for a while.
A work they just finished a 6 month HVAC upgrade but its not right. We renamed our cube space "the artic circle." Now I am armed with the Fangers Comfort Index and the ISO standards that will scare the crap out of our buidling engineers next time we call to complain about how out of balance the brand new system is. lol.
I really think you are good at organizing and making practical information. If there is a publisher within your trade that has enough sales volume to make it profitable I encourage you to contact them and consider writing pragmatic 'how to books.' Or even a yellow 'Home Heating & Air Conditioning guide for Dummies" to sell in Barnes and Noble.
Irascible I greatly appreciate your help. Whenever I can I will send some business your way.