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  1. #1
    Join Date
    Apr 2006
    Location
    Austin, TX
    Posts
    34
    We have a 2200 sf house with about 1800 down and 400 up. We have two systems, a 2.5 ton and a 2 ton (all electric heat). The ductwork system is not well designed, I think the previous owner took the path of least resistance in a remodel that almost doubled the square footage. In an earlier post I talked about wanting to go to one system. That does not look like it will be possible. The two duct systems are isolated from each other in side attics and their is just no simple or elegant way to connect them. The 2 ton unit (with a bad compressor) provides 80% of our heating and cooling. There is only about 400 sf (two downstairs bedrooms and a bathroom) that are soley dependent on the 2.5 ton unit, every other room either has ducts from both systems or ducts from the 2 ton system alone. We want to put in a heat pump and are looking for a whole house solution as opposed to a stop-gap measure. The one contractor who has come out so far wants to replace the 2 ton w/ the bad compressor with a 2 ton heat pump and just leave the 2.5 ton unit as an auxillary unit. Size wise he said just replace it with the size you've got because theres no way to do load calc. with this much overlap between the two systems. For all I know 2 ton may be the right size, but I don't want it to be just a guess when it's our primary system. Is there anyway to do this right? It seems like if we want all heat pump we need a unit sized to provide the bulk of our heating and cooling, and then a small unit to provide for the space not covered by the first, but is there a way to accurately load calc. that? Contractor #2 comes out today so I'll see if he has any better suggestions. Thanks for any thoughts, advice, opinions....

  2. #2
    Join Date
    Oct 2003
    Location
    Washington
    Posts
    7,405
    Of course there is. Here is what they need to do. They need to perform a block load for the entire structure. Based on that they can divide up the BTU's as needed to design the system whether it be 1 system or 2. If the contractor was telling you he couldn't do it, it either means he "couldn't" or he just doesn't want to. You can drop him off the list.

  3. #3
    Join Date
    Apr 2006
    Location
    Austin, TX
    Posts
    34
    Well, contractor #2 just left. Wants to do "rule of thumb" sizing. Told me I could do manual J if I wanted to, or if I was willing to go to kinkos and scan blueprints and email them to him he would do it. Good thing I have a bunch of companies lined up because it looks like its going to be a chore to find someone to do it right.

    This fellow told me that the Trane builder grade series (xr) versus the xl is pretty much the same, just more bells and whistles on the xl. He said this isn't true with all brands, but with Trane it is. Any thoughts on this?

    Also, is a variable speed air handlers main advantage to handle humidity? I've never given a second thought to the humidity level in our house so I guess I've always been comfortable enough. If humidity is not an issue are you just as well off with a non-variable speed or are there other advantages to variable speed air handlers?

  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,048
    Good riddance to #2.

    Trane doesn't make the XL any more. The XB is builder grade, XR is standard in 13 & 14 SEER, XLi is deluxe in 14-19 SEER. As you step up you get more bells & whistles and a quieter unit with better warranty. Getting away from 13 SEER gets rid of the old recip and into a scroll except in some smaller sizes.

    Variable speed if used right can reduce summer humidity. So can the right size unit

    Where are you? Maybe one of us is nearby and you can get some GOOD info!

  5. #5
    Join Date
    Apr 2006
    Location
    Austin, TX
    Posts
    34
    I think I meant XLi. None of the XB units will get me the electric company rebate (they want 14 seer)so I'm not really sure what he was talking about. It looks like either XR14 or XL14i are going to get me that. XL14i has higher SEER, is energy star, quieter, and has a better warranty and ironically seems to be priced lower than the XR14 according to the 06 dealing invoice pricing I've seen. How is that possible?

    Figuring out the right size seems like it's going to be a much bigger problem than picking the brand or style. The two companies that have come out so far are biggies in my city (Austin, TX). I've really liked both guys on a personal level, both funny and informative and have been in the business a long time, but neither had the slighted interest in doing the load calc. Theres always tomorrow and friday when the next guys are coming out. Or, I can just do it myself. I've gotten one recommendation for an austin contractor from someone on the board but he's busy and might make it out next week. At the rate I'm going I'll still be looking next week so it might work out.




  6. #6
    Join Date
    Oct 2003
    Location
    Washington
    Posts
    7,405
    Originally posted by chris7


    Figuring out the right size seems like it's going to be a much bigger problem than picking the brand or style. The two companies that have come out so far are biggies in my city (Austin, TX). I've really liked both guys on a personal level, both funny and informative and have been in the business a long time, but neither had the slighted interest in doing the load calc. Theres always tomorrow and friday when the next guys are coming out. Or, I can just do it myself. I've gotten one recommendation for an austin contractor from someone on the board but he's busy and might make it out next week. At the rate I'm going I'll still be looking next week so it might work out.



    Click the red tab, upper right corner and do the load calc yourself! Then, not showing them the results, compare what they want to put in with your numbers. Good luck.

  7. #7
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    VS

    Is it on-topic to discuss the variable speed air handler with a 2-stage furnace? Because with many houses it is a real pleasure to get very quiet airflow along with enough heat to get the job done. In my own house it is a Trane/AS brand, one almost cannot tell when the low speed heat is running (and there is enough furnace oversizing that I *never* have seen high speed run).

    In the case of single stage AC, you can apply a dehumidistat to the system, and I believe it will control the air speed varying from approx. 300-350 cfm/ton when humidity control is needed, to perhaps 450 cfm/ton when not. One benefit would be perhaps 6% better energy efficiency with the latter mode. This would take a long term to pay off in pure dollars and cents analysis, but you may be attracted to the long term benefits (and simply the idea too) of energy efficiency. Some good thinkers on this board have written essays about the benefits of using variable speed air handlers with single stage systems. Again since you say there is no subjective discomfort, the whole matter of humidity control may be moot for you.

    I think we can rule out the case of two-stage AC systems, which *require* that variable speed air handler. But in your case, you may have stumbled into a pretty good design by accident. The idea of getting 80% of your cooling from one hard working system, is actually pretty close to the ideal of the 2-stage single system. Count your blessings! That is, if your duct system is free of grievous flaws.

    I lament how hard it is to get an AC tech to actually do a Manual J calculation. It isn't that difficult to do it yourself, just lots of measuring and entering numbers into the HVAC-CALC program. If you can locate a tech who can do Manual J and certain other "sophisticated" measurements such as measuring external static pressure, you will probably have found someone in the top 5% of all AC techs.

    Best of luck -- Pstu

  8. #8
    Join Date
    Apr 2006
    Location
    Austin, TX
    Posts
    34
    I do not know if it's a grievous flaw, but our duct system is badly in need of sealing and that will be done when we change systems. The other day it was cooler in our attic than in the rest of the house so we're currently paying good money to air condition our attic. The two fellows who have been here so far (who I won't be working with) said they'd seal them with mastic. One said he would not do any "fancy" diagnostics, just seal it. The price the other guy quoted was so high that I certainly hope it included fancy diagnostics to confirm that all the leaks were sealed. Any thoughts on the testing that can be done to locate all the leaks and then confirm that they've all been sealed? I don't remember the names of the equipment and tests, but I've read about them. Is that standard practice, or should it be, or can you can a good job done by just eyeballing it?

  9. #9
    Join Date
    Oct 2003
    Location
    Washington
    Posts
    7,405
    Originally posted by chris7
    I do not know if it's a grievous flaw, but our duct system is badly in need of sealing and that will be done when we change systems. The other day it was cooler in our attic than in the rest of the house so we're currently paying good money to air condition our attic. The two fellows who have been here so far (who I won't be working with) said they'd seal them with mastic. One said he would not do any "fancy" diagnostics, just seal it. The price the other guy quoted was so high that I certainly hope it included fancy diagnostics to confirm that all the leaks were sealed. Any thoughts on the testing that can be done to locate all the leaks and then confirm that they've all been sealed? I don't remember the names of the equipment and tests, but I've read about them. Is that standard practice, or should it be, or can you can a good job done by just eyeballing it?
    As long as they apply the mastic properly I see no problem in just doing it without the fancy shmancy equip. Really not worth the cost as long as they do it right. I recommend you inspect how they do it and make sure they don't cut any corners. Also, they may need to replace/install insulation in places it is needed. Like I said, climb up there and inspect it for yourself and note any uninsulated/unsealed ducts. good luck

  10. #10
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,271

    Overall Integrated Approach


    Originally posted by chris7
    I do not know if it's a grievous flaw, but our duct system is badly in need of sealing and that will be done when we change systems. The other day it was cooler in our attic than in the rest of the house so we're currently paying good money to air condition our attic.
    I usually offer to provide some real guidance .. but there are way to many issues to get to a Good solution quickly on this one.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  11. #11
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Opinions

    I am an amateur, but this is what I think: Sealing that duct system is a high priority. In all likelihood the high price quote for duct sealing, would include an air leakage test -- there are two types I have heard of, don't know which is better myself.

    Probably one can feel cool air leaking from several points in your existing duct system. That would be a low-tech means to identify where to apply mastic and seal those leaks.

    Regarding the duct *design*, a test for external static pressure would be one valuable tool to help identify whether your duct system is OK, or needs to be replaced. If replacement is in order, of course why seal the old one? I would definitely consider this or another test before deciding which way to go.

    In this industry a whole lotta guys want to go and fix things without any instruments to measure their results. I guess it can work sometimes... but being a tech oriented person myself, I would much prefer to see some kind of before-and-after testing.

    Hope this helps -- Pstu

  12. #12
    Join Date
    Apr 2006
    Location
    Austin, TX
    Posts
    34
    That's another thing i'm having a hard time getting these guys to address. I ask them if the ducts are sized right for the unit they are proposing and they just kind of brush it off and say they're fine. #3 will be here this afternoon so we'll see how this one goes.

  13. #13
    Join Date
    Apr 2006
    Location
    Austin, TX
    Posts
    34
    Well, #3 has come and gone. I will say he was much more thorough than the first two. He was the only one who really looked at the duct work and told me that it might be sufficient for the 2 ton we have but would have to be replaced for anything else b/c it's just to small. Also said the size of the ducts leading to registers in main living areas were tiny, the size they normally put in bathrooms. No suggestion of load calc. Said that 90% of their installs are the bryant evolution systems. Specifically for us..3 ton heat pump w/ 2 speed condenser (M#69813), variable speed fancoil (M#FE4A), backup heaters, evolution control, media air cleaner and drain overflow sensor. When I asked if they do air balancing at the end of installation he said they would put in a manual damper that we/they could tweak to control how much air was getting to certain areas. Detailed explanation of duct upgrade but I'm not going to type that out unless someone thinks it's relevant. I kind of got the impression that the system suggestion was kind of cookie cutter, that that's what they reccomend to everyone. From what I've read it's a pretty top of the line system w/ all the bells and whistles. He seemed to suggest that w/ a two stage you don't have to worry about sizing so much because 80% of the time it will be running at the lower speed and only when it's needed will it kick up to the higher speed. I have high hopes for the guy coming out tomorrow. The company web site specifically says that the only way to size a unit is with manual J calc.'s and that's how they do it. We'll see. I'll prob end up spending the $50 and do it myself. Oh yeah, he did not address the second system, said just let it keep on going until it conks out since it's only 6 years old. But then to replace it with a 2 ton which would put total tonnage on our 2200 sf house at 5.

    [Edited by chris7 on 04-13-2006 at 06:12 PM]

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