Results 1 to 11 of 11
  1. #1
    Join Date
    Jul 2005
    Posts
    10
    Looking at replacing two, builder grade, 18 year old heat/air split systems in suburban Atlanta.

    Have received several quotes. Like the Rheem/Rudd systems. Proposals are for Modulating furnace in lower unit (basement/1st floor) and 80% 2-stage variable speed unit for attic (second floor).

    Contractors do not recommend 90+ units in unconditioned space. Said the flue gas could freeze up and cause the system to shutdown on the coldest days of the year.

    How realistic is this problem? The payback for a 94% unit is only about two years based on the quotes in front of me. Plus, it seems the Modulating furnace is newer technology.

  2. #2
    Join Date
    Apr 2004
    Posts
    7,680
    In Atlanta this is probably not an issue but in any case a simple heat tape to the drainline will be enough to prevent any problem (per the installation instructions)this of course would include garages, attics, unconditioned crawl spaces etc. The mod has been aound for a good 8 years and has only improved with time. You will probably not find ANY complaints by anyone who owns one. The efficiency is pretty good but you are buying comfort with the mod in addition to the fuel efficiency. The 80+ VS 2 stage is a good compliment for the second floor.

  3. #3
    Join Date
    Mar 2004
    Posts
    1,042
    You use very little heat on the second floor when there is heated space below it. You aren't going to get the kind of payback you expect from a high efficiency unit on the second floor, especially in this climate. When you look at your heating bill, it's not 50-50 between the two existing furnaces; it's probably 75% for the lower one and 25% for the upper.

    The modulating furnace is great stuff, but it's not immune to this issue. It's not the flue gas that freezes, it's the condensation that's produced. Combustion produces some water vapor. In an 80% furnace, the flue gases are hot enough to carry the steam out the flue without any condensation. All 90+% efficient furnaces are so efficient that the flue gases aren't hot enough to carry the steam out, though; instead, they catch the condensation and get rid of it through a drain. That drain line is what will freeze up in an attic or other such unheated space, and when it does, the result is usually a flooded furnace, drywall damage, or both.

    You won't get any payback at all if you end up with wet drywall. It's just not worth messing with in this case. So yes, they're on the right track. Just get the nice 80% unit for that location.

  4. #4
    Join Date
    Mar 2004
    Posts
    1,042
    I wish you were right about the risk of freezing in Atlanta, doc. There are people in my neighborhood that found out that they had humidifiers in their attics when the pipes froze and burst.

    Heat tape can be a workaround. My issue there is that the time when the attic is most likely to freeze up is when the power goes out for a day or two (which is typical for Atlanta ice storms)... and a heat tape won't help you there.

    I must confess that I just put a humidifier in MY attic in Atlanta, with a heat tape, but I also plumbed it up so that I could drain the lines if the power went out for long enough to be of concern.

    By all means, though, the modulator is a great choice for the system where freezing is not a concern.

  5. #5
    Join Date
    Apr 2004
    Posts
    7,680
    If the power is out there shouldnt be any condensate flowing through the pipes. (yes the trap is full but its in the return and if the duct is insulated, I dont think it would be an issue.

    humidifier is a whole nuther ball game. Again, I think hes on the right track by installing the mod in the basement and the high end 80+ in the attic. I thought I said that?

  6. #6
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,744
    i had a 90+ pulse furnace in the attic of house i just sold since 1987 in n.j. never had any problem with freezing. heat on drain line straight drop down the buried in insulation. i think there are more problems running 80% furnaces in unconditioned space because they are not made to condense but do in extreme cold weather, rotting out heat exchangers. j.m.o.

  7. #7
    Join Date
    Mar 2004
    Posts
    1,042

    Thumbs up

    Fair enough. I just wanted to make sure joanr and I understood your first message properly (thanks!).

  8. #8
    Join Date
    Apr 2004
    Posts
    7,680
    No biggie but you'll have a hard time convincing anyone above the mason dixon line that atlanta sees extreme cold weather. 20 degrees is not cold. -20 is.

    By the way, Im north of Atlanta (a few hundred miles at the least) and I have a 90+ in my garage.

    [Edited by docholiday on 07-11-2005 at 05:17 PM]

  9. #9
    Join Date
    Mar 2004
    Posts
    1,042
    Oh, no doubt... 20 isn't cold. Our design temp is 22, big whoop. That said, once in a blue moon we do see single digits. I went looking in the weather data a couple years ago, and was surprised myself to see that the all-time record low here is about -5. We still burst plenty of pipes, though, 'cause we are conditioned to not try as hard to protect them.

    A garage is vented when the door is open, which you minimize when it's cold, and otherwise it's closed up. A good attic is at ambient temperature in the winter, though. And a leak in a garage just lands on a slab and goes out the door.


  10. #10
    Join Date
    Jul 2005
    Posts
    10
    Thanks for the helpful responses. I think we'll stick with the 80% 2-stage variable speed for the attic. I mad a mistake in may payback analysis (I used the savings for both units) so assuming half the savings from the attic, the payback is four+ years and if I assume the attic unit is only 25% of my heating bill, then it is more like eight years!

    On to a follow -up question. We are looking at the 75K BTU model with a 2-1/2 ton AC unit and a 3 ton high efficiency coil (it is the Rheem/Rudd recommended match). One contractor proposed the 3/4 hp blower motor model while the other spec'ed the 1/2 hp version.

    The rationale for the larger motor is that it would be set up to run at low speed (1000cfm) while the 1/2 hp motor would be running at a higher speed to get the same cooling 1000 cfm. The price is basically the same.

    Being an engineer (although female!), I like the idea of extra horsepower! Any reason not to go with the 3/4 hp model? (I noticed the 50K btu furnace has the 3/4 hp motor, too)

  11. #11
    Join Date
    Dec 2004
    Location
    Derby City
    Posts
    3,964
    I would strongly advise you at least 'looking'at the use of a dual-fuel system. You would install a high-efficiency gas fired furnace for the first floor and lower level, and a high efficiency heat pump system for second level. As indicated earlier, the main source for your homes' heating will be the first floor system. By using the heat pump upstairs, you eliminate the need for gas piping or venting, and the only condensate you have to worry about is from the drainline. An auxillary drainpan should be installed beneath any water producing equipmentin the attic. the newer generation heat pumps discharge a warmer air than earlier models are much more accepted (at least in this area) than when they were first introduced in any numbers (back in mid '70s.) I have used this type of system extensively, and with very good success and high satisfaction among customers. Good luck.
    Everyone has a purpose in life..........even if it's to be a bad example.

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