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  1. #1
    Join Date
    Dec 2008
    Location
    Central New Jersey
    Posts
    8

    High Efficiency Furnace in the Attic

    Hello Everyone,

    I recently had a new HVAC system installed in my attic. It is a Carrier heat pump system with a propane furnace backup. The furnace is a 58MVB120-120 high efficiency unit. After the installation was done, I found out that these high efficiency furnaces are not recommended in the attic because the condensate trap can freeze. The contractor has put heat tape on the condensate trap to keep it from freezing. He also plans to put a foam board box around the furnace to help keep the condensate trap from freezing.

    In the short time that this furnace has run during cold nights, it has used a LOT of propane. I have asked the contractor to come and check this out. Does anyone know if putting this high-efficiency furnace in the cold attic makes it run less efficiently? Other than the risk of the condensate trap freezing, does anyone see any other problems with putting the high efficiency furnace in the cold attic?

    I appreciate any feedback.

    Thanks,
    aghahary

  2. #2
    Join Date
    Aug 2008
    Location
    Lansing, MI
    Posts
    40
    You're right, it's not typically recommended to install a 90+ furnace in an attic due to the possibility of freezing the condensate trap or any other place in the furnace water might collect in and freeze in the off cycle. The heat tape should keep the trap from freezing, reducing the risk of damage to home or equipment.

    The furnace itself should run as efficiently in an attic as in a conditioned space, but you will lose some heat through the furnace jacket and the duct. Is the duct well sealed and well insulated. Is there a door on the filter access?

  3. #3
    Join Date
    Dec 2008
    Location
    Central New Jersey
    Posts
    8
    TechIntense,

    Thanks for your reply. The duct work has R8 insulation on it, and it seems fairly well sealed. Other than the condensate trap, is there any other spot I should be worried about the water freezing in the off cycle? Do you think the foam board box around the furnace will help with the heat loss? Do I have enough of a reason to make big fuss with my installing contractor over this, or are these and some other solutions good enough to get around this problem?

    The filter access is separate from the furnace. I don't remember if it is a door, or I just pull out the filter. Should I be worried about this as well?
    Last edited by aghahary; 12-02-2008 at 12:38 AM. Reason: Adding a few more comments.

  4. #4
    Join Date
    Aug 2008
    Location
    Lansing, MI
    Posts
    40
    AS long as the furnace is properly leveled and all the condensate drain lines are connected, it should drain. The problem is that if the secondary heat exchanger doesn't drain then you could have a much bigger problem than a broken drain trap. That's why they are not typically installed in unconditioned spaces. The foam box should help keep the furnace warmer, but the heat tape is still necessary. It sounds like the contractor is acting in good faith to address your concerns. It just makes some of us a little nervous.

    As far as the filter is concerned, it's a good idea to have a door on it to prevent excessive unconditioned air from being pulled from the attic.

  5. #5
    Join Date
    Aug 2008
    Location
    Lansing, MI
    Posts
    40
    The heat pump should be doing it's fair share of the heating as well. I don't know where you live but in Michigan we've only had a few times that the propane furnace would have ran. The balance point may need to be adjusted.

  6. #6
    Join Date
    Dec 2008
    Location
    Central New Jersey
    Posts
    8
    The contractor has put an automatic shut-off switch in the secondary condensate line in the event of an overflow condition. Is the secondary condensate line the same as the drain for the secondary heat exchanger? Would the automatic shut-off switch turn off the whole furnace in an overflow condition?

    As for the heat pump, it does an OK job at low temperatures. I have the 25HNA960 from Carrier. We are located in NJ, and a couple of nights when it got close to 20 degrees outside, it was struggling. After I saw the propane furnace burn too much fuel, I lowered the balance point to about 22 degrees. Is that too low?

  7. #7
    Join Date
    Feb 2007
    Posts
    25
    Don't know about the carrier unit but the Rheem manual says it is allowed to be installed in the attic as long as the drain line is wrapped in heat tape.

  8. #8
    Join Date
    Feb 2008
    Location
    Dallas & Longview, TX
    Posts
    629
    Quote Originally Posted by aghahary View Post
    The contractor has put an automatic shut-off switch in the secondary condensate line in the event of an overflow condition. Is the secondary condensate line the same as the drain for the secondary heat exchanger? Would the automatic shut-off switch turn off the whole furnace in an overflow condition?

    As for the heat pump, it does an OK job at low temperatures. I have the 25HNA960 from Carrier. We are located in NJ, and a couple of nights when it got close to 20 degrees outside, it was struggling. After I saw the propane furnace burn too much fuel, I lowered the balance point to about 22 degrees. Is that too low?
    The auto shut-off shuts down the unit if triggered. I don't think it will allow the blower to run thus it keeps the HP from running but I may be wrong on this.

    The balance point for the HP vs. propane is based upon the cost of elect. vs. propane in your area. This is also balanced with comfort as the output temp of the HP will drop as the external temp drops. The HP on 2nd stage will heat to an outdoor temp close to 0* but vs. a high efficiency furnace may not be the best.

    Post your cost for elect. and propane.

    Regarding freezing problems due to the attic, I also have an Infinity HP dual fuel and love it. It is at my 2nd home in a closet install. My main house in Dallas has 3 units in the attic so I was going to put 80% NG furnace dual fuel systems there. I was talking to a guy regarding foam insulating my attic and he mentioned that enclosing the attic eliminates combustion air for furnaces. Due to the 3 furnaces and 2 gas tank water heaters I would have to replace the water heaters with direct vent water heaters and the 3 furnaces with high efficiency furnaces. This would be very expensive upfront for me but if you foamed your attic it would eliminate the concerns and vastly improve your houses envelope. You already have the right furnace. Any other furnaces or water heaters in your attic? Just a thought.

  9. #9
    Join Date
    Jul 2005
    Location
    SE PA
    Posts
    183
    Quote Originally Posted by Daltex View Post
    The auto shut-off shuts down the unit if triggered. I don't think it will allow the blower to run thus it keeps the HP from running but I may be wrong on this.

    The balance point for the HP vs. propane is based upon the cost of elect. vs. propane in your area. This is also balanced with comfort as the output temp of the HP will drop as the external temp drops. The HP on 2nd stage will heat to an outdoor temp close to 0* but vs. a high efficiency furnace may not be the best.

    Post your cost for elect. and propane.

    Regarding freezing problems due to the attic, I also have an Infinity HP dual fuel and love it. It is at my 2nd home in a closet install. My main house in Dallas has 3 units in the attic so I was going to put 80% NG furnace dual fuel systems there. I was talking to a guy regarding foam insulating my attic and he mentioned that enclosing the attic eliminates combustion air for furnaces. Due to the 3 furnaces and 2 gas tank water heaters I would have to replace the water heaters with direct vent water heaters and the 3 furnaces with high efficiency furnaces. This would be very expensive upfront for me but if you foamed your attic it would eliminate the concerns and vastly improve your houses envelope. You already have the right furnace. Any other furnaces or water heaters in your attic? Just a thought.

    to perform that calculation (HP vs. fossil fuel...and balance point) do you need the COP and/or HSPF of the heat pump? Can you show me the calc?

  10. #10
    Join Date
    Feb 2008
    Location
    Dallas & Longview, TX
    Posts
    629
    Quote Originally Posted by im4snow2000 View Post
    to perform that calculation (HP vs. fossil fuel...and balance point) do you need the COP and/or HSPF of the heat pump? Can you show me the calc?
    Your HP's COP will decrease as the outdoor temp decreases so if you plug in your rates (total w/ tax delivery etc.) then keep lowering the cop until it breaks even with the fossil. This is the point at which it is cheaper to go with fossil. Rates change and the HP may not be able to deliver the required BTU's you need at colder temps. Get your contractor to tell you the temp at which the COP is breakeven according to the calculator. http://www.warmair.com/html/fuel_cost_comparisons.htm

  11. #11
    Join Date
    Aug 2008
    Location
    Lansing, MI
    Posts
    40
    Quote Originally Posted by Daltex View Post
    Your HP's COP will decrease as the outdoor temp decreases so if you plug in your rates (total w/ tax delivery etc.) then keep lowering the cop until it breaks even with the fossil. This is the point at which it is cheaper to go with fossil. Rates change and the HP may not be able to deliver the required BTU's you need at colder temps. Get your contractor to tell you the temp at which the COP is breakeven according to the calculator. http://www.warmair.com/html/fuel_cost_comparisons.htm
    The comfort balance point is not as important if you have a thermostat that allows fossil fuel backup based on balance point or droop. Then if the economic balance point is too low and the temperature drops too far below setpoint, the compressor will be turned off and the furnace will come on.

  12. #12
    Join Date
    Feb 2008
    Location
    Dallas & Longview, TX
    Posts
    629
    Quote Originally Posted by TechIntense View Post
    The comfort balance point is not as important if you have a thermostat that allows fossil fuel backup based on balance point or droop. Then if the economic balance point is too low and the temperature drops too far below setpoint, the compressor will be turned off and the furnace will come on.
    To me the comfort balance point is more important than the economic balance point.

    I am luck to have a great install on my HP and I have no issues but I have recently been in a house that had poor ducting and when the HP runs on 2nd stage full blower to keep up, the draft is not comfortable. I changed the lockout temps and now this house has gas heat above the economical balance point but the homeowner is estatic that the drafty high velocity "wind" is gone.

    He realizes that there is an extra cost for this comfort but doesn't want to renovate the ducting yet.

  13. #13
    Join Date
    Aug 2008
    Location
    Lansing, MI
    Posts
    40
    Quote Originally Posted by Daltex View Post
    To me the comfort balance point is more important than the economic balance point.

    I am luck to have a great install on my HP and I have no issues but I have recently been in a house that had poor ducting and when the HP runs on 2nd stage full blower to keep up, the draft is not comfortable. I changed the lockout temps and now this house has gas heat above the economical balance point but the homeowner is estatic that the drafty high velocity "wind" is gone.

    He realizes that there is an extra cost for this comfort but doesn't want to renovate the ducting yet.
    The point of using an air-source heat pump with a furnace is to save money. There are times when exceptions have to be made for comfort. But, in this case it sounds like this customer should have saved the extra money and just bought a furnace/AC.

    I stick by my point. If you've got a properly sized unit on an adequate duct system, you can save more money by setting the changeover at the economic balance point or a moderate amount above and letting a high quality thermostat take care of the comfort changeover.

    If comfort becomes an issue then move the setting up in small increments.

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