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  1. #1
    Join Date
    Jun 2008
    Posts
    7

    Two Systems into One

    Greetings,

    We have a 2600 sq/ft house (1500 on main, 1100 on upper) currently serviced with two separate furnaces. One of the two units has a 2 ton A/C to heat & cool the upper floor. No A/C on the main floor via the second system. The main floor ductwork is sized proportionally larger as compared to the upper floor ductwork.

    We are evaluating options for replacement of the system with High Efficiency units. The recommendations range in order of the highest cost to lowest as follows:

    Option 1: Replace two furnaces with a single unit, sized for the entire house, with new condenser and zone control system. Changes required to merge ductwork are somewhat challenging but not extremely difficult. Contractor has installed similar systems with reportedly good success. The downside is total cost about 40% higher than lowest cost option.

    Option 2: Replace two furnaces with two new furnaces and one condenser sized for upper floor only. Essentially replace exactly the same configuration with the downside being that we do not gain A/C on the main floor which is desired (temperature variance between conditioned upper floor and unconditioned main floor is significant). Control over the zones is excellent with the two separate furnaces and thermostats, but A/C only on upper floor.

    Option 3: Replace two furnaces with one new furnace and one condenser sized for entire house. No zone control, but consider manual dampers in trunks to adjust for seasonal requirements (more flow to upper during A/C season and more flow to main for Heating season). This is the lowest cost option, but only about 10% less than Option 2 and would not provide any automated zone control.

    All three contractors are experienced and highly regarded, but have different opinions on the solution.

    Our concerns are with Option 3 providing much by way of zoning efficiencies, but we expect that the fan running 24/7 on low speed will maintain even temperatures throughout the house.

    Our concern with Option 2 is not gaining of A/C on the main floor, unless there exists a condenser that may be able to service two independent coils. If such a solution were available then this scenario would be a good cost/benefit solution. Installing separate condensers for each furnace-A/C unit would be impractical for space limitations and would become as costly as Option 1, but would provide absolutely independent and fully programmable zones serviced by their own system and ductwork.

    Our concern with Option 1 is the overall cost and the actual benefit of utilizing zoning to save costs during heating season by lowering the temperature of the unused zones at the appropriate hours. Can the extra cost ever pay for itself in dollars or in level of comfort assuming that Option 3 can circulate and maintain even temperatures throughout the building?

    I'm interested in comments or concerns with the recommendations.

    Thanks!

  2. #2
    option 3 will not give you the even temperatures through the house that you desire. no if, and's or but's about it.

    option 1 is your best bet since your not satisfied with the current set up you have(rules out option 2)

    A zone system, when done correctly, will make you think you had the second a/c system installed on that downstairs unit.
    Last edited by jrock52766; 06-19-2008 at 01:55 AM. Reason: spelling-its late :-)

  3. #3
    Join Date
    Apr 2008
    Location
    Alberta Canada
    Posts
    2,246

    Zoning is good

    Zoning get expensive and problematic after years of service, variable speed gets noisier. Top floor will be hot and bottom floor will be cooler. I would go for 2 furnaces and 1 air conditioner with variable speed blowers and run fan 24/7 if installed right they are quieter. Lower cfms means lest noise as long as your duct work is sized properly.
    Do it right the first time.

  4. #4
    Join Date
    Jun 2008
    Posts
    7
    Thanks Jrock...

    For option 1: I know you said "no ifs, ands or butts"....but, is there no amount of airflow (ie. even on high) or balancing of dampers that could equalize temperature throughout? Or is it just not worth pursuing? Funny, contractor for Option 2 stated you'll never be happy merging into a single unit....and wouldn't recommend doing it.

    For option 2, is there any condenser that you are aware that can service two coils, ie. two compressors in a single frame or any sort of pressure valves that can switch between one or the other coil on low stage and serve both on high stage? I'm assuming two coils in series couldn't work as the non-called coil wouldn't be seeing any airflow....

    For option 3, looking like where we have to go (sigh with $$$). Do they really work that well...you sound very certain. What should I be watching for to ensure that the system is properly installed? i.e. should the returns be dampered as well?

  5. #5
    jeff,

    even if the the temperature in a two story house could be kept even with one system using manual dampers they would still have to be adjusted twice a year for heating season and cooling season. So you would be paying someone to do it or crawling around your attic and hoping you got it right.

    The only single systems i have seen with 2 cooling banks(2 compressors and 2 coils) are in commerical package units.

    The zoning systems do a great job as long as they are installed correctly and the duct sizing is correct. A key point is to pay attn. to the warrenty on the dampers that are used as they are the primary component to fail in zone systems. Several zone brands have 5 yr zone board warrenties but on 1 year on the dampers. I do know that Aprilaire has 5 year warrenties on all the products they make. Heres a link to the website that will give you more info http://www.aprilaire.com/

  6. #6
    Join Date
    Jun 2008
    Posts
    7
    Quote Originally Posted by 21degrees View Post
    Zoning get expensive and problematic after years of service, variable speed gets noisier. Top floor will be hot and bottom floor will be cooler. I would go for 2 furnaces and 1 air conditioner with variable speed blowers and run fan 24/7 if installed right they are quieter. Lower cfms means lest noise as long as your duct work is sized properly.
    Thanks 21Deg....

    Based on your Location information, you share the climate that we are dealing with.....-40c in winter and occasional +40c in summer.

    I'm a "fan" of running the blowers 24/7 for circulation and temperature. But is there a practical method that we can mix the airflow between the two floors to provide sufficient cooling in summer from the single A/C unit?

    As it works now, the A/C unit for the upper floor cools appropriately. But the temperature of the main floor can be 10+ degrees higher on hot summer days. This doesn't seem efficient to me, where part of the house is 85 deg with high humidity and the other is 72 deg/low humidity. Somehow, I'd like to gain A/C for the entire house, but with reasonably consistent temps.

    These days, there are so many 2500 to 3500+ sq/ft houses being constructed....how do they equalize temperature with the single furnace-A/C installations? Or don't they?

  7. #7
    Join Date
    Jun 2008
    Posts
    7
    Duplicate post....sorry.
    Last edited by jeffr_ca; 06-19-2008 at 02:32 AM. Reason: Duplicate post

  8. #8
    Join Date
    Mar 2004
    Location
    NJ
    Posts
    1,214
    Option 1 sucks, 2 systems are always better than 1. 1 of 1 systems breaks your f'd. 1 of 2 breaks --sucks but you can get by until the other is fixed. U do the math.
    thehumid1-------I live in NJ, a state where it's free to come in but you have to pay to leave!

  9. #9
    in respone to that last question you asked, jeff, i know in atlanta it is code to have a zone system on any two story house with one hvac system in new construction. That is what they are doing there and heat and hunidity are a major concern in that city, and the zone systems are keeping the houses comfortable last i heard.

  10. #10
    Join Date
    Jun 2008
    Posts
    7
    Quote Originally Posted by jrock52766 View Post
    jeff,

    even if the the temperature in a two story house could be kept even with one system using manual dampers they would still have to be adjusted twice a year for heating season and cooling season. So you would be paying someone to do it or crawling around your attic and hoping you got it right.

    The only single systems i have seen with 2 cooling banks(2 compressors and 2 coils) are in commerical package units.

    The zoning systems do a great job as long as they are installed correctly and the duct sizing is correct. A key point is to pay attn. to the warrenty on the dampers that are used as they are the primary component to fail in zone systems. Several zone brands have 5 yr zone board warrenties but on 1 year on the dampers. I do know that Aprilaire has 5 year warrenties on all the products they make. Heres a link to the website that will give you more info http://www.aprilaire.com/
    Jrock...

    We actually have a full basement where the furnace(s) are installed, so access is easy. We anticipate some experimentation to determine a "winter" and "summer" position for manual dampers, but re-setting them each season would be easy.

    Thank you for the link on the Aprilaire products. All the systems offered so far are completely Lennox equipment (including the zone control). Any concerns there? I will confirm warranty coverage on the dampers, etc. (furnace and A/C are 10 years parts/labour). I'm assuming clean air/filters could affect life of damper mechanics.

    Please confirm my understanding in that the zone system would behave as though there were two furnaces-A/C units depending on the call from the two thermostats. Zone "A" makes a call - Zone "B" dampers close. Zone "B" makes a call - Zone "A" dampers close.

    If both zones make a simultaneous call, all dampers would be opened.

    If both thermostats have "fan" enabled, all dampers would be opened and we would gain full circulation throughout the house through single blower (that's good, as it would help equalize temperatures).

  11. #11
    Join Date
    Mar 2004
    Location
    NJ
    Posts
    1,214
    Again stay with 2 independent systems. Do not convert to one and zone it.
    thehumid1-------I live in NJ, a state where it's free to come in but you have to pay to leave!

  12. #12
    Join Date
    Jun 2008
    Posts
    7
    Quote Originally Posted by thehumid1 View Post
    Again stay with 2 independent systems. Do not convert to one and zone it.
    Hi Humid....

    Perhaps it's more common to have two systems than I've experienced.

    I understand what you're saying about redundancy in having two systems regarding failures, but most houses don't have any type of backup system even in the cold climate where we live. We have to be somewhat dependent on the reliability of the systems we have installed. There are 24 hour emergency services available.....so long as someone is home to know that there's an emergency.

    Is it common to have two A/C condensers (one for each furnace/coil)? Adding the second A/C system would give us the ultimate in zone control and redundancy, but it seems excessive for the 8 weeks of a year that we may require A/C. This would take the cost to about equal that of the zoned system.

  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,741
    2 heating cooling systems for one house is not unusual.

    So option 4 is the best.
    1 funace and A/C for the second floor.
    1 furnace and A/C for the first floor.
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