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Discussion Forum

2 Furnaces VS zoned system

piobuilder | Posted in General Discussion on February 4, 2009 10:48am

Hi y’all, My name is Kurt Piotrowski.    I’m somewhat new to the breaktime forum.  I’ve read quite a bit hear and there but, this will be my first post.  I own a remodeling business in Michigan. 

My question actually has to do with my own house.  To give you some brief history, we decided to add a second floor to our existing ranch style home.  After demo and prep, we realized that the existing floor system and walls were not suitable to build on, so we tore it down to the foundation, we basically have a brand new house, saving only about 10% of the original frame.  The house has been framed and we are in the process of putting the roof on.  We are getting ready to start the mechanicals.

I am trying to determine the heating and cooling system for our house.  The house is built on a crawlspace.  About 1100 sq ft on the main floor and about 930 sq ft on the second floor. I would say about average amount of glass, no huge picture windows and not alot of north and east facing glass.   We are locked into a forced air system.  The furnace(s) will be located on the main floor somewhat central in the house (the crawlspace is not an option due to access).  The second floor is framed with 16″ floor trusses, so running ducts or trunks lines can be somewhat easier.  The walls will be insulated with spray in cellulose about r-15ish.  The ceiling will be r-50 blown-in.  The foundation walls will be insulated with 2″ foam(r-10) and the bond/rim will be insulated with sprayfoam and fiberglass batt.

My buddy who works for a HVAC distributor has looked and the plans and has done a walk through the house.  He feels that 2 furnace systems would be the best.  I agree for the most part, but I also think that it may also be overkill, it seems that our house isn’t big enough to justify 2 systems, but I really have nothing to compare it against.  The other way he mentioned would be to serve the first floor overhead and the second floor from the floor with one system, but I’m not sure about the comfort factor of overhead heating.  I guess I am looking for some input as what you guys are seeing or doing for houses of the same size and setup. Thanks

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  1. DanH | Feb 04, 2009 11:23pm | #1

    Our house (about 2000 sq ft in two levels, southern MN) was originally built (1976) with two furnaces, but when it came time for replacement we got a single zoned system. Basically the two approaches work pretty much identically. The tradeoffs (other than cost) are that the single furnace takes up much less space while the double give you a "backup" should one fail.

    I see no point in serving the lower floor from overhead, when you have the option of doing it "right". It should be possible to zone a system with both floors served from below. (If they could do it with our house they can do it with any house.)

    The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith
    1. DaveRicheson | Feb 05, 2009 03:02am | #4

      Is your system two stage? I know you have talked about the savings on the dc variable speed blower motors in the past. I was just wondering how it is configured for the a/c.

      I like a zone system. depending on load dynamic, I think a two stage system with zoning is the cats meow.

      Another question for you. Can you explain how the zone dampers, t-stats, and the vs motor all work together to keep the temps the same in each zone? Do you use static air perssure and analog logic ?

      Maybe I'm just over thinking this, cause I use to large commercial syste. I'm guessing the residential systems are a lot simpler, so that is why I'm asking?

      1. DanH | Feb 05, 2009 03:21am | #5

        Yep, it's a two-stage setup with a variable speed DC motor. 96%. The furnace has a "smart" 2-stage timer that attempts to adjust to bring on the high stage a little sooner if it thinks it'll be needed, or a little later if it thinks it won't be needed, based on recent history.The zoning is done via a separate box (though I believe some furnaces have the function built in). It's a computer board that the thermostats feed into, and it controls the furnace and AC. It also has outputs to operate the electric dampers.The zoning is really pretty simple. If either zone calls for heat the furnace fires, and the dampers divert the air to one zone, the other, or both, as indicated by the thermostats. The furnace still decides on its own when to go to high & kick up the fan.AC is done essentially the same -- the zone computer turns on the AC and tells the furnace fan to come on, then selects the zones.I don't know what would happen if you had one thermostat calling for heat and the other calling for AC -- never really wanted to try that.
        The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith

        1. stevenplane | Feb 05, 2009 03:42am | #6

          Would you rather have two cars to carry a family of seven or one car big enough for all? 

          That's kind of what you are asking.  Just get one unit from someone who uses actual load calculations to size your unit...not the old 600 sq. ft. per ton "rule".

          One is easier to maintain than two.  Also easier to replace one when the unit eventually goes bad or becomes outdated...one of which will eventually happen.

        2. DaveRicheson | Feb 05, 2009 05:34am | #7

          Thanks Dan

        3. User avater
          BillHartmann | Feb 07, 2009 06:39am | #9

          That is exactly what I want to do.My house is only about 1700 on 2 stories.But I ended up with 2 furances because that is the only way that I could get any one to put in floor level registers in the first floor. It is on a slab and the back side is 4ft underground. So every one said "we will put a couple of regiters in the ceiling to heat the 'basement'".But they are both side by side. And both very oversided. One drastically.And I would also like to add a heat pump to make it dual fuel.And replace the water heater with a direct vent and then close off the external combustion inlet to the room.The area did not get sealed from the rest of the house like I intended..
          William the Geezer, the sequel to Billy the Kid - Shoe

          1. DanH | Feb 07, 2009 07:03am | #10

            I don't understand. If they can put in floor registers with two furnaces, why not with a single zoned system. It's just ductwork.
            The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith

          2. User avater
            BillHartmann | Feb 07, 2009 08:15am | #11

            Well it was not done at the time the house was built because zoned FA systems where not common.And now the question is if there is enough space to make the connections..
            William the Geezer, the sequel to Billy the Kid - Shoe

          3. DanH | Feb 07, 2009 03:35pm | #13

            If the two furnaces were essentially side by side it should be simple.
            The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith

          4. User avater
            BillHartmann | Feb 07, 2009 04:55pm | #15

            The potential problem is that one is updraft and the other is down draft.So they need to make a lot of direction changes in a small area..
            William the Geezer, the sequel to Billy the Kid - Shoe

          5. DanH | Feb 07, 2009 07:06pm | #16

            That's what ductwork is for. Seems to work for us.
            The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith

  2. rawood | Feb 04, 2009 11:54pm | #2

    Kurt,

    Dan is right.  One furnace with a zoned system is probably the best way to go.  You will save big money on energy costs through the years also.  If you have someone do accurate heating and cooling load calculations, you may be amazed how small the furnace can accually be.  Many people oversize furnaces "to be safe". Also, don't let anyone talk you into running a main trunk line into the upper attic and install insulated supply lines that dump air from ceiling registers into the rooms. That method saves money on installation, but you will pay for it in discomfort and higher energy costs as long as you own the home. Sealing ducts can also reduce the size of equipment and save energy. Creative design and planning of the duct system will pay off in the future and save you money.

    Bob W.

    1. cudavid | Feb 05, 2009 12:04am | #3

      Hi,

      I have 2296 sf 1 level and two Trane hi eff condesning furnaces, 1 for rooms used at night (bedrooms,bath, 1 for rest of house.

      I love it, when 1 crapped out a few years ageo(warrenty) we still had heat, Agreed more $ likly to install, but I like.

      House is R60 attic, R21 walls,  have a Honeywell HVRC (installed in 1993, I was ahead of the curve!) both units 40K btu, and 2 ton ac each, gas fired,

      Dave

  3. User avater
    Matt | Feb 07, 2009 06:22am | #8

    My HVAC contractor recommends 2000 sq ft as the cut off point between one system and 2.  Having 2 (smaller) systems costs about $1k more than 1 zoned system.

    Right now I have 2 spec houses sitting empty side by side.  A 1696 sq ft 2 story house with one system (2 zones) and a 2112 sq ft 2 story house with 2 systems.  They both have gas forced air.  When I was going over the bills lately I noticed that the smaller house had a higher gas bill - by maybe 15%.  I went and checked them out - all thermostats were set to 64 degrees...  No windows cracked open or anything else I could find.  Both gas water heaters are set to "Low".  Can't really explain it.  Just reporting what I found.   I think both are insulated equally.  The smaller house does have a 2 story foyer that is maybe 10'x 10' and then includes the stairway.  Maybe that is it...

    We have a 3 zone system in our house.  It's ~2700 on the main level (with 2 zones) and ~400 on the 2nd floor which has the 3rd zone.  I'm happy with the way it works.

    1. LIVEONSAWDUST | Feb 08, 2009 08:11am | #20

      I had simialar experience...1) 1900 square foot house with single system(75000 btu)

      and                                     2) 2600 square foot house (including walkout lower level

                                                   with 900 sf finished) with two 50000 btu systems

      cubic feet used was about 20% less on the house with two systems, my contractor claims that it was because the lower unit was doing most of the work with a smaller unit and the upper unit only ran occasionally,

  4. danski0224 | Feb 07, 2009 03:01pm | #12

    If you do a load calculation... and do the ductwork right... you could do the whole house with one system. Therein lies the problem with typical residential work... cap-n-tap "fittings", lack of duct sealing, lots of flex...

    The 16" deep trusses can work to your advantage- especially if someone thought ahead and put a larger mechanical chase in the web design (that gets messed up when the truss is set backwards, though). That 16" space could eliminate the need to run ductwork in the attic. Getting return ducting up there too will probably be a challenge (assuming an upflow configuration).

    There is nothing wrong with "overhead heat". Supplies high, returns low... not a big deal. The problems are with poor installation. You lose/gain a lot of heat in an attic system (or unconditioned crawlspace system), and if it is done poorly (there is the sealing and insulation again), performance suffers.

    Zoning adds just as much complexity as a second furnace... more if it is done poorly.

    I am not yet familiar with a zoning system that will modulate between zones without cycling the equipment off and on (and I am not familiar with all brands of zoning systems). That means your single HVAC unit will have twice as many starts and stops as two distinct systems. The equipment has a designed number of starts and stops, and those starts cost you money each time motors/compressors turn on. It costs much more money to start a 4 ton ac unit vs a 2 ton unit- look at the amp draw.

    Ideally, the system would be sized for the largest zone, and not the whole house... but therein lies another problem- you will not have the capacity to heat/cool the entire space at the same time. Sizing to condition the entire home, then zoning results in severe overcapacity and therefore, waste. Residential zoning and equipment does not work like medium pressure commercial VAV systems with variable speed fan drives. Multi stage equipment is a crutch for this fault, but multistage AC units are expensive and complicated. Variable speed compressors, like in mini split systems, are not yet common.

    Two smaller systems will work better than one larger, zoned system... even if one is in the attic. 

    1. DanH | Feb 07, 2009 03:37pm | #14

      Ours is a pretty vanilla system, and the zones can go on and off without stopping/restarting the furnace. I can't imagine why that wouldn't be the norm.
      The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith

      1. danski0224 | Feb 08, 2009 01:07am | #18

        What zone system do you have?

        Last Honeywell and Aprilaire stuff I saw cycled the equipment between calls for heat/cooling.

        1. DanH | Feb 08, 2009 01:13am | #19

          EWC Controls Model NCM 300.
          The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith

  5. frenchy | Feb 07, 2009 08:35pm | #17

    please stop!

        Look at in floor radiant rather than forced air.

      You will be much more comfortable and the potential exists to save money over forced air..

       What you might consider doing is a combination..

      In floor radiant with a back up of forced air  to the windows..

     (windows are your major heat loss area)

     

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