Single Large Zoned System Vs Multiple Seperate Systems - HVAC - DIY Chatroom Home Improvement Forum

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Old 08-18-2008, 12:37 PM   #1
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single large zoned system vs multiple seperate systems

I live in a old Victorian home and am starting to plan the replacement of the furnace/AC. The furnaces are going on 20 years old and the AC system is going on 15 years and I would also like to add HVAC to the 3rd floor. I will be adding zoning to the system regardless on whether it is a single unit or 3 separate units.

Here is what I have now:
-3 floors plus basement, roughly 30' wide and 45' deep
-first 2 floors are solid brick, 18" thick with the interior plaster directly on brick, no insulation

-all windows are double pane vinyl
-3rd floor is a finished attic like space that will be insulated
-1st floor: 1300 sq/ft with 280 sq/ft of windows/doors
-2nd floor: 1300 sq/ft with 200 sq/ft of windows/doors
-3rd floor: 900 sq/ft with 80 sq/ft of dormer windows
-two identical furnaces, one each for first and second floor. 3 ton AC and 100k BTU gas heat each which going by comfort level and run time, they are sized close to properly for their efficiency ratings

Is it better to go with one large system that covers all 3 floors or to have a separate AC/furnace for each floor?


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Old 12-04-2008, 04:27 PM   #2
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I am building a new house and would also like some input on this. I'm debating whether to get a zoned unit for the basement and main level or two separate units. I know that two separate units seem to be more efficient and you get more cost savings in the long run, but a zoned unit seems to be cheaper up front. I was wondering if anyone had any good estimates on how long the pay back periods would be for going with zoned or standalone units is as well as if there is a general consensus in the industry as to whether standalone is better than zoned.


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Old 12-04-2008, 04:44 PM   #3
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Brian: I doubt if anyone makes a furnace large enough in BTU's to do what you want. The largest Lennox high efficiency unit is 135,000 BTU's. I have installed a high end Lennox Harmony 111 system with a G61V furnace and understand their product. It may be quite difficult to balance your airflows if the furnace is very far from the 3rd floor. I would get separate systems and make sure the installer has installed quite a few zoning systems. It requires professional duct sizing, commissioning and most companies don't specialize in that type of work. Very good results when done properly but expect to pay a good $$ for it. The Lennox system will modulate the furnace firing rate/fan velocity and do all kinds of neat functions when set up properly. Make absolute sure that they do a proper heat load/loss calculation B4 anything else.

Squiggie: Similar info but I would add that the initial cost of the zoning panel, wiring, commissioning will add a lot of cost. 2 separate units may be better. The zoning system may be more comfortable if setup/installed/sized properly.

Some info for you guys:

Last edited by yuri; 12-04-2008 at 05:45 PM. Reason: fix spelling
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Old 12-04-2008, 10:50 PM   #4
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If you've got the money, do additional systems. Otherwise you'll push a single system to work extra hard to heat and cool a large area. it's like 1 horse trying to pull a ton of bricks VS 3 horse trying to carry the same load. Which analogy works more efficiently and effectively. Yea, the 3 horses cost more out of the stable, but if you're gonna stick in the house for a while (5+ years) you'll make the money back on your utility bills.
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Old 12-05-2008, 06:18 AM   #5
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Use 2 units atleast.
Why use a furnace 2 to 3 times the size needed for a small area, when your duct work is already set up for 2 units.

You believe they are sized right. They may be oversized.
Do your own load calc, and see what size each floor takes.
You might find that instead of needing 2 100,000BTU furnaces, that you can use a 60,000BTU on the first floor, and a 80,000 for the second and third floor.
Even old farm houses seldom need 60BTUs output per sq ft for heating.
Try this sizing program, it could save you a lot of money in both installation cost, and operating cost.

Also keep in mind. That newer furnaces require more air flow then older furnaces did.
So if you use the same size as you have now, your duct work could be very noisey with the new furnace trying to move 30% more air.

Same with A/C.
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Old 12-05-2008, 07:47 AM   #6
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If you want more info on the benefits of heat loss see this link.


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