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Old 09-06-2010, 12:24 AM   #16
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In Trouble need 150,000 BTU's


Twinning 2 furnaces doubles the initial cost and requires EXTENSIVE return duct modifications which may or may not be physically possible due to space constraints. Both units need equal amounts of return air and ductwork or each fan will fight the other one and be unbalanced in the long run. NOT as easy as it sounds. Easy to interlock them, hard to balance them. Would require 2X as much venting and holes in the wall for the venting. Unless you have done a job like twinning there is a LOT more logistically to it than it sounds. Reducing the heat load is a lot more cost effective and comfortable in the long run but most people don't think of that idea.

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Old 09-06-2010, 02:26 AM   #17
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What is so hard about balancing a twined. One furnace will activate on low demand...the furnace not being called on turns its blower onto keep air flow volume in the duct uniform and stops it from revolving in reverse.
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Old 09-06-2010, 04:21 AM   #18
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In Trouble need 150,000 BTU's


If he runs a load calc with HVAC CALC. He can do what ifs on his insulation and air sealing. And see if he can reduce his heat load. So a nice single 2 stage furnace can be installed. And keep his upfront cost on the furnace down. And still save money on his heating bill.
Of course, if he also has A/C, the insulation and sealing will also reduce that bill too.
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Old 09-06-2010, 01:28 PM   #19
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If he runs a load calc with HVAC CALC. He can do what ifs on his insulation and air sealing. And see if he can reduce his heat load. So a nice single 2 stage furnace can be installed. And keep his upfront cost on the furnace down. And still save money on his heating bill.
Of course, if he also has A/C, the insulation and sealing will also reduce that bill too.

It's more about having the additional blower power then anything.

like I said, when one big furnace is used in situations like this the air distb is not good at all.

You get more air flow with a twined system. I know what you Pros are thinking , "Clover's off his nut again. he's not considering the the higher static pressure with a higher air flow.."

Well it's the lesser of two evils.
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Old 09-06-2010, 03:41 PM   #20
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It's more about having the additional blower power then anything.

like I said, when one big furnace is used in situations like this the air distb is not good at all.

You get more air flow with a twined system. I know what you Pros are think, "Clover off his nut again. he's not considering the the higher static pressure."

Well it's the lesser of two evils.
Your right. Lots of those big houses have really undersized duct work. And many contractors will use twinned systems to move the extra air a new same BTU furnace needs to move.

A smaller furnace that moves less air. Will heat the house just fine. After the duct system has been balanced.
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Old 09-06-2010, 08:38 PM   #21
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In Trouble need 150,000 BTU's


I am talking about right sizing a twined combo.
Been, we are talking about duct system that is so sub standard that there is no fix for other than pushing more air than the original system.

I am going to give you an example of what the typical 3500 to 4500sqft home with one furnace is like around here.

It's always a colonial. The single furnace is always 150k or 200k (arcoaire made 200k resi furnaces in the 90s).

There are take offs on top off the side and off the bottom of the supply trunk. Branch runs feeding the second story that run 30 or40 feet making left and right turns before heading up to the second story.

You use a furnace with lower airflow in this situation the air will not have enough volume to fill the trunk let alone the branch lines.

Lower airflow velocities would hit terminal point before getting to a register.
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Old 09-07-2010, 04:05 AM   #22
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Where does the OP say he has a duct system like that.
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Old 09-07-2010, 05:11 PM   #23
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he does not need to.

The size and design of homes over 3000 -3500 sqft dictates the layout of the trunk system. When used with only one furnace the results are going to be an overextended duct system in any case.
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Old 09-07-2010, 05:26 PM   #24
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Got lots of 3600 plus sq ft homes here. With single furnaces(Had GE's in originally).

Duct systems aren't over extended. Head room in the basements is often lacking though.

I gots lots of head aches before I learned to duck low enough in some of them.
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Old 09-07-2010, 05:37 PM   #25
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Got lots of 3600 plus sq ft homes here. With single furnaces(Had GE's in originally).

Duct systems aren't over extended. Head room in the basements is often lacking though.

I gots lots of head aches before I learned to duck low enough in some of them.

We have complete abortions here. The emphasis has always been to put the mechanical systems to one side of the basement so the HOs have more unobstructed basement room to finish off to living space.

Have not seen many that worked well no matter how well installed.

Always more hot and cold spots in single furnace systems.
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Old 09-07-2010, 06:02 PM   #26
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In Trouble need 150,000 BTU's


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We have complete abortions here. The emphasis has always been to put the mechanical systems to one side of the basement so the HOs have more unobstructed basement room to finish off to living space.

Have not seen many that worked well no matter how well installed.

Always more hot and cold spots in single furnace systems.
Agree....

If you are going to to do a split level... either zone it or go with 2 "seperate systems"

Remember that 99.9999% of the time the stat is on the lower level.

so the upstairs allways suffers
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Old 09-08-2010, 02:24 PM   #27
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In Trouble need 150,000 BTU's


Well......I hate to say this, but I had a heating contractor over today, and when he looked at the furnace, he said Holy Crap, this monster is 200,000 BTU's, not sure what I was thinking when I said 150,000 anyway, my bad, it is 200,000 he is going to work up some prices and ideas, however he did say the only way to get a new system is to get 2 of them, nothing is easy for me, nothing.
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Old 09-08-2010, 02:32 PM   #28
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Quote:
Originally Posted by hvaclover View Post
I am talking about right sizing a twined combo.
Been, we are talking about duct system that is so sub standard that there is no fix for other than pushing more air than the original system.

I am going to give you an example of what the typical 3500 to 4500sqft home with one furnace is like around here.

It's always a colonial. The single furnace is always 150k or 200k (arcoaire made 200k resi furnaces in the 90s).

There are take offs on top off the side and off the bottom of the supply trunk. Branch runs feeding the second story that run 30 or40 feet making left and right turns before heading up to the second story.

You use a furnace with lower airflow in this situation the air will not have enough volume to fill the trunk let alone the branch lines.

Lower airflow velocities would hit terminal point before getting to a register.

You will see a huge difference in comfort. Won't be cheap, but repairing
a botched job with quality work and materials pays for itself faster than you think.

Your gas and electric will dip substantially.
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Old 09-13-2010, 06:52 AM   #29
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In Trouble need 150,000 BTU's


Clover described my situation almost to a "T". 200,000 BTU's, 2 guys said and pointed out to me that they tapped into the main trunk to feed the up stairs, and I get poor airflow up there, even with the size of that monster furnace. poor ducting is an under statement, looks like I'm leaning toward 2 units, what should I be aware of when I hire someopne do to this?
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Old 09-13-2010, 02:37 PM   #30
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Clover described my situation almost to a "T". 200,000 BTU's, 2 guys said and pointed out to me that they tapped into the main trunk to feed the up stairs, and I get poor airflow up there, even with the size of that monster furnace. poor ducting is an under statement, looks like I'm leaning toward 2 units, what should I be aware of when I hire someopne do to this?
Hire the best pro for the job. Choose somebody who has done a few jobs like yours. Does not have to be in the hundreds. Get references and call the customers to find out if they were happy with the out come.

CAUTIONARY NOT: The opinions will vary but look for the overall consensus that there was improvement in comfort.

Because your duct work is the way it is you will never get the same results
as a system with two dedicated duct system.

But with a knowledgeable Pro and two furnaces you will get a worthwhile and more comfortable out come.

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