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In Trouble need 150,000 BTU's

13K views 59 replies 9 participants last post by  turbomangt 
#1 · (Edited)
Our furance is 20 years old and needs replacing, the problem is our house is just over 4200SF and the builder did not 2 zone the air and heat we have a 150,000 BTU furance and so far the 2 guys that came over for an estimate says they do not make a high efficientcy furance in that size, one guy said he would try connecting 2 smaller units in parallel (however he never did that before) the other guy wants to redo the duct work and 2 zone it, (way too much $$$, we will never recop that cost ) my question is what can do if we just want to replace furnace with a better one,
 
#4 ·
A 150,000 BTU input 80% efficient furnace, is 120,000 BTUs output.
A 120,000 BTU input 95% efficient furnace is 114,000 BTUs output. Doubt you'll miss 6,000 BTUs.


A load calc will probably show that a 120,000 95% input furnace is more then enough heat for your house. You can do your own load calc http://hvaccomputer.com/talkref.asp and find out what size you really need.

2 furnaces can be twinned to get the size you need, if it would turn out that you need the full 120,000 BTU output. The new furnaces will have instructions how to twin them. Done right, you'll end up with a nice 2 stage heating system. And probably save a lot of money on your heating bill.
 
#5 · (Edited)
A 150,000 BTU input 80% efficient furnace, is 120,000 BTUs output.
A 120,000 BTU input 95% efficient furnace is 114,000 BTUs output. Doubt you'll miss 6,000 BTUs.


A load calc will probably show that a 120,000 95% input furnace is more then enough heat for your house. You can do your own load calc http://hvaccomputer.com/talkref.asp and find out what size you really need.

2 furnaces can be twinned to get the size you need, if it would turn out that you need the full 120,000 BTU output. The new furnaces will have instructions how to twin them. Done right, you'll end up with a nice 2 stage heating system. And probably save a lot of money on your heating bill.

Gotta put my two cents in here.
At that size furnace there is no wiggle room. The biggest they come is a 120k. and if for some reason the furnace does not push enough air(some of the 150k furnace came with a 1 hp motor) you won't have any recourse with the 120k even if it has a 1hp blower.

A twined system would be two stage with both blowers going at the same time, but only one furnace working under low demand needs.

This gives more flexibility and does not limit your options as a single system would.

as an example a Nordyn standard 90% at 120k has a 1 hp motor.

Two Nordyne 902% 72k have 3/4 hp.

The reason I am suggesting this route is that the size of the OPs home should have had two furnace on separate duct work from the beginning.

Having seen more than my share of bigger home with only one furnace.
I can say the duct work in every case did not distribute well, which is the reason two systems were needed on separate duct systems. The second story of the home was always too warm or too cool.

The twined system has more to offer in correcting the bad air flow.

It reduces electrical consumption on the blowers and only one furnace will run in mild weather with addtional fuel saving.
 
#9 ·
Another idea is that if you upgrade windows and add some attic insulation you may get away with a smaller furnace and save LOTS of Dollaros $$$ and be more comfortable. There may be some home improvement/add insulation grants from your local gov't and or utility co worth finding out about.
 
#10 · (Edited)
Depends if he intends to stay there for a long time and appreciates less drafts and more comfort. Insulation and triple pane low e-argon windows are a wise investment. LOTS of rebates where I am for those improvements so why not take advantage of them. I hear it gets real cold in Chicago at times so those improvements will pay for themselves.
 
#11 ·
Maybe soooooo....

But he was asking about a furnace.... Not windows or insulation:wink:
True.

But, new windows can cut his heat loss enough that a new smaller furnace can save him enough on his heating bill. that they both pay for themselves in a short time.

Only thinking of a new furnace. Without giving consideration of what can be done to the house to save on heating and cooling cost. is a mistake many make.
 
#12 ·
Agreed
New windows, doors & sill insulated in basement cut my heating bill in 1/2
Just the 3 basement windows & sill insulated made a huge difference
Cost for the basement maybe $250

I've continued to update the insulation in my house
Added a 250 sq ft sunroom & heating cost still dropped
 
#13 ·
Personally I'd go with the twin setup. The nice thing with that is if one furnace breaks down, you have redundancy. It may not be as effective, but it's better then no heat at all. Guessing they need to be wired in a special way so the blowers run at same time though, otherwise you'd just be pushing air in the return of the other furnace if one blower is off, or is there some kind of damper? Would be interesting to see a setup like this. I've been in a house that had two, but never noticed if the ductwork was connected in any way. It may have been two zones.
 
#15 ·
Its all part of a load calc on a house
Right ?

If someone did not take this into account when replacing my system then I'd be stuck with an oversized 130,000 BTU system instead of the 68,000 system my house needs

Like it or not this will come up

I need someone I can trust, referrals welcome, need advice...
Sounds like he asked for advice.....
So he is getting advice

Advice is advice
As long as it is all concerning replacing the furnace
We would be remiss to NOT suggest reducing the heating system he needs
THIS IS ON TOPIC
So STOP repeating to keep on topic
 
#16 · (Edited)
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.
 
#18 ·
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.
 
#19 · (Edited)
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.
 
#21 · (Edited)
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.
 
#28 ·
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.
 
#24 ·
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.
 
#25 ·
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.
 
#27 ·
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.
 
#29 ·
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?
 
#30 ·
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.
 
#31 ·
Had two contractors out both said opposite things,
guys one, wants to put in 2 units with 2 stage motors, 80,000 btu's each.
guys two wants to put in one 95% unit(lennox G71MPP0 with what he called a mogillating (not sure of spelling) continuos motor, he said he guarantee's it will do the job, he also said you cannot have 2 units with both 2 stage motors and have then run in sink, now I'm more confused, everyone says they are right, but no one agrees.
 
#32 ·
BIG NO ON MODULATING FURNACE! Same for two stage furnaces.

THEY CAN NOT BE TWINNED OR USED IN TANDEM.
Modulating and two stage furnace blower motors will not run at the same speed consistently which is necessary to prevent the individual blowers from over powering the return air ( that could be said better but that's the best way I know how to put in plain terms).

Yuri you are the resident lennox expert. How about a little input?
 
#33 · (Edited)
[Edit]After HVA's kind words below, I thought harder.... I can't find the article I was referring to, but on reflection I think it was about sizing AC units not furnaces. I'm still curious if anyone has done a J-manual load calc as opposed to a thumbnail guess.[edit]


A lot of comments about "load calc".

So these guys come over, eyeball the existing unit, and just sort of look around, then say "you need ___x____ BTUS", right? I mean, did anyone ever spend considerable time inspecting the building and punching numbers into the load calc software?

If that never happened, then there is a very strong chance their eyeball guesstimate is 'way more BTU than you need. There was a good article in FHB magazine awhile back. A common practice is for contractors to size HVAC gear for the extremes of the location. But that's only for a very small number of days in the year. The rest of the time they cycle off and on in some way that is inefficient. I forget the tech reason its inefficient hte rest of the year. The eyeballing-style contractors do that to make sure the customer is comfy and prevent complaint calls that one or two days each year that hit the extreme temps. The author argues that the CORRECT BTU output is for based on need during the bulk of the rest of the year. Anything else, they said, was oversized and inefficient.

So.... if you are able to swing it.... consider a real load calc for the building as-is, and then tweak it for various insulating/sealing/venting upgrades. That might shed a lot of light on your options and best bang for buck.

SteveEl
 
#35 ·
A lot of comments about "load calc".

So these guys come over, eyeball the existing unit, and just sort of look around, then say "you need ___x____ BTUS", right? I mean, did anyone ever spend considerable time inspecting the building and punching numbers into the load calc software?

If that never happened, then there is a very strong chance their eyeball guesstimate is 'way more BTU than you need. There was a good article in FHB magazine awhile back. A common practice is for contractors to size HVAC gear for the extremes of the location. But that's only for a very small number of days in the year. The rest of the time they cycle off and on in some way that is inefficient. I forget the tech reason its inefficient hte rest of the year. The eyeballing-style contractors do that to make sure the customer is comfy and prevent complaint calls that one or two days each year that hit the extreme temps. The author argues that the CORRECT BTU output is for based on need during the bulk of the rest of the year. Anything else, they said, was oversized and inefficient.

So.... if you are able to swing it.... consider a real load calc for the building as-is, and then tweak it for various insulating/sealing/venting upgrades. That might shed a lot of light on your options and best bang for buck.

SteveEl

Excuse me while I wipe the excrement from shoes I stepped in from your post.

Do not attempt to confuse the OP further. He already knows a load calc needs doing.

If you had any credentials you sure invalidated them with your post.

You're saying if, say in Mi, the design temp is five degrees and the load calc says a 60kbtu furnace is required, the home would not need so big a furnace because it seldom gets down to five degrees.

Horse Hockey Pucks!

You get OD ambient of 20*F with a strong wind blowing you are going to need every bit of that 60Kbtu of heat.

Three thing effect the speed of heat transfer: size, temp difference, and BTUH content.
 
#34 · (Edited)
Clover is right. May not be too pretty but he knowz hiz stuff. :thumbup: I just checked the Lennox G61V and the G71MPP and neither has a twinning terminal on their circuit boards for the reasons he said . I would use 2 G51's and get a PROPER heat loss calculation done to buy the correct sized units. NO eyeballing in your situation. Lennox makes an optional twinning kit for the G51. Very good tried tested and true furnace, no fancy bells or whistles, easy to fix and get parts for. I have one in my house.

Good Luck:)
 
#36 ·
Turbo, can you take a half dozen or more pics of the furnace and ductwork so we can see what it looks like. May need modifications to the return duct and we would like to see what you have now to help better. Post them at photobucket and put the links here or make small jpegs and post them here.
 
#37 ·
No VS blower unit, no matter what brand it is, is approved for twinning. proper twinning requires that both units be single stage units. And PSC blowers.

A mod can serve you well on your duct system. Since it has low static in its lower modulations. it tends to have more air come out at the far registers, then at the near registers.
 
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