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Old 01-31-2012, 01:41 PM   #1
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BTU/h estimates


I'm about 99% ready to sign a contract for a new furance install... going with a modulating furnace...

there are major swings in the BTU/h ratings though...

right now it has a 125k BTU/h furnace which overheats...

1st bid wanted to put in a 60k BTU/h furnace
2nd bid wanted to put in a 120k BTU/h furnace (for some reason i dont think they did the manual J)
3rd bid wanted to put in 90K BTU/h furnace


when I calculated out the manual J myself I got 84K BTU heat loss...

does the fact its a modulating with 98% AFUE play into this? right now we have a 92% AFUE single stage..

the 1st and 3rd bidders did do a manual j they said as part of the bid, so not sure why there is a 30K btu/h swing... and I got 84K myself...

the house is 1,500 sq ft 1st floor, 600 sq ft 2nd floor and 1,500 ft below ground finished (minimal heat loss here, completely insulated and all below grade)

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Old 01-31-2012, 01:59 PM   #2
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They dont make a 84,000 btu furnace....you have to go with the closest one you can get......go with the 90,000......it will be damn near perfect .What brand did you decide on?

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Old 01-31-2012, 02:14 PM   #3
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They dont make a 84,000 btu furnace....you have to go with the closest one you can get......go with the 90,000......it will be damn near perfect .What brand did you decide on?
lennox slp98v... i dont like the installer company but what you going to do.. well its 3 people on their crew i dont like, the rest seem to be with it
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Old 01-31-2012, 03:38 PM   #4
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BTU/h estimates


The 60,000 is probably the right size.
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Old 01-31-2012, 04:09 PM   #5
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lennox slp98v... i dont like the installer company but what you going to do.. well its 3 people on their crew i dont like, the rest seem to be with it
You have made a well educated choise.....you will be happy with the out come.
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Old 01-31-2012, 06:04 PM   #6
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BTU/h estimates


Hopefully they make sure your duct is properly sized as well. Usually you find undersized ducts and an oversized furnace
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Old 01-31-2012, 08:39 PM   #7
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The 60,000 is probably the right size.
yeah, it just seems small for the size of the house to me after the current furnace was sized at 125k BTU/h cut the size in half...

but I also did heat gain and got 30,433 BTUH, 2.5 Ton AC which is exactly what they came up with... my heat loss numbers might of been too high (1st time doing this myself so to get at least close is neat) I came up with 85,000 BTU/H loss they came up with 60,000, I'm sure I missed something when I did it

shouldn't the furnace have a communicating thermostat? I thought they used IAQ or some version of ComfortR, not sure what Lennox used for modulating, they just quoted it as a standard non-programmable digital thermostat gona have to ask them about that since I requested a programmable one at minimum...
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Old 02-01-2012, 02:29 AM   #8
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Lennox would be the Icomfort stat.
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Old 02-01-2012, 05:50 AM   #9
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You can use a single stage program or non program but that will give you 3 stage operation. a 2 stage programmable like any of the 3-4 Lennox has will give you modulating BUT they have to run another wire to the w2 on the board and I would get 6 wires installed so you have a C and don't need to change batteries. The IComfort is good with 4 wires and is communicating but is probably another $300-500 upgrade. I would go with the 90,000 as it will VERY rarely ever ramp up that far and with a mod it will probably mod between 30-70 % most of the time. will go higher in my climate and on the few severely cold days you have. you do need the extra capacity for those days. comfortsense 5000 and 7000 are very good. 7000 has most of the features of the Icomfort without the nice blue 3D screen and price tag.

http://www.lennox.com/products/comfort-controls/
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Old 02-01-2012, 08:09 PM   #10
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Are you getting new a/c too?
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Old 02-01-2012, 09:49 PM   #11
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If you have a heater that overheats it doesn't necessarily mean it's sized improperly. A faulty or ill-placed thermostat is generally the reason for an overheating situation. Could be poor duct placement or lack of return air. What if the efficiency of the current furnace? A 100,000 btu furnace with a 80% efficiency is equivalent to 80,000 btu with 100% (not that you will find a 100% efficient furnace) Your furnace may need replacing, but it's not all you need to consider.
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Old 02-01-2012, 09:57 PM   #12
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Excuse me for going rude here.This thread is all over the page and leaves everyone asking what the ****.
Your house has one heating need.Going too big is going to cost you extra everry time it runs.It wuill be on for a very quick timew then go off and then it will have to come back on extra timews to make up the differance.You will be heating up 1,2,or even 3 extra sections on the heat exchanger and your blower mtr and venting mtr will be bigger (read costs extra to run) than it needs to be.,,You will have more service problems and less effiency.
Too small and you get cold.
Two manuel J 's will not or better should never be way off.
Now putting in a modulating furnace because it will only ramp up to need is a lie.A well sized furnace will have longer runs and will run fewer times .Thats part of the effiency thing.
Throwing in a mod furnace is doing you no favor at all.Afterall its not the installers house.
Do this thing right.You will be living with this for the next 20 years.
Now if your name is Mitt and you only pay 13.9% tax then you can forget what I just wrote,but most of us can't afford to be wastefull.
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Old 02-02-2012, 04:34 AM   #13
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but I also did heat gain and got 30,433 BTUH, 2.5 Ton AC which is exactly what they came up with... my heat loss numbers might of been too high (1st time doing this myself so to get at least close is neat) I came up with 85,000 BTU/H loss they came up with 60,000, I'm sure I missed something when I did it

Check the info you used for your heat loss. Such as design temps, and infiltration rates. Don't pad it.
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Old 02-02-2012, 07:17 AM   #14
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Excuse me for going rude here.This thread is all over the page and leaves everyone asking what the ****.

Define "everyone", i was and still am very confident that i gave the correct answer.


Your house has one heating need.Going too big is going to cost you extra everry time it runs.

I guess you have never had the pleasure of seeing a mod furnace operate

It wuill be on for a very quick timew then go off and then it will have to come back on extra timews to make up the differance.You will be heating up 1,2,or even 3 extra sections on the heat exchanger and your blower mtr and venting mtr will be bigger (read costs extra to run) than it needs to be.,,You will have more service problems and less effiency.

again this statement begs the question......do you know how a mod furnace operates?


Too small and you get cold.

again.....there is no such thing as a mod thats "too small"


Two manuel J 's will not or better should never be way off.
Now putting in a modulating furnace because it will only ramp up to need is a lie.

So they run past the t stat set points? not the ones we install



A well sized furnace will have longer runs and will run fewer times .Thats part of the effiency thing.

please explain the effiency thing in more detail, you have me scratching my head


Throwing in a mod furnace is doing you no favor at all.

Except cutting big bucks off the yearly heating budget.........money savings are so over rated now a days.....who needs it?



Afterall its not the installers house.
Do this thing right.You will be living with this for the next 20 years.
Now if your name is Mitt and you only pay 13.9% tax then you can forget what I just wrote,but most of us can't afford to be wastefull.
He has been given excellent advise by sevral people on this thread......why throw BS statments out and confuse the OP?
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Old 02-02-2012, 09:03 AM   #15
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If you have a heater that overheats it doesn't necessarily mean it's sized improperly. A faulty or ill-placed thermostat is generally the reason for an overheating situation. Could be poor duct placement or lack of return air. What if the efficiency of the current furnace? A 100,000 btu furnace with a 80% efficiency is equivalent to 80,000 btu with 100% (not that you will find a 100% efficient furnace) Your furnace may need replacing, but it's not all you need to consider.
the entire system is being replaced, furnace, ac, duct work, thermostat, eveything... ducts where undersized furnace was oversized, thermostat was no where near a heat source, it was centrally located also... furnace now was 92% AFUE, but it was basically junk.. they are redoing the entire system

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