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Old 01-17-2013, 12:44 PM   #31
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Any harm in increasing the control limit temp value?


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Originally Posted by beenthere View Post
The 600 sq ft apartment probably doesn't need anywhere close to 100 BTUs output per sq ft.

1700 sq ft 2 story, maybe a 60,000 BTU input 80% on extreme cold nights.

I do 2400 sq ft 2 story 1930s with 70,000 90%ers, and they have room to spare at -6 outdoor temp.

The OP would need to do a load calc to find out for sure what size his home needs. But he isn't getting the full output from his furnace.
And how well insulated are these 1930s home?

OP has said nothing bout how tight his home is, only that it trips on limit.

It is premature to talk about the size of the furnace as we have yet to nail down the limit trip cause yet. And don't forget he is getting 58* return air.

That is an issue that still needs to addressed also.

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Old 01-17-2013, 01:38 PM   #32
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Any harm in increasing the control limit temp value?


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Originally Posted by hvac5646 View Post
And how well insulated are these 1930s home?

OP has said nothing bout how tight his home is, only that it trips on limit.

It is premature to talk about the size of the furnace as we have yet to nail down the limit trip cause yet. And don't forget he is getting 58* return air.

That is an issue that still needs to addressed also.
2-8" flex supplies with only a 12" flex return pretty much indicates a low air flow issue on a 75,000 BTU input 80% furnace.

18" stone walls with plaster, and 8" blown insulation in the attic.
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Old 01-17-2013, 01:40 PM   #33
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Any harm in increasing the control limit temp value?


58F inlet is spot on because that's what I set my temp at when I'm not home and it's cold in Boston It doesn't take more than a few hours for it to drop from 68F to 58F inside.

The house is old, built in 1900, has huge vinyl windows and really tall 8-10ft ceilings so its a decent space to heat. I also don't think there is insulation in the walls, just horse hair plaster. I'll add foam insulation when I re-do the siding.

I definitely agree though, this is in the realm of experience and knowing how to setup the basics of a furnace which is totally out of my expertise. I've been making some calls for an experienced HVAC professional to come out and assess my house because everything I've read and seen makes me think that whoever installed these was a joke. I've found some gaping holes and leaks on the outlet side of the furnace were the heat is just spewing into the basement giving my tenant lots of free heat. This needs some serious TLC.

I also noticed the downstairs unit has THREE modern outlets, all 8 or 10". My unit is 2x the size and only has two outlets that are 8". What the hell! I need to figure out where I can jam a third duct up to the second floor.

Again, thank you everyone for all the input and advise. I've learned an insane amount about my furnaces! I welcome anymore input you folks have

Last edited by CzarDestructo; 01-17-2013 at 01:46 PM.
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Old 01-17-2013, 01:55 PM   #34
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Any harm in increasing the control limit temp value?


Would I learn anything meaningful if I opened up the doors on my back stairway, removed one of the ducts from my output plenum and let the furnace pump out lots of extra air through my stairwell instead of the ducts to see if it stops over-heating? Wouldn't this prove that it's just starving for air?
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Old 01-17-2013, 02:08 PM   #35
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Any harm in increasing the control limit temp value?


Not really. Since you would still only have 2 supplies. It could stil trip the limit, or move just enough that it doesn't, no way to tell from here.
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Old 01-21-2013, 05:49 PM   #36
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Any harm in increasing the control limit temp value?


For those interested, had a wonderful technician out to my house today. Told him most of what I tested/did and he found indeed that there isn't enough airflow. To compensate for it he turned the fan speed up to its maximum setting (which I could have sworn I checked but apparently it was on low) and lowered the gas pressure which was apparently over 30% higher than it should have been. Everything has been working great since he left but he still told me I really need to run a third duct up to properly fix it as the furnace is starving for air.

Again, thank you everyone for your help and input

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