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Old 08-28-2020, 06:43 PM   #16
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Re: Furnace and AC sizing question


Your existing equipment may still be fine and last another 5+ years. Fix the house/ducts before bothering with new equipment.

If you do it the other way around, you will end up with oversized equipment plus any problems caused by your duct system as it will not be fixed.

Most of your problems will probably go away with proper distribution and equipment operation*.

*Furnace airflow and fuel input properly set, a/c airflow and refrigerant charge.

With an undersized furnace, the house should still be comfortable most of the time.

It sounds like the house has a lot of problems - replacing the furnace without figuring out everything that's wrong is malpractice.

You may not be able to solve every problem at once, but you will not be happy with the results if you just get a bigger furnace/ac.



Quote:
The problem is that part of the house the floor is only 6-8" above the dirt with a field stone foundation and no insulation in the floor with
Is the house above a cold crawlspace?

The best way to do with a crawlspace - encapsulate the floor and insulate the walls, close the vents, get a little heat in there.

Insulating floors is less effective.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.

Last edited by user_12345a; 08-28-2020 at 06:46 PM.
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Old 08-29-2020, 12:02 AM   #17
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Re: Furnace and AC sizing question


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Originally Posted by roughneck View Post
So you were planning on going from 2.5T/80k BTU to 5T/110k BTU with no duct changes?
We are planning on increasing the size of 3 of the 7 runs on the old part of the house. The other 4 are maxed for available space. We want to add 2 more runs but they will have to wait until we gut those rooms.
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Old 08-29-2020, 12:08 AM   #18
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Re: Furnace and AC sizing question


Room by room calc results need to guide everything done to the ductwork - don't just randomly decide to add/increase duct size runs.

Required cfm is calculated based on heat loss/gain of each room as well as airflow requirements of equipment.

Trunk ducts also need to be sized to feed the branches.

You will have horrible results if you proceed like this - throwing more capacity at the problem and just upsizing branch ducts.

Please heed previous advice - need to verify what you have is working properly.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.

Last edited by user_12345a; 08-29-2020 at 12:13 AM.
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Old 08-29-2020, 12:12 AM   #19
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Re: Furnace and AC sizing question


Quote:
Originally Posted by user_12345a View Post
Your existing equipment may still be fine and last another 5+ years. Fix the house/ducts before bothering with new equipment.

If you do it the other way around, you will end up with oversized equipment plus any problems caused by your duct system as it will not be fixed.

Most of your problems will probably go away with proper distribution and equipment operation*.

*Furnace airflow and fuel input properly set, a/c airflow and refrigerant charge.

With an undersized furnace, the house should still be comfortable most of the time.

It sounds like the house has a lot of problems - replacing the furnace without figuring out everything that's wrong is malpractice.

You may not be able to solve every problem at once, but you will not be happy with the results if you just get a bigger furnace/ac.





Is the house above a cold crawlspace?

The best way to do with a crawlspace - encapsulate the floor and insulate the walls, close the vents, get a little heat in there.

Insulating floors is less effective.
The existing system started failing last year and died at the beginning of this summer. We have been surviving with window AC. We have had insulating experts out they recommend against insulating the "crawl space"(you would have to be a bug to crawl in there), they said it would just turn into a rodent nest. They also recommend that we wait until we remodel/gut the old part of the house.
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Old 08-29-2020, 12:19 AM   #20
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Re: Furnace and AC sizing question


I know you’ve already proceeded with your plans, but for hvac I’d suggest scrapping any ideas and plans, sell the equipment you have and start from scratch. The information you’ve gathered so far is guesswork and assumptions.
Start fresh, do it right and you’ll not have any more issues to worry about. And you will not have to redo everything a second time.
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Old 08-29-2020, 12:20 AM   #21
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Re: Furnace and AC sizing question


I think after all comments our best option is to sell the new system and attempt to recoup some of that money.

Then maybe do what we wanted to do back in 2004 and have 2 systems installed 1 for the old part of the house and 1 for the new part. We have another month to get this figured out before the cold gets here.
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Old 08-29-2020, 09:03 AM   #22
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Re: Furnace and AC sizing question


That would be wise.
Do a load calculation and plan out everything before making a purchase. Measure twice, cut once as they say.
If duct is a problem, or the house is very cut up, you could do minis or even VRF. Can be pricy though.
Just keep in mind, all parts of the system, meaning the sizing, equipment and duct, must play nice together.
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