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Old 08-28-2020, 02:53 PM   #1
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Furnace and AC sizing question


Hello,

I have a question about sizing and we need a new furnace and AC. The current system was install in 2004. I think its a 90% efficient 80BTU with a 2.5 ton AC. about our house it's 3000 sq st 2 story, however the original 2 story was built in 1890 and good insulation in that attic but only 1/2 foil covered foam for the side walls(was like that when we got it) we put log siding on it if that matters replaced all the windows and doors. in 2004 we put the addition on so the old part is about 1600 sq ft and the new about 1400sg ft. We are in Minnesota and the system that was in stalled in 2004 never shuts off in the winter or summer, unless it's a nice day that you dont need heat or air. The system never kept the house warm enough or cool enough we always run space heater as well as window AC in the old part of the house. Needles to say not happy with the old system.

So we decided to buy the new system ourselves and have it installed. We based the size of the multiple different hvac sizing calculators and told the manufacture about our house and issues then decided on getting a 5 Ton 14.25 SEER Upflow 95% AFUE 110,000 BTU Complete Split System Air Conditioner with Gas Furnace. We just had the guy out to install it and he just informed us that it's to big both heat and AC. He then said we need the same size system that we had.

We would greatly appreciate any advise on this as it doesn't seem like a good idea to basically put in a newer version of what didn't work.

Thanks
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Old 08-28-2020, 03:14 PM   #2
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Re: Furnace and AC sizing question


There are no online sizing calculators or charts. Equipment isn’t sized via square footage.
Equipment is sized via manual J calculations.
www.loadcalc.net
It’s a poor idea to buy online and have the equipment installed. If you want to buy online then it’s best to install it yourself. Otherwise let a contractor supply and install the equipment.
Most consumers don’t understand or realize the lack of warranty they are getting with online equipment purchases.
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Old 08-28-2020, 03:41 PM   #3
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Re: Furnace and AC sizing question


I would suspect part of what the installer is concerned about is duct sizing. The new system will require at least double the airflow for cooling (assuming the furnace fan is sized correctly). It is obvious that you need more heating and cooling and I doubt the old system was properly sized and on top of that additions and changes. It sounds like you have done at least some of your homework for the new size, but doubling the AC seams a little much, either way you need to do some research on duct sizes (there are duct sizing calculators on line that can help, just remember the furnace fans are a very low SP).


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Old 08-28-2020, 04:04 PM   #4
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Re: Furnace and AC sizing question


Thanks for the reply's and info.

The blower is sized correctly for the use with the AC, it was sold a complete ready to install package. We understand warranty or lack there of. We just want to make sure that what we are being told is correct, as it does make sense to install the same thing again. The AC couldn't even keep to new part cool.

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Old 08-28-2020, 04:22 PM   #5
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Re: Furnace and AC sizing question


That could be a duct problem.
No, it doesn’t make sense to install the same size equipment unless the old existing system properly conditioned the space, and didn’t cycle on overly hot/cold days.
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Old 08-28-2020, 04:37 PM   #6
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Re: Furnace and AC sizing question


I will have to research the ducting to find out more info.

I'm not exaggerating when I said that unless it was nice enough to have windows open if it was cold the furnace would run 24/7 until spring and in the summer the AC ran 24/7 as well. So we do not want to put the same back in.

We cant return it even if we want to, it sat in the garage for months after we purchased it.
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Old 08-28-2020, 04:46 PM   #7
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Re: Furnace and AC sizing question


I would highly suggest you verify sizing.
Before you put it in and have problems, like mold from excessive humidity or the furnace cycling on limit.
The whole system, equipment and ductwork, is a puzzle that has to fit properly together.
Isn’t it going to get expensive redoing your ductwork to accommodate the larger unit?
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Old 08-28-2020, 05:25 PM   #8
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Re: Furnace and AC sizing question


Here are some links. Read-and understand-the information they have to offer.

http://efficientcomfort.net/assets/d...ty_Control.pdf

http://efficientcomfort.net/assets/d...ent_Sizing.pdf

http://efficientcomfort.net/assets/d...s_Properly.pdf

http://efficientcomfort.net/assets/d...ign_Issues.pdf

https://www.angieslist.com/articles/...vac-brands.htm
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Old 08-28-2020, 05:27 PM   #9
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Re: Furnace and AC sizing question


Many things can cause the system to not deliver rated capacity and you should never blindly upsize.

For example, the furnace could have been under-fired or on the other end, cycling on limit.

80k 90% does sound a little lite for Minnesota 3000 sq ft above ground when the house has poor insulation. It's a pretty brutal winter climate, right?

It's very possible you need 100k btu input - but without the calculations it's a guess. The ducts need to be handle the airflow required.

Same for 2.5 ton -> you may need 3 or 3.5.

I do favor upgrading insulation and air sealing over up-sizing.

One thing you should know is that running continuously is desirable, provided the house is comfortable.

The equipment is sized so that it almost never shuts off in the most extreme conditions you get. The less extreme it is, the less time should be spent on.
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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.
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Old 08-28-2020, 05:29 PM   #10
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Re: Furnace and AC sizing question


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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.
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Old 08-28-2020, 06:19 PM   #11
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Re: Furnace and AC sizing question


This is what the temp is like at my desk in the middle of winter if I don't have the baseboard electric heat on.


Furnace and AC sizing question-imag2694.jpg
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Old 08-28-2020, 06:21 PM   #12
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Re: Furnace and AC sizing question


probably don't have enough if any supply air going to that room.

you have more wrong than potentially undersized equipment. get a professional in to evaluate your system before you pull the trigger on replacement.

it can be well worth the money.

I would not jump to replace equipment from 2004 unless very expensive repairs are required. you may be able to get what you have working properly, then you can focus and ductwork and reducing heat loss/gain.

the furnace and ac should be checked to see if they're set up properly and delivering full capacity.

ducts need to be evaluated.
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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.

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Old 08-28-2020, 06:28 PM   #13
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Re: Furnace and AC sizing question


If the ducts are insufficient, adding larger equipment can actually amplify the issue. The room could wind up colder because of shorter run times. And the equipment life will be greatly shortened as well as make the equipment very unreliable due to lack of airflow.

***Do you plan on upsizing/replacing the duct work for this larger system you’ve bought?***
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Old 08-28-2020, 06:31 PM   #14
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Re: Furnace and AC sizing question


There's duct 8' from there. 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 plaster walls zero insulation except for 1/2" R-5.78 Rigid Foam Insulation that we added to the outside before residing. Our plan is to gut the old part someday but that is a long way off for now.
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Old 08-28-2020, 06:36 PM   #15
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Re: Furnace and AC sizing question


So you were planning on going from 2.5T/80k BTU to 5T/110k BTU with no duct changes?
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