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Discussion Starter · #1 ·
I’m in Colorado, elevation 8700 ft. Very little air conditioning here, but with the rising temperatures I’m considering adding central air to my central heating. My lower level in no way needs a/c especially with our very low humidity. I’ve had two estimates that figured 4 ton units which I understand might be needed for my total area, but no way I need that much to cool 1700 sq ft. Currently I have two 10,000 btu portable units and rarely need the second. I would prefer central over these portable units because of the noise and venting. I understand it becomes a furnace fan volume issue when I shut off the lower level but otherwise it’ll be an ice box if I don’t. With our kids grown, the lower level vents are closed in winter also. The Manual J calculator doesn’t allow me to pick a location even close to my area or elevation. Is there a way to make this work?
 

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By lower level, are you talking about a basement or a first floor, entirely above ground?

The calculation should include the entire house.
Ductwork permitting (allowing for sufficient airflow with some vent dampers partially or completely closed), you can balance.

You can manually input the design conditions - outdoor temp, etc.

Unsure of how high altitude is considered in load calculations though. Different air density can impact results when it comes to load attributed to air leakage.


With our kids grown, the lower level vents are closed in winter also.
You may be ruining your furnace doing this - particularly if the ducts were only sized for the amount of air the furnace needs to move in heating mode and nothing more.
Closing vents does not reducing heating bills, can actually increase.
 

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I’m in Colorado, elevation 8700 ft. Very little air conditioning here, but with the rising temperatures I’m considering adding central air to my central heating. My lower level in no way needs a/c especially with our very low humidity. I’ve had two estimates that figured 4 ton units which I understand might be needed for my total area, but no way I need that much to cool 1700 sq ft. Currently I have two 10,000 btu portable units and rarely need the second. I would prefer central over these portable units because of the noise and venting. I understand it becomes a furnace fan volume issue when I shut off the lower level but otherwise it’ll be an ice box if I don’t. With our kids grown, the lower level vents are closed in winter also. The Manual J calculator doesn’t allow me to pick a location even close to my area or elevation. Is there a way to make this work?
Have you considered a whole house attic fan?

Sent from my BE2028 using Tapatalk
 

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Www.loadcalc.net

Add your walls as square feet. Do the whole house as one calculation first, then post a screenshot of the results. It allows you to select the location. How many furnaces do you have? One?
 

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Discussion Starter · #5 ·
By lower level, are you talking about a basement or a first floor, entirely above ground?

The calculation should include the entire house.
Ductwork permitting (allowing for sufficient airflow with some vent dampers partially or completely closed), you can balance.

You can manually input the design conditions - outdoor temp, etc.

Unsure of how high altitude is considered in load calculations though. Different air density can impact results when it comes to load attributed to air leakage.




You may be ruining your furnace doing this - particularly if the ducts were only sized for the amount of air the furnace needs to move in heating mode and nothing more.
Closing vents does not reducing heating bills, can actually increase.
l
By lower level, are you talking about a basement or a first floor, entirely above ground?

The calculation should include the entire house.
Ductwork permitting (allowing for sufficient airflow with some vent dampers partially or completely closed), you can balance.

You can manually input the design conditions - outdoor temp, etc.

Unsure of how high altitude is considered in load calculations though. Different air density can impact results when it comes to load attributed to air leakage.




You may be ruining your furnace doing this - particularly if the ducts were only sized for the amount of air the furnace needs to move in heating mode and nothing more.
Closing vents does not reducing heating bills, can actually increase.
Lower level is a walkout basement (sloped mountain property). I mentioned altitude because my temperature zone is way different than front range cities which are 2500 ft lower in only 15 miles. The furnace is apparently fine since it’s twenty years old. The basement was unfinished when I bought. There was only one vent installed at that time. It was that way for ten years before I finished it. I’ve always wondered about the change after it was finished vs air flow. Based on my house size the 120,000btu furnace is correct from what I understand.
 

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Discussion Starter · #6 ·
Have you considered a whole house attic fan?

Sent from my BE2028 using Tapatalk
I actually have a whole house fan, which is great. We also deal with forest fire smoke and allergy issues on certain days so the house has to stay closed.
 

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Based on my house size the 120,000btu furnace is correct from what I understand.
Please read my recent posts. You can't size based on sq ft alone.

There are many junk charts/calculators on the internet.

A real load calculation is needed to see what is really required.

120k btu is absolutely huge for a house!
Most homes in north america, barring extremely cold parts (example - Minnesota) can heat on under 60k btu output. It takes a large house or poor insulation to even need 70 to 80k btu output.

Granted, the furnace has to be de-rated for altitude, so you don't get the nameplate btus

One issue is, if you only need a 2 ton or so, the blower in your furnace may not be able to be slowed down enough.
 

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Discussion Starter · #8 ·
Www.loadcalc.net

Add your walls as square feet. Do the whole house as one calculation first, then post a screenshot of the results. It allows you to select the location. How many furnaces do you have? One?
Guess I need to do more research on how to use the calculator. The preselected cities for Colorado are all completely different climates than where I live. They’re either front range or plains, which get way warmer or there’s one at 10,000 ft which never needs a/c.
 

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Discussion Starter · #9 ·
Please read my recent posts. You can't size based on sq ft alone.

There are many junk charts/calculators on the internet.

A real load calculation is needed to see what is really required.

120k btu is absolutely huge for a house!
Most homes in north america, barring extremely cold parts (example - Minnesota) can heat on under 60k btu output. It takes a large house or poor insulation to even need 70 to 80k btu output.

Granted, the furnace has to be de-rated for altitude, so you don't get the nameplate btus

One issue is, if you only need a 2 ton or so, the blower in your furnace may not be able to be slowed down enough.
Yeah, there is a vast difference here between heating and cooling requirements. Rarely does it get over 80 but there is smoke from forest fires and allergy issues at times that require a closed house. Plus we’re getting older and softer I guess. The portable units work fine, I just don’t like the noise etc. Altitude is apparently the factor on furnace btu. My neighbor with a comparable house just had his furnace replaced and it was 132k I believe. These houses are all well insulated etc. My 120k furnace rating drops to 108 at 6500 ft according to the tag. I’m 2200 ft higher than that. It runs for a good amount of time on winter mornings after a four degree nighttime setback so I don’t think it’s oversized. I know it’s not a good reference but I have 3200 sq ft of floor plus vaulted ceilings on the main level.
The vast difference between heating and cooling vs fan speed is my problem. I’m just afraid a 4 ton coil will be cycling too much when I’m only needing a 10-12 degree drop with low humidity.
 

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Yeah, there is a vast difference here between heating and cooling requirements. Rarely does it get over 80 but there is smoke from forest fires and allergy issues at times that require a closed house. Plus we’re getting older and softer I guess. The portable units work fine, I just don’t like the noise etc. Altitude is apparently the factor on furnace btu. My neighbor with a comparable house just had his furnace replaced and it was 132k I believe. These houses are all well insulated etc. My 120k furnace rating drops to 108 at 6500 ft according to the tag. I’m 2200 ft higher than that. It runs for a good amount of time on winter mornings after a four degree nighttime setback so I don’t think it’s oversized. I know it’s not a good reference but I have 3200 sq ft of floor plus vaulted ceilings on the main level.
The vast difference between heating and cooling vs fan speed is my problem. I’m just afraid a 4 ton coil will be cycling too much when I’m only needing a 10-12 degree drop with low humidity.
Just because a certain size of furnace is installed in your area, doesn't mean it is correct.

The furnace is sized to maintain, not raise the temperature in extremes. In the coldest weather you get, it shouldn't be shutting off much just to maintain. This is for comfort, to prevent short cycling the rest of the year - because even a properly sized one is too large 99% of the time.

Now, I'm not going to actually draw the conclusion that it is too large, I don't have enough information and you're not looking at changing the furnace now, right?

The furnace fan speed used for cooling can be reduced, but there are limits.
It probably has a 4 or 5 ton rated blower.


I doubt 2 tons will cut it but 4 is probably too much when it rarely exceeds 80f.
 

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Discussion Starter · #11 ·
Just because a certain size of furnace is installed in your area, doesn't mean it is correct.

The furnace is sized to maintain, not raise the temperature in extremes. In the coldest weather you get, it shouldn't be shutting off much just to maintain. This is for comfort, to prevent short cycling the rest of the year - because even a properly sized one is too large 99% of the time.

Now, I'm not going to actually draw the conclusion that it is too large, I don't have enough information and you're not looking at changing the furnace now, right?

The furnace fan speed used for cooling can be reduced, but there are limits.
It probably has a 4 or 5 ton rated blower.


I doubt 2 tons will cut it but 4 is probably too much when it rarely exceeds 80f.
I agree on your furnace assessment as I’ll defer that to more knowledgeable professionals.
No I’m not looking to replace the furnace unless it is going to be necessary to achieve a good result. It is 20 years old but has zero issues. That said, is it possible to run different fan speeds between heat/cool? Basically I’m open to any suggestions. I agree on the 2 tons and was thinking 3? All the contractors here are so busy they hardly want to talk, but that’s a good thing.
 

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I agree on your furnace assessment as I’ll defer that to more knowledgeable professionals.
No I’m not looking to replace the furnace unless it is going to be necessary to achieve a good result. It is 20 years old but has zero issues. That said, is it possible to run different fan speeds between heat/cool? Basically I’m open to any suggestions. I agree on the 2 tons and was thinking 3? All the contractors here are so busy they hardly want to talk, but that’s a good thing.
Yes, cooling can use a different speed.
Typically they came from factory wired to use high speed for cooling and medium-high or medium low for heat. (4 speed motor was the most common)




Speeds are field adjusted to deliver proper airflow in each mode of operation.

See if you do the calculation rather than guess.
 

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Discussion Starter · #13 ·
Yes, cooling can use a different speed.
Typically they came from factory wired to use high speed for cooling and medium-high or medium low for heat. (4 speed motor was the most common)




Speeds are field adjusted to deliver proper airflow in each mode of operation.

See if you do the calculation rather than guess.
I’m mainly reaching out because of the wait time here, plus I’m always about educating myself. This chat room has already benefited me in only one day.
I’ll check out the motor to see what I have, plus try to get educated on the calculator. And I’ll certainly get professional advice on tonnage etc.
 

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Guess I need to do more research on how to use the calculator. The preselected cities for Colorado are all completely different climates than where I live. They’re either front range or plains, which get way warmer or there’s one at 10,000 ft which never needs a/c.
Then ignore the city and add your design temps yourself.
 

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I’m mainly reaching out because of the wait time here, plus I’m always about educating myself. This chat room has already benefited me in only one day.
I’ll check out the motor to see what I have, plus try to get educated on the calculator. And I’ll certainly get professional advice on tonnage etc.
What's the model number of your furnace. We can look it up.
 

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I'm at 6000 ft, and just a little farther north. It hit 94 here today. My 1.5 Ton A/C kept my 1900 sq. ft. house at 72 just fine.
Without knowing how your house is built compared to the OP, it's incomparable. Especially so since you don't get too many cookie cuter subdivisions in the mountains.
 

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Discussion Starter · #18 ·
I'm at 6000 ft, and just a little farther north. It hit 94 here today. My 1.5 Ton A/C kept my 1900 sq. ft. house at 72 just fine.
I’m in Woodland Park, 8750 ft at my house. Good to know you kept cool. So I guess your system is somewhat similar to what I’m after.
 

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Unsure of how high altitude is considered in load calculations though.
Simply put, you need to move more air across the cooling coil and heat exchanger to maintain design delta T. Condensing units usually don't have a fan speed choice so there is a bit of capacity loss (best to check manufacturer data or call for derate). When checking pressures we generally see higher head and lower suction pressures. The good thing is, the OP is in a very dry climate and he could easily run a 2 ton condensing unit on a 3 ton coil (or larger pairings) to maintain air flow for the furnace. I live at 7000' in Salida. For outdoor design temps I use 90F for cooling and -5F for heating OP, choose Colorado Springs as your city, this sets the latitude, then manually change your design temps (don't get crazy) and altitude for your location.
 

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Without knowing how your house is built compared to the OP, it's incomparable. Especially so since you don't get too many cookie cuter subdivisions in the mountains.
Mine's no cookie cutter tract house, either. The only significant difference that would matter is if the OP's roof is very poorly insulated, since mine is fairly good in that respect. In that case, the heat gain there could be significantly larger. Other than that, with a max 10 or 12 degree required temperature difference between inside and out, the cooling load for 1700 sq. ft. should be quite small.

The page you linked to post #4 would provide a more exact answer, and I would recommend it.
 
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