Best options for replacing 23 year old gas/furnance & AC

[jpsmith1cm]

Sicofthis

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[udarrell]

2x4 downstairs, but the office will be 2x6. However, the return will have to pass through the 2x4 plate on the first floor so that would be the maximum volume for the air to pass through. And they are spaced 16" on centers. The office would have two supplies and one return, but given the responses here two returns should probably be used.

We do intend to have a filter installed on the return next to the furnace. However, I'm not sure I understand your response about need two larger air filter racks in the conditioned space.
Holy Cow! :-)

You might not be able to use filter racks in the conditioned area because you have too many ducted return grilles, etc.

For a 3-Ton system you could use two drops, one to each side of the furnace Return chamber.

Hart&Cooley Engineering Data, a 24X20 media type filter provides 2.3-sf (Ak) of free-air-area & will flow 713-CFM of air at 300-fpm velocity through it.

Therefore, it takes two of those filters @300-fpm to get above 1350-CFM, or 1426-CFM a little extra area is always better as filters load...

If you only used one of those filters at 1200-CFM / 2.3-sf (Ak) is 522-fpm Velocity through the filter; above 500-fpm debris blows through a fiber glass filter.

As a media filter loads the free-area becomes less & the velocity will soon reach 500-fpm limit with the low cost filters.

A one inch thick pleaded filter will have too much pressure ]drop & loads only on the surface area of the filter.

[beenthere]

The office will have two supplies is almost the same sq ft and in the same position of the existing house. So logically I think that's enough for the office and up to recently thought a single return was sufficient given this is also how the current master bedroom is done today. In short, the supply/returns were just going to be mirrored off the existing bedroom. Both rooms are on the back of the house and the only difference is that the office would have 2x6 walls instead of the 2x4 used on the bedroom. I don't want to deminish the need for running calculations but also think to some extent that it really shouldn't be this complicated if we care comparing like for like.

Then the office will have a higher R value insulation in its walls then the bedroom does, so it will lose and gain less heat then the bedroom does. Roughly 30% less heat loss and gain. Where the office adjoins to the bedroom, will eliminate the heat loss and gain that bedroom wall use to have.

The choices you make today, will be the electric bills you have to pay later.

[Z0neme]

You might not be able to use filter racks in the conditioned area because you have too many ducted return grilles, etc.

For a 3-Ton system you could use two drops, one to each side of the furnace Return chamber.

Hart&Cooley Engineering Data, a 24X20 media type filter provides 2.3-sf (Ak) of free-air-area & will flow 713-CFM of air at 300-fpm velocity through it.

Therefore, it takes two of those filters @300-fpm to get above 1350-CFM, or 1426-CFM a little extra area is always better as filters load...

If you only used one of those filters at 1200-CFM / 2.3-sf (Ak) is 522-fpm Velocity through the filter; above 500-fpm debris blows through a fiber glass filter.

As a media filter loads the free-area becomes less & the velocity will soon reach 500-fpm limit with the low cost filters.

A one inch thick pleaded filter will have too much pressure ]drop & loads only on the surface area of the filter.

You lost me here. I'm just not grasping how adding a second filter increases the flow per minute as I would think just the opposite since it's an obstruction. As a comparison, my current furnance has 1 filter running the entire bottom of the box at 18" x 28".

[Z0neme]

Then the office will have a higher R value insulation in its walls then the bedroom does, so it will lose and gain less heat then the bedroom does. Roughly 30% less heat loss and gain. Where the office adjoins to the bedroom, will eliminate the heat loss and gain that bedroom wall use to have.

The choices you make today, will be the electric bills you have to pay later.

That's correct and why I think the office would be fine with 2 supplies and one return. Agreed? I'm not the HVAC contractor though so my logic may be off.

[udarrell]

You lost me here. I'm just not grasping how adding a second filter increases the flow per minute as I would think just the opposite since it's an obstruction. As a comparison, my current furnace has 1 filter running the entire bottom of the box at 18" x 28".

It does not increase the flow per minute, it divides the amount of air in half passing through the same open-area of the filters & thus reduces the fpm velocity.

You're not adding the filter in one single Return Air stream for the entire home, you're splitting the Return Air in half by dividing the required amount of air flow between two return filters that enter the two sides of the furnace. This larger free-air-area also reduces air resistance & the total pressure drop of the RA filtering system.

The furnace normally has two knock-out areas on both (Right & Left) sides & the bottom so that RA can enter the furnace return chamber from three differing area sources...

If you were to use two filter-racks in the conditioned areas then you would never use a filter in the furnace as that would be two filters doubling filtering one air stream which would greatly increase the pressure drop resistance.

If you were using only 16X25 filter grilles @1.7625-sf of open-area flowing 680-CFM @300-fpm velocity = 1269-CFM, it would take two of them in different areas of the home to achieve over 1200-CFM of air flow @300-fpm velocity through those two filters.

Many 3-Ton furnaces will have knock-outs that will only receive a maximum of a 16X25 duct.

Therefore many will only have one entry to the furnace return air chamber using one 16X25 filter; @1200-CFM / 1.7625-sf open-air-area is 680-feet per minute (fpm) velocity through that new clean filter. Well, there would be NO filter grille restrictions so it wouldn't be quite that high a velocity.

However, it would need two entries with a filter in each, to efficiently flow 1200-CFM for a 3-Ton system at an initial 300p-fpm or less velocity through the filters.

This is an important area to gain system efficiency of air flow that is rarely done by the book; but we should try to achieve it whenever possible. IMO, It ought to be part of the code...

The 18X28 results in around 540-fpm velocity through a clean filter; if you're using a 4 or 5-inch thick pleated filter you can start at 500-fpm velocity through it.

[beenthere]

That's correct and why I think the office would be fine with 2 supplies and one return. Agreed? I'm not the HVAC contractor though so my logic may be off.

Still need to add a return somewhere because of the units need for more return.

2 supplies? How many CFM from those 2 supplies.

Don't be guilty of just buying a bill of goods.

[Z0neme]

It does not increase the flow per minute, it divides the amount of air in half passing through the same open-area of the filters & thus reduces the fpm velocity.

You're not adding the filter in one single Return Air stream for the entire home, you're splitting the Return Air in half by dividing the required amount of air flow between two return filters that enter the two sides of the furnace. This larger free-air-area also reduces air resistance & the total pressure drop of the RA filtering system.

The furnace normally has two knock-out areas on both (Right & Left) sides & the bottom so that RA can enter the furnace return chamber from three differing area sources...

If you were to use two filter-racks in the conditioned areas then you would never use a filter in the furnace as that would be two filters doubling filtering one air stream which would greatly increase the pressure drop resistance.

If you were using only 16X25 filter grilles @1.7625-sf of open-area flowing 680-CFM @300-fpm velocity = 1269-CFM, it would take two of them in different areas of the home to achieve over 1200-CFM of air flow @300-fpm velocity through those two filters.

Many 3-Ton furnaces will have knock-outs that will only receive a maximum of a 16X25 duct.

Therefore many will only have one entry to the furnace return air chamber using one 16X25 filter; @1200-CFM / 1.7625-sf open-air-area is 680-feet per minute (fpm) velocity through that new clean filter. Well, there would be NO filter grille restrictions so it wouldn't be quite that high a velocity.

However, it would need two entries with a filter in each, to efficiently flow 1200-CFM for a 3-Ton system at an initial 300p-fpm or less velocity through the filters.

This is an important area to gain system efficiency of air flow that is rarely done by the book; but we should try to achieve it whenever possible. IMO, It ought to be part of the code...

The 18X28 results in around 540-fpm velocity through a clean filter; if you're using a 4 or 5-inch thick pleated filter you can start at 500-fpm velocity through it.

A great explanation. So can you confirm for me the needed CFM using the specs I posted earlier for the potential system that will be used. I'm uncertain how you derive the minimum needed flow unless this is a standard spec for all 3 Ton units. Further more, I take it most installers won't split the return and rather will just use a single knock out. In fact, it's my understanding that the HVAC contractor has to add the filter shelf as the furnance doesn't come with it by standard. Lastly, my current filter is a washable sponge type material. It's no where near 5 inches thick but I would think it flows well.

[Z0neme]

Still need to add a return somewhere because of the units need for more return.

2 supplies? How many CFM from those 2 supplies.

Don't be guilty of just buying a bill of goods.

Well, the current supplies are 3.5" x 13" in size so what would those provide?

[beenthere]

Well, the current supplies are 3.5" x 13" in size so what would those provide?

Could be 100CFM, could be 150CFM, depends on length.

[Z0neme]

Could be 100CFM, could be 150CFM, depends on length.

And by length you mean the run from the supply in the room back to the furnance?

[udarrell]

So I'm in the middle of a whole house renovation which includes adding a 238 sq ft office to the 2nd story. My central AC unit died late last summer so our intention is to replace the whole HVAC system instead of just the AC unit itself.
We never had any issues with heating the home but the upstairs typically is about 6-7 degrees warmer than the downstairs during the cooling season which can make it difficult to sleep at night.
Our house faces the sun all day with almost no shade so it gets baked to say the least. All rooms have two supplies and one return except for the kitchen and bathrooms which have no returns and only one supply. The living room where the thermostat is located actually has two returns and one supply if that matters. In addition, there is no zoning of the house so all supplies are open when the HVAC is on. Our old AC was a 2.5 TON unit and the furnace is a Plus 90 Bryant 66,000 Btuh. The existing sq ft is 2,000 but will jump to 2238 sq ft after the office is completed.

That all being said, we are looking to have the house zoned. The recommendation is to pull the main trunk down and split it into two seperate trunks. One will be the first floor and the other will be the second floor. The office and basement will also be separate zones. Per the HVAC tech, we would not need to go bigger with the furnace or the AC unit, but they will bump the AC unit to 3 Tons.
From what I understand, the secret is in the duct work but I would still think a larger furnace/AC unit would be needed given the additional sq ft? I can see the system being fine when only a particular zone is calling for air but would it have enough flow to supply all zones at once?
Let me know your thoughts when you have time. What are others takes on those electrostatic filters compared to the normal paper filters.

If the system is zoned it might be a disadvantage to go from 2.5-Ton to a 3-Ton system which requires more airflow to handle per zone.

Well, the current supplies are 3.5" x 13" in size so what would those provide?

The 3.5" X 13" is around 0.316-sf duct area which is close to a 7.5" rd duct at around 0.307-sf area.

According to my airflow software; the 7.5" duct flowing 140-CFM on a long 50' run with (2) 90's, velocity is only 457-fpm, friction rate is 0.04" over the 50 foot run; or 0.06" FR per 100 foot.

Run a Return air duct at least that large back to the air handler & with a zoned system the 1st floor & 2nd floor zone thermostat will allow you to control to any temp you want within each zone.

With programmable thermostats on each zoned level you can save on your utility bills by programming the cooling & heating according to which zoned level you'll be on during specific hours of the day or night.

There are some real advantages with a properly zoned system.

[udarrell]

An accidental double post!