Replacing the AC unit - yep, another one!!

[Kbach]

So I'm curious to get some of the board AC guys input on this. I have a two story house, ~2200 sq ft with central air. The unit was installed in '96 and is 10 seer I believe. I'd like to install a newer more efficient unit when I do a master bath remodel so I'm in the information gathering stages.

The current AC works but won't get the house very cold, especially upstairs. If I have the temp set for 77 downstairs it's usually 81-83 upstairs by the end of the day. I'm wondering if I can do something with ducting and or dampers to get both floors closer in temp throughout the day. I probably can't change any first floor ducting but can obviously change second floor as well as the plenum if/when I change out the indoor unit.

Any input would be appreciated as well as info on which board peeps get out to Simi Valley. I'm not looking to do this during he summer so obviously wouldn't be during your busy season.

Thanks!

 

[Icky]

I added a Gable vent fan in my attic, made a noticeable difference in temp variation from down stairs to up stairs

 

[rlamoreaux]

Home Depot had a sale on blown in insulation and rented the blower for free. I feel it really helped the second floor to stay cooler.

 

[Justfishing]

I added a Gable vent fan in my attic, made a noticeable difference in temp variation from down stairs to up stairs

Yeah it sucked the air from the house to the attic. Which helped draw more cool air from the lower level. When you put an attic under negative pressure you will pull air from every air leak between the house and the attic.

We all know that heat rises but also cold air falls. When the AC runs the air inside the home is being cooled and condensed. This causes the air to fall to the lower level. The cool air leaks from the lowest points in the house. Hot air is being pulled into the house, guess where from? It's not leaking in and out from the same spot. Hot air is being drawn in from the top.

You need to seal all the leaks between the attic and the house

How much of a factor is hot attic air temps. Not much in a well insulated home. Radiant heat is the primary form of heat transfer from the attic, well actually the roof, to the house. Radiant heat travels through air but doesn't heat the air it heats solid objects. The radiant heat penetrates the insulation and heats it or possible the ceiling below. You will sense the radiant heat coming from the cieling.

The Florida solar energy center has studied radiant heat in attics. It found that radiant barriers were not the solution. Per there studiesradiant barriers were most effective in poorly insulated attics. With R38 or higher insulation they found they would never pay for themselves.

The third factor is there are probably HVAC ducts in the attic. You have very poorly insulated air ducts in the hottest area of the house. Seal up air leaks in the ducts and bury them in insulation.

I like cellulose as it is better at trapping air and more effective at stopping radiant heat. Pull the existing insulation back seal up air leaks and put the insulation back. Cover with cellulose to R60.

 

[C-2]

JustFishing - Thanks for writing the post, it is insightful, and helps us regular folks. Excellent :thumbsup

 

[RiverDave]

Home Depot had a sale on blown in insulation and rented the blower for free. I feel it really helped the second floor to stay cooler.

Did HD have this in havasu? I need to insulate my garage!

 

[HavaToon]

Did HD have this in havasu? I need to insulate my garage!

They had it a few years ago, when we did my parents house here.

 

[Kbach]

Yeah it sucked the air from the house to the attic. Which helped draw more cool air from the lower level. When you put an attic under negative pressure you will pull air from every air leak between the house and the attic.

We all know that heat rises but also cold air falls. When the AC runs the air inside the home is being cooled and condensed. This causes the air to fall to the lower level. The cool air leaks from the lowest points in the house. Hot air is being pulled into the house, guess where from? It's not leaking in and out from the same spot. Hot air is being drawn in from the top.

You need to seal all the leaks between the attic and the house

How much of a factor is hot attic air temps. Not much in a well insulated home. Radiant heat is the primary form of heat transfer from the attic, well actually the roof, to the house. Radiant heat travels through air but doesn't heat the air it heats solid objects. The radiant heat penetrates the insulation and heats it or possible the ceiling below. You will sense the radiant heat coming from the cieling.

The Florida solar energy center has studied radiant heat in attics. It found that radiant barriers were not the solution. Per there studiesradiant barriers were most effective in poorly insulated attics. With R38 or higher insulation they found they would never pay for themselves.

The third factor is there are probably HVAC ducts in the attic. You have very poorly insulated air ducts in the hottest area of the house. Seal up air leaks in the ducts and bury them in insulation.

I like cellulose as it is better at trapping air and more effective at stopping radiant heat. Pull the existing insulation back seal up air leaks and put the insulation back. Cover with cellulose to R60.

Ive heard this before, and it may be a stupid question but what are the areas that "leak" air into the attic? Only thing I can think of would be can light housings and possibly ceiling mounted light fixtures. Are there others possible major areas that I'm not thinking of?

 

[jet496]

So I'm curious to get some of the board AC guys input on this. I have a two story house, ~2200 sq ft with central air. The unit was installed in '96 and is 10 seer I believe. I'd like to install a newer more efficient unit when I do a master bath remodel so I'm in the information gathering stages.

The current AC works but won't get the house very cold, especially upstairs. If I have the temp set for 77 downstairs it's usually 81-83 upstairs by the end of the day. I'm wondering if I can do something with ducting and or dampers to get both floors closer in temp throughout the day. I probably can't change any first floor ducting but can obviously change second floor as well as the plenum if/when I change out the indoor unit.

Any input would be appreciated as well as info on which board peeps get out to Simi Valley. I'm not looking to do this during he summer so obviously wouldn't be during your busy season.

Thanks!

A 5 ton unit should cool your house pretty good (the model number will tell you the tonnage). It's a common problem for the upstairs to be a problem when on the same zone as the 1st Floor. You need to air balance hot & cold areas the best you can & also sounds like you may need to add or upsize some ducts upstairs. Size of the ducts matter.

If you can draw an asbuilt of your HVAC ducts with sizes & get your tonnage (it's the even number in the beginning of the model number on your condenser unit. If you see a 60, that's 5 tons) & post it, you'd get some good feedback. That's what I would do for starters.

 

[SHOCKtheMONKEY]

Yeah it sucked the air from the house to the attic. Which helped draw more cool air from the lower level. When you put an attic under negative pressure you will pull air from every air leak between the house and the attic.

We all know that heat rises but also cold air falls. When the AC runs the air inside the home is being cooled and condensed. This causes the air to fall to the lower level. The cool air leaks from the lowest points in the house. Hot air is being pulled into the house, guess where from? It's not leaking in and out from the same spot. Hot air is being drawn in from the top.

You need to seal all the leaks between the attic and the house

How much of a factor is hot attic air temps. Not much in a well insulated home. Radiant heat is the primary form of heat transfer from the attic, well actually the roof, to the house. Radiant heat travels through air but doesn't heat the air it heats solid objects. The radiant heat penetrates the insulation and heats it or possible the ceiling below. You will sense the radiant heat coming from the cieling.

The Florida solar energy center has studied radiant heat in attics. It found that radiant barriers were not the solution. Per there studiesradiant barriers were most effective in poorly insulated attics. With R38 or higher insulation they found they would never pay for themselves.

The third factor is there are probably HVAC ducts in the attic. You have very poorly insulated air ducts in the hottest area of the house. Seal up air leaks in the ducts and bury them in insulation.

I like cellulose as it is better at trapping air and more effective at stopping radiant heat. Pull the existing insulation back seal up air leaks and put the insulation back. Cover with cellulose to R60.

I installed a powered attic vent in my 2006 home and it made a noticeable difference in the upstairs temps. We don't run the air very often, usually only when its humid or in the higher 100 degrees. Hard to believe it's sucking all my cool air from the house to the attic through the penetrations in the ceiling. Attic has plenty of vents to feed powered fan.

My neighbor paid about 7k to have his system fixed to get even temps on both floors. I can live with a two degree difference.

 

[Flying_Lavey]

I installed a powered attic vent in my 2006 home and it made a noticeable difference in the upstairs temps. We don't run the air very often, usually only when its humid or in the higher 100 degrees. Hard to believe it's sucking all my cool air from the house to the attic through the penetrations in the ceiling. Attic has plenty of vents to feed powered fan.

My neighbor paid about 7k to have his system fixed to get even temps on both floors. I can live with a two degree difference.

That is exactly it. An attic fan doesn't produce enough negative pressure to pull any noticible amounts of air through the small leaks in the ceiling and framing (light fixtures, wire penetration, plumbing penetrations). An attic fan works well because it will move air THROUGH the attic. Attic temps can be 140 to 150 degrees on a 100+ degree day (I used to place a pocket thermometer in the attic as soon as I went up there when I was doing residential. Different temps I'd have different lengths of time I could stay there before having to come down), so pulling even 110 degree air through there helps DRAMATICALLY. Proper eve vents and gable/peak vents alone will do wonders and induce a cooling draft. Adding powered fans with good eve vents work even better.

To the OP though, the stratification you are getting is VERY normal. I'd suggest zoning your house but without seeing your duct layout, I don't know how feasable that is without extensive re-ducting. The best, quickest, and cheapest fix though would be to turn the fan on the thermostat from "Auto" to "On". This will keep the air circulating and reduce the stratification you are feeling. A lot of the more modern thermostats have a "Circulate", or "Circ", option as well. That should work also and that just cycles the fan periodically even when the temp is satisfied thus resulting in less stratification.

 

[SHOCKtheMONKEY]

I watched an Ask this Old House where they installed balloon like dampers in each duct that could inflate/deflate as required to even out temps in a two story So Cal home...
Cool and effective idea. No guess on cost....

 

[SHOCKtheMONKEY]

Here ya go:

Most of the A/c guys are bitchin' in the comments...

[video=youtube;F2EJLhZ939A]https://www.youtube.com/watch?v=F2EJLhZ939A[/video]

 

[Flying_Lavey]

Here ya go:

Most of the A/c guys are bitchin' in the comments...

[video=youtube;F2EJLhZ939A]https://www.youtube.com/watch?v=F2EJLhZ939A[/video]

That system is horrible!!! Would cause WAY more problems than solutions and would net saving zero $.

 

[Justfishing]

Ive heard this before, and it may be a stupid question but what are the areas that "leak" air into the attic? Only thing I can think of would be can light housings and possibly ceiling mounted light fixtures. Are there others possible major areas that I'm not thinking of?

Actually it is a lot of small leaks and maybe a few big leaks. If you had leaks in your boat would you only go after the big leaks?

Take 2x4 construction. There is drywall on each side of the studs and top plate. This seam varies a lot in size. In many areas you can easily get a business card or more through. Add up the length of all the walls and it adss to a big leak. Every light fixture on the ceiling is a leak. Can lights are even worse and there can be lots of them. Where ever a light switch is a wire runs up to the light. A hole is drilled in the top plate. The way soffits in a bath or kitchen are constructed lead to bigger air leaks. Plumbing vent stacks, flues etc tend to be bigger air leaks. I have seen chases for plumbing or flues that are open at the top, these can be as big as a small to medium window.

Air leaks are more pronounced with a larger delta between inside and out. So on the hottest days the leakage is the worst. If you are in a climate that gets cold it is the worst on the coldest days too. So when you need it the most to stay comfortable your home is performing at its worst straining the equipment more and driving your bills higher.

Fiberglass insulation is very air porous and doesn't stop the leaks it just filter the air. Blower door testing is done at 50 Pascal's. That is 0.01 psi. So we're are talking very small pressure differentials lead to large air leakage when added up.

I like cellulose since it does a better job of trapping air. Oh but it settles after being blown in. Yes it does but the charts take that into consideration. Settling actually improves the performance by trapping the air better.

In new home construction air tightness is a very high priority. Insulation does not work with air leaks. Air leaks let sound in, you will have a quieter house.

 

[Justfishing]

That is exactly it. An attic fan doesn't produce enough negative pressure to pull any noticible amounts of air through the small leaks in the ceiling and framing (light fixtures, wire penetration, plumbing penetrations). An attic fan works well because it will move air THROUGH the attic. Attic temps can be 140 to 150 degrees on a 100+ degree day (I used to place a pocket thermometer in the attic as soon as I went up there when I was doing residential. Different temps I'd have different lengths of time I could stay there before having to come down), so pulling even 110 degree air through there helps DRAMATICALLY. Proper eve vents and gable/peak vents alone will do wonders and induce a cooling draft. Adding powered fans with good eve vents work even better.

To the OP though, the stratification you are getting is VERY normal. I'd suggest zoning your house but without seeing your duct layout, I don't know how feasable that is without extensive re-ducting. The best, quickest, and cheapest fix though would be to turn the fan on the thermostat from "Auto" to "On". This will keep the air circulating and reduce the stratification you are feeling. A lot of the more modern thermostats have a "Circulate", or "Circ", option as well. That should work also and that just cycles the fan periodically even when the temp is satisfied thus resulting in less stratification.

Please read this. There are several other links in there. Georgia outlawed powered attic fans for the reasons I pointed out.

http://www.energyvanguard.com/blog/75600/The-1-Reason-Power-Attic-Ventilators-Don-t-Help

A good place to ask questions is at green buildings divisors.com

Buildingscience.com is a group out of Boston that does a lot of testing and study. Headed by a couple of phd's. Their studies are on the website and there is no charge to view.

 

[Icky]

All I cared about was it got the hot air moving out of the attic, which lowered the temp up stairs.

 

[Flying_Lavey]

Please read this. There are several other links in there. Georgia outlawed powered attic fans for the reasons I pointed out.

http://www.energyvanguard.com/blog/75600/The-1-Reason-Power-Attic-Ventilators-Don-t-Help

A good place to ask questions is at green buildings divisors.com

Buildingscience.com is a group out of Boston that does a lot of testing and study. Headed by a couple of phd's. Their studies are on the website and there is no charge to view.

I get what the article is saying and I agree that their options would work very well or possibly better. But they assume the attic fan doesn't have adequate make-up air. Every single example they used stated multiple fans in one attic and insufficient make-up air. 1 fan with about double the square footage in make-up air inlets nullify their argument almost entirely.

And Georgia outlawing them...... Well we all know how well the government "helps" citizens.

 

[Riverbound]

All I cared about was it got the hot air moving out of the attic, which lowered the temp up stairs.

Minimal unless you are able to draw dry, cool, clean air from out side. Unless you have little to no attic insulation attic temps don't play a huge role. In hearing the house up.

There are going to be exceptions to the rule but the above statement plays in to most situations.

 

[Icky]

Minimal unless you are able to draw dry, cool, clean air from out side. Unless you have little to no attic insulation attic temps don't play a huge role. In hearing the house up.

There are going to be exceptions to the rule but the above statement plays in to most situations.

In my mind it's working, don't screw it up

 

[Riverbound]

In my mind it's working, don't screw it up

There are instances where it helps drastically, but that's not the norm.

 

[Justfishing]

I get what the article is saying and I agree that their options would work very well or possibly better. But they assume the attic fan doesn't have adequate make-up air. Every single example they used stated multiple fans in one attic and insufficient make-up air. 1 fan with about double the square footage in make-up air inlets nullify their argument almost entirely.

And Georgia outlawing them...... Well we all know how well the government "helps" citizens.

You assume the air is smart and will only come in the vents. Given that attic venting is 1sf for every 300sf of attic floor and the air flow is based on passive air flow. I don't think they are up to the task of providing enough air flow.

If you have a 1500 CFM fan and let's say 80% comes from the vents that is 300 CFM coming from the house and it could be more. That's like having 4 or 5 bath fans running.

Even if the fan is not drawing inside air the cost to run it exceeds any savings. That doesn't even consider the cost of installing.

Also a fan does nothing for the primary heat flow from the roof. As we know hot air rises but you imply it is going down. Proper attic venting let's the air flow up and out. Air sealing and more insulation is the better fix plus it helps in cold weather too.


http://www.greenbuildingadvisor.com/blogs/dept/musings/fans-attic-do-they-help-or-do-they-hurt

 

[SoCalDave]

To the OP, keep in mind if you try to get more air upstairs for cooling purposes come winter you will have to deal with higher temps up there when running the furnace.

 

[C-2]

Great thread guys, learning a lot. :thumbsup

I think burning my Kaufman and Broad cracker box to the ground, and starting over, is the easiest way to fix all the issues discussed :yikes

 

[jet496]

Best solution is separate units for the upstairs & downstairs. Easier said than done, but if it's a real problem, might be worth it.

Multizone minisplit systems may be an option & are extremely efficient. I'm thinking of doing this for my house.

 

[Riverbound]

Best solution is separate units for the upstairs & downstairs. Easier said than done, but if it's a real problem, might be worth it.

Multizone minisplit systems may be an option & are extremely efficient. I'm thinking of doing this for my house.

Multi stage system with Zoning is the best option hands down. The tricky part is having a contractor that knows how to do it right.

 

[Kbach]

Great thread guys, learning a lot. :thumbsup

I think burning my Kaufman and Broad cracker box to the ground, and starting over, is the easiest way to fix all the issues discussed :yikes

I agree, I'm definitely learning a lot and am extremely grateful for all of the trade experts and diy homeowners sharing their knowledge and input. As a mechanical engineer I tend to over analyze everything but there are just some things you can only learn by experience.

I dug around in my attic a little today and noticed a problem as I see it. My eves are vented (between every few rafters) which in theory would supply "cool" air into the attic which can then vent through one of the two static roof vents. Problem is the insulation in the attic is so thick/deep that where it was stuffed into the space between the roof rafters and ceiling joists it has basically sealed off the vents. I'm thinking I can try and go up there to smash it down a little so it doesn't obstruct he vents but not sure if it will just bounce back up. If that's the case, I'll have to figure out how to wedge something in there which of course is gonna be a biyotch since it's so hard to reach that area when your up in the attic.

Im hoping to do as much as possible to minimize the temp difference between floors but I'm not too worried about a couple of degrees. If I had more efficient system, I wouldn't mind setting the temp downstairs a couple degrees cooler to get a more comfortable temp upstairs.

Again....thanks for all of the input thus far :thumbsup

 

[Kbach]

Multi stage system with Zoning is the best option hands down. The tricky part is having a contractor that knows how to do it right.

View attachment 580069

I'm all about this kinda system but saw you mention an approx price of $25k for a similar system. If we could move the decimal point over one place to the left I'm in

And thanks for all of your input on this and other AC related threads. It's much appreciated. :thumbsup

 

[SoCalDave]

HD has vent baffles just for that purpose...I installed 18 of them in my attic when I replaced my HVAC system and insulated the hell out of the attic space.

 

[DLow]

I agree, I'm definitely learning a lot and am extremely grateful for all of the trade experts and diy homeowners sharing their knowledge and input. As a mechanical engineer I tend to over analyze everything but there are just some things you can only learn by experience.

I dug around in my attic a little today and noticed a problem as I see it. My eves are vented (between every few rafters) which in theory would supply "cool" air into the attic which can then vent through one of the two static roof vents. Problem is the insulation in the attic is so thick/deep that where it was stuffed into the space between the roof rafters and ceiling joists it has basically sealed off the vents. I'm thinking I can try and go up there to smash it down a little so it doesn't obstruct he vents but not sure if it will just bounce back up. If that's the case, I'll have to figure out how to wedge something in there which of course is gonna be a biyotch since it's so hard to reach that area when your up in the attic.

Im hoping to do as much as possible to minimize the temp difference between floors but I'm not too worried about a couple of degrees. If I had more efficient system, I wouldn't mind setting the temp downstairs a couple degrees cooler to get a more comfortable temp upstairs.

Again....thanks for all of the input thus far :thumbsup

View attachment 580073

They make cardboard "chutes" that go between your trusses and slide into place to help keep your insulation out of the way, allowing your bird block to be clear and flow air. Pretty simple deal.

 

[Kbach]

See, it took a whole 2 minutes before somebody had a viable solution!:champagne:

 

[Flying_Lavey]

I agree, I'm definitely learning a lot and am extremely grateful for all of the trade experts and diy homeowners sharing their knowledge and input. As a mechanical engineer I tend to over analyze everything but there are just some things you can only learn by experience.

I dug around in my attic a little today and noticed a problem as I see it. My eves are vented (between every few rafters) which in theory would supply "cool" air into the attic which can then vent through one of the two static roof vents. Problem is the insulation in the attic is so thick/deep that where it was stuffed into the space between the roof rafters and ceiling joists it has basically sealed off the vents. I'm thinking I can try and go up there to smash it down a little so it doesn't obstruct he vents but not sure if it will just bounce back up. If that's the case, I'll have to figure out how to wedge something in there which of course is gonna be a biyotch since it's so hard to reach that area when your up in the attic.

Im hoping to do as much as possible to minimize the temp difference between floors but I'm not too worried about a couple of degrees. If I had more efficient system, I wouldn't mind setting the temp downstairs a couple degrees cooler to get a more comfortable temp upstairs.

Again....thanks for all of the input thus far :thumbsup

View attachment 580073

HD has vent baffles just for that purpose...I installed 18 of them in my attic when I replaced my HVAC system and insulated the hell out of the attic space.

They make cardboard "chutes" that go between your trusses and slide into place to help keep your insulation out of the way, allowing your bird block to be clear and flow air. Pretty simple deal.

That is precisely the set-up I was referring to about the eve vents and peak vents. Install the chutes/baffles or what not and it should help quiet a bit. Also, trim them so they only stick above the insulation a couple inches