[mnoone]

I recently purchased a home built in the 50s. I believe the ductwork (rigid, galvanized steel) is mostly original to the house. It is also in nearly perfect condition (besides the fiberglass insulation - which is falling apart and needs to be replaced). No rust or anything that I can see, not dented, etc.

I've had three companies quote me having new AC + furnace installed.

Company #1 thought the ductwork looked great but offered to seal it and reinsulate it.

Company #2 thought the ductwork looked great but said I should replace the main inlet with flexible ductwork as he said the rigid ductwork is much louder (he said it's not as important for the various outlets - only the inlet is important to have as flexible). He also wanted to seal and reinsulate it.

Company #3 said I'd never be able to get the rigid ductwork sealed properly and I should purge it all and replace with flexible.

Any opinions here? Searching the forums and lots of Googling has not let me to believe company #3. I haven't found any sources backing up the noise claim that #2 made but intuitively I believe it.

What do you all think?

 

[beenthere]

If your system currently works. Is it loud?

Company 3 sounds like they are more interested in making money, than your best interest.

 

[George2]

I would choose rigid over flex any day. Why change it if it's all in good condition? I would be more concerned with the size of the return air.

Ductwork in homes in the 50's were not normally designed for A/C.

 

[mnoone]

If your system currently works. Is it loud?

Company 3 sounds like they are more interested in making money, than your best interest.

I don't have a lot to compare to. I don't hear the HVAC system at my work. I definitely hear the fans for my system at home.

To be fair to company #3 - when quoting the work they got annoyed that my furnace controls weren't working quite right so they fixed them for me. No charge. They seemed nice but perhaps misinformed. I did get worried that he said that his inspector would pass the install, regardless of what the HERS test said (this was said in reference to if I wanted to stick with rigid).

 

[mnoone]

I would choose rigid over flex any day. Why change it if it's all in good condition? I would be more concerned with the size of the return air.

Ductwork in homes in the 50's were not normally designed for A/C.

Companies 1 and 2 both agreed that my inlet needs to be increased. It's currently 14" and there seems to be an agreement that I need 16". I won't pretend to fully understand why 2" makes that big of a difference (cross section increases by 25% - not that big of an improvement?)

What would need to be done differently to design ductwork for A/C?

 

[George2]

Companies 1 and 2 both agreed that my inlet needs to be increased. It's currently 14" and there seems to be an agreement that I need 16". I won't pretend to fully understand why 2" makes that big of a difference (cross section increases by 25% - not that big of an improvement?)

What would need to be done differently to design ductwork for A/C?

Just changing the 14" to 16" won't quiet the system unless the return air grille is enlarged. Is the filter at the grille or the furnace?

 

[mnoone]

Just changing the 14" to 16" won't quiet the system unless the return air grille is enlarged. Is the filter at the grille or the furnace?

Currently there is a filter at the furnace only (but I do have the option of installing one at the grille as well). But the furnace is soon to be replaced and the cold air return is getting moved (and likely increased in size). I think the goal of increasing the size was for airflow restriction reasons, not noise reasons.

So a larger grille will make things quieter? The new location for the air return could actually have grilles on three of it's four sides - would that be a good thing? On that note - that's assuming the grille is on the wall. I could also have it come from the ceiling - is that advantageous? The furnace will be in the crawlspace either way.

 

[George2]

Currently there is a filter at the furnace only (but I do have the option of installing one at the grille as well). But the furnace is soon to be replaced and the cold air return is getting moved (and likely increased in size). I think the goal of increasing the size was for airflow restriction reasons, not noise reasons.

So a larger grille will make things quieter? The new location for the air return could actually have grilles on three of it's four sides - would that be a good thing? On that note - that's assuming the grille is on the wall. I could also have it come from the ceiling - is that advantageous? The furnace will be in the crawlspace either way.

I was going to ask if the furnace was in the crawl space, but when you said the ductwork wasn't rusty I thought it must be in the attic. Must be a dry crawl space!

Yes, more or bigger return air grilles are the way to go. Quieter and needed for good system performance. How hard is it to get to the furnace to change the filter?

 

[George2]

What size is the A/C? This will tell us the RA requirements. One filter grille isn't large enough for a 16" pipe. You don't want 2 filters in the system.

High returns are favorable.

 

[mnoone]

What size is the A/C? This will tell us the RA requirements. One filter grille isn't large enough for a 16" pipe. You don't want 2 filters in the system.

High returns are favorable.

One quote I got is for Carrier condenser 24AHA4 (4 ton) and furnace 59SC5 (I think these are incomplete part numbers - but that's what the quote says). How large of a grill do you need for a 16" pipe? I'm assuming it's not the number of grills that matters, but instead the cross sectional area of the grill.

I was going to ask if the furnace was in the crawl space, but when you said the ductwork wasn't rusty I thought it must be in the attic. Must be a dry crawl space!

Yes, more or bigger return air grilles are the way to go. Quieter and needed for good system performance. How hard is it to get to the furnace to change the filter?

Yes everything is in the crawl space. Bone dry. I'm in California - we could use some rain... Is there any downside to massively oversizing the inlet ductwork? I have tons of space in the crawl space for it.

Furnace takes a solid 10 seconds to get to from the garage. That section of the crawl space has an 8 foot ceiling So I have no problems with changing filters there if that's the better place to have them. When I moved in (a few months ago) there were filters in both the inlet grille as well as in the furnace. I removed the one in the inlet grille because it was... scary. I admittedly haven't even looked at the other one yet - I should do that...

 

[George2]

A 4 ton A/C requires 1,600 cfm's That's a lot of airflow. One grille is not big enough. What size is the A/C now? You'll need multiple grilles.

2 cfm per square inch is the rule of thumb to give you a starting place. Ex: A 20 x 24 grille equals 960 cfm.

 

[mnoone]

A 4 ton A/C requires 1,600 cfm's That's a lot of airflow. One grille is not big enough. What size is the A/C now? You'll need multiple grilles.

2 cfm per square inch is the rule of thumb to give you a starting place. Ex: A 20 x 24 grille equals 960 cfm.

I have no AC right now. Just a 25 year old natural gas furnace.

My new air return duct will come up through the floor into a cavity that is 11" x 24" (as in, I think the duct will be 11x24). That duct can either go into attic and then curve back down into ceiling, or it can instead have registers on the side of the wall it is passing through. The duct will be up against the wall on 3 out of 4 sides of the duct (as in - I could have registers on three of its four sides). In theory that means I could have a ~24x96 and 2 11x96 ducts. Would it be beneficial to have registers on multiple sides? That would get me a register in the living room and two registers in the hallway. So I can definitely satisfy the 800 in^2 goal. How big of a duct do I need to get to the registers? I imagine it's a function of length and diameter? I'll have to measure this evening - but I think it's about a fifteen feet run from furnace to register.

 

[Poodle Head Mikey]

1. What area of the world do you live in?

2. How many square feet of heated / cooled floor space do you have?

PHM
------------

I have no AC right now. Just a 25 year old natural gas furnace.

My new air return duct will come up through the floor into a cavity that is 11" x 24" (as in, I think the duct will be 11x24). That duct can either go into attic and then curve back down into ceiling, or it can instead have registers on the side of the wall it is passing through. The duct will be up against the wall on 3 out of 4 sides of the duct (as in - I could have registers on three of its four sides). In theory that means I could have a ~24x96 and 2 11x96 ducts. Would it be beneficial to have registers on multiple sides? That would get me a register in the living room and two registers in the hallway. So I can definitely satisfy the 800 in^2 goal. How big of a duct do I need to get to the registers? I imagine it's a function of length and diameter? I'll have to measure this evening - but I think it's about a fifteen feet run from furnace to register.

 

[George2]

I have no AC right now. Just a 25 year old natural gas furnace.

My new air return duct will come up through the floor into a cavity that is 11" x 24" (as in, I think the duct will be 11x24). That duct can either go into attic and then curve back down into ceiling, or it can instead have registers on the side of the wall it is passing through. The duct will be up against the wall on 3 out of 4 sides of the duct (as in - I could have registers on three of its four sides). In theory that means I could have a ~24x96 and 2 11x96 ducts. Would it be beneficial to have registers on multiple sides? That would get me a register in the living room and two registers in the hallway. So I can definitely satisfy the 800 in^2 goal. How big of a duct do I need to get to the registers? I imagine it's a function of length and diameter? I'll have to measure this evening - but I think it's about a fifteen feet run from furnace to register.

I'm confused by your description of your situation. Sorry. The sizes of the ductwork don't register with me. Originally, you said something about a 14" return that the contractor recommended enlarging it to 16" (I assume we're talking about round pipe?) Anyway, try to keep the new returns grilles away from the furnace. This will help keep the system quiet. How large is your home? Is it a ranch or 2-story.

 

[mnoone]

1. What area of the world do you live in?

2. How many square feet of heated / cooled floor space do you have?

PHM
------------

1. San Francisco bay area
2. 1600 square feet, single floor

 

[George2]

1. San Francisco bay area
2. 1600 square feet, single floor

They recommended a 4 ton A/C for you! Wow!

 

[mnoone]

They recommended a 4 ton A/C for you! Wow!

It seems big, right? I've gotten recommendations ranging from 3 - 4 tons.

 

[George2]

It seems big, right? I've gotten recommendations ranging from 3 - 4 tons.

It's just a guess, a Manual J calculation will be accurate, but I would say a 2.5 ton is closer to the truth. Depends on insulation, windows and exposure to the sun.

 

[mnoone]

It's just a guess, a Manual J calculation will be accurate, but I would say a 2.5 ton is closer to the truth. Depends on insulation, windows and exposure to the sun.

Currently no insulation in walls, none below house, and only a modest amount in attic (~3" of blown in fiberglass). Single pane windows.

I'm working on fixing all of that - but it'll be a year or two before that's all fixed.

All three companies looked at my place and immediately told me a tonnage needed. Is that a red flag? Should there have been more calculations?

 

[George2]

Currently no insulation in walls, none below house, and only a modest amount in attic (~3" of blown in fiberglass). Single pane windows.

I'm working on fixing all of that - but it'll be a year or two before that's all fixed.

All three companies looked at my place and immediately told me a tonnage needed. Is that a red flag? Should there have been more calculations?

Yes, they should have done more, but almost no one does a Manual J calculation. Honestly, with your home's lack of efficiency, I would insulate and install new windows
first.

The worst thing you could do is to install an A/C unit too large, do the improvements, and then find out the A/C never runs (dehumidify.)

If you HAVE to install A/C now, I would feel safe to tell you a 2.5 ton is all you'll need 95% of the time. With the improvement you do down the road it'll be just fine.