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Discussion Starter · #1 ·
Hi All

My house has a converted attic that has very little headroom. I'm drawing up plans to remove roof, put in 8 foot walls and a new roof. All good on the framing and loads (spoken to a few architects and they say everything's good and I will have them draw up plans for permit).

I'm trying to finalize budget and need to to determine my HVAC approx costs

House is 3 floors. Finished basement, main floor, 2nd floor. Currently basement and first floor are about 1000 sq ft (basement has about 250 sq ft of unconditioned space for large mechanical room and where main stack is, so really only 750 sq ft).

2nd floor is currently about 640 sq feet, but will become about 1000 sq feet.
There is a single HVAC duct from basement to 2nd floor which is then split. AC on 2nd floor is terrible (largely do to poor insulation). I will be insulating roof to R49 and walls to R21

My furnace is AUH2C100A948VAA (100,000 BTU, 95% efficient, 2 stage).
My AC is Carrier 24ABB336A0031010 (2 TON?)

One of the architects suggested putting 2nd HVAC on 2nd floor or in attic. If i have to spend the cash on extra capacity, I will, but if I do, I'd prefer to keep it in basement (mechanical room is 160 sq ft so tons of space and I have the existing duct work to 2nd floor, which can be made bigger if necessary)

So I have 2 questions.

  • Are these units big enough for 2750 sq ft?
  • If I install HVAC for 2nd floor, will it be effective if I have furnace in basement and a single duct to 2nd floor attic and distribution from there?

Thanks

Mark
 

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Hvac is not sized via just square footage. It’s done through manual J calculations.
How big is the duct feeding the second floor? I suspect it’s likely too small. Is there a return duct?
Second floor should ideally have its own system.
That’s a 3 ton air conditioner. And that’s an enormous furnace. You must have no insulation to need a furnace that large.
 

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Get a room by room load calculation done during the planning stages.

If you don't mind a small temp difference between floors in cooling mode, you can absolutely use one system, the duct system just needs to be designed right.

Most duct systems are horrible so second floors don't heat/cool well, but they can.

In a retrofit application, best to run one dedicated trunk line up for the second floor. Keep it out of unconditioned attic though!

Zoning may be a very good option but equipment needs to be properly sized and 2-stage or efficiency is really sacrificed. (requires bypass duct)

100k btu @ 95% efficiency is huge and if the house is even half decent, 60 to 80k input may do the whole house with capacity to spare. Depends on construction and climate.
 

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Discussion Starter · #4 ·
Get a room by room load calculation done during the planning stages.

If you don't mind a small temp difference between floors in cooling mode, you can absolutely use one system, the duct system just needs to be designed right.

Most duct systems are horrible so second floors don't heat/cool well, but they can.

In a retrofit application, best to run one dedicated trunk line up for the second floor. Keep it out of unconditioned attic though!

Zoning may be a very good option but equipment needs to be properly sized and 2-stage or efficiency is really sacrificed. (requires bypass duct)

100k btu @ 95% efficiency is huge and if the house is even half decent, 60 to 80k input may do the whole house with capacity to spare. Depends on construction and climate.

Thanks for the reply.

I can live with variation between floors

climate is chicago (zone 4 I believe). 2nd floor will be well insulated. 1st floor original from 40's AFAIk

is the AC unit up to the task

What size dedicated trunk would I use?

I'm *considering* closed cell foam on underside of roof, so would that solve the problem of running duct work in the attic?

Is it feasible to run between joists (I have sistered 2x8's, so a void of 7 1/4 x 11 1/2 inches. What's the typical size of duct off the main trunk and is it Ok to run between joists?

Thanks

mark
 

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Mark


Simple physics hot air rises cold air falls. An AC has a very hard time pushing cold air even to a second story; not any chance of reaching the third. A properly sized system on the third floor should be able to cool floors 2 and 3. The one in the basement can handle floor 1.


Sizing must be done by a pro. The pros who hang out here will tell you the same thing.
 

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An AC has a very hard time pushing cold air even to a second story; not any chance of reaching the third. A properly sized system on the third floor should be able to cool floors 2 and 3. The one in the basement can handle floor 1.
This is a common myth perpetuated because people only have experience with bad duct systems.

The impact of air density on distribution when there's a fan is really insignificant.

Remember, the supply air is normally only 15 to 20F cooler than return in cooling mode.

If you were to measure the flow rate in heating mode, then a/c mode at the same blower rpm, the difference would be within the margin of error of the test instrument.

Static pressure testing can also confirm this; if cold supply air was much harder to move through air ducts than warm, the supply static would be much higher in cooling mode than heating mode flow rates being equal.

Second floors don't often cool well on one system mainly because the air ducts are not properly sized - inadequate supply and return.

Return is particularly important to pull the hot air off the ceiling.

(having a hot attic above is also a factor)

The air has to travel longer to get up to the second floor and the sizing factor, the duct friction rate needs to correct for the longest run. Balancing needs to done and duct joints should be sealed too.

In heating mode, natural convection in 2-story houses hides this problem.

In cooling mode, convection makes this problem even worse.
 

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Thanks for the reply.

I can live with variation between floors

climate is chicago (zone 4 I believe). 2nd floor will be well insulated. 1st floor original from 40's AFAIk

is the AC unit up to the task

What size dedicated trunk would I use?

I'm *considering* closed cell foam on underside of roof, so would that solve the problem of running duct work in the attic?

Is it feasible to run between joists (I have sistered 2x8's, so a void of 7 1/4 x 11 1/2 inches. What's the typical size of duct off the main trunk and is it Ok to run between joists?

Thanks

mark
To size the ducts, you need to see how much heating and cooling each section of the house will need and determine the total equivalent length.

I recommend finding a good designer who can do a room by room load calc and proper duct design.

One system is absolutely an option - you should get quotes and see what makes the most sense.

The duct system for the existing part of the house may require extensive modification to get the distribution right.

Without seeing the house and taking measurements, one can only guess.

It may be cheaper insulate the roof deck as you brought up, have a small mechanical room on the second floor and ducts in the insulated attic.

You wouldn't need very large equipment for the second floor - probably 30-40k furnace and 1.5 ton a/c.

Goodman/amana/daikin make a 30k furnace I believe.

Other may too, though they aren't common.

The existing furnace can be locked on low fire if it's severely oversized for just the main floor. (i'm sure it is!) When furnace and a/c get replaced, can be resized.
 

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Discussion Starter · #8 ·
To size the ducts, you need to see how much heating and cooling each section of the house will need and determine the total equivalent length.

I recommend finding a good designer who can do a room by room load calc and proper duct design.
Thanks for the reply

I definitely plan to use a pro, I know what I don't know :smile:

Just trying to figure out general stuff to figure out bigger picture - e.g.

  1. do I need to leave space for a mechanical room on 2nd floor - space not an issue, just concerned about noise in sleeping area
  2. Should I seriously consider closed cell foam $$$$ to be able to run duct in attic. Definitely don't want soffits in rooms, so only other option I see is between joists

What would ball park cost of the equipment only be that you mentioned?

Thanks

Mark
 

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Discussion Starter · #9 ·
This is a common myth perpetuated because people only have experience with bad duct systems.

The impact of air density on distribution when there's a fan is really insignificant.

Remember, the supply air is normally only 15 to 20F cooler than return in cooling mode.

If you were to measure the flow rate in heating mode, then a/c mode at the same blower rpm, the difference would be within the margin of error of the test instrument.

Static pressure testing can also confirm this; if cold supply air was much harder to move through air ducts than warm, the supply static would be much higher in cooling mode than heating mode flow rates being equal.

Second floors don't often cool well on one system mainly because the air ducts are not properly sized - inadequate supply and return.

Return is particularly important to pull the hot air off the ceiling.

(having a hot attic above is also a factor)

The air has to travel longer to get up to the second floor and the sizing factor, the duct friction rate needs to correct for the longest run. Balancing needs to done and duct joints should be sealed too.

In heating mode, natural convection in 2-story houses hides this problem.

In cooling mode, convection makes this problem even worse.
Thanks
 

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I don't know about costs in your area.

Where I am you can't get a central hvac system put in for much less than 10 grand canadian and that's with the cheapest furance/ac.

Decide what to do in the planning stages - get a good designer on board, then decide on insulation/mechanical room.

If you decide to have ducts in the attic, definitely foam the roof deck.

Running ducts just between the joists is just not practical - only the branch lines would fit.

It's a good idea to put sub-floor material down on the joists up there everywhere so there's no risk of falling through the ceiling if ducts need modification/repair.
 

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We made some changes to our 2 systems after moving in. Regarding the upstairs unit, we are in Houston Texas, we had them put return airs in all of the rooms. With rooms that have doors that get closed, this is a must IMO. Balanced the upstairs better. We increased the tonnage also.
 
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