Thank you for sharing. Yes, you are right, its two ACs or running it off one too. My concern is two fold - 1) is one system enough (and have good flow to the 2nd flr)?

2) is two systems in the basement a good approach (one dedicated to the 2nd flr and the other servicing the basement and main)? Most ppl I've spoken with say they have the 2nd system in the attic, few say basement, and a small number told me they have their 2nd system on the 2nd floor in a closet. Also, some said they don't prefer registers on the floor of the 2nd floor - the preference are ducts through the attic with vents in the ceilings - which is what our builder is saying is inefficient (they prefer to not cut into the ceilings?).

And I've asked for two quotes to see the difference. Standard in the plans is a one stage. Ive asked for variable speed for the one system quote and then two system quote. Im more concern with performance.

My house has two independent systems both of which are in the basement. This was an upgrade option when the house was built 35 years ago. The systems work very well and I believe have saved a significant amount money and have better temperature control when compared to the same house with a single system.

I don't recommend installing HVAC equipment in the attic unless it is conditioned. The floor vents in the floor work fine. The best option is to have the a system on the second floor, but the drawback it takes up floor space.

If you are going to finish the basement and want good temperature control then install a zone off the first floor system.

Thanks for your replies everyone. It sounds like everyone is in agreement that two systems are better than one.

Josh Dixon - do you think with two systems, we should look into a variable or two speed unit to service the second floor?

Anglophilia - I do like the 2nd system on the 2nd floor. I don't think our builder has a plan that routes the duct work through the attic. That's my impression with them insisting on units in the basement.

Mike_home - thanks for sharing and addressing vents in the floor. Someone told me there's a reduced air flow, but I wasn't sure if that dramatically different.

Thanks Josh D. Appreciate it - I've asked for a Carrier and Trane quote for variable and single.

I think mine are both variable speed. Since this was an addition, running duct work up from the basement, through the walls, would have involved tearing out plaster walls! We were not gutting the house. Running the duct work through the attic seems more efficient to me, especially when the AC is running. Cold air falls and trying to push it up seems ineffective to me, but I’m not an HVAC contractor!

I had a similar size house with 3 floors and 2 systems. Infinity carrier. Zoned the 2nd floor because of sleeping cold. We lived there 8 years and the repair cost and frequency of the Infinity was unacceptable. I can't say the troubles were based on the zoning - but it did increase the cost of repairs. Infinity had an issue with their staged compressors and had a discount replacement program that ran for awhile. They said the new ones are better....

1 system can handle a house that size - that is not an issue at all. 1 system needs 1/2 the maintenance. 1 system needs half the repairs.

Zoning without staging is not a great idea. Then with zoning and staging you double the cost of repairs. But you don't double the maintenance. Something to consider.

New house, we went with 3 systems (argh) even though it is smaller. We elected to minisplit our bedroom this time - rather than zone.

"thanks for sharing and addressing vents in the floor. Someone told me there's a reduced air flow, but I wasn't sure if that dramatically different."

If you are comparing a second floor vent that is connected to duct that runs down two stories to a basement to a ceiling vent connected to a duct work in an attic, then some of the floor vents may have less air flow. But the heating and cooling loss of a flexible duct in an unconditioned attic usually will not make up for the additional air flow.

We have had discussions on this forum about wall insulation for new construction ranging in R-15 - R-20. This is the barrier between the summer indoor temperature of 75 degrees and and outside temperature of 90-95 degrees. However flexible duct work installed in attics are R-8. The air in the duct work is about 55 degrees but the attic can reach temperatures of 110 degrees on a hot summer day. That big temperature differential creates a significant loss even for a short duct run.

Anglophilia - thank you, that makes sense.

David Cary - thank you for posting. For your new home, are you running one system for the basement and main (is it one system zoned?) and then mini splits on the 2nd floor?

Mike_home - thank you for your expertise here. That makes sense with the flexible duct work in the attic. Our plan has R21 for the 2X6 walls and its R49 in the attic (but not spray form - wonder if we should use spray foam in the walls and attic... what do you think?).

No basement. Main floor is 1 system with master having mini; 2nd floor - 2nd system.

Don't waste money on spray foam in walls. Rigid foam on the outside is a better use of money ... usually.

I don't know anything really about HVAC, but this is what my DH decided to do in our house. We have 2 systems located in the basement. One system uses floor vents for the main floor and the other system uses ceiling vents for the basement. I have the basement unit set at 65 degrees (unless someone is staying in the guest bedrooms down there, then 70) and the main floor at 70 degrees. I put in R19 in all exterior 2 x 6 walls. In the basement, I installed rigid foam board on the concrete walls, then insulated the furred wall over that with R11. I also put rigid board and R19 in between all the floor joist next to the rim on top of the concrete walls. The floor and ceiling have blown in insulation of R49.

Last month our lows averaged in the teens and the highs in the 30s and I had family over for Thanksgiving staying in the basement. My gas bill for last month was $41.00. I thought that was pretty good.

Thanks David Cary

Thank you Suru. Looks like David and you agree on rigid foam boards. And that is a really good bill last month. The house we are living in right now approaches $175 in the Nov/Dec months (1970s with limited insulation and upgrades).

Ours is a single floor home with full height unfinished attic and unfinished basement. (A future owner may want to create a second story.) House runs E to W, two rooms deep.

We have two two-stage Carriers with supply through the floor and two sets of air returns -- one for heat and one for A/C. Each furnace has a flow-through humidifier (no mold can grow on media) and an Aprilaire air cleaner.

We have lost heat once at each end of the house in 17 years. Yes, we have to pay to service two systems. Bulk of the expense is just the tech showing up.

I don't know the answer to your question but I can tell you that where you are located is very important and in a cold climate it is not wise to put any HVAC equipment or ducts in an unconditioned attic. In New England I have always used a hydro-air system wth one air-handler in the basement and another in a conditioned attic.

Thank you Sue and RES for sharing. Appreciate it - your posts and others have given us confidence we are taking the right approach.

We are thinking two stage systems as well. Can anyone share what's been a quality unit/brand - Carrier, Rheem, Trane?

Sue, I see you have a Carrier - reliable?

Seventeen years and going strong with zero problems. I credit the separate returns for keeping the house comfortable summer and winter (no 'layering').

I would sat that 17 years ago and today are not necessarily the same. I am firmly in the "Never Carrier" camp.

Uh, how big a single unit are they proposing for your 5000 (+/-) sq ft home and does anyone make a residential one that big?

Thanks Sue and David. sktn77a, I have to check. But I think we are leaning on two units with 2 stages both in the basement. I like the idea of one, but I'm not sure I want to take the risk of the contractor not being the best and building a zone system that doesn't work well.

In my experience, few general contractors have much knowledge about HVAC matters. They tend to pick their subs based on friendships or pricing and not necessarily based on expertise. I think you'll do much better having two separately controlled systems, both from a comfort standpoint and from an operating simplicity standpoint. Also, having two separate systems will allow each to be properly sized. Make sure they do load calculations.

Thank you for your recommendation!

@HU-676357264 - we are building similar sized home and facing similar choice...

Could you pls tell me what you ended up doing? And are you pleased with your choice? Looking back, is there anything you would've done different? what brand HVAC did you pick? TIA ...

If you can afford it, get two systems. Most HVAC companies don't know how to design a zoned system and it becomes an ongoing headache..

The biggest problem with HVAC zone systems is knowing when to choose it as a solution and when not to. Some HVAC companies it's about "sell, sell, sell" whether or not it works... there's more money in "sell, sell, sell" --- when it's only about money?

So does the comment above mean I do it for free? No. If you think that you are reading things that aren't there.

New construction? best to choose separate systems 9 times out of 10.

In order for a zone system to work, you have to know what you are doing. Most HVAC companies staff (the techs they send you) are 20-30 year olds. How much practical experience could they possible have?

It's best for HVAC zone systems to be an elegant solution. A HVAC zone system is a tool in the tool bag. Not all homes are suitable for zone systems. Some homes there is no other way, and in some cases complications to those problems will make an HVAC zone system design possibly more prone to problems.

You have a problem now, do you want more problems. These decisions among others are decided "in person" --- so considering that becareful what information you decide upon from the internet. Just because it's here doesn't necessarily mean it's true.

That said: Depending on equipment level chosen, how many different floors you have in your structure, how much room you have available... will all be factors that make you decide one way or another.

From there it's in your best interest to pick a contractor that is skilled: especially when it comes to HVAC zone systems. (there's a lot of garbage out there.)

(click to enlarge if necessary)

I service the Katy, Texas area.