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Old 03-15-2020, 08:13 PM   #1
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HVAC system location and efficiency.


My wife and I have been looking at model homes and speaking with home production builders. We are in the market for a new single home. The homes that we have been looking at, and would like to buy one day is about ~5500 square feet of finished living space. It has two floors above ground, and a basement.

We see a split in the middle when it comes to the location of the secondary HVAC system. The primary HVAC system is the same with all builders. The primary HVAC system sits in the basement, and server the main floor and the basement.

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As with the secondary HVAC system, some builders put that in the attic. Other builders actually put that system in the basement in the same space as the primary HVAC system. The secondary HVAC system only serve the 2nd floor, where all the bedrooms are.

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What are the advantages and disadvantages with having the secondary HVAC system in the attic space?

My best guess is that there would just be less duct work that need to be run. The initial cost (the cost to builder) would be lower. The drawback is that since the system is in the attic, which is an unconditioned space, this is less efficient.

Also, this system cannot have a humidifier. The water line would just freeze during the winter.

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What are the advantages and disadvantages with having the secondary HVAC system in the basement?

Since there are more duct work, it would cost more to build. Does it actually cost that much more to have this kind of system vs the one above? It takes up more space because you have a run a giant supply duct to the 2nd floor.

The benefit is that the duct system is inside condition space. Thus this system is more efficient because it doesn't always have to fight against extreme hot and extreme cold. This secondary system can have a humidifier because temperature would never be cold enough for water to freeze.

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If my above statements are wrong, please correct me. Please add anything important that I have not mentioned. I'm here to learn.

Greatly appreciate your help.

Last edited by 012abc; 03-15-2020 at 08:27 PM.
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Old 03-15-2020, 08:20 PM   #2
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Re: HVAC system location and efficiency.


The duct system should be kept in a conditioned space if at all possible.
Different floors require different loads and capacities as far as heating or cooling go. Zoned single systems or multiple systems should be used. Expecting a single unzoned system to condition multiple floors evenly isn’t expectable.
The basement is an entirely different environment then above grade floors and many times needs a small amount of heat and dehumidification.
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Old 03-15-2020, 08:27 PM   #3
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Re: HVAC system location and efficiency.


In 5,500 sq. ft. of living space, a builder should easily find approximately 40 sq. ft. of floor space on the ground floor for 2 units with nothing in the attic. If he cant he needs to re-align his ducks because they are way out of line.
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Old 03-15-2020, 09:32 PM   #4
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Re: HVAC system location and efficiency.


Keep the equipment out of the attic.

Doing multiple systems or zoning is more of an american thing - I've never walked into a house with multiple systems up here.

2 stories above ground can absolutely be heated properly with one system, the duct system just needs to be engineered properly and balanced.

There's an attic above, but if well sealed and insulated heat loss is minimal.

Basement without supplemental heat can be a little cold in the shoulder seasons though - proper wall insulation practices can help with this. (vapour barrier plus fiber or roxul is horrible for basements)

Wise to throw a gas fireplace down there if/when you decide to finish it.

Cooling is a bit more challenging but the first and second floor can be within 3F of each other on a good duct system.

Zoning can be very problematic especially if the builder cheaps out and uses single stage equipment with a bypass. Problems are compounded by undersized ducts.

Need at a minimum 2-stage heating and cooling with variable speed blower and a good zoning panel. The second stage is kept off until both zones are are calling.

The variable speed blower automatically compensates for changing duct pressure.

Best type of zoning setup would be manufacturer's proprietary system controlling modulating equipment but lots of $$$.

Whatever setup you end up with, be sure a room by room load calculation is done and everything is sized based on that.

Don't know where you are, but typical 3600 sq ft (plus basement) detached house with 8ft ceilings, R19 walls probably only needs like 80k btu input (@96% efficiency) and 3 to 4 tons of cooling in the northeast.

Yet there could be crappy builders in your area putting 100 to 120k input and 5 to 6 tons of cooling in.
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Old 03-15-2020, 09:50 PM   #5
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Re: HVAC system location and efficiency.


Quote:
Originally Posted by SeniorSitizen View Post
In 5,500 sq. ft. of living space, a builder should easily find approximately 40 sq. ft. of floor space on the ground floor for 2 units with nothing in the attic. If he cant he needs to re-align his ducks because they are way out of line.
basement was mentioned.

we all know builders cheap out on structural and mechanical so they can spend money on what sells houses - nice finish materials.
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Old 03-15-2020, 10:42 PM   #6
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Re: HVAC system location and efficiency.


Let's start with, where are you, approximately?
Production builders have often squeezed every feature down to minimum code level to maximize profits. Not all but you do give up some flexibility. Ask them if they pressure test new homes during construction and what number they guarantee?

Point being, a builder that is interested in selling comfortable energy efficient homes will be following and explaining the modern approach to quality and comfort.

If they can't do that, keep looking.

I'll wait to see what state you are in.

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Old 03-15-2020, 11:00 PM   #7
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Re: HVAC system location and efficiency.


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Let's start with, where are you, approximately?
Production builders have often squeezed every feature down to minimum code level to maximize profits. Not all but you do give up some flexibility. Ask them if they pressure test new homes during construction and what number they guarantee?

Point being, a builder that is interested in selling comfortable energy efficient homes will be following and explaining the modern approach to quality and comfort.

If they can't do that, keep looking.

I'll wait to see what state you are in.

Bud

Ahh. I did forget to include our location. We are looking at homes in Loudoun County, Virginia, USA.

NVHomes is the builder that put the secondary HVAC system in the attic, un-conditioned space. I did ask them about this in person, and they said that they didn't find any significant benefit in putting the secondary HVAC system in the same space as the primary HVAC system.

Stanley Martin is the builder that put the secondary HVAC system in the same area as the primary HVAC system.

The primary HVAC system design is practically identical for both NVHomes and Stnaley Martin.

Both builder pressure test their homes. It's actually the county that pressure test new construction home. Evidently the county set the standards for new construction homes.

Hope that clarify things. Thanks again for the help.
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Old 03-15-2020, 11:07 PM   #8
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Re: HVAC system location and efficiency.


Attic duct system should be a complete deal-breaker unless the entire roof deck is foamed.

They don't see a significant benefit because you'll be paying the utility bills. It's cheaper to them - requires less duct and no bulkheads.

Assuming the house will be heated by furnaces, keep in mind 90% furnaces can have their condensate lines freeze up in attics.

So you would most likely end up with 80% for second floor - between that and duct heat loss, could end up with much higher utility bills.

Can you get any info on the equipment they want to install and sizes?
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Old 03-16-2020, 12:43 AM   #9
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Re: HVAC system location and efficiency.


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Attic duct system should be a complete deal-breaker unless the entire roof deck is foamed.

They don't see a significant benefit because you'll be paying the utility bills. It's cheaper to them - requires less duct and no bulkheads.

Assuming the house will be heated by furnaces, keep in mind 90% furnaces can have their condensate lines freeze up in attics.

So you would most likely end up with 80% for second floor - between that and duct heat loss, could end up with much higher utility bills.

Can you get any info on the equipment they want to install and sizes?

"Attic duct system should be a complete deal-breaker" Why? I can understand that it's less efficient, but why a complete deal-breaker? When buying a new home, there's just so much factors to considered. HVAC system is a major factor but I don't think this system is significant enough to be a deal BREAKER.

"They don't see a significant benefit because you'll be paying the utility bills." ROFL so true. Didn't think about that. :-)

With regards to everything else, you are absolutely correct. We do have a 90% furnace in the basement and an 80% furnace in the attic.

Will try to get info on the equipment for both HVAC systems.
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Old 03-16-2020, 12:56 AM   #10
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Re: HVAC system location and efficiency.


After mortgage and property taxes, utility bills are the greatest expense. It's so much cheaper to invest more upfront during construction to save energy than pay for the energy.

Once the house is built, opening walls and running new air ducts from the basement is prohibitively expensive.

You only have one opportunity to get insulation, plumbing, electrical, hvac right.

You can always paint, replace countertops, install hardwood floors, etc later.

If the builder wants to cut corners on hvac, they will cut corners elsewhere.

The insulation on air ducts is normally only like R8 which is lower than walls in a modern house. There's also leakage.

It's crazy to get an 80% furnace in a cold climate in a new build - throwing away 20% of the heat produced vs 4-5%.

It's even crazier to put 110f-120F air through air ducts in a cold attic.

The temperature differential is huge, r-value poor so you can lose a lot of capacity to the attic.

Could easily cause anywhere from 10 to 33% capacity lost.

Need a bigger furnace and a/c to compensate.

The effective efficiency of such a system could easily be 60% between needing to use a 80% furnace and duct loss. In my country, they don't even sell furnaces below 90%, it's against the law to manufacture and import them.


There are other disadvantages:

Attic systems increase the number of penetrations into the attic ceiling resulting in greater leakage even if they're sealed off.

Poor comfort -> Having hot being pushed down from the ceiling.
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Last edited by user_12345a; 03-16-2020 at 01:06 AM.
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Old 03-16-2020, 02:08 PM   #11
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Re: HVAC system location and efficiency.


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Ok. I just checked with builder. Both system is 92%. I asked them about the condensation factor of the system in the attic and they said it's insulated, so no need to worry about condensation for the attic system.
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Old 03-16-2020, 03:16 PM   #12
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Re: HVAC system location and efficiency.


It's a bad idea to have anything with water in the attic even if insulated.

Not only due to risk of freezing but also risk of water damage.

It's really bad for the service technician who has to work on the equipment when it's in the attic.

It's a waste of energy.

Just say no. The way houses are still done in the us in this day and age is absolutely disgusting.

I'm betting the builder also wants to be two 60 or 80k furnaces with two 2 to 3 ton a/c units in.

Any word on the sizing?

Builders get away with crap they never would get away with in the rest of the civilized world.

If you want a separate system for each floor, keep everything in the conditioned space.

It's possible to put a furnace in a closet on the second floor and have an extra 1 to 2 feet above the ceiling for ductwork so there's no ugly bulkheads.

Really, two systems are totally unnecessary at least for heating if the air duct system is properly engineered, installed, sealed.

The heat loss of the main floor and second floor increase pretty proportionally as it gets colder outside.

For cooling, a small temperature difference between floors is normally unavoidable.
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Last edited by user_12345a; 03-16-2020 at 03:20 PM.
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Old 03-16-2020, 04:46 PM   #13
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Re: HVAC system location and efficiency.


Ideally I would put both in the basement. I would avoid having one in the attic. The upstairs unit in a second floor closet works, but adjacent rooms will hear it. A few weeks ago we stayed in a house with a second floor unit in a closet by our bedroom. That thing cycling all night kept me from getting much sleep. For quiet operation and ease of service, I would put them in the basement.
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Old 03-16-2020, 09:22 PM   #14
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Re: HVAC system location and efficiency.


I'm sure NVHomes will put the second system in the basement if it gets then the sale! Yes, the basement is the better choice but be very careful that the ductwork to the second floor is done correctly - once the house is built, there's no changing it (that's the one advantage of the attic location).
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