Thread: Only one system

Did a 4000 Sq. Ft. single family two story home today. Originally home had one HVAC system serving 3300 Sq. Ft. and now the finished basement adds another 700 Sq. Ft. The HVAC system is a Lennox 81% 132,000 btu forced air furnace. Around here most homes over 3000 Sq. Ft. have two HVAC systems. Is there a written standard how many systems are required based on Sq. Ft. of the home?

Originally Posted by Richard Roshak

Is there a written standard how many systems are required based on Sq. Ft. of the home?

No.

I would feel free to tell my client that getting balanced heating/cooling in this home with one HVAC system will be problematic if not impossible.

Originally Posted by Richard Roshak

Did a 4000 Sq. Ft. single family two story home today. Originally home had one HVAC system serving 3300 Sq. Ft. and now the finished basement adds another 700 Sq. Ft. The HVAC system is a Lennox 81% 132,000 btu forced air furnace. Around here most homes over 3000 Sq. Ft. have two HVAC systems. Is there a written standard how many systems are required based on Sq. Ft. of the home?

I am in a similar climate zone. There is no definitive standard because there are too many variable. A rule of thumb for ac is not to exceed 750 sf per ton. In some cases that number is too high. Based on that you need to jump to two systems at 3750 sf because a 5 ton ac is the largest size in residential equipment. Basements do not need much ac. A rule of thumb for heating in newer houses is about 20 to 25 btuh per sf. At 107,000 output and 4000 sf that is about 27 btuh per sf. I would not be too concerned if the ac is 5 ton. If it is 4 ton then it seems a bit small

Have you heard the expression: putting the cart before the horse?

If the Cu Ft area previously generously hosting the furnace has been reduced, and a significant portion (apparently 700 sq. ft. or so times X height) has been "finished" (suggesting restriction of prior sources of infiltration and reduction of cu. ft.'age hosting fuel fired furnace - I'd be firstmost concerned with sufficient volume and exhange of air for dillution (up the flue), combustion, and make-up air (also considering the likelihood of a fuel fired and exhausting clothes dryer in the likely-situated nearby laundry area.

heat recovery ventillator? unknown. Other forms of ventillation? unknown. Cold air return/exhange to furnace room? unknown. zoned damper system? unknown. Modulating unknown. multi-speed fan? unknown.

I likely missed it, but I recalled no reference to Central AC coil in the basement furnace airhandler in the OP, just furnace/heating?s.

Sub-soil basements require less btu than 2nd story glass/exposed generally. High mass subfloor and walls well insulated av. soil temp below frost line in mid 50s Y/R.

One system can work in a two story home. However, there will almost always be a balancing issue, unless a zoned system is installed where you can have thermostats on each level and controlled baffles in the ducts.

Will it work? Possible. Is it going to work well? Kind of unlikely. For that much square footage I'd want to see two systems.
However, it all depends on the house. Maybe the place has tons of insulation, maybe there are great windows, maybe wall penetrations have all been sealed really well, on and on.
I'm happy when I can do an HI on a home like this when its cool/cold. When a situation is questionable like this I like to verify conditions.
Turn system off for a few, document stat temp and start system, monitor system while I'm doing the inside of the house at 10-15 minute intervals and document temp rise on stat each time, document total temp rise after 1 hour.
This doesn't work so well at 50-60 degree outside temp but lower than that it works really well. As dumb as clients can be they aren't necessarily stupid. When its 30 degrees out and the furnace has only brought the house temp up to 65 from 62 after running for an hour they get the idea.
This works great in reverse for AC as well. Had one a couple months ago. After running AC full blast for an hour the temp dropped from 85 to 82. Outside temp was roughly 80, not a lick of insulation in a fully new rehabbed house by well known developer.
Obviously this may not be the most scientific approach but it gets the message across to the Buyer and Seller.

Originally Posted by Richard Roshak

Did a 4000 Sq. Ft. single family two story home today. Originally home had one HVAC system serving 3300 Sq. Ft. and now the finished basement adds another 700 Sq. Ft. The HVAC system is a Lennox 81% 132,000 btu forced air furnace. Around here most homes over 3000 Sq. Ft. have two HVAC systems. Is there a written standard how many systems are required based on Sq. Ft. of the home?

Not merely based on a simple square footage number nor just BTU max capacities, but yes.

ACCA Manual S which is provisional upon the ACCA Manual J Calculations (and in your example both Block and Room-by-Room calculations would be required) in addition to ACCA Manual D (and if necessary reverse calculations for in-place existing duct/trunk work) which further takes into consideration the specific and unique specifications and performance factors for the individual specific equipment considered/compared as incorporated into the overall system being designed.

ACCA (Air Conditioning Contractors Association of America) Manuals

To properly design a complete HVAC system, there are three fundemental procedures that should always be followed:
ACCA Manual J Load Calculation
ACCA Manual S Equipment Selection
ACCA Manual D Duct Design
In the order listed.

First: ACCA Manual J Load Calculations (Whole House {Block} Load Calculations and/or Room-by-Room Load Calculations)
ACCA's Manual J is the first step in the design process of a new or replacement HVAC system. By following the Manual J methodology, HVAC designers are able to accurately determine the total amount of heat tht is lost through the exterior of a home during the cooler months, and the total amount of heat that is gained through the exterior of a home during the wrmer months. Through a complex series of calculations and inputs, the HVAC designer is able to analyze all aspects of the thermal charcteristics of every wall, floor, ceiling, door and window. In addition, an HVAC load calcultion also takes into consideration other factors such as the home's geogrphic location, orientation to the sun, envelope tightness, duct leakage, lights and appliances. ACCA's Manual J even calcultes the mount of heat nd humidity that each occupant of the house will add to the interior of the home.

There are two types of Manual J load calculations:

Whole House (Block) HVAC Load Calculations Whole House or Block HVAC Load Calculations provide the heating and cooling loads for the entire home. This type of load calculation is used by itself when there is no need to design or modify an existing duct system. Whole house load calculations are commonly used to determine the correct HVAC equipment size and match-up that is required when replacing the previously properly functioning HVAC system in an existing home (with no modifications (insulation, windows, coverings, etc., additions, changes in occupied space/living space - including compartmentalization or de-compartmentalization) when changes will be/have been made and/or not previously functioning correctly/comfortably, Room-by-Room Load Calculations must also be made).

Room-by-Room Load Calculations. Room-by-Room Load Calculations provide the heating and cooling loads for each individual room within the home. In addition to the information to the information produced by a block load calculation, the Room-by-Room method also determines the mount of air that is required to heat and cool each individual room. This information is critical when determining the individual duct sizes as well as the size nd overall layout of the duct system.

Next: ACCA Manual S Equipment Selections.

Once a Manual J HVAC load calculation has been completed, the HVAC designer will have the fundamental informtion required to accurately select the proper HVAC equipment based not on its size, but on other performance traits such as the equipment's total capacity to remove heat and moisture from the air as well as how much total air, and at what pressure, the system can produce. This is important to note because, for example, one 3 ton HVAC system can perform significantly different than another 3 ton HVAC system.

Then: Manual D Duct Designs.

Manual D is the ACCA method used to detrmine the overall duct lay-out including the individual duct sizes. To design a duct system, the HVAC system designer must hve completed a Room-by-Room Manual J load calcultion as well as a Manual S equipment selection. All to often, duct systems are created using 'rule-of-thumb' methods in lieu of using Manual J, Manual S and Manual D. This practice is the predominant reason for complints of temperture differentials throughout home as well as complaints of excessive noise caused by air velocity that exceeds the maximum allowed by Manual D.

Furthermore:

Energy Sheets (REScheck & EnergyGauge Energy Code Compliance Reports)
Most states require that energy code compliance of a residential construction project must be reported in order to qualify for the appropriate construction permits. In addition, most state energy codes require that an ACCA Manual J heat load calcultion be performed on the construction project, and that the heating and cooling equipment must be sized per ACCA Manual S.

While energy codes vary from state to state, in general there are 2 different paths that can be followed to determine if construction project meets your state's energy codes (generally in existing construction/remodeling/additions the Performance Method is utilized).

Prescriptive Method. The Prescriptive Method has pre-assigned minimum values such as, thermal resistnce (R-value), thermal transmittnce (U-value) and solar transmittance (SHGC), for each component of the building. This approach is quick and easy to use, but many users find it somewhat restrictive becuse the requirements are typiclly bsed on worst-case ssumptions and all requirements must be met exactly as specified. Energy code complince using the prescriptive method is usually not the most cost effective path to follow to chieve energy code compliance.

Performance Method. The Performance Method allows more flexibility by allowing one energy saving mesure to be traded for nother. Each energy sving mesure is assigned points or credits. The totl points/credits for ech are must meet the minimum total points required to qualify for the building permit. Typiclly, this method is less restrictive than prescriptive approaches because components that exceed the requirements can compensate for those that do not meet the code.

When you think about it (your closing question), square footage alone would be far from adequate to calculate requirements for volume. Hopefully the explanations and information provided above will help you understand why generic 'rule-of-thumb' numbers are completely useless, especially when comparing performance of different models without any specifications whatsoever other than unknown vintage maximum "efficiency" numbers and max Btu at sea-level numbers.

HTH.

P.S. the old "A" key is touchy and I occasionally miss correcting for the missing "a"s. Fill in as necessary.

Originally Posted by Richard Roshak

Did a 4000 Sq. Ft. single family two story home today. Originally home had one HVAC system serving 3300 Sq. Ft. and now the finished basement adds another 700 Sq. Ft. The HVAC system is a Lennox 81% 132,000 btu forced air furnace. Around here most homes over 3000 Sq. Ft. have two HVAC systems. Is there a written standard how many systems are required based on Sq. Ft. of the home?

Many of the posts raise good points at to whether one system will or will not work, or work well. And of course there are standards for sizing heating and air conditioning systems. And we all know that this goes well beyond home inspection standards. I think the main point to consider is to observe what are typical sizes of systems for typical houses in your area. Then at least you may get a feel for when something seems out of line (like your example above). That is about the best you can expect to do during a home inspection.