I did an inspection on a newly built home with 3200 square feet and there were two heat pumps with a total of 5.91 tons of heating and cooling capacity. I usually use 500 square feet per ton to calculate tonnage requirements. I calculate the system can heat/cool 2955 square feet, so the system is undersized. Are the tonnage requirements different for the new high efficiency systems?
More importantly, what did the ASHRAE load calculations show as appropriate for THAT particular home?
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^^^^^ This. Also what did the construction plans say was designed for that home?
The newer built houses for the most part are tighter. Better insulation, better windows etc. So the tonnage will be adjusted accordingly.
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Some general rules can be 700 sf per ton… There are alot of factors that go into determining the sizing.
None of which would be your responsibility to figure out.
I have done a couple brand new homes lately that are 3000 sf with just a 3 ton unit… Obviously, this seems too small, but it depends on the layout of the home, the air return location, etc.
However, I do put this in my report as a concern, and let the builder verify.
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Well, that is totally wrong to begin with…
Load calculations require more than measuring some square foot number and divide by…
Read this…
If you need to ask this, there is no reason to be discussing this in your inspection. A 3 ton deals with 36,000 Btu/hr, regardless of its efficiency.
High efficiency has to do with cost to operate and the units’ ability to function above and below design conditions for your region.
By the way, these design conditions are derived from the load calculations you can not determine with Sq/ft divided by 500.
(b) Home inspectors are not required to:
2. Calculate the strength, adequacy, or efficiency of any system or component
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Hey David, glad you replied…
What would you say about the 3000 sf home with just a single 3 ton unit?
I have had a few of those lately with builders, and the client always has that concern. But the only thing I can say is to verify with the builder/hvac contractor… (So obviously, the builder wont be doing anything about it).
-Its typically a two story home with the return upstairs at the AHU.
Only thing I have thought of is to tell them to meet a neighbor and ask how their unit performs.
I ran into the same thing on new homes. The builder provided the load calculations and that is what it called for.
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“Depends”…
As you pointed out, load calculations are complex. The building has more to do with performance than the equipment to condition the space.
In this thread, the OP indicates it’s undersized. Well, does that matter? Sizing has to do with the “sensible heat ratio”.
An undersized unit removes more humidity, meaning that “comfort” can be achieved at 68F or 80F. When the outdoor heat load goes above regional design factors for your region, all equipment becomes undersized. Are you still comfortable with this temporary undersized condition? Yes, most of the time.
You are in Fla and have lots of humidity. As where I am, humidity has the greatest load concern with the sensible heat ratio consideration. Oversize is a bigger issue than undersized. Just because it is mostly hot there does not mean you need a bigger system.
Equipment size is not determined by the hottest/coldest day of the year. Every city in the US has a design temperature which determines variations in the size of the equipment for your side of the street.
This is more important than the size of the equipment!
Where is that return located?
Where do you live?
Is heating or cooling the primary factor.
What are your heating/cooling days?
For Fla, you need more cooling than heating. Hot air rises, so a high return is better for cooling but causes air stratification in the heat season. It is more important to put the returns in the right locations and the supplies. To condition the air, you must remove it and send it to the equipment to be processed. If you just blow cold air into the room without a return, it will not cool to comfort levels (indicated by ASHRAE Psychometric calculations).
Let me say this, HVAC Installers often use SF/500 to size equipment by rule of thumb based upon the local regional weather, how houses are constructed there, what seems to work (from previous load calculations). This is where all this came from. However, all houses constructed the same do not have the same load calculation. Orientation, exposure to wind, color of the walls and roof, trees etc. change the load.
Building code does not accept SF/500, so why should a Home Inspector when it is not in the scope of their job in the first place?
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As mentioned above…Please explain where the SOP says determining heat pump, or any equipment size is required. Stick to the SOP.
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X2 - No upside, LOTS of downside to going down this road.
Want to have some fun? Go into an HVAC message board and watch guys fight about sizing equipment. It’s their equivalent to, “What’s the best report writing software.” 
I went down this rabbit hole once in my early years of inspecting and it turned into a disaster with everyone fighting and me in the middle. I’ll concede it’s good info to be aware of and understand, and a lot of the info posted here is great… I just personally don’t get into it in my inspections.
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Douglas - That’s the first and best answer! 
To others - Problem is the need for Inspectors to be bigger and better than the next guy. This leads to a lot of the “false advertising” seen on the Web.
INACHI puts out a lot of additional training for inspectors, and the SOPs’ say you can go beyond the “Requirements” of the SOP (hopefully, if you’re qualified to do so).
You can know stuff, it helps you do your job better. It helps you answer clients questions. But does it belong in the report?
Knowing stuff helps you make the right call and know if the alleged professional you deferred to is really a professional. I talked to my clients about the stuff I posted above all the time, but it isn’t in the report. It is to help them make their decisions, not show that I know more than the contractor and code inspector.
Every time you step outside your SOP, you’re on your own.
Many times in giving deposition, lawyers questioned me why I didn’t do stuff like all the other inspectors do (I work with a lot of lawyers on cases). “Because it is not correct or a required standard”. But if we go the other way and I step outside the Standard, they will ask why I stepped outside. My answer, because I’m qualified to. Then I have to prove it.
Can you prove SF/500 is correct? No, because it is wrong and not a recognized standard. 
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Outstanding post!!
Every HI should be required to memorize and write this on a chalk board (Bart Simpson style) 100+ times before being allowed to perform an inspection!
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You can have a professionally designed system, and the new buyers preference for how hot, cold or humid they like to keep the house will vary. My wife and I can’t agree just on the temperature. She likes to keep the house at 68 I prefer 72. For several years when she was having hot flashes our house never got above 67!
I really appreciate all the replies. I understand discussing the tonnage requirements is beyond the standards of practice. I live in Arizona and my employer wants me to review the approximate tonnage requirements for the heat and cooling systems. This paticular home had 2 air returns and three thermostats. I am going back for a reinspection and the builder is going to be discussing with me and the buyer the load calculations. I understand that the insulation and several other factors determine the precise sqare footage requirements for heating and cooling systems. This forum is awesome for helping an inspector expand their knowledge and get different perspectives!
From what I remember, I calculate 20 btu’s per (1) single square foot of space, if you want to get a rough idea of how much cooling capacity you need for a home.
A 6 ton package unit or 2x3 ton spilt into 2 zones are ‘more effective’ in distributing and circulating energy.
This sounds like a zoning system. Which should also be considered as far as “tonnage requirements”.
I had a conversation with a buyer recently during an 11-month warranty inspection that proves sometimes things aren’t always what they appear and may get us in too deep when discussing HVAC sizing. I didn’t do the inspection prior to closing, but the buyer called me to ask about an undersized unit. I explained HI’s don’t do that and he should speak to his builder and if desired contact an HVAC contractor for further information. His concern was the first-floor unit was undersized and the second story unit was oversized.
When I showed up to do the warranty inspection I asked him about his sizing concerns, and if he ever contacted an HVAC contractor. He said he didn’t need to, when he addressed his concerns with the builder, they explained to him the first floor-unit only served a portion of the first-floor square footage, and the second floor unit (oversized) served the remainder of the first floor, (a separate casita space) along with all of the second floor.
I didn’t see that coming, but the builder’s explanation made sense, although I’m not sure why they built it that way. Seems like overly complex ductwork, but to be fair A/C seemed balanced throughout the home and all rooms were comfortable in the Texas heat.
Ok, so you have little to no Latent Heat Load in your zone.
Your systems should be sized for the Sensible Load entering the house.
Important !!!
When you zone a system, you use smaller tonnage because it is designed not to run all zones at the same time. That is one of the reasons to use zone controls. Smaller equipment, lower operational cost.
Reinspect what? Hopefully something other than this. You should not be involved with this and your client should not be in control of this situation when it affects you!
This is the alleged “comfort zone” set by ASHRAE! Good luck with that working all the time! 5*C and 20% Rh is a big span for one person to feel the same comfort level.
It’s different for age, gender, ethnicity and psychological state. No two people in the same room have the same comfort level. 
I had a job in an office building up in New England where the thermostat was in the bosses’ office. She was in her 50’s. Everyone else were in an open cubical area. There was a huge conflict going on. My solution was to move the thermostat to the open office space and install a thermostat with a bathroom fan above the ceiling in the bosses’ office. She could play with her t-stat and be in charge. As long as she could hear the fan run, she was a happy camper. Just a jumper duct with no temperature control.
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The reinspection is for all the items that were not yet completed. Through this forum I now have a greater understanding of what goes into the calculation of heating and cooling requirements. I will be discussing with my employer the risks of using simple square footage to calculate the requirements.
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