A/C design

As home inspectors the design and efficiency of the heating and cooling system is beyond the scope of what we do.
I had a home last week 1969 model that I could not help my self but to write a comment. It had one 3 ton split system condenser with a down flow forced air furnace 100K BTU input located in the garage closet. The downstairs supply air ducts were beneath the slab foundation, The upstairs supply came off the back side of the furnace plenum traveled to the upstairs attic with one 10 inch trunk feeding 3 bedrooms and one bathroom. The return air was in the downstairs ceiling with no upstairs return except down the stair case.

Some would say that I should have just recommended further evaluation by a HVAC contractor. It was a contractor that screwed the pooch on this home in the first place and my thought is they should be called out.

Some would say that I should of just operated the installed system as installed and moved on.

My question to the mass is what would you have done.

My statement below is what I wrote in my report.

I probably would have just wrote what I observed in the report, then gave the client my two cents about the design verbally.

I would stay within my competency. If my competency enabled me to speak definitively about such a system design, I would not hesitate to inform my client of it. The SOP is a minimum standard, it’s open-ended on the top.

If its a design issue, which is what Charlie describes, I suggest further evaluation by a person qualified to design HVAC, or whatever I am referring to. I will also say it does not appear to designed properly, and give some reasons like Charlie did if I know enough about what I am seeing to be able to say so.
Never refer to contractors. Smaller contractors usually don’t have design staff. Large ones may have their own, but there is no guarantee a contractor will have design qualified people.
A qualified designer does not have to be an engineer in most cases, they are usually called technologists where I am. Large wholesalers and or dealers of construction materials or equipment may have design staff as a service they provide to the small contractors who do not.

Well, seeing as you asked…

Home Inspectors report their observations, knowing things are wrong and provide an “Opinion”.

Unless you are doing the load calculation or can take measurements and document that the system is not functioning as intended, you should refrain from opinion.

You do not have to explain your opinion seeing it is just an opinion. Use your IR to show inadequate comfort conditions which is the primary concern.

Your not going to correct the condition by insulation or window modification so that is not pertinent. What is, is that that one unit (not zoned) will not provide adequate comfort conditions (regardless of its size or other building component condition). That fact with IR scans of stratified air temps in the house identifies the problem and the only concern. How it is fixed is to be determined (by others).

There is no reason a 3 ton could not be adequate for this house (with design modification).
You are comparing a/c size by floor sq. ft. thus a smaller unit up and larger down. Depending on house design (is the second floor completely closed off from the first), your local heating and cooling days, and several other factors to be determined by load analysis, we really can not formulate a proper recommendation which will only create a conflict between you and the HVAC designer.

We see a lot of things reported that are not code or mfg complaint and call them out. But few Inspectors document the actual occurrence of defect due to the non-compliance which in turn changes things from a “Further observation required” to “Requires further evaluation by others”.

Just the way I approach it, for what it’s worth.

We have to disagree a 3 ton unit on that home would run non stop from 2PM until 11:00 PM the way it presently is in Okla summers.

As I said, just my opinion.

You say “would run”. I would say “It ran…”.
Information collected by data logger.

What’s wrong with that?
I had a 1.5 ton AC unit that would run continuously from morning to night and it was very comfortable in the house. When I replace it with a 2.5 ton heat pump, it would run intermittently and it was not as comfortable.

A properly designed and balanced system should cycle off every 15 to 20 minutes of run time. That should be adequate time to remove the humidity from within the home. If your system had to run non stop to be comfortable you must of had a humidity problem and a high electrical bill;-):wink:

#1 Short cycle times reduce life expectancy of the equipment. Like light bulbs which blow out when you turn them on (not while they are on) they are only designed to turn on just so many times.

#2 Short cycle times indicate a larger capacity system which will run at a higher Sensible Heat Ratio than a smaller one. Thus will not control moisture and actually promote mold issues.

#3 The RLA of a smaller unit is much lower than a larger one. Thus less power consumption during run time.

#4 LRA (power used during start-up) is 800% greater than RLA. So it takes more to start than run.

#5 A smaller unit running all the time has a constant load to work against (lower amperage draw). Cycling the larger unit causes increased load and power consumption for the first 50% of the run cycle till it reaches equilibrium at which time it is ready to shut off.

My statement that a 3ton could do the job is based on the principals of using a zoning system which is not just to have multi set-point temps in a single building. It allows smaller equipment which is cheaper to operate. Which seldom shuts off. Which has a better sensible heat ratio. Which is more comfortable.

Cheaper to buy. Cheaper to operate, More comfortable.

I disagree. Having my older AC unit run non stop was much more comfortable than having the replacement heat pump cycle off every 15-20 minutes. OF course in heating mode the heat pump running continuously is much more efficient thus the drawback in cooling mode.
As for my utility bill, it’s actually lower due to the heat pump benefits…:wink:

Were not comparing apples to apples look where you live as compared to Okla how many days per year do you get above 100 F. What is secondary heat source and at what temp does it kick in

So you like your system to operate all day and 1/2 the night, not my choice I like two systems no zones and that is what I see most of. Zones are used here in very large homes which usually have 2 or 3 system and each system is zoned separately. Usually at least 3 zones on each system.

In most all cases, this is the case. But there are other options.

I am describing the zone option and the things that happen differently than individual zoning.

Posting here that they must cycle every 15-20 min to be correct may turn into “An Old Charley Tale” (which superceeds The Old Wives Tale"). We have enough of them to deal with…

Why does a unit run all the time, sometimes? (a point of consideration for others):

  1. The length of run time is controlled by a sensible heat thermostat.

  2. Depending on where you live (New Orleans vs. Phoenix) the equipment must be properly sized to control the Sensible Heat Ratio, not the Temperature. The longer a unit runs, generally the more moisture is taken out of the air.

  3. A thermostat can not measure humidity, it measures sensible temperature. As the unit runs the first thing it does is remove moisture. The temperature will not come down till a corresponding amount of moisture is removed from the IA.
    The thermostat then shuts off. The thermostat and the hvac equipment do not speak the same language, they just happen to be able to work together.

If the unit is not big enough, it may never pull down to the sensible set-point temperature. If too big, it will leave behind excessive moisture which may decide to condense on something inside the house that the oversized hvac unit made cooler than the dew-point temperature of the IA (ie. air duct and registers, sheetrock).

  1. This only applies when equipment and buildings were constructed properly in the first place. If the building leaks air, it lets in moisture and the unit runs more. This is a building problem, not an hvac problem and should not be confused as such.

You must know the equipment performance, not just how long it runs…
Therefore it is safe to say that Home Inspectors can not make this call based on visual observations.

How does one determine equipment performance?

  1. You must know how the unit is operating for the conditions. Is the unit properly charged? Is there proper airflow through the equipment? Does the air duct leak? Is the air duct properly insulated?

  2. What are the properties of air entering and leaving the equipment?
    A hygrometer is needed to determine at least two points on a psychrometric chart. From there we can determine everything we need to know about performance;

air temp in
air temp out
sensible heat ratio
relative humidity
amount of total BTU removal (sensible-latent)
weight of the air
evaporator by-pass factor
apparatus dew point
air flow cfm
dew-point temperature
grains of moisture/lbs of air
BTU’s per pound of air

With this information we can now start to make a SWAG guess at how the unit is operating and why.

As you should be able to comprehend from this, you can not determine anything with Delta-T thermometer measurements or visual observation of the installation. But many of you will continue to wander around in the dark with your Home Inspector School, Old Wives Tales and think you can evaluate HVAC performance.

If you run the equipment in heat and cooling and write in your report that the units appear to be operating adequately your fine… To say anything more without proper testing is a flat out lie.

Dave a client does not give a squat about all that mum bo jumbo what matters to them is how they feel to hot or to cold. I will stand behind my run time statement as it is very close to what a home owner will be satisfied with. How you or me derives at that is irrelevant.

Just for giggles I am going to start posting the tonnage of the A/C units installed on my inspections just so you can see a trend of what happens in Okla. Ya need to get out of Tenn more.

Today’s inspection 2 story 1980 model 3K SQ FT 1500 up and 1500 down split system both levels had a 3 ton each which = 500 SQ FT per ton on average. Standard Insulation in the attic and Original windows. The front of the home faced west

What was the Delta-T on them? :cyclops:

I don’t do Delta T;-):wink:

Thanks for the insight… How do variable speed units enter into these equation?

Today’s investor special 875 SQ FT with 1.5 ton split system HP made in China a mini split with coil and blower mounted on the inside wall. No difference than a window unit on a 95+ day the bedrooms will be hot as the coil was mounted in the kitchen???. It was a doozie the GFCI in the utility room when tripped shut off the over head light in the utility room and the refrigerator in the kitchen.

The kitchen GFCI near the cook stove when tripped shut off the dishwasher, the microwave and the igniter for the gas cook stove.

So many things play into the comfort factor on a home’s HVAC system that I would not be inclined to go very far beyond my observations (was it cooling or not, did it seem adequate at the time).

I think it is right and proper to raise concerns, even put them in the report to CYA, but would refer the concern to a licensed HVAC tech rather than try to speculate outside my area of expertise.