home a/c efficiency

They are salespeople because they are good at selling. They don’t need to know many technical details because few customers do.

 

Our salesman worked at carrier for 25 years, and not working on his selling skills. He spent 3x the time of the other potential suppliers investigating and figuring and answering our questions .

I’m a chemical engineer who works around refrigeration for a bulk of my professional career. I can see when the wool is trying to be pulled out and attempted to be placed over our eyes.

Not all salesman are created equal and the reason why we went with not the lowest cost supplier .

 

I know that is the conventional wisdom......but I would like to have some reserve capacity because the "hottest days" are becoming hotter and more frequent.
That also gives you some margin for the inevitable slow reduction in total capacity.

Bisco: It almost sounds to me like you don't have really good insulation......but there are limits to what you can do when "remodeling" an older building.

 

Home AC or heat pump systems are calculated/designed to maintain 75F and 50% humidity levels at the design outdoor temperature based on your location. (Boston obviously has a different design temp than Columbia). I think this is the 99th percentile for temps. When outdoor conditions reach design conditions, the AC/Heat system should run 24/7.

Heat is similar, but uses 70F and 55% relative humidity if I remember correctly.

Here's a little update. Bostons design temp for cooling is 88F, Columbia is 95F. At that temperature, a perfectly designed cooling system will run 100% of the time and maintain 75F indoors at 50% relative humidity. If anyone is curious about their local design temp, here's a link.

https://higherlogicdownload.s3.amazonaws.com/ACCA/c6b38bda-2e04-4f93-bd51-7a80525ad936/UploadedImages/Outdoor-Design-Conditions-1.pdf

A LOT of information and design go into a properly installed/setup heating and cooling system. Significantly oversized systems will lead to more problems than a homeowner ever wants to deal with. Short run times fail to remove adequate humidity/moisture from the air, resulting in a lack of comfort and possible mold buildup in the system. Air velocity of the system makes a huge difference in how the air mixes or stratifies in the building. You can't just say, I'm gonna change my 2 ton system to a 3 ton because I want it to be colder. It DOESN'T always work that way. Indoor and outdoor units (most residences use a split system) need to match capacities. Line sets need to be properly sized for the refrigerant type and flow rates. Ductwork needs to be properly sized for correct flow velocity and quantity. There's a lot of hacks out there that just throw a system together. It may work, it may not. The difference between a "working" system and a properly working system is YUUUUGE.

 

The one thing you could do is FLIR the ceiling before you insulate.

This will show you where the opportunities lie.

That’s what @Pedal Logic did for his house and it showed where to go and the after effect was impressive .

 

Trying to close the thread before we get to 50 posts?

Glad you made it through.

 

For me, AC is the most wanted necessities in the home when the summer knocks on the door. A well-maintained AC is capable of providing suitable temperature in the room. I think regular maintenance is the best approach by which one can increase its efficiency.

 

I’m visiting our beach cottage in MA this week. It’s off-grid, everyone there has a generator and nobody can find a good service company.

It has become a pretty normal thing for us to have to call our neighbors and say “Hey, we know you’re in Brazil this week but your generator started itself again. Just letting you know so it isn’t a surprise to find the propane tank empty… Would you mind getting it fixed so it doesn’t keep everyone up all night?”

 

That's the second time you have said that........and it is NOT necessarily true.
It depends on your definition of "perfect".

Given the atmospheric trends lately (never mind the reasons), anyone who had a system designed like that a few years ago might well find out NOW that it won't keep up. So then it depends on how much discomfort you can stand.

I still vote for a little extra capacity........regardless of what the "purists" might say.

 

Sam,

That's the second time you have said that........
Yup, because it's true. It's fact. It doesn't depend on definition, because that is the design parameter.

Feel free to vote however you wish. These are the factual design parameter considerations utilized for designing a heating/cooling system for a dwelling. A HVAC engineer doesn't just guess. A set of parameters based on design temperature, 75F and 50% RH (on the cooling side) is utilized to design a complete system. I'm not just randomly throwing a number out there to see if it sounds good. A Manual J calculation is used for load requirements, a manual D is used for Duct sizing requirements. Design Temps are based on locality. Do the manuals have a bit of fudge factor? Absolutely. I'm not saying any specific design is perfect for every individual. I'm saying when all design parameters are taken into consideration, a "perfectly" designed system, in a perfect world, with perfect results, the unit will run 24/7 and maintain the dwelling inside temperature at 75F and 50% RH when the outside temperature is equal to the design temperature.

It's no different than saying I'm going to design a car that will go exactly 100 miles an hour when at it's mechanical limits (even though we know a car will only spend <1% of it's lifetime ever going that fast. I would look at the engine HP curve, the aerodynamics, the transmission ratios, the final drive ratio, the wheel diameter and any other "design consideration". So in theory, my car will hit 100 mph exactly at it's engine redline. The remaining 99% of it's life....just cruising along.

Now, that being said, you can have anything you want built for your dwelling. Many companies will do whatever you want. A good company will explain to you why or why not it's a good idea. There's more to a HVAC installation/repair/replacement than just temperature. A good HVAC company wants a customer to be comfortable, not necessarily just cold. A good company will perform the manual calculations (at least the J) to ensure the system is accurately sized for current conditions, even if it's just a replacement of a condenser unit. Unfortunately, there are 10 sh*t companies out there for every good company. And 100 sh*t customers for every good customer when it comes to understanding HVAC.

Design temperatures can change every year. That's one reason the manuals have a bit of fudge factor. If you take the fudge factor and now add in a homeowner who insists on jacking up the size of the unit, you're heading toward problems. A reputable company would likely refuse the job, as they know it's going to be the same clown that keeps calling them back because the house isn't as comfortable as they think it should be and they blame the installer. Bigger is not better. It's a hard concept to understand sometimes.

 

[QUOTE="sam spade 2, post: 2918860, member: 163338”] I still vote for a little extra capacity........regardless of what the "purists" might say.[/QUOTE]

I’m with you on that. Maintenance might get missed, or done incorrectly. Oxidation on the coil and fins can’t be blown out like dust. You might chop down a tree that was providing cooling shade to that one part of the house. Stuff happens.

This is not to say you should pack a 10-ton chiller for a 4-bedroom home, but a little extra capability goes a long way in the grand scheme of things.

 

Excessive cycling of the unit on/off - this wears the components faster than running continuously. It also limits the amount of time that air in the home has to flow over the A-coil on the inside unit and remove moisture, so the inside humidity isn't well controlled and you "feel" warmer at the same actual temperature. Depending on what got oversized (did you add a big pad mount unit and not change the blower and ductwork inside the house), air flow could be inadequate to heat the A-coil, resulting in coil freezing. Even if a bigger blower were installed in the house, if the ductwork was not sized properly then you still don't get adequate airflow.

Every part of the system has to be matched for the system to work properly and efficiently, as previously well described by @TMR-JWAP.

 

Probably not. You may cycle the unit less, but you also will have the house colder, increasing the heat gain. For an oversized system, you can do the equivalent of pulse and glide. Let the house heat up a bit, run the system to cool it, then repeat. Even if the low temp is lower than normal by a few degrees, if the high is also higher then the average will be the same as now.