home a/c efficiency

For anyone that wants to learn a bit about heating and cooling :

www.hvac-talk.com

and bisco.....your question about whether to change your set points has been discussed there several times. It's probably in the dead horse category, so it should show up well on a search.

 

Dense.
You can not design the outside temperature.
A system that runs 24/7 has no reserve capacity and putting in something designed like that is ***NOT*** doing your customers any favors.
It is NOT perfect.
Rant and rave all you want.
I quit.

 

NO, not unless the humidity inside is actually too high.
You are better off setting it where you feel comfortable.......and not obsessing over it.

 

You are free to put in whatever size unit you choose. But professional engineers do the calculations and then put in the correct size unit, which is not undersized.

 

If the calculations were done with the ambient temperature data from 5 years ago............it is almost guaranteed that indeed it WILL be........IF the calculations were designed to produce what is being referred to here as "perfect".

Do you REALLY not understand that ??

 

And 100 sh*t customers for every good customer when it comes to understanding HVAC.

Those 99 always think they understand it. Sam, it is you who don't grasp it. I realize it's hard for you to believe, but it's true. I've tried to explain it the best I can. You're wearing blinders and aren't comprehending what's being written. You're making yourself look like a fool.

 

I understand HVAC very well. The sizing calculations have stood the test of time and any climate changes in the past 5 years don't suddenly make every system installed obsolete and undersized.

Since you claim to have an understanding of this issue, how much bigger do you think HVAC systems should be sized to account for climate change in the last 5 years or even what is predicted in the next 10 years? We could simplify all the calculations and just write this number into all the reference material.

Here is one example of a now hopelessly outdated manual:
https://www.energystar.gov/sites/default/files/asset/document/Design%20Temperature%20Limit%20Reference%20Guide%20-%202017-06-01_clean_0.pdf

 

The calculations are based on various published tables of typical values for heat loss / heat gain through various construction materials, styles of construction, types of window glazing, sun exposure, square footage, etc., etc., and typical geographical-region figures for heating and cooling degree days, and minimum and maximum design temperatures.

Those of us with houses older than we are, and incomplete documentation of their construction methods, sometimes have to check the assumptions against reality. When I replaced the heat here some years ago (haven't replaced the A/C yet, it was added later and is still doing fine), I actually rigged the old furnace to a chart recorder for a few days when the nights were forecast to reach our building-code-specified minimum design temperature, so I could see what fraction of its output was in fact needed at that design limit.

I should probably have done the same with the A/C last weekend.

One nice thing about my house is it's small enough that nobody makes a furnace small enough to be right-sized for it. The closest I could get was a modulating unit that can throttle down and run near-continuously on the coldest nights. That's way more comfortable than the old unit, which was about 3✕ oversized, which meant it would kick on, spike the temperature, kick off, the air would stratify and feel chilly for 40 minutes, kick on ... argh.

Likewise, when the time comes for my A/C, I probably won't have to worry too much about getting a unit that's undersized. Will probably return to the trick of finding something 2-stage or modulating, just to avoid being crazy oversized.

 

When I replaced the HVAC in my house, I realized that the OEM furnace was way oversized. It would blast out heat, then shut off quickly. This didn't get any flow over the humdifier and didn't mix the air in the house well. I re-ran the sizing calc and reduced the furnace output from 115 kBTU to 85 kBTU, with a lower stage of 60 kBTU. I also put in a heat pump that can provide about 36 kBTU above 20F. This has greatly improved winter comfort. Our 99% heating design temp is 2F, but it has gotten as low as -14F and the heating system has maintained the desired heat setting.

 

A misting system could give a bump to the systems efficiency with evaporative cooling; some industrial systems use water towers for the same reason.

This is a DIY link, but there are several retail systems available.
A/C Mister: 7 Steps

 

Misting systems have been a well discussed topic in the professional HVAC forums also. General consensus is that it does produce short term gains, but with significant long term equipment degradation and damage. A misting system would need a significant purification system, which kind of kills any overall cost savings. There are many gimmick systems out there, but you'll likely hate yourself (and the person who suggested it) a year or two or three down the road when you need a new condenser.

Here's a basic run-down....

Can Mistbox Lower Cooling Bills? Not So Fast. - Energy Smart Home Performance

 

No you don't.
Not if you claim that a home system that runs 100% of the time all summer is somehow "perfect".

 

Sam,

You continue to not comprehend and make yourself look foolish.

Please READ the earlier posts instead of just guessing. DESIGN temperature varies by locality. It is the 99th percentile temperature for that area. That means the temperature in that area will reach the DESIGN temperature (or above) 1% of the run-days of the equipment. So, we can estimate the design temp will be reached 1 day out of 100. At that point, the system will be running 100% of the time to maintain the design requirements of (did you guess it yet?) 75F and 50% RH.

At any temperature less than DESIGN temperature, the system will cycle. If your thermostat setpoint is 75F, and it's 74 degrees outside, the unit may not run at all (depending on heat generation inside the home). As temperatures rise, the system starts cycling. Maybe at 77 degrees, it runs for 10 minutes and is off for 2 hours. As temperatures rise more, those on/off cycle times will get closer to 50%. As temperatures continue to rise, run time will exceed off time. Up until design temperature is reached, where run time will be 100%.

Do you understand yet?

By design, every cooling/heating system has reserve capacity. At DESIGN temperature, that capacity is theoretically used up. Once again, for a perfectly designed system that reacts and operates exactly per theory.

Do you understand yet?

 

If you read my comments, you will find that I did not claim that a home system should run 100% of the time all summer.

 

And I'll just throw this little tid-bit out there also....

For decades, the HVAC industry has been moving toward longer run times. Longer run times equals more consistent temperatures, more consistent humidity and more consistent air quality. We've gone from single stage systems with orifices, then txv controls, then two stage, three stage, variable speed air handlers and now to (almost infinitely) variable capacity systems running on variable speed frequencies controlled by inverters.

Why?
It's all about efficiency and comfort.

Single stage systems are all or nothing. A 3 ton single stage runs at one condition...and that's 36k btu. But what if the dwelling doesn't need 36kbtu of cooling? Too bad, because that's what you're getting. That can lead to very short run times and very poor dehumidification on a poorly installed/sized system.

Enter the multi-stage/multi-speed systems. Now you essentially have two systems in one. For simplicity, we can think of it as just two 1.5 ton systems backing each other up. The first 1.5 ton system works just like the single stage except it only provides 18kbtu. Now your run times have lengthened and your humidity control has become better. Air handler fan speed is able to switch between 2, 3 or 4 speeds as needed. If btu requirements exceed 18k, then the next stage kicks in. More people are happy.

Now we're seeing inverter type units. Feel free to check out the Bosch Bova series. These systems are fully communicating and probably know more about your home air conditions than the owner does. Everything is variable speed. The units obviously have a maximum btu rating, but they can modulate down to about 1/3 of that or less. Why? To completely optimize run times/temperature control/humidity control/air flow control and anything else you can think of. At some point, we very well may see HVAC systems that run 100% of the time, regardless of indoor/outdoor temperature.

 

That may not be what you meant but that IS what was said by somebody......more than once.
Part of the problem, as it often is, is the lack of details.

NOW......even given the details in the previous few posts......the word "estimate" was in there a few times.
So, as a practical matter, a system can NOT be deemed "perfect" if it will run 100% of the time under any conditions.
Except maybe in the middle of a forest fire.

The whole POINT of a home cooling system is to keep the occupants comfortable.
And I don't consider one to be adequate if there is any outside remote chance that the weather can do anything that will cause it to leave me less than comfortable. Forget perfect, it is not even adequate.

I know what you "engineers" are saying but I am saying that those "facts" have no bearing in designing a system for ME............at least not the way you are trying to apply them.

This has gone way too long.
Ignore mode ON.

 

One of our 10 year old, room A/C went kaput this morning. A plastic squirrel cage 'rapidly disassembled itself.' I've ordered a new, replacement that should be here when the hot weather returns.

Bob Wilson