Furnace options

[ultrarunner]

I did a complete central air install at brothers home... he was newly married and wife said no way was she living without A/C. Home only had a wall furnace.

Several local companies provided quotes and they were budget busters and all included Electrical Service Upgrade.

I told them I would spec the system and do the install as a wedding present... lots of fun in 130 degree attic.

By every bid my system was undersized... 3 vs 4 or even 5 ton.

I had 30amps to work with so bigger units out.

They lived there 17 years and thrilled with the system... Tempstar variable speed, multistage etc... neighbors liked it too as some had wind tunnels and noise or lost floor space

My question is just how much faith can be put in contractor recommendations as all specified much larger units?

17 years through week long heat waves and freezing spells seem to contradict...

 

[ponytug]

That sounds like a great wedding present.

In my limited experience, HVAC installers seem to have lots of eyeballers, at least doing home installation. Very few seem willing to drill down and do a manual J computation. A few have left me wondering if they have ever done it. Don't get me wrong, I think it is a fair amount of work to get the correct cooling capacity close, and heat calculations have so many ways that they can be off.

So, I think that the HVAC installers tend to eyeball an oversized compressor to have enough capacity, and people freeze, and are happy with the ability to have blast freezing capabilities, not realizing the energy inefficiencies nor the issues with humidity control and lifetime reduction due to rapid cycling.

All the best,

Peter

 

[ponytug]

Handy HVAC estimator;

https://www.energy.gov/sites/prod/files/2013/12/f5/webinar_hvac_calculatingloads_20110428.pdf

 

[Torvy]

@Sigarms . Thanks, our city place is about 25 years old. We've been here 15. Put in the Trane about 5 years ago. He explained it when we bought it. Our yard is small, there is really nothing to protect it from. Only use a cordless mower near it and no rock or anything to kick into it. It has saved us $100 a month and we keep the house at 74 in the summer. I am the miser. We used to keep it at 78 or more, wife was tired of being too hot and being of a certain age and married this long...she won. The new place will be better insulated and about 2/3 the size of this one. I would like a similar unit...I figure if you find something that works as advertised keep it. I'm sure you already know, but if others are reading this, the AC runs at a very low speed most of the time. It keeps the humidity under control and reduces the likelihood that it needs to run full blast. The old system was either off or on. The Goodman we just put in about 2 years ago. Only about 250 SF upstairs and we knew we would be moving soon.

 

[ultrarunner]

Could be no one ever got fired oversizing...plus they can hedge saying extra capacity if you add on...

I think the mark of a good system is one you don't notice....

 

[Sigarms]

Handy HVAC estimator;
View attachment 711188
https://www.energy.gov/sites/prod/files/2013/12/f5/webinar_hvac_calculatingloads_20110428.pdf

That's an old one.

I actually made up a formula that sounds pretty convincing because it requires the same kind of picture and instructions, but I included 4 different math formulas completely as a joke. When you involve math, it really sounds more legitimtate LMAO

Here is the reality with most HVAC contractors an (and this is what a lot do)...

Take the square footage of the house, and divide by 600. That is your tonnage of AC.

If it's an older house, add 1/2 ton to the above. If it's an older farmhouse, add 1 ton to the above (exact age that determines if it's "old" or not up for interpretation LOL). If the house is "new" and looks "tight" with good windows, subtract 1/2 ton.

What's required in NC is a block load for a basic change out. Garbage in, garbage out. Honestly, depending on the garbage inputed, the quick formula listed above will generally work more more often than naught.

Here is the thing though, when it comes to even North Carolina west of Greensboro, the heating load will generally exceed the cooling load. This is why guys generally oversize the the gas furnace. If you need 3 ton of air, guys will generally go with a 60,000 or 70,000 BTU gas furnace and only look at the input, not realizing that the AFUE will actually dtermine the actual output of the gas furnace (when you only need 36,000 BTU/h of cooling).

The hardest thing a contractor has to learn is to input the right date per insulation factors and other variables such as window types.... and trust the information he's inputting. What guys end up doing is changing the input data to what they "think" the house really needs. I've caught myself doing this as well when I first started doing loads, not trusting the program.

On a sidenote, I know who installed the system in my own home when we bought it around 2004 (house build in 1988, decent insulation). Contractor is one of the better ones in my area and on new homes, they due a load for everything, even back then, and I trust his manual D's and J's. 3,500 square feet, and a total of 7 ton was installed. First floor took more load than usual due to all the windows (loaded with windows, not the norm).