Is this equipment selection appropriate?

[lakebilly]

Referencing the attached documents and assuming I got the load calculation right, would the following Goodman equipment be a reasonable selection to add on to an existing 80K furnace. I've used Goodman in this example because the data is ready available. Actual equipment will likely be different, but I just want to know if I understand this or not.

Condensor: DSXC160241A 2 Ton 2 Stage
Coil: CAPF3636B6D

Load
Total= 23,889
Sensible = 20,690
Latent = 3,199

From the expanded data, S/T ratio = .81 at 800 CFM making sensible BTU = 19,444 / Latent = 4,560

At 900 CFM S/T ratio = .84 making sensible BTU 20,160 / Latent = 3,840.


I've had several contractors look at this now and two of them have suggested 3 TON units. My existing unit is 2 1/2 TONs and it we tend to overcool things in an effort to keep the humidity down. This approach really drives up the electric bill and makes the house uncomfortably cool.

I would really like to get the humidity down close to 50% without the overcooling.

I appreciate the input.

[BaldLoonie]

Hopefully it is a matched Goodman variable speed furnace if you are adding the high SEER 2 stage unit to it. Be sure a TXV is installed.

The sensible/latent split does vary with conditions so if the numbers line up, you should be good. With an under 2 ton load I sure wouldn't recommend a 3 ton, even 2 stage.

[rickboggs]

Mr lake,
You just did what many a/c folks don't have a clue.
I'm posting some expanded data for another piece of equipment just for comparison.

Just for fun, when selecting equipment, you can subtract the latent load of the structure from the latent capacity of the equipment, divide the left over latent load by half and add what's left to your equipment sensible capacity. In other words, the equipment uses half the left over latent as sensible. Doesn't add much in your case, but can send you way over the top when going to the next size up equipment.

expanded data.pdf

[lakebilly]

Mr lake,
You just did what many a/c folks don't have a clue.
I'm posting some expanded data for another piece of equipment just for comparison.

Just for fun, when selecting equipment, you can subtract the latent load of the structure from the latent capacity of the equipment, divide the left over latent load by half and add what's left to your equipment sensible capacity. In other words, the equipment uses half the left over latent as sensible. Doesn't add much in your case, but can send you way over the top when going to the next size up equipment.

expanded data.pdf

Thanks for the feedback. I've complained about the performance of the current system for about 17 years now, so I want the new system to be sized right if at all possible. Fortunately my leading contractor is a friend and won't feel that I'm an overbearing homeowner. Thanks for the attached data as well. That data is presented very nicely, less sifting through multiple pages.

Have a great day.

[udarrell]

Great work! I wish everyone would do that.

Fort Wayne 2.5% summer design, showed 89-db 75-wb, the calc software yielded the below results.
I ran some calculations: the outdoor design humidity is 52.7%, with 109.21 gr/moisture per/lb air.
Indoors, at 75-DB & 63-WB (close to 50% RH) yields 67.34 gr moisture per/lb air

You might want to operate the airflow at 350-cfm per/ton, that will provide a little longer runtimes with less sensible & a little more ratio of latent.

Just wondering what the square footage of your home is?

[Rodney28334]

The 2 Ton will do it, but I can kind of see where the contractors are coming from. With 2 stage equipment you do not have 1/2 Ton intervals, and the price difference equipment wise between a 2 Ton and 3 Ton is not that significant. Some contractors are scared to death of undersizing a unit so most of time might go up 1/2 Ton which isnt grossly oversized and the design temps used in manual j are lower than the high temps we are see now a days but during peak heat most people are away from home so why size for peak temp, but going into the contractors mind it could be a older retired family or stay at home mom who will call the office complaining everytime the indoor temp is a few degrees above her set point of 76 at 1pm on july when its been 105 degrees out side for two weeks straight and that the unit runs all day (and you try to explain to her that it is supposed to at this ODT) but she goes on about how it will make her electric bill higher (and you try to explain that if the unit was bigger it would cost her more as it would short cycle and cause higher humidity which would make her have to lower temp to stay comfortable making it run more) Sorry for rant. BTT If this was a lower seer unit and they installed a 2 1/2 ton not a big deal, but because higher seer units do not do 1/2 tons a 3 ton would be way oversized, as it (if a 2 stage scroll) would be putting out about 2/3rds ie 2 Tons on low stage which is really all you need 95% of time anyway so you waste the extra money you paid for a 2 stage unit. Dual compressor 2 stage units Like Trane 20XLi will stage at 1/2 capacity. So in summary, go with the 2 Ton but see where the guys are coming up with 3 Ton even though they are incorrect.

[lakebilly]

The 2 Ton will do it, but I can kind of see where the contractors are coming from. With 2 stage equipment you do not have 1/2 Ton intervals, and the price difference equipment wise between a 2 Ton and 3 Ton is not that significant. Some contractors are scared to death of undersizing a unit so most of time might go up 1/2 Ton which isnt grossly oversized and the design temps used in manual j are lower than the high temps we are see now a days but during peak heat most people are away from home so why size for peak temp, but going into the contractors mind it could be a older retired family or stay at home mom who will call the office complaining everytime the indoor temp is a few degrees above her set point of 76 at 1pm on july when its been 105 degrees out side for two weeks straight and that the unit runs all day (and you try to explain to her that it is supposed to at this ODT) but she goes on about how it will make her electric bill higher (and you try to explain that if the unit was bigger it would cost her more as it would short cycle and cause higher humidity which would make her have to lower temp to stay comfortable making it run more) Sorry for rant. BTT If this was a lower seer unit and they installed a 2 1/2 ton not a big deal, but because higher seer units do not do 1/2 tons a 3 ton would be way oversized, as it (if a 2 stage scroll) would be putting out about 2/3rds ie 2 Tons on low stage which is really all you need 95% of time anyway so you waste the extra money you paid for a 2 stage unit. Dual compressor 2 stage units Like Trane 20XLi will stage at 1/2 capacity. So in summary, go with the 2 Ton but see where the guys are coming up with 3 Ton even though they are incorrect.

I figured this was where they were coming from. I'm sure that your average Joe really thinks he's getting more for his money when he gets a bigger unit. From the contractors side, too big is better than too small. Nobody want's to under size a system by a 1/2 ton and have an unhappy customer.

[beenthere]

Since your design temp is 89. You can extrapolate the performance by plotting the data for an outdoor temp of 85 and outdoor temp of 95.

[lakebilly]

Since your design temp is 89. You can extrapolate the performance by plotting the data for an outdoor temp of 85 and outdoor temp of 95.

Thanks for the tip. I'm really hoping that a 2 ton, 2-stage takes care of the humidity. Teddy Bear has all but sold me on a whole house dehumidifier. I think I'll wait until the new system is installed and only spring for the WHD if necessary.

[lakebilly]

Great work! I wish everyone would do that.

Fort Wayne 2.5% summer design, showed 89-db 75-wb, the calc software yielded the below results.
I ran some calculations: the outdoor design humidity is 52.7%, with 109.21 gr/moisture per/lb air.
Indoors, at 75-DB & 63-WB (close to 50% RH) yields 67.34 gr moisture per/lb air

You might want to operate the airflow at 350-cfm per/ton, that will provide a little longer runtimes with less sensible & a little more ratio of latent.

Just wondering what the square footage of your home is?

udarrell thanks for the input. Your suggestion may further help with the humidity issue as well as the overcooling. It's not uncommon to see my wife with a blanket while watching TV in July. Keeping the house at 69 in order to control the humidity is not fun.

I guess I left out some pertinent information. The house is about 1450 on the main level and 1400 in the walk-out basement. Most of the exposure is toward the north with a lot of trees. It's also lakefront.

[motoguy128]

At some point you get diminishing return by overcooling to dehumidify. What your really doing is trying to increase run times and get the coil colder (intentionally waste energy) to maintain comfort. But as you drop the dry bulb temp 1F, you need continue to drop the dew point even further to achieve the same RH%. Lets say it's 73F and 55% RH. IF I want it 50%RH, I can jsut simply add enough heat to raise the temp to 75F. Dewpoint stays 55F. OTOH, if I overcool the space to 69F. I have to drop the Dewpoint to 50F to reach 50%RH.


Here's a example from my work. I have a small block building with a big piece of industrial processing equipment that's basically a big open pool of 90F water. Guess what happens when it's below 40F outside? Yes, it's rains indoors as the moisture condenses under the roof. Why don't we use an exhaust fan? Because there's not enough heat in the space to bring in that much fresh air, the the giant exhaust fan gets shut off in winter.... which si the worst tiem of year ot do that. What happens? I have to spend the price of a nice entry level luxury car on a new roof deck and steel roof joists. So why not add a big dehumidifier. I could, but it would cost again... the price of a nice entry level luxury car. What else could I do? Install heated make-up air (outdoor air is nice and dry in the winter) year round. Then add enough heat even in summer to maintain <60% RH (point where corrosion becomes a problem) without getting above 90F indoors. So other than a couple of the most humid days out of the summer. If I add enough heat and fresh air I can keep it under 60%RH.

You could do the same in a home, but most people don't want it 90F indoors.