What size A/C unit, please?

[mikeinomaha]

I'm a consumer in the market for a new HVAC system.

Price is important but only secondary to getting the correctly-sized system for my house installed properly. If I can get the correct size installed professionally, then I figure it'll cost what it costs.

I live in Omaha, NE in a 21y/o 2 story house(original owner) with approx. 2650 sq.ft. I'm currently getting bids to replace the funace and A/C which are 21y/o as well. Both units are working, but I would rather get a new HVAC system with a methodical approach rather than under duress if one or both units went down.

I've had the advantage of reviewing a multitude of discussion threads on this message board, learning and understanding quite a bit BEFORE working with the contractors for bids.

I've purchased the HVAC Calc and ran the numbers on my house. I've been fairly meticulous, having the original detailed blueprints of the house plus knowing how the house was built.

My current HVAC system is Trane XE 70 furnace 140,000BTU with a 4 ton Trane XE 900 A/C. It has a Honeywell EAC. All were original to the house. Only significant repair was an evaporator coil 10years ago that was leaking from a small hole.

The furnace seems to heat OK, A/C cools but on most summer days, ice-cold drinks will sweat in the house. If I recall, the A/C will cycle during most summer afternoons except for when temp > 97°F. There's only my wife and I, we don't generally cook much and only do laundry once/week.

None of the contractor so far have offered to do a Manual-J and when prompted, they've all said it wasn't necessary....that their "Rule of Thumb" (500-800) sq.ft./ton was adequate or their "years of experience" tells them what size to chose, was the only answer I get from them. Another was just going to use the same size that was originally installed. One actually wanted to "up-size" the A/C from 4 ton to 5 ton without a Manual-J calc, even after showing him the article on how A/C units are frequently over-sized.

These are considered some of the A-List contractors in Omaha. All have been chosen because they advertise NATE-certified pros. I realize I'm dealing with the sales people and not the installers but I'm getting frustrated.

My only peculiar need is that I like to keep the house very cool year-round. By that I mean I keep it 62°-68° in the winter and 70° in the summer. I also like to keep either the A/C or the heat on most of the time rather than open the windows (good reasons, trust me). That means on one extreme, I try to maintain 70° when it's 99° outside along with good dehumidification, and on the other extreme, run the A/C when the outside temp dips in the low 50°'s. I've asked about a "low-ambient" kit(?) for such times the outside temp drops into the 50°'s. I know my expectations are extreme, but I can at least ask, right?

The HVAC Calc shows:

Total Heat Gain = 24,879 (2 tons)
Total Heat Loss = 48,583
Sensible Gain = 21,646
Latent Gain = 3,233

So far one bid has been on Trane XL16i for the two-stage compressor but he wants to increase(!) the size from 4 tons to 5 tons. Another bid was from a Lennox dealer who I just didn't like but wanted to maintain a 4 ton. I'm waiting to get bids for Carrier and Rheem later this week.

Questions: With HVAC Calc results shown and my peculiar performance expections, what size A/C would a pro chose for their house?

I'm a consumer with little knowledge, but I'll be happy to provide additional info if I can. Thanks in advance.

Mike

[hvacgreg]

From the information that you have given you like the temperatures pretty cool in the summer.I whould go with a 4 ton two stage unit.It will remove alot of humidity and keep temps more consistent thoughout your home.But still have the capacity to keep it at 70 at higher outdoor temps.I would not raise capacity if existing has been keeping up ok and it is a 4 ton.

[mayguy]

Have you updated your windows?

You said it cycles on and off till it get 97 outside, does it keep up with temp?

How often does it get 97? Otherwise, what is your avg temp in the summer?

As for heating, I'd go 60k two stage, and 3tons XL16i

[Edited by mayguy on 10-09-2006 at 08:25 PM]

[Freezeking2000]

Originally posted by hvacgreg
From the information that you have given you like the temperatures pretty cool in the summer.I whould go with a 4 ton two stage unit.It will remove alot of humidity and keep temps more consistent thoughout your home.But still have the capacity to keep it at 70 at higher outdoor temps.I would not raise capacity if existing has been keeping up ok and it is a 4 ton.

Ditto on going back with at least a 4 ton!
Why do you keep the home so cold?

[mayguy]

,

[mikeinomaha]

Originally posted by mayguy
Have you updated your windows?

You said it cycles on and off till it get 97 outside, does it keep up with temp?

How often does it get 97? Otherwise, what is your avg temp in the summer?

As for heating, I'd go 60k two stage, and 3tons XL16i

[Edited by mayguy on 10-09-2006 at 08:25 PM]

About the windows.....they are all double-pane/low-e casements. They've all been checked for leaks, and those that lost their seals were replaced last month. I replaced the siding on the house and added Tyvek house wrap. When the house was built, I personally placed the vapor barrier and it was placed thoroughly and correctly.

About the cycling, yes, the unit has easily attained any temp set by the T-stat. This last summer, recovery has "seemed" slightly slower. Humidification has always seemed less than adequate.

Omaha's average summer high temp is 87.5°. I think according to the HVAC Calc, the Design temp for Omaha is 95°.

Thanks for the response.

Mike

[mikeinomaha]

Originally posted by Freezeking2000


Ditto on going back with at least a 4 ton!
Why do you keep the home so cold?


Thanks for your recommendation.

Actually, I think 70° is "cool" not cold, but of course, it all depends on whether a person is "hot-blooded" or "cold-blooded". I'm hot-blooded and used to working in hospital operating rooms all day which are like meat lockers (just asked the patients).

In most every other aspect of my life, I'm fairly energy-conscious. Keeping my home cool and dry year round is my vice. It's partially balanced by keeping the house quite cool in the winter.

Thanks,

Mike

[comfortdoc]

When you performed the load calcs did you enter your desired indoor design conditions (70 summer/68 winter)? With your personal comfort desires I would definitely recommend 2 stage cooling and 2 stg or modulating heat. If you ever sell the house the equipment will not be sized properly for a "normal-blooded" person.

[mikeinomaha]

Originally posted by comfortdoc
When you performed the load calcs did you enter your desired indoor design conditions (70 summer/68 winter)? With your personal comfort desires I would definitely recommend 2 stage cooling and 2 stg or modulating heat. If you ever sell the house the equipment will not be sized properly for a "normal-blooded" person.

Yes, I did enter my desired indoor conditions.....very flexible program that HVAC Calc.

Yes, the two-stage makes lots of sense, and I will surely go with them. I just wasn't confident on the size. I was afraid to under-size for obvious reasons, and was also concerned about the common problem of over-sizing after reading these threads.

You're correct about not being properly sized if I sell the house. I did think about it and decided that it's a judgement call. I plan to live here until retirement (15yrs). I figured since I'm paying for it, and it's my house for now, I get what I want. I'd be silly to buy the system for the next guy and be dissatisfied for 15 years. Also, by that time, it might be approaching end-of-service anyway.

I'm glad you pointed that out though, because had I been planning to move sooner, it might have been an issue. We consumers don't always think of that stuff.

Thanks,

Mike

[hivacer]

I would get the multistage units and have the first or low stage meet your Manual j calcs, that way your second stage is there for the abnormal days when you want it to be 70 and its 105 outside.

It sounds like a 3 ton unit with 2 stage would suffice. Is this unit doing multiple floors? Or do you have more than one system?

[comfortdoc]

Originally posted by mikeinomaha
....The HVAC Calc shows:

Total Heat Gain = 24,879 (2 tons)
Total Heat Loss = 48,583
Sensible Gain = 21,646
Latent Gain = 3,233

....

Is it possible to get a professional to verify the load calcs? Unless I'm missing something the A/C they are installing is double the size caculated.

If you were my customer I would crunch the numbers and size a multi-stg/modulating system that would satisfy your needs now and allow for typical design temps (as close as possible) if ever desired. But yes you are paying for it and first consideration should be what you want it to do.

[Edited by comfortdoc on 10-09-2006 at 09:57 PM]

[Freezeking2000]

I have a 2 ton unit on a 2000 sf colonial in Connecticut and keep my thermostat set to 78 degrees. I believe once you have a continously operating compressor with a 2 stage unit you will de-humidify to the point you will raise the temp. I do not know for sure, but i bet with the 4 ton unit it only runs 30% of the time except for the hottest days. The 2 stage will increase the run time and dry your home.

[mikeinomaha]

Originally posted by hivacer
I would get the multistage units and have the first or low stage meet your Manual j calcs, that way your second stage is there for the abnormal days when you want it to be 70 and its 105 outside.

It sounds like a 3 ton unit with 2 stage would suffice. Is this unit doing multiple floors? Or do you have more than one system?


The A/C unit does both floors of a 2-story house. The contractors that have look at my HVAC system have stated that the ducts appear very well sized. Anecdotely, the upstairs bedrooms have always been comfortable(save for less than adequate dehumidification), even the master bedroom which is furthest from the blower.

I have rechecked all my input numbers for the HVAC-Calc to be certain of the results. Your suggestions help immensely.

Since (as a consumer) I know little about the reliability of such things, as professionals, do you all find the HVAC-Calc acceptable? Assuming I can find a company that will do a Manual-J, can I have confidence that my results should be close to the contractors? Of course I realize my input numbers need to be correct. (GIGO...garbage in, garbage out).

Thanks again,

Mike

[comfortdoc]

Yes. I've been using this program for 8+ years. With proper input it's on the money.

[mikeinomaha]

To comfortdoc and Freezeking2000,

Thanks for your input. I'm gaining much more confidence in the type of system I need. This is quite a nice forum, that simple consumers can learn from experts donating their own time.

With the right size and type of system, I guess I can get efficient a/c and get better dehumidification at the same time. I've gotten much more thoughtful advice from you all than the contractors that are bidding. Thanks.

I need to verify my HVAC-Calc numbers, but I believe I'm getting the jist of what your stating.

Thanks,

Mike

[Paul Pippin]

Mike,

If they won't do a load calculation now do you think they will be there when you need there services? I wouldn't even consider it usless they had done that first. It can't be priced without knowing what size of equipment you need.

[mikeinomaha]

Originally posted by Paul Pippin
Mike,

If they won't do a load calculation now do you think they will be there when you need there services? I wouldn't even consider it usless they had done that first. It can't be priced without knowing what size of equipment you need.



Paul,

You're exactly right! I'm very frustrated because the companies bidding on the job are considered to be on the A-list for Omaha with NATE-certified personnel, etc. When I push for a calc, they all act like I'm be foolish. They tell me "not necessary", "haven't done one for years", "nobody ever does them", "waste of time", and on.

I guess I'll start contacting the smaller companies. I'm not against smaller companies, just don't know most of them. Again, price is only secondary. Whatever system is correct for me, installed correctly....it'll cost what it costs. I like to get paid well for what I do, I figure other professionals do as well.

Thanks,

Mike