Size for new system

[larrytbm]

I’m planning on replacing my 11 yr. old heat pump split system and having problems deciding on the right system size. First, the house is a 1400 SF double wide, but unlike most mobile homes, I spec’d it for efficiency, 2x6 R19 walls, R30 ceiling insulation and R22 floor insulation. Double pane vinyl frame windows and foam core doors. Exterior is cement board. Roof is now metal over the original shingles with a ridge vent, full soffit vent and 3/4 inch venting between the shingles and metal. No Southern exposure due to attached garage but major East and West exposure.

Existing heat pump is 3.5T and definitely too big, the installer just used what they normally use w/o considering the specifics of the house. Like most mobile homes, attic ducting is marginal with limited air flow to the master BR, but there is no practical way to solve this problem. Even now with outside temp at 105 F, the old unit will only run about 20 min. with inside temp set on 77 F and a discharge temp of about 64 F. So, it isn’t running very efficient.

While I haven’t run the industry std. analysis for the house, two other pretty detailed models have given me heat gain numbers of 21,556 BTu’s and 19,822 BTu’s. Downsizing to 3T seems like a no brainer, but going all the way to 2T makes me a little nervous. So, 2.5T might be the right size. It is EZ to find a 3T, 18 SEER dual stage heat pump with a variable speed air handler, but I’m not having any luck identifying a similar 2.5T unit. Does anyone know which brand(s) definitely have 2.5T, 18+ SEER units?

My other question is which would be a better choice, the 3T, 18 SEER system or a 2.5T, 15 or 16 SEER single stage heat pump and maybe w/o the variable speed handler? Something just good enough to get the Tax Credit. Up front cost diff is obvious, but which will cost more to operate and is it significant? What about comfort level, will the benefits of dual stage and variable air flow with short cycles be better or worse than a longer running single stage, fixed speed system? One final point, regardless of which system I install, I might add a 7000-9000 BTu window AC in the master BR for more independent temp control. I’m experimenting now with a portable AC unit in the BR. At 10 pm with outside temp of 95 F, I can shut off the main unit and temp inside the rest of the house will rise to 82 F by 6 am while outside temp drops to 78 F, then it takes less than 1 hr. for the main system to cool the house back to 77 F.

[udarrell]

We really can't advise on unit sizing from here, too many unknown variables.

Even being there, they may have missed some of the variables.

Tell them if you want it sized for a 75 or 70-F indoor temp at 50% or less humidity.

[sktn77a]

Couple of general questions:

That 21,000 BTU is both sensible and latent? How much sensible, how much latent?
Now is the time to look into options for that attic ducting to the MBR - some of these HVAC guys amaze me what they can do!
A two stage high eff HP with VS motor will be much more comfortable as it will run on low most of the time and dehumidify the inside air better.
Can the window AC idea unless it's an absolute necessity! ;)

[beenthere]

Yep. Need to know the sensible and latent load break down.

[larrytbm]

Couple of general questions:

That 21,000 BTU is both sensible and latent? How much sensible, how much latent?
Now is the time to look into options for that attic ducting to the MBR - some of these HVAC guys amaze me what they can do!
A two stage high eff HP with VS motor will be much more comfortable as it will run on low most of the time and dehumidify the inside air better.
Can the window AC idea unless it's an absolute necessity! ;)

The one report lists sensible at 15,248 BTUs and 1.3X for moisture removal for total of 19.822 BTUs. The attic on a MH is only about 24 inches high, sloping to maybe 15 inches in the center of the room. With insulation, lots of trusses and no direct access, running larger ducting would require exterior side wall removal best case and roofing removal worst case. I've lived with the MBR problem for 11 yrs. If reduced cycling (thru 2 stage and VS or thru smaller unit) doesn't improve the problem, then a window AC/HP won't be an issue for me.

[beenthere]

A 2.5 on can meet that sensible at those design conditions. With a little to spare.

[larrytbm]

A 2.5 on can meet that sensible at those design conditions. With a little to spare.

OK, what brands make a 2.5T dual stage 18+ SEER heat pump + VS? I'd like to really focus on those dealers.

Then, if availability, price or other issues make that choice questionable, which would be my next best choice, moving up to a 3T unit or staying at 2.5T but only single stage and 15-16 SEER? Forget about the up front price diff., which will help my marginal ducting more, a VS blower or an oversized dual stage HP?

[motoguy128]

Nobody makes 2 stage units in 1/2 ton sizes that"m aware of. You'll need to go with a 3 Ton 2 stage unit. You'll want a 2 compressor model (usually these are the 19-21+ SEER units)which will reduce capacity by about 40%. The unloading scroll compressors (usually the 15-18 SEER units) only reduce by 25-30%, which means a 2 ton system on low won't be much lower than a 2.5 ton unit.

[beenthere]

Trane is the only one that makes a 2.5 ton 2 stage.

Their 19i, I guess now its their 20i.

[larrytbm]

Trane is the only one that makes a 2.5 ton 2 stage.

Their 19i, I guess now its their 20i.

Thanks, so if only Trane has a 2.5 Ton 2 stage unit, that makes it more important to understand my other options better. Would a 3 Ton 2 stage unit run on low much of the time? Like at night and more moderate days? And therefore have lower operating cost than a 15 SEER, 2.5 Ton single stage? Which would have better humidity control? Or will the smaller unit work better IF it includes a VS handler?

During the summer here in central Texas, humidity is not as bad as say, Houston, but it is still not so low that nighttime temps drop off significantly. Yesterday's high was 104 F and low was 78 F. However, during early Spring and definately during the Fall season, nighttime temps fall low enough that I can keep the windows open at night to cool the house, then not run the AC during the day. So, maybe I'm not likely to use the low stage that much.

[beenthere]

Depends what temp you set your thermostat at.
It will have long run times in first stage.

A 3 ton 2 stage probably won't cost less to operate then a 2.5 single stage unit.

[DeAngel]

Carrier's Infinity™ Series Heat Pump, Model No. 25HNA9 is available with
2.5 Nominal Tons; a 2-stage scroll compressor and a 19 SEER Rating.

[teddy bear]

Thanks, so if only Trane has a 2.5 Ton 2 stage unit, that makes it more important to understand my other options better. Would a 3 Ton 2 stage unit run on low much of the time? Like at night and more moderate days? And therefore have lower operating cost than a 15 SEER, 2.5 Ton single stage? Which would have better humidity control? Or will the smaller unit work better IF it includes a VS handler?

During the summer here in central Texas, humidity is not as bad as say, Houston, but it is still not so low that nighttime temps drop off significantly. Yesterday's high was 104 F and low was 78 F. However, during early Spring and definately during the Fall season, nighttime temps fall low enough that I can keep the windows open at night to cool the house, then not run the AC during the day. So, maybe I'm not likely to use the low stage that much.
Air tight home needs fresh air?? I find 60 cfm of fresh air when occupied is an important part of good indoor air quality and should be considered as part of the load. Any concerns? Regards TB

[beenthere]

Carrier's Infinity™ Series Heat Pump, Model No. 25HNA9 is available with
2.5 Nominal Tons; a 2-stage scroll compressor and a 19 SEER Rating.

You got the whole model number to that unit. That you can post it.

Like to see the ARHI rating saying its 2.5 tons nominal.

[DeAngel]

Condensing Unit Model Number 25HNA936A0010

Please refer to the Combination Ratings for the indoor unit on the product data at http://www.docs.hvacpartners.com/idc/groups/public/documents/techlit/25hna9-4pd.pdf to achieve 2.5 tons.

The information below came from http://www.ebuild.com/product-detail/hvac/heat-pumps/549725.hwx

Carrier Corp. Infinity™ 19 Model 25HNA9 Two-Stage Heat Pump with Puron® Refrigerant Product Specifications:

Manufacturer Model No. 25HNA9
Product Name Infinity™ Series Heat Pump
Nominal Tons 2
2.5
3
3.5
4
5

SEER Rating 19
HSPF 9
Cooling Capacity Output (max. MBtu/h) 24
MBtu/h Output (max.) 60
CFM (Max. Airflow) 800 CFM
Sound Level 72 db
Compressor Type 2-stage scroll
Refrigerant Type R-410A
Electrical Requirements 208/230 V, 60 Hz
Energy Star® Compliant Yes
Warranty Period 10-year compressor limited warranty
5-year parts limited warranty

Notes 11.5 to 15.0 EER
Shipping Weight 378.00 lb.

[beenthere]

If you check. I think you'll find thats a typo on the website.

The 2 ton, doesn't get better then 26,200 with any listed match up.
And the 3 ton, doesn't drop below 34,200 with any listed match up.

All the condensers listed for match ups are listed as 2, 3, 4, or 5 ton.
So no 2.5 ton nominal unit.

[dash]

Carrier's Infinity™ Series Heat Pump, Model No. 25HNA9 is available with
2.5 Nominal Tons; a 2-stage scroll compressor and a 19 SEER Rating.

Can you post those model numbers??

[larrytbm]

Trane is the only one that makes a 2.5 ton 2 stage.

Their 19i, I guess now its their 20i.

Since Trane uses dual compressors with the smaller one 50% of the large compressor, I'm assuming that they both don't run at the same time, correct? So, the large compressor would be 30,000 BTUs and the small one 15,000 BTUs, right?

Also, what is the logic for dual compressors vs. a two stage compressor? Seems like if either compressor failed, then the entire unit would not run, correct? On the other hand, maybe with less operating time and fewer duty cycles on each compressor, they will last longer?

[pstu]

Since Trane uses dual compressors with the smaller one 50% of the large compressor, I'm assuming that they both don't run at the same time, correct? So, the large compressor would be 30,000 BTUs and the small one 15,000 BTUs, right?

Also, what is the logic for dual compressors vs. a two stage compressor? Seems like if either compressor failed, then the entire unit would not run, correct? On the other hand, maybe with less operating time and fewer duty cycles on each compressor, they will last longer?
An interesting thing about the Trane 19i sizing (I am a homeowner with one, don't have similar info on the 20i)...

Assuming outdoor dry bulb 95F and indoor conditions something like 77F, 46%RH (i.e. 63F w.b.),
the 3.0 ton sizing has 35,200 btuh capacity in 2nd stage and 18,700 or 53% in 1st stage.
The 2.5 ton sizing has 32,000 btuh capacity in 2nd stage and 18,700 or 58% in 1st stage.

Yes you read that right. 1st stage is exactly the same between the two models. Both are rated at 1300 cfm 2nd stage, 700 cfm 1st stage. So getting the 2.5 ton sizing may not yield the benefits you might expect. This info is from a manual titled "Split System Cooling Product & Performance Data".

Hope this helps -- Pstu

[beenthere]

Since Trane uses dual compressors with the smaller one 50% of the large compressor, I'm assuming that they both don't run at the same time, correct? So, the large compressor would be 30,000 BTUs and the small one 15,000 BTUs, right?

Also, what is the logic for dual compressors vs. a two stage compressor? Seems like if either compressor failed, then the entire unit would not run, correct? On the other hand, maybe with less operating time and fewer duty cycles on each compressor, they will last longer?
Correct they don't both run at the same time.

As far as why 2 compresors, Its just Tranes method.

[larrytbm]

An interesting thing about the Trane 19i sizing (I am a homeowner with one, don't have similar info on the 20i)...

Assuming outdoor dry bulb 95F and indoor conditions something like 77F, 46%RH (i.e. 63F w.b.),
the 3.0 ton sizing has 35,200 btuh capacity in 2nd stage and 18,700 or 53% in 1st stage.
The 2.5 ton sizing has 32,000 btuh capacity in 2nd stage and 18,700 or 58% in 1st stage.

Yes you read that right. 1st stage is exactly the same between the two models. Both are rated at 1300 cfm 2nd stage, 700 cfm 1st stage. So getting the 2.5 ton sizing may not yield the benefits you might expect. This info is from a manual titled "Split System Cooling Product & Performance Data".

Hope this helps -- Pstu

Pstu,
That is good info. If I understand your point, there will be no difference in operating costs or performance during 1st stage operation or the amount of time the system will operate in 1st stage. The only benefit will occur during 2nd stage operation. But the 2.5 ton SHOULD be cheaper to buy IF it is readily available?

[beenthere]

If your house only needs 2.5 tons.
Then the 2.5 is still better to use then the 3 ton.

Or, as the outdoor temps get hotter. You could end up with higher humidity when it goes into second stage.

[pstu]

This one is a mathematical oddity all right. Most 1-stage sizing choices are 1/2 ton difference and apparently the industry considers that close enough to be correct. You can call that a 6000 btuh gap that is not worth filling, otherwise you would see manufacturers making units available in 1/4 ton sizes. So why does Trane make something with 3,200 btuh different size? That is only 10%. The biggest benefit of doing so seems to be marketing not engineering.

But consider this: most other 2-stage models have a gap approximately 12,000 btuh from model to model. A size jump much bigger than 10%. Are they setting up homeowners for problems due to having 11,000 btuh extra capacity in some houses? I think all manufacturers would be producing more sizes if their engineers thought so.

I have seen research that challenges the idea that oversizing is all that important in humidity control or energy use. That research seems to tell us that other factors are more important, that making the simple change from oversized to right-sized may not pay the benefits that the simple theory tells us. Almost everything Beenthere tells us is spot-on, however... in this case it is very hard for me, a homeowner, to see the significance of "oversizing" between these two models.

Best of luck -- Pstu

[beenthere]

Did you read how the 19i is suppose to work to control humidity?

Don't forget, when it goes to second stage. Its no different then a single stage.

Check some of the other coil matches for it.

[udarrell]

This is the way I approach the sizing issues.

There is a proper order of sequence that I would always follow with my presentation & also in respect to the work that must be properly performed.

First, when you go for up-sizing or oversizing, always do a heatgain-heat-loss calc., then always do a ductwork system & airflow analysis.

Never try to go above what tonnage the existing ductwork will handle, unless you're willing to spend the money to make all the requisite changes; - go for an equipment match to the ductwork. Equipment tonnage can be considerably less, but ought never to be more than the ductwork will properly handle!

Second, always remember that if you undersize some, a lot of cost effective ways can be done to bring the heatload in-line or below selected equipment size, -then you get the smaller more airflow efficient & lower power usage blower motors & compressors.

Third, that also is great for reducing the heat-loss in cold weather & results in a win-win investment equation.

Fourth, the most critical procedure is the installation & setup of all the matching components of the total system of which the house is a first order critically important part.

Fifth, the proper presentation sequence, has potential advantages toward getting you the job. ha...:)

Sixth, always ask them all the questions that will indicate what they want & then illustrate the most cost-effective & human comfort way to get there.

Seventh, make sure you let them know that SEER level is meaningless if all the proper sequential procedures are not followed.

Eighth, your proper sequence presentation, if it is authentic backed with integrity, has the potential to blow the other guys out of the running. Your only concern is what is best for your prospective client.

In the end, you have to go with what they want. - Darrell