Phoenix heat pump sizing...Am I nuts?

[nerdsite]

I currently have a zoned 5 ton, 9.5? seer unit in my 2 story phoenix-area home. Works okay, but short cycles on smaller zones (has bypass damper). Heating has never been a problem even in the coldest weather. I'm having a floor added over the family room (~240 sq ft), but it changes no interior space, just floor space. Every contractor here says 400ft^2 per ton and I don't believe them. Since I have to add some ducts I've thought about replacing the 19yr old unit to a 2-stage ~16 seer unit. I know Carrier has some nice zoning systems (infinity/evolution) but that's another discussion. Since zoning has been important to me I want the smallest unit (2 stage) that will do the job, otherwise, I wouldn't care much just like the other 99% of over-sized-unit customers.

My goal is to figure out if I need to stick with a 5-ton unit or a 4-ton will work. Every contractor says I need two systems (more than 5), but then they ask me how the 5-ton works and I say just fine (except for short cycling due to zoning, etc). I ask my contractor to do a load calc and he told me to talk to his distributor who said to run a manual j and compare the total BTU's (not sensible because we're in AZ). I ran an HVAC Calc with the builder's R values and came up with my 2500 sqft, 37500 sensible, & ~38500 heat gain (31900 heat loss). If I include the recently installed 90% exterior sun screens (guestimated 55% shading - why, because that was the lowest setting before none), it drops the sensible/total BTUs to 31,724 & 32,644 respectively.

Using the distributors proposed 5 ton unit it handles 48539 total btus (derated), and 37707 sensible @ phoenix design temp. I'm sure this would be adequate. The 4 ton version (same air handler) does 40773 total btus and 31892 sensible. The distributor said the latent heat is converted anyway so use the total btus (which also says it's adequate). Is this true??? HVAC calc says there's no safety factor, but I'm banking on the shade screen values as the safety factor.

I hate doing the job my contractor should be doing but they seem pretty clueless when it comes to sizing except for their rule of thumb (unless I'm the one who's nuts). And I'm not afraid to run some numbers. Given my zoning requirement (goal for lower CFMs in zones), is the 4 ton too wimpy for AZ? Do I compare the sensible btu to sensible btu or total for a dry arid climate per the distributors direction (except in the monsoons it can be rather humid). If the 2-stage unit were around 50-60% of output on stage 1, it would be less of an issue. But it seems to be only 80% so it doesn't drop it that much. Thanks!

And if anyone has any input on the Carrier Infinity Zoned system I would like some feedback; how well does it do on smaller zones (300-400 sq ft) especially as it could relate to this 4 or 5 ton discussion (given adequate duct work). Thanks again.

[motoguy128]

You've done your homework. You discovered the reality, that with zoning, smaller is better and rarely do ALL zones (especially in a soalr heat gain dominant region) need full capacity at once.

I think you numbers are good. Definitely don't go more than 4 tons. 4 ton should be just about right since in your climate, you capacity at desing temps is below nameplate.

CAn you get away with 3 tons since you have multiple zones. If you can live without using setbacks, or carefully staggering setbacks between zones and can live with the possiblity of it falling behind by 1-2F in one or two zones when it's really hot out.... then 3 tons will work. No reason a well insulated, well shaded home even with a 110F design can't be cooled with 3 tons. I can cool my 3200sqft larger home with less insulation because it has good shading and a lot of mass. Thermal mass (solid high desity building materials like paster, stucco, tile, stone, brick, concerete) allow you ot "ride through" peak temperatures. But my design is only 93F, but it does have humidity. It's also only fell behind by 105F.

You really don't want to oversize with zoning. However, in you dry hot cliamte, oversizing has less of a penalty.

You might look up the specs on a Infinity Air handler with a 3 ton 2 stage Infinity 19 heat pump with with the larger 5 ton air handler with the a hi stage airflows of 1300 and see what the capacities are. You mgiht get the 37k you need even without shading.

4 ton definitely shouldn't be too wimpy. 5 ton sounds like overkill and is causing you issues wiht small zones. Infinity/Evolution zoning and other modulating zone systems resolve this issue somewhat by dumping capacity ot other zones when only the smallest zone is calling. However, best practice is to size ductwork when possible so it can handle minimum airlfow in the smallest zone on low stage. With a 3 ton Infinity system I think that's 450 CFM, 600 with a 4 ton.

For reference, a signle 8" duct with a round diffuser or a better quality 6x14" register can handle 300CFM without being too noisy. You need a pair of 6" ducts with 4x12" registers otherwise.

[motoguy128]

FYI, I don;t think your using the product performance tales right in your climate. The distributor is right, you will have a dry coil in your climate, so you use total capacity. Again, you also can factor in using higher airflows if your borderline, but with Infinity, I would use comfort airflows. When humidity is low it will increase airflwo anyway if needed. It adjusts CFM by looking at indoor temp, humidity and outdoor temperatures.

I have a 2 ton Infinity 19 at home. Its' insanely quiet outdoors on low stage cooling. I think some refrigerators are louder. I think it's rated at 65db on low stage. My 68db Infinity 17 next to it is almost loud by comparison. The Greenspeed is I think 58db.

[motoguy128]

Using a wet bulb of 57F (46F dewpoint, 75F indoors), 1600CFM, and 115F outdoor temp (design is actually 108F in Pheonix), I get 39,460 BTU's capacity. FWIM, that's a 9.8EER in those conditions. THsi was a 25HNB9 with a FE4ANB006. So an oversized air handler.

WIth hte 3 ton and still a FE4ANB006, @ 1200CFM and same conditions, capacity drops to 27,610BTU's.

Will Infinity zoning manage thsoe small zones? Better than using a bypass, but at some point a small zone is a small zone.


Another option is a Greenspeed heat pump. They cost more, but a 4 ton unit can drop down to I think about 350CFM on the lowest speed. A 4 ton 2 stage would be I think 600CFM. You would never, ever needs heat strips, even if you used setbacks with that unit.

[dan sw fl]

You can likely use total capacity for most of AZ,
however, you should also know the actual A/C output AT ~ 110'F.

You would have to know/determine Time-of-Day Heat Gain per zone to
judge whether a 4-ton is going to be adequate WITH the Added space.

Knowing window orientation, heat gain for each room could be added to
determine maximum Zone and Block loadS.
_____________ ... ... ____________

[nerdsite]

Thanks for your input. It's more clear than mud now

I've always used the 63 WB (I thought this doesn't need to be derated to 75F--I could be wrong; I'm really not sure on all of this). Anyway @ 1400CFM & 115F I get 38.37K. Hopefully enough (this is a 16 seer 2-stage. I haven't gotten any availability or pricing on the 19 seer as described above. I wouldn't go with a 3 ton. I also saw that the scroll compressors can do 220 CFM/ton, so that would be 880CFM min for such a unit? It's about twice the amount for my zones. I'm drooling over the greenspeed capabilities, but again, don't have $$$ and avail.

Dan, you have a good point about where the sun is and how it relates to my calcs. I don't know how I can run this w/ HVAC Calc, or maybe this is worse-case scenario at peak load. Input anyone? I used # from my builder, but I'm sure I don't get R30 for the ceiling and R23 for the walls, although maybe this is less important than the windows.

The added square footage is really within the current envelop space. It shouldn't make any difference to my performance, and maybe improve it by adding a new zone (more CFMs). I usually had air moving in that space too, so it really wasn't too much of a heat pocket. Plus I'm getting rid of a 9ft diameter 1/2 circle window and replacing it with 2 low e 366 windows (whatever that means), although similar sq ft. I'll put shade screens on that too. But that's already figured in my calcs.

I guess the worse case scenario is I hat the 4 ton condenser and replace it with a 5, sell the 6mo old 4 and take a loss, but hopefully I won't need to do this if my calcs are correct.

Thanks.

[nerdsite]

One more data point (of confusion), from HVAC Calc, he gives and example:

"suppose you live in Nevada and your total cooling load is 30,000 BTUH. You may think, great, I need a 2 ½ ton unit. Wrong, if you look more closely at the HVAC-Calc printed report, you will notice that the Total Heat Gain = 30,000 and the Sensible Heat Gain = 28,000 and the Latent Heat Gain = 2,000. Now, look at the published capacities of the 2 ½ ton unit. They may be Total Capacity = 30,000, Sensible Capacity = 20,000 and Latent Capacity = 10,000. The unit is too small; it will not remove enough sensible heat. You will need to find a larger unit with a higher sensible cooling capacity. If you can not find the published capacities, a rough guide is that 72% of the total capacity is sensible."

So I've seen both both opinions that sensible outputs must be matched and others that says total is fine (give they're close for btus, but from the equipment perspective, sensible is far lower than total). Sensible output on the 4 ton unit is around 30-31K btus. So where am I going wrong?

Thanks!

[motoguy128]

You don't get humid enough for a 63F Wet Bulb. 57 is a little low. Your probably closer to a 60. But either way, it's a dry coil in your climate.

I think the 19 has a little more capacity and after Carrer rebates I found the increase in price was pretty small. I think it was around 2% of the total installed cost. It looks like it's about 10% more efficient.


I'm seeing 38,120 BTU's with a FV4*005 at 1400CFM. The correction factor should be about 0.98 for capacity. So your number appears to be correct.

I wouldn't worry about that last 1000BTU. 115 is actually above design anyway. You should take the 105F number and the 115 and split the difference. I get 38,150.

Again, I'd be suprised is 4 tosn doesn't cut it in a zoned system in a house that size. But as I mentioned before, you can use a 8F setback and then wonder why it doesn't recover on a 110F day in the afternoon. Setbacks in cooling save little if any energy. They can save soem money with time of use electricity, but at hte expense of comfort.

My home for example. I have the 2 ton Infinity 19 on my 1700sqft downstairs, plus a full basement with attached garage in 1/2 of it (doesn't add any significant heat gain if I keep the garage door closed.). Even when it's 65F outside yesterday (no extior heat gain at all) it still took over 1 hour just to cool the space off by 0.5F. It dropped the humidity from 53% to 42%RH in that time frame, stayed in 1st stage the whole time (Infinity controller si really, really good about holding a lower stage as long as it's making some forward progress) But according to my controller it was only using 1050 Watts. So it cost me a whopping $0.08 to cool off the downstairs. I'm not sure how accurate it is on hte energy calculation. I also have very, very low static pressure because I downsized from a 3.5 ton unit that was there.

Hmmm... lets see, run around and open and close 4 or 5 windows and let in cool, damp air along with noise and losing privacy.... or push a button and spend $0.08?

[ReferYankee]

Greenspeed compressor would ramp for demand.

Sent from my SCH-I500 using Tapatalk 2

[dan sw fl]

Thanks for your input. It's more clear than mud now

I've always used the 63 WB (I thought this doesn't need to be derated to 75F--I could be wrong; I'm really not sure on all of this).

Anyway @ 1400CFM & 115F I get 38.37K. Hopefully enough (this is a 16 seer 2-stage. I haven't gotten any availability or pricing on the 19 seer as described above. I wouldn't go with a 3 ton. I also saw that the scroll compressors can do 220 CFM/ton, so that would be 880CFM min for such a unit? It's about twice the amount for my zones.

Thanks.

CFM set-up needs to approach 450/ ton in hot, dry, elevated climate
+++++++++ __________________

[penderway]

How to select equipment for sensible and latent loads (or lack of in your case) is all well explained in the ACCA Manual S

[jimj]

CFM set-up needs to approach 450/ ton in hot, dry, elevated climate
+++++++++ __________________

X2

I would be frightened to use any contractor that would not do a manual J on my home. On a zoned system a room x room ( not a block load) load is MANDATORY, along with a manual D.

[nerdsite]

Can someone explain why I can use the total btu output from an equipment manufacturer (given the design conditions) in an arid climate like AZ, when I see that Manual S apparently says I should be matching sensible output? Since my latent loads is almost negligible (~1000), total/sensible requirement is almost the same. BUT then derated sensible/total outputs from the equipment is NOT the same. What makes phoenix so special that I don't have to match sensible output loads to sensible requirements? If I need to match sensible output then a 5 ton is required, otherwise a 4 ton might work. Big difference. I guess that's my biggest disconnect with the Manual S equipment selection. It seems I've been given two conflicting answers. Thanks.

[penderway]

Can someone explain why I can use the total btu output from an equipment manufacturer (given the design conditions) in an arid climate like AZ, when I see that Manual S apparently says I should be matching sensible output? Since my latent loads is almost negligible (~1000), total/sensible requirement is almost the same. BUT then derated sensible/total outputs from the equipment is NOT the same. What makes phoenix so special that I don't have to match sensible output loads to sensible requirements? If I need to match sensible output then a 5 ton is required, otherwise a 4 ton might work. Big difference. I guess that's my biggest disconnect with the Manual S equipment selection. It seems I've been given two conflicting answers. Thanks.

I'm glad to help, but I need to understand more. What are the "derated" outputs? Are you speaking of the actual equipment performance data versus the "AHRI rated" performance?

AHRI ratings are for consumer comparison and not used by design professionals for equipment selection.

Let me know if this is helpful or if I need more info.

[SBKold]

I'm glad to help, but I need to understand more. What are the "derated" outputs? Are you speaking of the actual equipment performance data versus the "AHRI rated" performance?

AHRI ratings are for consumer comparison and not used by design professionals for equipment selection.

Let me know if this is helpful or if I need more info.

I think that he is derating the nameplate for 115 ambient. Which is still sizing the equipment for the 1%ers.

What do you think wayne?

[penderway]

I think the OP is justifiably concerned about proper sizing and knows enough about it to question the recommendations he's been given.

Unfortunately, even if this was a closed pro forum It would be too difficult to walk someone through the whole design process.

I suggest the OP hire a residential design specialist who can take all the measurements, crunch all the numbers and provide him with a "written" report that once is explained is trusted.

A long time ago, engineers did all the design work and contractors installed it. Now it's expected to be provided by the contractor or distributor for free.

As the saying goes...you sometimes get what you pay for.

[catmanacman]

l would like to see what the real btu output is on the current system as most 5 tons are way short on airflow.

[Conor Ricakrd]

This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

You can find the rules for posting and qualifications here.

Additional infractions may result in loss of posting privileges.

[jpsmith1cm]

Conor Ricakrd

This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

You can find the rules for posting and qualifications here.

Additional infractions may result in loss of posting privileges.

[Conor Ricakrd]

This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

You can find the rules for posting and qualifications here.

Additional infractions may result in loss of posting privileges.