PDA

View Full Version : home owner with HVAC-Calc questions

romulus
06-13-2011, 02:30 PM
Hello All,
I am a home owner and I live near Pasadena, CA. I have 1300 sqf single story house. The current central air system is a roof mounted package system from Carrier dated 1981. I believe is a 4 ton based on the tech who worked on the furnace side last winter. I am looking to replace this aging unit with a split system since I need to re-roof the house any way. Also, thinking in replacing the ducts as well. The system performed marginal during summer (78 degrees inside with unit on all day) It is even worst during summer gatherings with the addition of guest. So before I talk to HVAC contractors , I decided to do a load calculation myself to verify if the current system was sized correctly. So, I purchased the homeowner license of HVAC-Calc. I also read the instructions, and watched the videos. I tried to search for the answer here but no luck.

My question is the following: In the design conditions, once loaded the location. The author mentioned in the video that some people like to increase 5 degrees in the outside summer temp. What about inside summer temperature? Is there any problem in changing the summer inside temp a bit lower?
The default inside summer temp is 75 degrees. It seems ok but I like it cooler at 65 degrees or so. Plus my annual summer parties my guests would be comfortable (average 12-18 people). I know that I will be using more power plus the cost of the equipment is higher. I don’t have problem with humidity here since it’s pretty dry here.

I just finished doing both whole house and room by room calculations. They were both within 250 BTU/h difference. Not too bad if you count some slight errors in the measurements.
The results are the following using 100*F outside temp; 61 grains of moisture with:
75*F inside results= 40.4K BTU/h
70*F inside results= 46.7k BTU/h
65*F inside results= 57.2K BTU/h

Also, I tried to research about 4 1/2 ton units but manufactures jumps from 4 to 5 tons. Is there any particular reasons?
Thanks. Any feedback is appreciated.

pstu
06-13-2011, 03:09 PM
Hello All,
I am a home owner and I live near Pasadena, CA. I have 1300 sqf single story house. The current central air system is a roof mounted package system from Carrier dated 1981. I believe is a 4 ton based on the tech who worked on the furnace side last winter. I am looking to replace this aging unit with a split system since I need to re-roof the house any way. Also, thinking in replacing the ducts as well. The system performed marginal during summer (78 degrees inside with unit on all day) It is even worst during summer gatherings with the addition of guest. So before I talk to HVAC contractors , I decided to do a load calculation myself to verify if the current system was sized correctly. So, I purchased the homeowner license of HVAC-Calc. I also read the instructions, and watched the videos. I tried to search for the answer here but no luck.

My question is the following: In the design conditions, once loaded the location. The author mentioned in the video that some people like to increase 5 degrees in the outside summer temp. What about inside summer temperature? Is there any problem in changing the summer inside temp a bit lower?
The default inside summer temp is 75 degrees. It seems ok but I like it cooler at 65 degrees or so. Plus my annual summer parties my guests would be comfortable (average 12-18 people). I know that I will be using more power plus the cost of the equipment is higher. I don’t have problem with humidity here since it’s pretty dry here.

I just finished doing both whole house and room by room calculations. They were both within 250 BTU/h difference. Not too bad if you count some slight errors in the measurements.
The results are the following using 100*F outside temp; 61 grains of moisture with:
75*F inside results= 40.4K BTU/h
70*F inside results= 46.7k BTU/h
65*F inside results= 57.2K BTU/h

Also, I tried to research about 4 1/2 ton units but manufactures jumps from 4 to 5 tons. Is there any particular reasons?
Thanks. Any feedback is appreciated.
I believe you are at liberty to choose the indoor temperature you want. If you personally want it extremely cold, then OK. Just be wary of going to extremes. I understand many people add 5 degrees or so to the outside temperature, I did that myself with Hvac-Calc. Not sure if this was wise or not, but I just worried about above-average years. The design recommendation for Houston TX was 94F, we just saw a couple days of 105F official temperatures, so I just made the choice to design for 99F. Many would criticize that choice.

The ideal is for the AC to run hours on end when the design conditions are exceeded, but the #1 reason for that is humidity control. Of course your planning for many people in party conditions, implies the need for a compromise toward a bigger AC.

Your existing system may be half broken already, and a properly functioning AC might cool much better than the one you have. You are already at 325 sqft per ton, which is a very aggressive figure for AC sizing. My 1989 house near Houston TX gets by with half that sizing, I always thought Pasadena was a mild climate?

I'm not a pro, but a homeowner. I sure wish you could get one of the more expert AC technicians to your house to take a bunch of measurements and find out what is wrong at present. One possibility is poor ductwork prevents your AC from putting out the full 4 tons of cooling. Another possibility is AC charge or TXV (thermostatic expansion valve) is wrong, my AC had a bad TXV and it really cooled AND dehumidified better after that fix.

Hope this helps -- Pstu

P.S. Since you are thinking of replacing ductwork, that is a rare opportunity to get them well sealed and sized. You might write into your contract that you need ESP to be measured at 0.5 inches w.c. at full airflow, or some other number compatible with your blower.

romulus
06-13-2011, 05:03 PM
Thanks Pstu,
I will check with the AC contractors for their opinion. As for the results, it might seems too high for the house (1947) However, the second worst offender is my family room in which faces North, West and South and it has cathedral ceiling with exposed beams and no insulation. Anyway, the total heat gain is already 12.9k BTU/h.

Generally Pasadena is pretty mild climate but it has a high swing of temperature during the day. The official design summer temp is 95 but I think is 98 in average. There were few occasion that we hit 110 for couple of days during Santa Ana winds (off-shore desert winds). Like in any desert type of climate, the humidity drops to near 0%.

You are probably right that the AC is half broken already. I could have it repair it, but as mentioned I need to re-roof . I need to take out 5 layers of shingles (illegally done by previous owners??) ; too heavy for my house specially in earthquake country.

pstu
06-13-2011, 06:11 PM
I checked and learned Pasadena is not as mild as I had thought <g>. Keep in mind please that you already have a darn high sqft/ton ratio, if you go up to 5 tons it will be extreme. In my opinion put the additional people (from your parties) in your Manual J model, and if it says you need less than you expect, it probably is right. Perhaps one of the AC technicians will consult with you on how to properly account for that -- it's beyond my ken.

After all that, if you ensure your new duct system is up to the job, I cannot think of too much bad consequence. If you use 2-stage AC, then 1st stage can be 55-70% of the total, that will mitigate the short cycles with strong blasts of cold air. I have 2 systems on a larger house, one is Trane XL19 in 3-ton size. For that model 1st stage efficiency rating is seriously better than 2nd stage. And smaller AC units have better efficiency than larger ones in the same model line. I think I know something about your electric rates, and efficiency may pay off to some extent in reduced bills.

Best of luck -- Pstu

btuhack
06-13-2011, 06:13 PM
Wait,
Wait,
Wait,

Ok, so by now, you're aware that with what you are requesting, you'll need 8-10 tons of cooling(with -0- duct leakage~ not your house) to meet your calcs/specs for a 65 degree/1300 s/f house.

Plus, you'll need to circulate the appropriate volume as well...lots of big ducts and grills~ where will the ducts go/fit and where will all the supplies/returns be cut into.

Plus you'll need ample power available and the willingness to pay the bill generated.

98% of the time, the average home has to much capacity based on outside design conditions. You will be off the charts and risk having high indoor humidity issues

Those exposed beam ceilings are big loads and could be better managed with proper insulation, and I'd guess that the shortcomings of current 4 ton are due to high duct leakage rates and building infiltration excesses.

Give some thought to better managing what cause the load extremes, tighten up your losses/gains,raise your 65 degree expectation, then revisit the load calc. Bigger is not better.

I would question a contractor who is willing to promise and deliver, much less entertain, a solution to meet your specs.

Best of luck

pstu
06-13-2011, 07:02 PM
It is worth saying that Hvac-Calc's statement of tonnage is naive and incomplete. There is a whole ACCA Manual S to select an AC which meets the latent and sensible requirements. If you work with a technican who is able to use ACCA methods, he will be able to explain that and make things right.

If for some reason you must cut corners and not use Manual S, I am told usually the sensible capacity is the limiting factor. And for the amount of unused latent capacity, that about half of that can be counted as additional sensible capacity.

Hope this helps -- Pstu

udarrell
06-13-2011, 08:07 PM
Hello All,
I am a home owner and I live near Pasadena, CA. I have 1300 sqf single story house. The current central air system is a roof mounted package system from Carrier dated 1981. I believe is a 4 ton based on the tech who worked on the furnace side last winter.

I am looking to replace this aging unit with a split system since I need to re-roof the house any way. Also, thinking in replacing the ducts as well.

The system performed marginal during summer (78 degrees inside with unit on all day) It is even worst during summer gatherings with the addition of guest. So before I talk to HVAC contractors, I decided to do a load calculation myself to verify if the current system was sized correctly.

So, I purchased the homeowner license of HVAC-Calc. I also read the instructions, and watched the videos. I tried to search for the answer here but no luck.

My question is the following: In the design conditions, once loaded the location. The author mentioned in the video that some people like to increase 5 degrees in the outside summer temp. What about inside summer temperature? Is there any problem in changing the summer inside temp a bit lower?

The default inside summer temp is 75 degrees. It seems OK but I like it cooler at 65 degrees or so. Plus my annual summer parties my guests would be comfortable (average 12-18 people). I know that I will be using more power plus the cost of the equipment is higher. I don’t have problem with humidity here since it’s pretty dry here.

I just finished doing both whole house and room by room calculations. They were both within 250 BTU/h difference. Not too bad if you count some slight errors in the measurements.

The results are the following using 100*F outside temp; 61 grains of moisture with:
75*F inside results= 40.4K BTU/h
70*F inside results= 46.7k BTU/h
65*F inside results= 57.2K BTU/h

Also, I tried to research about 4 1/2 ton units but manufactures jumps from 4 to 5 tons. Is there any particular reasons?
Thanks. Any feedback is appreciated.

LA, Pasadena, CA., 2.5% summer design is 89F dry bulb; 70F wet bulb; 39% RH.

A mere 1300-sf single story house in that low humidity climate & you need 40.4 to 57.2 BTUH. Is it all windows & doors?

If those M-J numbers are correct, you need a Home efficiency audit, & retrofit what is wrong.

Where I live in SW WI summer design is 89F dry bulb with 48% RH; I cool around 1500-sf per/ton/cooling with window units. It's a 1937, 2-story home with with a deep basement, & a lot of non updated windows.

We can cool up to 2400-sf homes with 2-Ton systems in this area... I know one who is doing it.

You should never need an indoor temp of 65F.

At 65F; 60.95 grs. moisture per lb dry air; 65.66% RH.

At 70F; 61 grs moisture; 55.42% RH.

You should be comfortable with air movement at 75F & 50% RH.

pstu
06-13-2011, 08:32 PM
It's not exactly splitting hairs to say my Manual J book copyright 1986, has Pasadena design conditions at 95 db, 68 wb. Six degrees is something.

Best wishes -- Pstu

udarrell
06-13-2011, 11:46 PM
It's not exactly splitting hairs to say my Manual J book copyright 1986, has Pasadena design conditions at 95 db, 68 wb. Six degrees is something. Best wishes -- Pstu
Mine numbers come from an ARI Text, but I'm wondering if my source is a misprint, as they have Sacramento, CA at 98F db & 70F wb. (?)

kinghomes
06-14-2011, 12:32 AM
I believe you should follow the manual J. Tighten envelop and insulate Valted Ceiling. are you sure its not insulated? You could put a false ceiling between the exposed beam. maybe sheet foam covered by a decorative acoustic material? Also, since you are replacing the roof, chose the lightest color roofing material your eye can stand. My home is most comfortable when unit two is not used and unit one is under greater load - moisture drops way down. Wish my contractor had even thought about manual j. Why second guess.. If you want more cooling for party lower temp setting the day before. How often do you party? are they invite only? If you go for total no chance ever hot comfort, you will miss. I would would follow man J and party down - no fudge - If your worst nightmare transpires - take party to the pool or you could set up a bypass for a bed room and shut it off when worst case puts a damper on the party. regards.

wolfstrike
06-14-2011, 04:53 AM
it's unlikely you'll find anyone in this neck of the woods who will do a load calc.

definatly do not increase to a 4 1/2 or a 5.
most likely you have much better air flow just from the unit being new.

living in Pasadena you probably have an older house.
a split system will require the heater to be in a roomy attic, a closet, or in rare cases under the house.
it's probably not worth all the trouble.
changing to a split system will greatly increase your replacement costs.

1) have someone verify the size of the system.
(or you can post the model and seriel number here)

2) have someone check the size of the ductwork.
if everything in the house is less than 8" , than you should consider enlarging.

if your system is a 4ton, than stay with a 4 ton.

if your old system is not working properly, than it might not be removing enough humidity.
there's a big difference between a humid 65 , and a not so humid 65.

dan sw fl
06-14-2011, 05:49 AM
Hello All,
I am a home owner and I live near Pasadena, CA. I have 1300 sqf single story house. The current central air system is a roof mounted package system from Carrier dated 1981. I believe is a 4 ton based on the tech who worked on the furnace side last winter. I am looking to replace this aging unit with a split system since I need to re-roof the house any way. Also, thinking in replacing the ducts as well. The system performed marginal during summer (78 degrees inside with unit on all day) It is even worst during summer gatherings with the addition of guest.

I tried to search for the answer here but no luck.

The default inside summer temp is 75 degrees. It seems ok but I like it cooler at 65 degrees or so. Plus my annual summer parties my guests would be comfortable (average 12-18 people). I know that I will be using more power plus the cost of the equipment is higher. I don’t have problem with humidity here since it’s pretty dry here.

I just finished doing both whole house and room by room calculations. They were both within 250 BTU/h difference. Not too bad if you count some slight errors in the measurements.

The results are the following using 100*F outside temp; 61 grains of moisture with:
75*F inside results= 40.4K BTU/h
70*F inside results= 46.7k BTU/h
65*F inside results= 57.2K BTU/h

ANY feedback is appreciated.

One might expect about 30+K price tag for this SPECIAL Craziness.

Paraphrasing:
" ...I think I'll have a party (total 20 people) when it's 100'F outside
and maintain 65'F inside. "

Two Systems - Equipment + Ductwork -
one which runs normally and another redundant system
which runs ~ 50 hours per year.

... Just trying to bring you - back-to-Earth on
THE Extremes that you wish to address.

[ Rent a hall for your parties - much less expensive in the long run ].

mchild
06-14-2011, 09:11 AM
A variable capacity system from one of the two US manufacturers - Carrier or Nordyne or from one of the Japanese manufacturers - Daikin, Mitsubishi, Sanyo, and Fujitsu would help you meet those dramatic load differences. They use an inverter driven compressor to ramp up and down as the load changes. But, if you need 2 tons under normal conditions and 4 tons under the extremes you mention, you may still have some comfort issues at either end of the extremes even with these systems.

pstu
06-14-2011, 10:17 AM
Romulus, are you dead set on having that indoor temperature target? Things would be so much more conventional using a higher number. For one thing the fundamental research by Carrier puts your target outside the human comfort zone.

Regards -- Pstu

kayjh
06-14-2011, 12:35 PM
I wouldn't size my AC system for a "party load" unless you get a 2 stage system that can run at 1/2 capacity when you aren't having parties. My contractor ignored a Manual J and sized 6 ton of capacity for our home when the "J" said 4.2 - 4.5T. We had 75 people over for a supper party 3 years ago just after the system was installed. It was 75F outside and it was comfortable inside. But, after everyone left and we continued to use the AC system, we began to understand how terribly wrong the installation had gone. We were way over sized and have been uncomfortable ever since. Now the contractor is coming back to rip everything out and put in the 4 - 4.5T we should have had. I'm tired of the on/off cycling every 10 minutes and the accompanying blasting cold air and 65% RH that the oversized system has delivered. I'd rather be a bit undersized for comfort's sake.

romulus
06-14-2011, 02:55 PM
Wow… Thank you all for you feedback. I am surprised that a simple question prompted a good discussion.
Granted, I read everyone responses and of course there are few items/ terminology that I don’t understand but in generally I understand the overall message. I have read about wet/dry bulb temp and I think that has to do with psychometric (spelling?) chart in which has to do with humidty??

Pstu: No, I am not set about the inside design summer temperature. From what the pros are suggesting that inside summer temp of 75F is good for the area. I am kind of leaning toward 73 or 72 F.

Btuhack: At first I didn’t understand your feedback. Thanks for the link, know I understand the efficiency of the system goes down when the inside design temp is below 80F. But as one poster in that link, “when does it matter for a homeowner?” “HVAC comfort is art + science” I have no doubt about it. You all pros have all my respect.

Udarrell: The reason of these high numbers it that I only have single pane windows of average size plus a large front one. Sliding glass door in the West side, etc…. My only assumption is that I do not know if there any insulation inside of the exterior walls. Since it’s a old house, I assume that they are not insulated. The family room walls are insulated (I was able to confirm it) since it was an addition back in the early 80s (same time when the current system was installed). So, now I see that inside temp of 65F is going to be too humid. So 75F should be good with 50%RH. Did you get those numbers with psychometric chart? In any case I see your point. If I remember my HS biology and physics class, we sweat to cool our body down, but in reality the evaporation of sweat is what actually does the cooling.

Kinghomes: Yes, the cathedral ceiling in the family room is not insulated. It consist of a 6 x 18 main beam, 4 x 4 rafters at 48” center to center spacing with 2x 6 tongue and groove as sheathing. I have thought in insulating the ceiling but my wife and I are not willing to cover it up because of the character that the timbers have. They have aged gracefully. Besides, how often you see in today’s constructions a solid 6x 18 wood beam. However, we are planning to have the roof with white shingles (although it looks like light gray to me).

Wolfstrike: Since you are in L.A. you have better idea of the kind of weather we have here. Your recommendation to stick with 4 tons (even though is oversized ) is well taken. I have to agree, I have talked to friends and a few random contractors and they always says manual J??? what is that? That is when I just walked away from them.
I don’t have the model # handy, I think that it is receipt when the heater got repaired last year. I try to look for it.
I have close to 6’ of height in the attic space at the peak of the roof and it centrally located so it should have enough space for a split unit. I understand that will cost more to change from roof mounted to a split unit. But again, servicing the unit on top of the roof is never been fun when you have to worry about falling off the roof. As I type my responses, I started to believe that the lack of performance has to do more with the air flow rather than unit capacity. I think ducts are not sized correctly. As far as I can tell those are not flex and it has insulation. Maybe it is less than 1” I think. The bedrooms ducts I think that are 6” ; living/dinning room is 12”?? I don’t know about the return duct size but the grill is 20” x 25”. Too small for the capacity?

Dan sw fl: The more I read your paraphrasing, the more stupid this request/question is by thinking to make it cooler during parties. Besides, I realized that it is unlikely (plus I haven’t experience it) that all the guests would be inside the house the entire event. I guess that I got carried away based on last year experience when the unit was not performing well ? In any case, thanks for bring me back to earth…. :)

kayjh: I feel your pain. You have first hand experience of what I thought would be ok. I will research about the two stage units to see if this is the solution. I think would be a better way of having the unit sized right plus couple of ceiling fan should move enough air to cool everyone inside.

So, at least I have some conclusions and directions base on all of your responses.
-Inside design temp of 65F is not good for the area. IDT of 75F is good, perhaps 72 or 73 F.

-The lower IDT from 80F the less efficient the system becomes.

-The current unit already oversized 4 tons (need to confirm the model #)

-Raising the IDT reduced the require load. as for the current calc. Bumping the IDT to 72F yields 44.5k Btu/h. Changing the current windows to double pane low-e; yield to 39.9k Btu/h. That is pretty much close to 4.5k Btu/h. Right now it is assumed that the exterior wall of the original built does not have insulation. If it does have insulation, the total load drops it to 35.8K Btu/h. So, I can see now that the current unit is oversized.

-Most likely the lack of performance of the unit is due to undersized ducts with little insulation.

-Modern equipment outperform better than older units of same tonnage based of variable blower speed, variable load compressor, two stage (if equipped). I am guessing that I need to educate my self who makes what and what is out there. That will match the requirement.

Well, sorry to this lengthy post. There is a lot to learn here. Thanks….

pstu
06-14-2011, 04:03 PM
That is one of the nicest and most gracious posts I have seen. -- Pstu

romulus
06-14-2011, 05:58 PM
I never thought that way. I guess that I was being humble in which everyone took their time to respond my questions and concern. There is a lot of stuff gets involved than just slap and go
So, is there a model two stage can be recommended here?. Granted, I am aware that no matter how good the equipment is, a bad install any good equipment will perform bad.

pstu
06-14-2011, 06:29 PM
Some models from Trane have the advantage of about 55% of the total capacity on 1st stage. Easier to get long runtimes than other models which have 65-70% on 1st. I have a Trane XL19 and like it, not to say there aren't other fine choices.

Carrier's Infinity deserves a look, it might have some features you want, that are unavailable on others. But every brand makes some really good equipment.

Even if you go 1-stage, I think you would benefit from a VS (variable speed, not just 2 speed) blower.

Hope this helps -- Pstu