After talking to lewis heating and air for weeks I've determined that my hvac is functioning properly. According to them it is sized based on manual j. I don't really know anything about it except the ac unit can drop the inside temperature 15-20 degrees below outside temperature. My unit is closer to 15 so when it is 90 outside I can't get my inside temperature below 75. Even if it is 70 degrees when I wake up the temperature will slowly creep up to be no less than 15 degrees below outside temperature. I personally think this is crap and don't think that I should have to sit in an 80 degree household when the temperature hits 95. Are there any other common calculations used aside from unit j that I could possibly suggest my builder use? The house is still under warranty and is one year old. I know I can add attic fans / blinds / window tint to help cool the house but I'm more concerned with making sure the ac is the correct size for my 2000 sqr foot house. According to the builder the cooling load for my home is 27,588 BTUH which they said is well under the cooling capacity for the 3-ton (36,000 BTUH) system currently installed in my home.

9/3/2008 10:30:33 AM

Mechanical systems are usually designed to produce 75 degree air @ 50% RH on a "design day"


which in Raleigh is 94 degrees F dry bulb 72.4 Degrees F wet bulb or about 35%RH

9/3/2008 10:40:06 AM

source?

9/3/2008 10:42:11 AM

ASHRAE

9/3/2008 10:42:49 AM

Manual-J is the national standard used to calculate residential heating loads. However, it makes some assumptions about the quality of construction and the materials used to factor in heat loss. A 3-ton unit should be fine for a 2000 sqft house so long as you're properly insulated. A professional can come analyze air flow rates in each room to make sure that the duct work was designed properly and is performing properly. You should verify that all the duct is insulated to code and properly sealed.

In my opinion, you need to focus on improving your home insulation. Start with making sure your attic and floors are properly insulated. Make sure your windows and doors all have good seals and I highly recommend low emissivity glass if you don't already have it. Painting you house a lighter color can help. Sources of shade, like trees, can also help. During the day, make sure you have blinds and/or curtains for your south facing windows. Make sure your air filters are clean (a very common problem) and change them at least every 90 days and inspect them monthly.

[Edited on September 3, 2008 at 10:45 AM. Reason : air filters]

9/3/2008 10:43:29 AM

Residential loads are done haphazardly in my opinion... I'm a commercial hvac designer and have seen how residential loads are run and was like yall didn't do shit.

^ also get the cheap filters they flow air better good filters filter better but airflow will suffer because most fans aren't sized for them.


[Edited on September 3, 2008 at 10:50 AM. Reason : .]

9/3/2008 10:49:49 AM

Why south facing windows?

9/3/2008 11:11:45 AM

well we are in the northern hemisphere so there is a southerly component to the suns angle relative to our position on the earth

it becomes more southerly in the winter due to the 23.5° tilt of the earth on its axis

you will notice that most of the sunlight that enters your home does so through the southerly facing windows, with the southwest (winter) and more westerly facing (summer) windows getting most of the action in the afternoon

therefore, blocking those windows will keep the heat out

9/3/2008 11:35:03 AM

Ok. I have a huge window I really need to cover. Would you suggest tint or curtains?

9/3/2008 11:47:21 AM

http://products.construction.com/portal/server.pt/gateway/PTARGS_0_2_493_204_249_43/http%3B/205.142.52.224/swts_content_files/510/E785095.pdf


haha sorry i love those though... light colored curtains would work ok but make the house dark
dark tinted windows keep the sun out of the house but heat up themselves.. ^ that stuff is like magic... (almost like a mirrored glass but not) lets tons of visible light through but blocks out lots of invisible solar energy.


[Edited on September 3, 2008 at 11:56 AM. Reason : .]

9/3/2008 11:53:54 AM

do any of your neighbors have this problem?

9/3/2008 11:54:07 AM

Either is fine. I've actually got 1/4" sheets of styrofoam insulation in my southern windows.

9/3/2008 11:57:13 AM

^^

One guy I know does. Of course I get pissed if the temp gets above 70. I don't think other people are as hot natured. Also they may be "green" / cheap and keep the thermostat high in the summer.

9/3/2008 12:02:08 PM

Quote :

"I can't get my inside temperature below 75."

welcome to summer, man up

9/3/2008 12:05:02 PM

Quote :

"Ok. I have a huge window I really need to cover. Would you suggest tint or curtains?"

i have used the tint with great success

you can put it up yourself...just take care to keep the window completely clean and take your time during installation

9/3/2008 12:34:14 PM

I had a similar problem. Not only was my house warm and couldnt hold a setpoint of 73 once the outside got above 90, but my electricity use was ridiculous.

I had a 3 ton Carrier.

I had already brought my attic insulation up around R-40, installed and attic fan, zoned the upstairs, and added expensive tint to my windows. It helped a little, but it didnt solve the issue.

I am no pro, but I would recommend:

1) Get someone to read the temperature at the register.
2) Get someone to read the temperature as the cooled air leaves the A/C
3) Get the delta between the two (it should drop the temperature of the air by at least 10 degrees [i think thats the minimum]). If you are only dropping the temp 10 degrees or less, then the unit isnt transferring enough heat (see step 7)
4) Get the temperature of the air as it leaves the registers (upstairs and downstairs). This will tell you if you are losing the chill of the air through lack of insulation. If the air coming immediately out of the unit is 50 degrees, and the air coming out of the upstairs registers is more than a couple degrees off, then you probably have an insulation problem. Otherwise, its probably not the source of your problem.
5) Get air flow readings. How much air are you getting upstairs? Is the air cold?
6) Examine your supply and return line sizes. From what I know, you should be running 14 inch main lines (minimum) to flow 3 ton air properly to each duct box. If you are running 12 inch lines, then you probably arent pushing enough air. Conversely, if your return duct lines are 12 inches or smaller, you probably are restricting the amount of air your unit can push (might want to resize a return or add an additional return to improve air flow so the unit can breath in enough air to exhale).
7) Make sure that your coils are clean. If you havent done this already, you need to do this before everything else (I am not renumbering everything but this should have been first)

I had assorted problems with my POS Carrier unit. I ended up dumping it for a higher end 3 ton Trane XL-14C Dual Fuel unit. My supply ducts were undersized 12 inchers. I had new 14 inch supplies ran to a new enlarged plenum for the downstairs. I increased the 12 inch run for the upstairs to 14 inches as far as I could (even increasing just for the part that you can enlarge that runs under the house will increase airflow). I installed a new Honeywell Zone Controller with electronic alternating bypass (needed for variable speed fans).

I now hold a setpoint of 73 degrees every day (the house would get over 80 degrees on hot days). In the two months since I have had this unit installed, my per day energy use has dropped almost 40%. The air coming out of the registers has a delta near 30 degrees (return air is about 73-74, and the supply air is around 45 degrees, but can be as low as 42 [unit will cycle off it is gets any colder]).

The lesson learned is that even though I installed the same size unit, making adjustments to supply sizes and increasing the plenum sizes were enough to dramatically affect the cooling of my house. Since your unit is new, its unlikely that needs replacing, but you may need to have the system re-engineered a bit. If your house was not custom, its likely the lowest bid got the contract, and that came with shitty engineering.


[Edited on September 3, 2008 at 12:48 PM. Reason : /]

[Edited on September 3, 2008 at 1:06 PM. Reason : .]

9/3/2008 12:43:13 PM

sumfoo1 is pretty knowledgeable so i can back up everything he is saying.

also what darkone said about insulation and also making sure the ducts are sealed is key too. I dont deal with residential stuff at all (all commercial and industrial) but to me, that seems ridiculous that your unit can only cool your place to 15 degrees below outside air temperature.

where are your supply registers? floor? ceiling? If its ceiling, the duct could be improperly insulated and its picking up a LOT of heat before during the summer before getting to the register if this is the case.

Just a note, the general rule of thumb I have always heard for residential in the Carolinas is 1 Ton of cooling per 500-600 sqft of house. Using 1 ton/500 sqft youd need a 4 ton unit. Even using 1 ton/600 sqft youre looking at a 3.5 ton unit. And thats based around low 90 outside air temperature in these areas. Now I am not a residential HVAC expert so I cant say that is fact but based on some of the office buildings that get attached to the tech facilities I design for, that sounds about right.

^ lot of good stuff there too. the difference between your supply and return should be like 15-20.

[Edited on September 3, 2008 at 1:13 PM. Reason : ]

9/3/2008 1:11:50 PM

Thanks man, i appreciate that

9/3/2008 1:18:28 PM

Quote :

"Of course I get pissed if the temp gets above 70."

How pissed? It's gonna come down to whether you want to put in a whole new system just because you need it 70 degrees or below (really friggin cold imo).


oh and not to hijack, but i have a 17 y/o trane heat pump and i tried to put in a new Hunter thermostat, and I ended up popping the fuse in the air handler (even though I've installed 4-5 thermostats in conventional systems in the past) so i put the old one back in there. Any advice on how to get a newfangled thermostat installed? do i really need a trane branded one? if so do i need to call one of the local HVAC companies to procure/install it? I think the biggest deal was there were 3-4 wires that were marked differently on the old thermostat vs the new one.

9/3/2008 1:22:26 PM

Ashrae winter/summer comfort chart...
(colder hatched area is winter warmer is summer due to seasonal clothing)
Basically what i'm saying is its not the engineer's fault that they design a house that keeps 95% of people comfortable 95% of the time and you fall into the last 5%. To get yourself comfy 100% of the time you'll end up spending twice as much or more on the equipment (because you'll need staged cooling otherwise the equipment will cycle on and off so much during the fall/spring that you'll kill it)

^ do you have any model info on your old unit... the control wiring voltage may be different 24v vs 12v or something weird


i know nothing about hunter fans' equipment but i know trane,york,carrier are usually interchangeable.



[Edited on September 3, 2008 at 1:36 PM. Reason : - foo1 ]

9/3/2008 1:27:59 PM

you haven't mentioned how well your house is insulated.

do you have an attic fan and more than the usual insulation up there? an attic fan will help A LOT.

9/3/2008 1:39:37 PM

The house and ac are new. I'm not getting rid of it. I have insulation in the attic but no fan. It is dual zone but there is only one unit outside. I had a home inspection recently and they checked to make sure the air coming out of the vents was the correct temperature. The supply registers are in the ceiling. I told them I wanted a 4 ton unit but they said per their calculations a 3 ton unit would be plenty.

Carzin, I'm not sure what good would it do to get all that information if the unit is functioning properly i.e. 15 degrees below outside temperature?

9/3/2008 1:49:41 PM

^^ indeed.. an attic fan negates a LARGE portion of the roof's solar load.

^ the unit is functioning correctly and is capable of removing 36000 btus from the house... but if you minimize the number of btus coming into the house you can get closer to your desired temperature.

What happens is the ac blows are at around 55 degrees F into the house... the heat energy that enters your house (from your body heat, oven, fridge, sun etc.) then warms that 55 degree air up to (hopefully) your desired room temperature. So, if you minimize that heat you can get that final room temp down a little lower...

Things you can do to help are :
Attic fan, window treatments or better glass, make sure every grille in your house is wide open, keep your doors closed, minimize the amount you run exhaust fans, switch to compact florescent light bulbs, turn lights off as soon as you leave a room, Running fans will make the room feel cooler but will actually cause the thermostat to read higher anyone else feel free to add to this list.

Any energy you save in the house is less energy you have to remove via HVAC.





[Edited on September 3, 2008 at 2:04 PM. Reason : .]

9/3/2008 1:50:50 PM

Is it unreasonable to expect them my builder to pay for an attic fan to be installed? The home is brand new and there is now way 95% of the people are comfortable when it is 80 degrees inside.

9/3/2008 1:53:21 PM

you could do it yourself for $100 or so

payback would take about 3 or 4 months

9/3/2008 1:57:18 PM

^^ if your house is over 75 degrees more than 5% of the year yes, they messed up.
and i'm not saying they didn't mess up... most builders have 1 hvac system designed and put it in the same floor plan house no matter what. When the orientation of the house makes a rather large difference in the solar loading of the house.

For example you said you have a large window on the south face of your house, if the hvac system for the house was designed with that window facing north there would be a large discrepancy between the designed solar load and the true solar load.

[Edited on September 3, 2008 at 2:10 PM. Reason : .]

9/3/2008 2:08:04 PM

$100 isn't bad. How hard is it to install?

How can I find out and prove if they messed up?

9/3/2008 2:15:10 PM

hire me to run a load on your house but it'll cost you more than your other solutions
Hiring a mech. eng. would only be profitable if you do, in fact, need a new unit... and if you do need a new unit I'm willing to bet your duct work will also be undersized and this would be a nightmare to resolve.

9/3/2008 2:21:17 PM

Quote :

"Carzin, I'm not sure what good would it do to get all that information if the unit is functioning properly i.e. 15 degrees below outside temperature?"

I dont think I understand your question. A properly installed A/C unit should be able to do much more cooling than 15 degrees from the outside air temperature. If someone is telling you that is the case, you need to find a new contractor. However, once it gets above 95 degrees, the ability is decreased. My new unit will hold 73-74 on a 95 degree day without any problem (20+ degree spread).

Running through those steps I gave you should identify the sources of the problem. The likely source for such a new unit is probably not mechanical fault. Its probably in the design.

BTW, if your unit is zoned, how much air is dumping back into the return? You could be losing air flow if the barometric setting on the damper wasnt set properly, and its dumping a lot of supply air back into your return.

My attic temps, btw, for comparison peak at 120 degrees on the hottest of days (I have a digital thermostat to monitor). I never baselined the attic before the fan, however.

[Edited on September 3, 2008 at 3:06 PM. Reason : /]

9/3/2008 2:59:17 PM

Then did I misunderstand this statement?

"the difference between your supply and return should be like 15-20."

9/3/2008 3:03:25 PM

The air inside your house (return air) doesnt equal the air temp outside unless you have windows open. The A/C is using a closed loop that is getting no air from the outside. It strictly uses the air from inside your home. The outside air temperature, however, will affect the ability of the unit to do its job past a certain point (mid 90s). If air in your house is 80, then the air coming out of the registers should be something like 55-65. My unit has been doing a 30 degree delta (return is 73, air out of registers is 45ish). The more cold air you pump it should replace the hotter air, bring down the return air temp, and lower the register temp (so if the return temp was 72, then you should see register temps in the low to mid 50s).

The point of getting air temperatures at various points in the cycle is that is will help pinpoint the problem. In other words, if you are seeing air temps at the register in the low 50s, but the return air is 80, then you know its not a problem with the A/C cooling the air. The next place would be to look at the flow rates coming out of the ducts. However, if the air temp delta is small, then it should tell you where to look.

The pros can tell you if Im crazy or not, but I think Im right on this./


[Edited on September 3, 2008 at 3:14 PM. Reason : .]

[Edited on September 3, 2008 at 3:16 PM. Reason : sorry for all the edits. not happy with my post]

9/3/2008 3:11:53 PM

^ he is right

^^ yes you did

supply = high 40s low 50s
return = indoor ambient temp.

9/3/2008 3:14:45 PM

Ok, so how does the manual J come into play? I just need some proof that the ac unit should be able to get my house cooler than it currently is.

9/3/2008 3:18:20 PM

This is a pro question, but as I understand it, its simply going to tell you how large of a unit you will need to displace heat in your house in a PROPER INSTALLATION. If your install sucks, you could have a 5 ton unit in there, and your house is still going to be hot.

Imagine trying to drink a milkshake through a small straw. It doesnt matter how much you try, its going to take a long time to drink it. However, if you have a large straw, you'll drink it a lot faster and with more ease. So, applying this to a shitty designed HVAC system, a big unit with small straws still isnt going to push much air (if that is your problem, but I dont think you've provided enough information to decide either way).

What type of system do you have? Whats the make/model? Is your system really zoned, or do you have a system for the downstairs, and another system for the upstairs (this is really a dual system, not a zoned system)?

[Edited on September 3, 2008 at 3:25 PM. Reason : .]

9/3/2008 3:23:26 PM

Manual J = Ashrae fundamentals for dummies. It tells you how much of that 45-55 degree air your house will take to absorb the heat that is coming into the house and end up with the desired set point temperature.

9/3/2008 3:35:51 PM

to maybe clear up what we are trying to say for the OP. Its not just the supply temperature, its also the amount of air AT that supply temperature that is important.

If you take the basic thermo equation Q=m*cp*dT where Q is your energy in BTUH, m is your flow in CFM, CP=1.08 and dT is the difference in your supply temperature and your room set point/return air temperature.

Assuming your heat load is 28000 BTUH like they told you (Q) and lets assume they used calculations when your unit is pushing 1750 CFM (a little less than 1CFM / sqft at 2000 sqft of house). If you use these numbers you could solve for dT which would be 15 F. Meaning, if your desired room temperature was 70, the supply temperature would have to be 55 F to actually get 28,000 BTUH of COOLING to cancel out the 28000 BTUH of heat gained.

Now, lets say they undersized the fan or duct and you are only getting 1250 CFM max into your space. All of a sudden now, even though your unit is providing air at 55 F your room temperature will be 76 degrees. This is because with less supplied, the air would have to be 50 degrees now to maintain 70. And if 55 is the lowest your air could go for example (just example numbers here. i know your unit is sized for more etc but im just explaining a point) then youre never going to get down to 70 when you are maxing out or surpassing maximum design conditions on those peak summer days.


I assume thats what the last 2 posts were suggesting. Im not sure if that was more or less confusing for you though.

The other thing to note, similar to things we have already said in this thread, with ceiling supply and/or return, poor insulation on the duct could really affect these numbers. If they did their calcs with 55 degree supply air but have it coming off the coil at 55 degrees and it picks up 3 degrees before it gets to your registers, it will never cool the amount it is supposed to.

[Edited on September 3, 2008 at 4:28 PM. Reason : ]

9/3/2008 4:21:53 PM

Quote :

"What type of system do you have? Whats the make/model? Is your system really zoned, or do you have a system for the downstairs, and another system for the upstairs (this is really a dual system, not a zoned system)?"

I don't know the type/make/model offhand but it is one unit with a thermostat downstairs and another one upstairs.

Is there some way I can prove it is not working as efficient as it should. Like I said I, it is still under warranty? They said they were willing to get some sensors set up in your home for a day or so which are slightly more accurate than thermostats and can also read humidity levels in order to make sure your system is functioning as it was intended to do so. Would that be at all beneficial?

9/3/2008 4:26:49 PM

Honestly not very easily

bitching is your best bet but logic and planning will probably show that they did some pretty good CYA and you will have spent a bunch of money to prove it...

But if you bitch enough they may just cave and do something ( like swap it for a 4 ton at another job site) but then your duct might whistle and all kinds of other stuff.

My guess is you should try to make your current system and house as efficient as possible and learn to like 75 degrees. hahah

9/3/2008 4:32:10 PM

Well, it comes down to the house is not cooling to an acceptable level. I think keeping a 70 degree set point is a bit crazy, myself. However, if the house could not hold 72-74 upstairs and downstairs on days that were simply 90 outside, there is a problem.

However, the issue may not be a 'warranty' issue as nothing may be technically 'broken'. It simply may be that the system was not designed appropriately to begin with.

You need to pay someone to come out and look at your system, tell them what is happening, and to look at the duct sizes, return sizes, air flow, and air temps. It sucks that you may need to do this, but I'd get on it. HVAC is seriously expensive to redo down the road. I just spend close to $6500 on my system, and the system should have really costs close to 9k with all the stuff I got done [was done with an independent that normally just does design, not install, so I got all equipment at cost at high volume discount and a flat labor fee of $750]

[Edited on September 3, 2008 at 4:36 PM. Reason : .]

9/3/2008 4:34:58 PM

I don't think they are going to cave. The only thing they have offered thus far is to setup those sensors. Should I have them give that a shot?

9/3/2008 6:14:28 PM

Quote :

"Imagine trying to drink a milkshake through a small straw. It doesnt matter how much you try, its going to take a long time to drink it. However, if you have a large straw, you'll drink it a lot faster and with more ease. So, applying this to a shitty designed HVAC system, a big unit with small straws still isnt going to push much air (if that is your problem, but I dont think you've provided enough information to decide either way)."

If your duct work is too large you won't be able to pressurize it and then you won't move any air. Just remember that the door swings both ways so to speak.

Quote :

"Ok, so how does the manual J come into play?"

The manual J is just a guide with a bunch of coefficients and data for calculating heat loads based on building materials, window area, facing direction and location of the house, etc... It's a guide an HVAC system designer will use when calculating what size system your house needs and what volume of air needs to go to each room. These days, all of this is built into load calculation software. To calculate the heat loads for a house, you enter in the room, window, and door dimensions and construction information and the software calculates the information the HVAC guy needs to design the system. If you contact your builder, you can probably get the heat loading summary and the design criteria for the HVAC system in your house. All good HVAC companies will have this information. If they just used "rules of thumbs" you had a less than great HVAC installer.

Quote :

"What type of system do you have? Whats the make/model? Is your system really zoned, or do you have a system for the downstairs, and another system for the upstairs (this is really a dual system, not a zoned system)?"

Based on the OP's descriptions of his system, it sounds like he has a zone kit installed. i.e. He has computer controlled dampers in his system that will shunt air to just one or both levels of his house depending on what the individual thermostats are calling for. When these kinds of systems work correctly, they tend to be rather efficient, because they heat/cool only where it's needed so you don't over heat/cool one level of your home.

Quote :

"I just need some proof that the ac unit should be able to get my house cooler than it currently is."

Get a thermometer and measure the temperature of the air coming out of your registers and them measure the temperature of the air at the return. There should be at least a 15 degree different between the two. If there isn't, either your system isn't producing cold enough air or you have heat loss issues in the duct work.

9/3/2008 8:57:01 PM

Where do I go to measure the registers?

9/3/2008 10:34:54 PM

put a thermometer at the vent, its not that hard

9/3/2008 11:22:01 PM

^ Exactly

9/4/2008 1:05:30 AM

Quote :

"Get a thermometer and measure the temperature of the air coming out of your registers and them measure the temperature of the air at the return. There should be at least a 15 degree different between the two. If there isn't, either your system isn't producing cold enough air or you have heat loss issues in the duct work."

there can be a 15 degree delta and still not have enough CFM to drive it down to the set point temperature

9/4/2008 8:02:34 AM

^^^ Ok, so where do I measure the air return?

9/4/2008 9:41:26 AM

Stick the thermostat in the box that you install the filters. Those are your returns. Your return air is probably just a degree or two above your ambiant temperature.

9/4/2008 9:43:03 AM

^^^ True, but it's more common that you're gaining/losing more heat to the exterior of the house than the system was designed to compensate for.

9/4/2008 12:24:38 PM

david, let us know what temps you get....

9/4/2008 2:49:07 PM

mehh check the temp and i'll point out the alternate solution later...


do you know if you have a heat pump or a DX gas system?

[Edited on September 5, 2008 at 1:34 PM. Reason : .]

9/5/2008 1:33:14 PM