I am having trouble sorting out all the advice I have on cooling my house in the summer. <!—-><!—-><!—->
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House is a 1500 sq ft with walk out basement. 90% finished, 5 bedrooms, 3 baths, 28X26 great room up, 24×14 family room down and 14×15 multipurpose down. I have lots of shade on the west and north side and there is a covered deck with 8’ overhang the entire east side It is located at the lake in southern mid Missouri. We had a cold winter and my highest monthly bill was $170. I think that was great for an all electric home. The problem is when the temp gets in the upper 80s. I set the temp on 70. When I get up the temp is 70. Yesterday the outside was a very humid 88. By 4:00 the temp in the great room was 76 and humidity was 48%. <!—-><!—->
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Have had 2 highly recommended contractors come out. Both said the 3.5 ton, 10 year old unit was under sized. I have good insulation. They both said I needed 5 ton. One said I need more return in the lower level. I only have one 14×8 grill there. You can usually hang meat in the lower level as you might expect. <!—-><!—->
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I have spent hours on the net trying to figure this out. The expert at our electric company said 3.5 were plenty and there must be another problem. I know over sizing is as bad as under sizing. No, neither guy did an energy calc, that’s just not available around here. All of the quick sizing guides on the net indicated 5 ton. <!—-><!—->
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Then of course there is what seer (both said to go with 13 seer) to go with two stage or single stage compressor. I could go on.<!—-> <!—->
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Any and all opinions would be welcomed.<!—-><!—->
Growing old is inevitable, Growing up is optional.
Replies
in the past has it seemed to do ok? if so then something else has changed.
i'm in kansas i understand the load calc thing,everybody here talks about them but good luck finding a hvac guy that knows how to spell load calc,in the midwest,they all shoot from the hip.
your bills sound great,just a thought on if you could put in one of those mini split systems in the upper great room and be able to cool it down with out freezing out the basement.
i had a house one time that we couldn't decide if the return was the problem or what so i took the cover off the fan and wired the switch open,bam every thing cooled down .might give that a try for grins and gigles. larry
if a man speaks in the forest,and there's not a woman to hear him,is he still wrong?
I am sure I need more return in the lower level. Think I will try that and see what happens. Growing old is inevitable, Growing up is optional.
Guess I was wondering what you guys install in a 2 story with 3,000 sq ft finished. Growing old is inevitable, Growing up is optional.
Where is the house located? What is the climate like?
1500 square feet is a small house. Assuming midwest temps - 50F to 95F in the summer, relative humidity up to 80% or so - 3.5 T is probably oversized.
For reference, we did a 2000 sq foot masonry home with NO insulation and no shading, and the load calcs came out at 3 T.
A couple of things to try:
1. Turn the AC fan on ( i.e. the fan switch on your thermostat) and let it run for at least 24 hours. Can you cool the house to 72 F everywhere, and is the temperature within +/- 5F throughout the house? If not, is one room much colder? If you find that the temperature is fine after running the fan for 24 hours, the AC unit is fine, though maybe oversized. There are thermostat adjustments that can help if this is the issue. If one room is really cold and other rooms are too hot, that sounds more like an airflow problem - and you would want to revise the ductwork to solve that.
2. What is the temperature at the closest vent to the AC unit when it is running? If you are not getting at least a 20F differential (e.g. 80F house temp, vent should be 60F or less), then the AC unit is not working right. Have you had the freon level checked, and had the AC unit serviced? It may be that is has a slow leak and is not cooling as much as it should.
Hope this helps.
Where is the house located? What is the climate like?
Lake of the Ozarks, Missouri. Ideal climate for heat pump.
1500 square feet is a small house. Assuming midwest temps - 50F to 95F in the summer, relative humidity up to 80% or so - 3.5 T is probably oversized.
For reference, we did a 2000 sq foot masonry home with NO insulation and no shading, and the load calcs came out at 3 T.
Guess I didn't word it right. Here at the lake it is considered a 2 story, 3000 sq ft.
A couple of things to try:
1. Turn the AC fan on ( i.e. the fan switch on your thermostat) and let it run for at least 24 hours. Can you cool the house to 72 F everywhere, and is the temperature within +/- 5F throughout the house? If not, is one room much colder? If you find that the temperature is fine after running the fan for 24 hours, the AC unit is fine, though maybe oversized. There are thermostat adjustments that can help if this is the issue. If one room is really cold and other rooms are too hot, that sounds more like an airflow problem - and you would want to revise the ductwork to solve that.
I never turn the fan off.
2. What is the temperature at the closest vent to the AC unit when it is running? If you are not getting at least a 20F differential (e.g. 80F house temp, vent should be 60F or less), then the AC unit is not working right. Have you had the freon level checked, and had the AC unit serviced? It may be that is has a slow leak and is not cooling as much as it should.
Had to replace the a-coil last summer. It is working fine as it goes. Growing old is inevitable, Growing up is optional.
I am in KC on a small lake. Myt house is 800 sq ft up and 800 down.But only a small part is under ground. 3 ft at the back quickly tappering to zero on the sides.You said that yours was a walk out basement so I am assuming that much more is under ground.Because all of the HVAC kept saying that I had a "basement" and that would stick a could of vents in the ceiling I realised that won't work. So I have dual systems. But only AC on the 2nd floor. 2.5 ton cools both.My was built in 79 and tried to be energy eff such as using R-11 in the walls and 5/8 polyiso sheets (aroub r-3). but I know LOTS of details wheren't done right and so I believe that it is only average eff.So I am thinking that 3.5 tons should be enough.You might want to go to http://www.hvaccalc.com/ amd get thier calculator. IIRC is $40 for HO use..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Sure! You need a bigger engine!
If 3.5t in not big enought and it should be 5t then you need 1.5t of energy saving. you should check you present system for operating correctly and if its working the best it can, then look to improve insulation and other areas to reduce load.
Nice place!
Edited 6/26/2008 11:36 am ET by USAnigel
Thanks. I gutted it in and out when the existing unit was installed. Well insulated. Growing old is inevitable, Growing up is optional.
gourmand
I cool a 5500 sq.ft. house on the lake with a lot less A/C than that. The units size is large enough. It needs to function properly.
The pic is the east side, right? You get a fair amount of morning sun it looks like. What is going on ... on the south side ... windows? Shading?
3,000 sqft for 3.5 tons. My normal rule of thumb in commercial buildings is 300-500 sqft per ton ... low side for older less efficient ... higher side for modern efficient systems (and envelopes). Maybe someone else can help here ... don't know if you can use the same rule of thumb on residential. My rule would say you're unersized, but it really depends on lots of specifics.
Basement is cold and the RA grill is small. Maybe a bit of an air balance problem ... someone said ... more return air. Main level and lower level returns should be roughly the same ... maybe a little smaller in the lower level.
The lower level has a relatively small cooling load ... no roof and connection w/ the ground (is this a 'real' basement or just lower level?). A little air balancing to ensure more air goes upstairs.
Supply air temp ... did you check it? If really low (e.g. under 50 degF), the fan may not be kicking in high gear for the cooling mode.
At the conditions you stated, the system seems to be cooling you about right, though (88 degF) ... are you worried it won't do the job at higher temps?
With the 8" deck and overhang not that much sun. Fan is wired on high. When it gets in the 90s, company over and cooking will get to 80 even though set on 70. Going to try the RT test before deciding. Growing old is inevitable, Growing up is optional.
"When it gets in the 90s, company over and cooking will get to 80 even though set on 70. Going to try the RT test before deciding."If that is a frequent condition then you need to account for tht in the load calcualtions. Bodies had lots of cooling load.But you don't want that all the time. You want a 2 stage system or possibly 2 units.Has anyone tested the unit? Is this problem different from previous years?.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Remember an 8 ft deck and overhang do little for shading during the first few hours after sun rise ... if you have no trees shading the horizontal sun, you could have a significant early morning solar gain. Overhangs/eaves/etc do well on south orientations ... a little 12-24" eave can go a long way if properly placed.
If you have a plan of your house most furnace companies offer a service to their dealers to do the load calculations for them. They would have to note the orientation of the house on the plan and get some basic details such as insulation levels and quality of windows and doors. They would have to also supply attic ventilation information. It would take a good furnace guy all of 10 minuets to get the info he needs.
Some companies just have sales guys and installers. Other companies also have an engineer on staff that will size everything and give the installers detailed installation instructions. You need to determine who you are dealing with.
Oversizing the units is no solution. Go to http://www.energystar.gov and locate an energy star HERS rater that serves your area. Have them do an energy analysis, blower door test and duct blaster test and recommend a solution based on what they discover.
Otherwise your just cruising the internet.
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"You cannot work hard enough to make up for a sloppy estimate."
Heating is easier than cooling.
Air conditioner specifications and ratings are based on an indoor temperature of 80* F.
You want 70*- that is a huge difference.
You must derate the AC capacity by 850 Btu's for each degree below 80*F... so you are losing 8500 btu's just to meet your desired indoor temperature. That is, effectively, one full ton of sensible capacity.
If you factor in duct losses from leakage (air not going where it is supposed to- like leaking out of the basement duct instead of upstairs through the registers) and internal duct losses from low bid residential work (fast and easy to install, bad for airflow), there is more lost capacity.
It is a fact that unsealed slip and drive metal duct systems leak on an average of 20% if they are installed reasonably well. If they are installed poorly, then that number goes up quickly.
The clear majority of residential work I have seen is in or near the poorly installed side. It takes time to do it right. Builders and homeowners do not get quality work by squeezing subs and shopping for low bids.
Subs encourage sloppy work with incentive programs (get the job done under hours, and the worker gets a few extra bucks). That problem is difficult to spot until the work is done.
Homeowners do not generate a set of system specs to ensure comparable bids. One company might cut every corner and underbid the company doing a 100% better job by $50. The homeowner just understands dollars.
I have witnessed a 30% increase in airflow from changing a few bad fittings. That is 30% more air (for "free") that you are paying to condition that is otherwise lost to internal friction within the ductwork.
I bet your duct system is undersized now.
I know it *will* be undersized for sure if you go from 3.5 to 5 tons of airflow. Heck, the air handler might be set now to deliver 5 tons of airflow in cooling mode. If those companies do not have all new ductwork in the bid, then walk away. If you are too cheap to pay for it, then you will throw good money after bad.
You can go to http://www.hvaccomputer.com and do your own load calculation by spending $50 for a limited time homeowner license. The program is simple and easy to use. There is a flash how-to demo on the website.
Your current AC unit may not be charged properly. It takes time to do it right, and the sales oriented HVAC companies do not allow their techs the time to do it right- even though they charge enough to do so... provided the "tech" sent to do the job isn't a "salesman" and knows what he is doing.
Unfortunately, HVAC systems will heat or cool under a wide variety of installed conditions.
If you want something that performs as designed, has a long service life and is as efficient as possible, you must exercise due diligence and seek a pro instead of a hack.
That is difficult because it can't be done simply by looking at dollar amounts on a 3 line estimate. Big, well known companies can hack more than the guy operating out of a van.
Lots of luck.