Consequences of getting a system with more capacity than necessary?

[IOCTL]

Is there a significant drawback (besides cost of installing & operating) to having a system with "too much" capacity?

I am getting a 3.5 ton installed to my 1500 square foot house (old uninsulated plaster walls throughout most of it.) Ideally I would have got it insulated first, but I don't have the money to do that AND get AC installed, and I am just not willing to go through another summer of coming home from work and stepping into a sauna.

Also wondering about the implications given the variability of temperature during months where A/C is useful. Some days will be 78 degrees and only mildly humid. Then we have August with its 106 degree heat waves with high humidity.

If there is more capacity than necessary, can't it just run for shorter periods? Or is it going to be freezing for five minutes until the thermostat catches up? If I install insulation next year, am I going to find the thing running five minutes an hour and not removing enough humidity?

There are humidifiers that can be installed in central units, of course. Are there central dehumidifiers that can be added to the HVAC system, which can take over when the compressor is off?

[hackmaster]

The main problem with over capacity is lack of humidity removal. When its severe enough then this leads to mold issues especially in high humidity areas. When your system is over sized the run cycle is far too short and although your tstat reading will show it has brought the temperature down it may feel stuffy and hot still. When a system is sized correctly and especially with the newer systems you can actually leave your tstat temp up higher yet it will feel cooler and this saves you on energy cost. I do not know of an add on dehumidifier as Ive never had that issue on a system. Your system itself is the dehumidfier so it makes little sense to make something as an add on. I dont think your situation warrants much of a concern however because if your august temps avg 100 or better and you have a 1600 sq ft house I think your close enough without having this issue. You also arent insulated very well so your prob ok.

[beenthere]

Oversized equipment tends to short cycle and ost more to operate. Plus the humidity problems mentioned above.
Yes, there are whole house dehumidifiers. And sometimes they are used to correct oversized system problems.
You might want to check on the cost of installing one before you oversize your A/C. And then rethink oversizing.

[craig1]

if you are sure your going to get insulation next year, have your contractor size the system based on that. It might not be big enough this year, but then it will be properly sized for the rest of its life (which will be a longer life than an oversized system). even if its not big enough to handle the current heat gain of the house, it will still be alot better than no A/C. and once you get the insulation it will be perfect

[Airmechanical]

There are humidifiers that can be installed in central units, of course. Are there central dehumidifiers that can be added to the HVAC system, which can take over when the compressor is off?

why not use a variable speed air handler, with a IAQ t-stat to dehumidify?

what size system did you have that could not keep up, was that system running properly?

was there a heat load calc done for your house?

dont just guess at what size to puy in!



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[Superdark]

why not use a variable speed air handler, with a IAQ t-stat to dehumidify?



.

This is a question I would like the answer to as well.

It seems all the multi-stage/speed compressor units I am interested in only come in whole-ton increments, which would lead to "oversizing" a solution (load calc'ed for 2.5 tons, but have to install 3 ton to get 2 speed).

Does the variable nature of the blower and compressor in these systems compensate for the half-ton oversize?

/popcorn.

[BigJon3475]

There should be some good answers to that....there was just a thread on this subject.

http://i68.photobucket.com/albums/i25/BigJon3475/popcorn.gif

[beenthere]

I say, yes it does.
With in limitations though, and some settings to compensate. A 1/2 ton size increase can be comensated for.
In a high humidity area, may have to set the blower to 350 a ton, even if using a stat that controls blower speed. When this is done, you are lowering the efficiency of the unit. You have to (or should ) give more thought to coil selection, and use a coil with the lowest SHR that you can.

Installed several that are 1/2 ton over sized, they were matched to coils for best latent ability. No problems.

[alex_in_fl]

Drawbacks of over sized unit:
Higher initial costs, shorter run times, higher temperature swings, increased electricity costs (for larger unit and start-up inefficiencies), less humidity removal, less overall comfort.

Benefits: cools house faster, makes contractor that sold you the unit more profit, electric company gets more $$ too.

Ask the contractor about getting a 3 ton unit and installing a radiant barrier - or get a two speed 3.5 ton unit. Even a partial radiant barrier will help.

[IOCTL]

Thanks for your answers so far, this is a great forum! :cool:


Ask the contractor about getting a 3 ton unit and installing a radiant barrier - or get a two speed 3.5 ton unit. Even a partial radiant barrier will help.What kind of radiant barrier? Every window (even the sliding glass door) is double-paned so I don't expect there is much to gain by putting mylar on them.


why not use a variable speed air handler, with a IAQ t-stat to dehumidify?

what size system did you have that could not keep up, was that system running properly?

was there a heat load calc done for your house?

dont just guess at what size to puy in!The current system is two window air conditioners :)

The proposed system is a two-speed Bryant 187A 3.5T compressor with 4T coils. Load calc has not been done yet. (From reading other threads I understand they don't like to do that until after you've accepted their bid, which I just did yesterday.)


There should be some good answers to that....there was just a thread on this subject.

http://i68.photobucket.com/albums/i25/BigJon3475/popcorn.gifIt's true, there are a ton of threads about A/C capacity. I started this one because I wanted to know what I'd be getting myself into in summer '09, if I get the house insulated before then, and also because I was curious about how an air conditioning system optimized to handle 100+ days would operate when the temps were merely in the upper 70s.

It's my hope that the two-speed compressor, coupled with a humidity sensing thermostat, will allow the system to handle the job properly both this summer and next.

[beenthere]

Carrier doesn't have a 2 stage 3.5 ton unit. So if your contract says 3.5 ton, its single stage.

[IOCTL]

Carrier doesn't have a 2 stage 3.5 ton unit. So if your contract says 3.5 ton, its single stage.It's a Bryant 187A. Capacity on their website is listed as 2-5 tons.

[alex_in_fl]

Radiant barriers go in the attic. Lay them atop existing insulation or staple to rafters (better in the long run).

A 3 ton unit running on low is about like a 2 ton unit so I agree with Beenthere.

[beenthere]

It's a Bryant 187A. Capacity on their website is listed as 2-5 tons.
2 ton, 3 ton, 4 ton, 5 ton.

Thats 2 to 5 tons.

[Airmechanical]

2 ton, 3 ton, 4 ton, 5 ton.

Thats 2 to 5 tons.


thats some fancy calculating you have done there:)



.

[beenthere]

thats some fancy calculating you have done there:)



.
LOL...
I only know of one brand that currently has a 1/2 ton size in 2 stage equipment.

But it seems like most manufacturers list their 2 stage equipment coming in sizes from 2 to 5 tons, with out always list, its even tonnage only. :(

[jerryd_2008]

Sorry, I finally found how to create a new thres. Please ignore this random intrusion.

I'm a newbe. Live in Northwest Arkansas, not south where it is hotter. Zip code 72704.

I have a 14 year old house that is fairly energy efficient for the 5,000 sf under roof and 3,700 sf heated size. We bought it used with, I believe, a 4-ton 10 SEER air conditioner and 80% natural gas furnace, also gas hot water. I am trying to evaluate if I should convert to a duel fuel (hybrid) heat pump for heat and AC. I have a few questions:

1) I'll need 220 power to the inside air handling unit, right? Roughly what woud it cost to bring 220 from a wall panel, across a 3-car finished garage with floor trusses in the ceiling and back about a car length (40-50 feet???)? Who should do this for cost, an electrician or the HVAC guys?

2) Since everybody asks and I don't know, how do I figure out what my cost per kWh of electricty really is? Here is the latest bill:

Energy+Fuel Adjustment+Debt = $0.078387
Taxes = 0.0123541

Total = $0.0907411

Looks like 9.1 cents/kWh but that doesn't include the fixed $14.00 Customer Charge. If spread across the 480 kWh we used that would add about another 2.9 cents/kWh. Then again the tax is probably applied to the customer charge already.

What would you quote our cost at, 9.1 or 12 cents/kWh or ????

The electric company phone lady quoted $0.05... /kWh. Hmmm, that sounds convenient and at least half of the true value.

3) We can have what I would call winter weather on and off here from about early December through maybe mid March with a few days here and there before and after "winter" where we use modest to no heating and air conditioning. This lasts for a couple months before and after "winter". What furnace efficiency would be pratical if we like to have it nice and comfortable as well as economical?

Thank you for your assistance. Any information and suggestions are welcome.

[teddy bear]

Is there a significant drawback (besides cost of installing & operating) to having a system with "too much" capacity?

I am getting a 3.5 ton installed to my 1500 square foot house (old uninsulated plaster walls throughout most of it.) Ideally I would have got it insulated first, but I don't have the money to do that AND get AC installed, and I am just not willing to go through another summer of coming home from work and stepping into a sauna.

Also wondering about the implications given the variability of temperature during months where A/C is useful. Some days will be 78 degrees and only mildly humid. Then we have August with its 106 degree heat waves with high humidity.

If there is more capacity than necessary, can't it just run for shorter periods? Or is it going to be freezing for five minutes until the thermostat catches up? If I install insulation next year, am I going to find the thing running five minutes an hour and not removing enough humidity?

There are humidifiers that can be installed in central units, of course. Are there central dehumidifiers that can be added to the HVAC system, which can take over when the compressor is off?

During wet cool weather, 65^F-75^F, oversized or undersized a/cs do not operate. Assuming that you are getting minimal fresh air infiltration, which is an air change every 3-4 hours, your home will become humid. A few days of this high humidity is enough time to grow mold and dust mites. ASHRAE, Am. Medical ASS., EPA, Am Lung Ass etc. suggest this as a minimum amount of fresh air and suggest maintaining 50%RH. As you can tell from the posts, some of this group never heard of the problem, some feel a full featured a/cs is capable of taking care of humidity regardless of the cooling load. Others recognize the need for supplemental dehumidification and even suggest it. I come from the ventilation industry. We found out quickly that in green grass climates, a properly ventilated home becomes a damp home during cool wet weather. Damp homes grow undesirable biologicals that adversly affect some. As a result, I have promoted supplemental dehumidification along with mechanical ventilation for the last 15 years. Today, there are 10 major mfgs. of equipment offering supplemental whole house dehumidification systems. Some offer a fresh air ventilation feature. Yet many contractors cling to the concept properly sized a/cs that you can maintain <50%RH during typical weather.
A fringe benefit of supplemental dehumidification is not operating your a/c when you are not in your home. This is an energy saving concept. Check my post for more info. Regards TB

[beenthere]

Jerry, best ot start youor own thread.
But no, duel fuel would not need another electric line ran to the furnace.

[Glennhvac]

A lot of focus on insulation. What's your glass gain? Take the same house with 300 sq feet of west glass and rotate it 90 degrees and see what happens. In every heat gain calc I have ever done glass direction is the one that most influences sizing.

[IOCTL]

A lot of focus on insulation. What's your glass gain? Take the same house with 300 sq feet of west glass and rotate it 90 degrees and see what happens. In every heat gain calc I have ever done glass direction is the one that most influences sizing.Plenty of glass, but it's double insulated. I just touched the outside & inside panes of one of them on the west wall of the house. The inside pane feels about 10-15 degrees cooler than the outside pane. (For reference it's about 90 here right now. Inside pane feels it's in the 70 degree range.) All of the windows on the south/west have window coverings (regular curtains, paper blinds, aluminum foil, or some combination of these.) Additionally, the windows on the south side have metal awnings.

I'm concerned about the old walls being uninsulated plaster, as well as one of the rooms on the west side of the house that has no attic above it.

teddy bear, I guess this (Los Angeles) is a "green grass" climate, but only because we ship water in from other places. :) Today's RH is 8&#37;. The climate is Mediterranean, and without landscaping we'd be either desert or chaparral here. The worst humidity we have here in the middle of the day is perhaps comparable to Washington, D.C. sometime after midnight.

I will research your other posts for information about whole house dehumidification. If I don't go with that I might just put down $150 for one of the room sized units for the worst days in August/Sept.

[teddy bear]

LA is green grass climate only very close to the coast. Otherwise forget of humidity problems in your region. Regards TB

[speedymonk]

Jerry, I can only speak to the cost of your electricity. Simply take the total amount of the electric bill (everything associated with it including taxes and customer charge) and divide it by KWH. That is your effective cost.

[Hydrotopia]

I know two different people who had old houses and did the insulation and extra studs themselves on weekends.

[rponson]

Bryant 187 is 2 stage so it must be 3 or 4 ton

[beenthere]

The question is which size.

[DanW13]

IOCTL,
I don't know what the outside of your house consist of but what I did with my 100+ yr old home is strip my old aluminum siding off right down to the T&G sheating and apply 1/2" foam bd insulation and on top of that I put on the Tyvek house wrap and sealed all around my windows with caulking with all windows that are double pane glass, I then built a wrap around porch from the front of the house that is facing the south around to the west side of my house. My home with out all that I have added was no insulation in most of the walls except where I added some when I remodeled in certain places but otherwise none except for a 1/2" fiber bd stuck in the middle of each cavity opening was what they considered insulation 50 yrs ago. I just recently had installed a Carrier Infinity Mod furnace 3 stage and I run my fan 24/7 on low to circulate the air and granted the work I have done to date is alittle over the top for most homeowners but I will get paid back within 5-10 yrs for all the material I have purchased to date. But the main point was that if you have no insulation you may want to consider putting on new siding and add the 1/2" foam bd. insulation on the outside for the time being will help more than you could imagine, I am glad I had it for this past winter was realitively colder than usual being in the minus digits for most of Jan/Feb and had I not had the faom bd. on the outside I would have literally froze on the inside just that alone made a huge difference. My home is 1500+ in size 2 story, I also added a cold air return in my up stairs landing area which helped alot as well, so there's alot oyu can do yourself if your handy at all with a hammer and alittle know how to help you be more comfortable in your home, remember what you do now you will get back over a short period of time and at the time when you decide to sell.

[coolmen]

if you are sure your going to get insulation next year, have your contractor size the system based on that. It might not be big enough this year, but then it will be properly sized for the rest of its life (which will be a longer life than an oversized system). even if its not big enough to handle the current heat gain of the house, it will still be alot better than no A/C. and once you get the insulation it will be perfect

I agree craig.It may be a tad under sized until insulation is installed but at least you will have some cooling not perfect but livable.