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Question about AC Unit..

We are looking at buying a new house in Texas that is 2650 Sq Ft.

I am concerned that it has only one 5 ton 14 seer unit. It is zoned though. I live in a 2500 sq ft home now and it has two units.

Can anyone give me some information on what kind of questions I should ask the builder or point me to a web site that has some info?



Re: Question about AC Unit..

I live in a 2500 sq ft home now and it has two units.

There are many factors that affect the sizing and specifications of your system, including square footage, insulation, window surface and configuration, geographic location of your home, duct sizing and arrangement, and many others.

It's difficult to compare the home you're presently in to the new home you will be moving in to. While the square footage may be similar ... your present home is likely older and may not be as well insulated and sealed ... having a higher heat gain/loss than the new home will have.

Considering the home is currently being built ...... the size of the HVAC is based on the projected heat gain/loss loads. The important thing will be proper installation and balancing of the HVAC.

Hope this helps.:)

Re: Question about AC Unit..

The above posters are correct.

It is not clear from your post if you are talking about a house already built, or one still in the process.

If the house is already built, the best method is to go to the house & actually turn on the unit & see how it cools.

As canuk notes, there are many complex factors that determine if an AC unit is properly sized for the geographical location, amount of insulation, amount of window glass etc.

These factors are typically fed into a computer program called a MANUAL J HEAT GAIN CALCULATION, which considers all these factors, and computes a recommended amount of cooling btu/hour for the AC, based on the above factors.

Some of the free sites below will help you calculate the amount of AC needed for the new house, but you will need a lot more than just the square footage.

You can also Google such phrases as "sizing an air conditioner", "heat gain calculation", "AC sizing", "sizing air conditioner", etc.

One ton of AC should cool between 500 sq.ft. and 700 sq.ft. of residential floor space; 1 ton = 12,000 btu/hr cooling capacity.

If the house is located in a very hot, humid climate, the number would be closer to 500 sq.ft.; if the climate is less humid & hot, & there is extensive insulation & tight windows in the house, the number would be closer to 700 sq.ft.

One ton of AC = 12,000 btu/hr cooling = 2650 sq.ft./500 = 5.3 ton needed X 12k = 63,600 btu/hr needed.

One ton of AC = 12,000 btu/hr cooling = 2650 sq.ft./700 = 3.78 ton needed X 12k = 45,360 btu/hr needed.

The AC in the new house seems to be within parameters, but this is just a rough estimate & doesn't consider the proper sizing of the ducting (Manual D), or the other factors mentioned.


Re: Question about AC Unit..

I would just get general knowledge like you are doing and jack and canuk are helpful all the time.I would ask or test the zone area- some zones aren't set up correctly as respect to duct pressure and or tstats placement- if the home feel comfortble as you walk through, look at the Tstat set point and temp.If you see that the set point is 65 and it feels 70- you need to question why does it need to be on so low- is it to compensate for a poor zone set up. or visa versa---You get the idea.
If you have an inspection have an HVAC company check it out also.
Use you instinct for temperature and humidity you will feel a difference if there is one as you walk around..

Re: Question about AC Unit..

Thanks for all your replies. We are currently having the house built. It is in the Houston, Texas area which is notoriously hot and humid.

I am most concerned that it has only one unit. I am worried that the ac bill will be huge.

I will take a look at the sites you guys posted on here and hopefully that will ease my mind a little!

Thanks for all the information.

Re: Question about AC Unit..

I didn't notice what size the 2 units you presently have, but it is typically more efficient to run one unit of a certain size than two units that equal the one larger unit.
Two units would provide a backup in case one breaks down, but that wouldn't be high on my list given the cost of two units & the electricity to run them.

Re: Question about AC Unit..

2 correct size units are ALWAYS better than 1 unit if the bedrooms are on their own zone, why cool your living room, kitchen,etc, at night when you don't have too.

Re: Question about AC Unit..

You indicate the house is a new build. Is the home builder's HVAC contractor doing the work or are you contracting that out separately? If the contractor's HVAC sub is doing the work, then find out who that is, and talk with them about your options. NOW is the time to do this.

The contractor should have no objections to your doing this as after all, this is going to be YOUR home.

For all you know, the HVAC contractor may have already suggested zoning the HVAC and the home builder decided against it. That is why YOU need to be involved in the process.

Try to set up a time to meet with your HVAC guy, and ask these questions. See if he did in fact do a load calc on the home.

You certainly shouldn't go on the basis of what your current home has in deciding what your new home needs.

All the best, Irishmist

Re: Question about AC Unit..

I must respectfully disagree with HVACGUY, two systems are NOT always better than one, a properly set up and installed zoning system can accomplish the same thing, at a lower cost many times.

In Houston you certainly do not want to be over-sized either, the humity problems will not be fun.

Re: Question about AC Unit..
hvacguy wrote:

2 correct size units are ALWAYS better than 1 unit if the bedrooms are on their own zone, why cool your living room, kitchen,etc, at night when you don't have too.

That is exactly the set up that I have . At night , I run one 2.5 ton unit versus a 5 ton .

Bob Gabrilson
Re: Question about AC Unit..

I would zone with a 5 ton 2-speed. Remember, you can always have more than two zones (with zoning). I have 3 zones in my house with a 3 ton 2-speed A/C. Sweet.


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