Summer Cooling Issues - lots of details inside - help needed

10 years ago

I live in a 2 story townhome with units attached on both sides. It was built in 2007, but I bought it 2.5 years ago. I live in north texas and for the past two summers, the AC just can't keep up. The system only gets down to 78F only at night and only if we aren't cooking. During the day, it struggles to get below 80F. All bedrooms are upstairs.

There are two systems. The system for upstairs is 2 ton. The system for downstairs is 1.5 ton.

Some things that may be helpful to know:

1) Manual J tests indicate both units are undersized by .5 ton each (I need a 2 ton and a 2.5 ton).

2) The condensers are misinstalled. They got switched so that both condensers are operating with the wrong air handlers.

3) Both units have a single return for each. The returns are both upstairs and are apparently undersized. Since both are upstairs, they really support each other. There is no supply vent near the thermostat upstairs.

I've been presented with three different options from three different contractors:

a) Upgrade both units by .5 ton. Add a supply in the hallway at the top of the stairs (same hallway where the upstairs thermostat is - ceilings are tall and hallway is very open). Add a return and supply in the master bedroom (there's only one supply in this large, west facing room).

b) Replace the 2 ton currently running upstairs with a 3 ton and move the 2 ton in place of the 1.5 ton. (In other words, buy one 3 ton system and make better use of the existing 2 ton).

c) Replace only the downstairs system so that there are 2x 2-ton systems. This contractor feels that this is the only way to keep humidity down. He seems to think that bigger upgrades will result in high humidity.

I'm inclined to go with option b plus add a return and two supplies as suggested from option a.

Will this size unit result in high humidity? Will skipping the additional return(1x) and supplies(2x) make for airflow bottlenecks?

This post was edited by elixir75 on Mon, May 13, 13 at 14:43

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Comments (9)

User
10 years ago
I'm sure you'll get good sound advice here. I'm just a homeowner, but I have to ask why do you want a supply near the thermostat? Seems that would cause the thermostat to conclude the house was cooler than the representative temperature for the entire space and cause the other rooms to be warmer than desired.

In my two story colonial there is no supply duct in the upstairs hallway where the thermostat is. In fact the upstairs return is within just a few feet of the thermostat.
elixir75
Original Author
10 years ago
The supply near the thermostat was suggested by one of the sales people. It makes sense to me because the rooms always feel cooler than that hall where the thermostat is. The thermostat is at the top of the stairs so it really is probably the warmest part of the house at any time.
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Jeez, I hate the idea of soft insulation on the interior of ducts. No matter what you do, it is going to get dirty and a lot faster than a hard surface. Well I guess you have to understand the problems of the past with older methods that were tried in this area of the country prior to air conditioning era or introduction if you will around the early 1960's and mostly on into the 1980's in which the major retrofit of non-air conditioned buildings were converted. A lot of these structures had whole house fans, in which the indoors during the summer was more or less and effective wind tunnel. I've seen the indoor fans in older homes, they are huge monstrosities that would take indoor home air and expel it to the attic of the home effectively creating a 'breeze' --- but they were noisy and windy. The Heating systems during that era were typically ducted in and thru the attic with hard metal pipe and insulated on the exterior of the pipe. In many cases over the years of other problems (in the attic) like running TV cable lines, fixing plumbing issues and the like the ducts would either get smashed, busted apart or accessed enough in which the insulation would fall off the duct. Once the insulation falls off due to the climate here, the ducts would sweat. The water would drip down and leak into the ceiling. Humidity problems are real here and can occur upwards of 90% of year. By placing the insulation on the interior of the duct system you avoid those kinds of problems. So as you can see, the solution you propose has it's negatives just like anything else. If the ducts are in the interior, we don't need insulation for thermal reasons. We don't have the condensation problems either. Well a high humid climate is special. You only know what you can see right? As in the ducts are hidden in the interior of the structure.... Ducts are on the interior: Yes they do exist to some extent in this market but mostly relegated to the apartment market. The duct systems in these cases are typically duct board, mainly for sealing purposes. Because in a almost always humid climate if you have an air leak to the attic (think thru some protrusion like electrical or plumbing run) if humid air has the potential to infiltrate the duct system supply or return you have the potential of black mold growth. This was all the rage in this area a good 10-15 years ago. Experts often talk without considering different climates fully and rationally (it's either that or people think they can do what an expert recommends for one area and think it's best for every area or market) no two HVAC markets are the same. Each one has unique challenges. Typically the interior mounted ducts in this market are fur down type systems. Typically these systems are limited to 3 ton and lower. So in larger homes these make for a poor use. Fur downs are relegated to electric heat only and are lower end equipment designed for small foot print installations. I have a question. What is the mean time before failure for indoor vs. outdoor ducts? Indoor meaning attic and outdoor meaning run under a home in a crawl space or in the case of mobile home application? Because there is a difference in materials chosen for attic ducts versus ducts chosen for crawl space and / or mobile home market. I want to make sure you understand that. If you put flex designed for attic installation in crawl space or under mobile home application it will fail much quicker than it should. Attic installations can easily last 20-30 years (given the pictures I showed above, may make you want to replace them sooner.) I know this has gone off topic, but I think this discussion is important to realize that certain climates have unique challenges. HVAC is not a one size fits all market. As I have said many times, there are two sides to any coin. I service the Katy, Texas area.
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User
10 years ago
Let me understand. Your upstairs is too warm in summer so the solution is to make it seem cooler at the thermostat so that the thermostat will sense that it has done its job of cooling and reduce the A/C in an already too warm house?

Makes absolutely no sense to me. Does it make sense to you? lol
elixir75
Original Author
10 years ago
What I'm saying is that I'd want to add a supply only if it would provide a more accurate reading of temps to the thermostat. I agree the vent shouldn't be too close to the thermostat for the reason you mention. What I'm trying to avoid is having the system running when the rooms are plenty cool, which happens when we close our bedroom door - the cold air accumulates in our room and the temp in the hallway is significantly higher. During winter we have a similar problem with the heat.

There are only 5 total supply vents upstairs (3 bedrooms, 2 baths). With all the warm air coming up the stairway, I'd think there should be at least one supply in that hall.

But I'm not the pro :) What's recommended/customary in this case?

Edit: changed the original post to better describe the suggested supply location.

This post was edited by elixir75 on Mon, May 13, 13 at 14:44
mike_home
10 years ago
Why is the contractor suggesting a 3 ton unit for upstairs, when the Manual J calculation indicated a 2.5 ton load?

Did the contractor who proposed increasing the downstairs to a 2 ton condenser explain how this will solve the problems on the second floor?

I am not getting a good feeling about either of these contractors. Did they present you with written proposals with the details of the equipment and services?

Here are some questions:
What is the area of the first and second floors?
How much insulation is in the attic?
Did you get a copy of the Manual J calculations and can you summarize the results?
Do you currently have a humidity problem during the summer?
elixir75
Original Author
10 years ago
Mike, he's thinking a 3 ton will be an adequate way to compensate for an undersized downstairs system (since both returns are upstairs this sounds logical to me). The summer months are problematic on both floors.

First floor is ~800 sqft. Second floor is ~1200 sqft.

I'll check attic insulation height tonight.

I didn't get a copy of the Manual J. If it's needed to get a right sized system, I'll gladly pay someone to give me a proper report.

I do not have a humidity problem in the summer.

Question: What is the downside of having a system .5 ton larger than needed? I ask because this AC work is getting into pricey territory and I really want to avoid getting a system that doesn't have the headroom to handle entertaining people (extra heat from bodies as well as cooking). Last summer I had the oven running with a house full of people going in/out and it was sweltering inside.
mike_home
10 years ago
The duct work must be large enough to accommodate the AC air flow. Having a larger AC with undersized ducts is not going to work well. If the AC is over sized then it may short cycle. The AC will not run long enough to dehumidify the air.

Did one of the contractors do a real Manual J calculation? They should have least shown you the result. Contractors sometimes claim they have done a calculation when in reality they have not.

I would think the 1.5 ton could handle the first floor. Your problem may not be due to insufficient size. Is the second floor unit in the attic? It may have leakage and poorly insulated duct problems.

I don't understand why there is no return on the first floor. What is the location of the first floor unit?
elixir75
Original Author
10 years ago
Both units are in the attic. There's no leakage according to everyone who's inspected it. Although there's a bad kink in one of the upstairs supply ducts (almost 90 deg) and a minor kink in another.

The option 'a' contractor recommended increasing the return duct sizes and adding a plenum to each return duct grill to increase return air flow (there's none on either).
User
10 years ago
Time to get an estimate from Contractors C & D.

Summer Cooling Issues - lots of details inside - help needed

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