Home AC advice

[elcom]

Thanks -- that's pretty specific. I will ask when I get back to town.

 

[brewchief]

Anyone in the Houston area know of an expert that I can call to evaluate the situation? The only other option that I can think of is to call the other contractor that was bidding on this job whose bid was slightly higher.

I can't think of other options...

Bring in a third party, look for an air balance contractor, most will be heavily biased towards commercial work but if you talk to a few you should be able to find someone who will take a look at a resi job. They will be able to measure TESP and measure the air volume at every register.

danski's advice on upsizing all the ductwork and properly sealing the runs on each end is pretty much exactly what I think it will take in the end to get the proper performance out of the system.


It may make danski cringe but I'm going to throw out a couple rules of thumb that I use on occasion to determine if a system has enough ductwork.

#1 400 cfm of air flow per ton of A/C, sometimes a little less will work but never plan on it.

#2 figure a 6" metal heat run of reasonable length at 100 cfm, 7" 150 and 8" at 200.

#3 subtract at least 25% airflow when flex duct is used, more if it has a ton of tight bends, is squished into awkward spots or is more the 25' long.

These are rough and ugly rules of thumb that shouldn't be used for design but more as an aid to see if the existing duct system is at least close to what it needs to be.


You now have a 5 ton system so that means 2000 cfm, you really need somewhere in the neighborhood of 25 6" heat runs or 12-13 8" runs to even have a chance at it being happy.

If you have problem areas the consider changing the register boots out when you upsize the flex and use the largest grill practical, a 14x6 grill is going to be less restrictive then a 10x6. Seal the boots tight to the drywall, many people don't get that the airflow from the A/C can draw a bunch of attic air from around the boot if it's not sealed up tight.

I would use R-10 or better flex, may have to order it but it's not like you will be changing it out every few years, a properly reworked duct system may outlast several A/C units.

 

[CNGsaves]

OP . . . sure hope the wanna-be HVAC Trane guy has NOT been paid everything "owed" to him for mediocre result ??

Taking out a working unit, and "upgrading" with system that can't even match what was replaced . . . is plain wrong or incompetent, or whatever you want to call it. Since your air handlers are in attic that he had to haul up there and look at those octopus duct mess, he KNEW DARN WELL up front that it was longshot at any improvement in your situation. Every day working in your attic, he was looking at octopus mediocre setup . . .no way he can claim ignorance of the facts. He keeps giving you "lipstick on a pig" speech as he should have brought up ideas to improve ducts from the get go, IF he knew what the heck he was doing.

My bet is he's already re-sold your working system he pulled out of your house !! He's hoping for cool weather or that OP will give up.

I'd want 20% refund NOW and use that money to put towards reworking the ductwork with HVAC guy who will DO IT RIGHT !! OP you might want to hire Danski0224 as consultant to put together SPECIFIC Details of RESULT (testing, sizing, materials, etc.) that needs to happen !!

 

[mustangacman]

You could try these folks. I am pretty sure they do leak test.

http://www.foxenergyspecialists.com/services/residential-energy-testing/index.html

 

[elcom]

Update: I have been out of town for several weeks and have asked for an independent evaluation. The contractor suggested asking Trane tech support to come out.

Among other things, the Trane tech did a TESP and it was 1.21 inches of water (high). Interestingly, he thought that the Honeywell filter put in by the contractor was the major problem, not the duct(s). He measured 0.60 inches of water across the filter.

The bottom line: the Trane tech suggested

1) expanding the primary return (there are actually two returns) from 20x30 to 20x40 (which the contractor will do)

2) decreasing the amount of refrigerant slightly (contractor will do)

3) taking out the filter media at the and using filters at the return grate

Will try these relatively simple maneuvers and see what happens...

 

[danski0224]

Either the system is charged correctly, or it's not.

For proper installation, the furnace needs 2 returns and therefore 2 filters. Relocating the filter to the 2 grille openings feeding into a single opening at the furnace is a stopgap measure. It may actually backfire because as the remote filters get loaded, the fan will pull more dust through the unsealed duct connections and push the dust into the secondary filter: the air conditioning coil.

The Trane tech sounds like a hack. Good luck

 

[elcom]

Perhaps there is something that I am missing, but as I understood it, the issue was not the location of the filter(s), but the fact that the Honeywell 20x25x5 filter was too restrictive and therefore the system is "starving for air" (Trane tech's words).

Also, if the ducts were leaking substantially, wouldn't TESP be lower?

 

[danski0224]

You need 2 of those filters in parallel to lower the pressure drop at your required CFM.

A TESP measurement is an initial diagnostic tool. All it tells you is total external static pressure. It is NOT an indication of duct leakage. It is not possible to determine duct leakage with TESP.

A duct leakage tester is needed to determine duct leakage, and when used with a blower door, duct leakage to the outdoors can be quantified.

Duct leakage makes TESP lower by providing "make up" air.

Modifying that return is a waste of time and effort.

 

[pop pop]

elcom, feel for you man. You got a **** for a contractor. There should be qualifications for contractors in HVAC to weed the hacks out. But unfortunately there are not.

You've gotten several ideas and sucked into an argument you don't fully understand.

May I suggest you back up (I know, can't unring a bell) and remember you are supplying the location and funding. Your contractor is supplying the equipment, installation and knowledge. If he fails, he doesn't get your money. Stop trying to solve this for him as you know less than him (apparently not much less though). You've got to bring pressure on him to stop guessing and for HIM to find some help. I'm guessing you no longer hold any money? May as well dial up another HVAC firm. This one is no good, not knowledable and already have been paid. You have no leverage, he doesn't know what to do.

HVAC and refergeration is a SCIENCE. There is no guesswork involved (if you know the science!).

retired registered PE

 

[Mike007]

Danski is pointing you in the right direction. He suggested checking the TESP, and look what happened when it finally was checked, off the chart. And he is correct in regard to the filters. 5 tons requires a lot of airflow. 2 air cleaners at the air handler is the ideal solution. Putting filters at the return air grille just causes leaks in the return ducting to become bigger leaks as the filer gets dirty. And these leaks bring unfiltered air into your equipment.

Unfortunately you are going to be stuck with whatever solution Trane and your contractors come up with. The TESP needs to be brought in line and then the CFM verified. Good luck. I hope this gets resolved for you.

 

[Trey T]

Sounds like they're pointing you into the right direction.

 

[elcom]

I agree completely, the contractor has the knowledge and my role is "location and funding" as pop pop wrote.

Prior to hiring this contractor, I interviewed multiple local contractors (had ~5 visit and give estimates) and chose the one with the highest ratings that I could find.

So, if I were to hire another contractor, at additional expense, I would be hiring someone with fewer, or lower, ratings than this one... Very frustrating.

Notably, none of the contractors that came out said anything about the duct work. One (and only one) wrote an additional return into the quote. This contractor did that for free, but only after I complained about the 4 ton unit (now replaced with 5 ton unit) not working well.

That having been said, now that the temperatures have dropped a bit into the high-80s to mid-90s (it's all relative, folks), the upstairs unit seems to be doing just fine. Maybe not as efficient as it could be, I just can't tell...

 

[danski0224]

Among other things, the Trane tech did a TESP and it was 1.21 inches of water (high).

Attached is part of the specifications from a variable speed drive furnace. While it isn't a Tranewreck you should get the message.

Note that the specs call out a bottom return with filter.

The delivered airflow at 0.5" wc is 2005 CFM.

The delivered airflow at 1.0" wc is 1525 CFM and you are at 1.2" wc. Following the chart progression and extrapolating, somewhere around 1250 CFM should be expected (at a higher amp draw, BTW).

Given the "400 CFM/ton" rule of thumb, 2000 CFM = 5 tons, 1525 CFM = 3.8 tons and 1250 CFM = 3.1 tons.

You are paying for 5 tons of airflow, but only getting 3.1 tons of airflow (a 38% reduction if I did the math correctly). Yup, cooling capacity is significantly reduced, without accounting for additional efficiency/system losses through leaky ductwork.

Just on airflow losses alone, your system capacity is down ~38%. A 50% reduction factoring in duct losses (supply side) and duct gain (return side) is not inconceivable.

Some enthalpy measurements and some math can determine system capacity.

If 5 tons = 60,000 btu, then 3.1 tons = 37,200 btu.

But wait, it ***** even more.

You don't have a 60,000 btu coil. It's 52,000 btu... right? Taking 38% off of a 52,000 btu coil leaves you with 32,240 btu *at best* out of a "60,000 btu" system... and that is sensible + latent capacity.

Oops.

If the ductwork was right, you could install a 3 ton system and get better performance.

Damn, that bites.

Here are some additional notes copied over from another page:

5. All airflows of 1880 CFM or less on 21” and 24.5” casing size furnaces are 5% less on side return only installations.
6. Airflows over 1800 CFM require bottom return, two−side return, or bottom and side return. A minimum filter size of 20” x 25” is required.
7. For upflow applications, air entering from one side into both the side of the furnace and a return air base counts as a side and bottom return.
8. Airflow not stable at this ESP.

 

[Trey T]

I agree completely, the contractor has the knowledge and my role is "location and funding" as pop pop wrote.

Prior to hiring this contractor, I interviewed multiple local contractors (had ~5 visit and give estimates) and chose the one with the highest ratings that I could find.

So, if I were to hire another contractor, at additional expense, I would be hiring someone with fewer, or lower, ratings than this one... Very frustrating.

Notably, none of the contractors that came out said anything about the duct work. One (and only one) wrote an additional return into the quote. This contractor did that for free, but only after I complained about the 4 ton unit (now replaced with 5 ton unit) not working well.

That having been said, now that the temperatures have dropped a bit into the high-80s to mid-90s (it's all relative, folks), the upstairs unit seems to be doing just fine. Maybe not as efficient as it could be, I just can't tell...

If the outdoor temp is about 90degF, how low can your upstair unit cool to?

My down stair cools to 75degF but the upstair cools to 77degF easily. When you check, make sure to take several temp in different area upstair to be representative because often the thermostat can give a false reading. My t-stat is located right below the return and it reads about 2degF higher than other locations.

 

[elcom]

If the outdoor temp is about 90degF, how low can your upstair unit cool to?

My down stair cools to 75degF but the upstair cools to 77degF easily. When you check, make sure to take several temp in different area upstair to be representative because often the thermostat can give a false reading. My t-stat is located right below the return and it reads about 2degF higher than other locations.

Not sure how low I can get the temperature upstairs on a 90 degree day. Have not tried this.

I have tried to measure the temperature in various rooms upstairs; seems to be reasonably consistent (within 2-3 degrees) and the temperature at the supply grills. There was one duct that seemed high (80s when the temp outside was over 100 and the other ducts were much lower). I closed the grill.

That having been said, it has cooled down in Houston and the house is comfortable.

I gather that the system is not operating efficiently. Therefore, I have asked the contractor (politely, but firmly) to do what is necessary to analyze the situation including ducts and provide a plan to fix the situation.

 

[Trey T]

I see. I kept thinking that the problem has been resolved.

 

[elcom]

I guess problem has not yet been resolved based on the TESP), but frankly I am not entirely sure how to determine whether the system is operating properly or not. In other words, it should not be a matter of opinion, but what would constitute sufficient "proof" of proper operation without comparing electricity usage during months with similar temperatures...

 

[danski0224]

I guess problem has not yet been resolved based on the TESP), but frankly I am not entirely sure how to determine whether the system is operating properly or not. In other words, it should not be a matter of opinion, but what would constitute sufficient "proof" of proper operation without comparing electricity usage during months with similar temperatures...

I'd turn the outdoor unit off at the circuit breaker and set the thermostat to cooling and crank the temperature down so the fan stays on. Wait 7-8 minutes so that it is out of the low flow mode... or set the DIP switches to disable that feature.

I would then try a plain fiberglass filter to get the TESP reading within the published manufacturers data without extrapolation. Then you know how much air the fan is moving in the cabinet... or do you?...

I would then measure airflow at all supply and return openings with a flow hood. This way you know how much air is going back to the furnace and how much is being delivered. Yes, it could be wildly different from the assumption based upon the TESP reading and fan chart due to leakage.

If possible, take a traverse at the supply and return to corroborate the other measurements. Average them if necessary.

It takes a while to do this.

Once you know what the system is delivering without the AC on, turn on the AC and take another airflow measurement wherever it is easiest to do so, and correct the reading in a similar fashion to the other set of measurements. The wet coil will alter the airflow. Make sure that the system is not in low flow mode and the condenser is in high stage.

Yes, airflow measurements are a PITA and you are trying to hit a moving target.

After all of this is done, you will know how much air is moving through the AC coil... or at least a very good guesstimate.

Then turn on the AC unit again, wait for it to get out of low flow mode or disable it. Make sure that the condenser is in high stage.

You now need some wet bulb and dry bulb temperature readings from the ductwork and an enthalpy chart. With those numbers and the measured CFM, and a formula, you will know exactly how many BTU's your system is delivering at the equipment at that moment. The temperature readings need to be completed closely (time-wise) because the conditions change when the AC unit is on.

Just for S&G's, I would measure the pressure drop across the AC coil when it is dry (fan at cooling mode speed) and when the coil is wet.

There is diagnostic equipment available that can do all this (calculate capacity) in real-time.

This is the only way to get "proof" of how the system is operating... and it is just the beginning step of corrective action.

The engineering data for the evaporator coil will list capacities at a given CFM and return air wet bulb temperature.

If you do not have the BTU's at the equipment, then you need to correct the equipment mismatch problem (engineering data), refrigerant charge problem or airflow problem. It could be a combination of any of these.

If you have the BTU's at the equipment and good airflow, then you are losing it in the ductwork.

Comparing electrical usage would be pointless because the system is not the same.



Ok, so now you have the "proof", but what do you do with it or what good is it? By hand, it takes a lot of time to gather all of the data. The equipment that does it for you is not a casual purchase for most.

I know that the system isn't working because you don't have the airflow. I can tell that from here.

Without the airflow, you don't get the BTU's, because the formula uses CFM. Low CFM = low BTU's- always.

You also have equipment that is installed outside of manufacturer specifications.

It is impossible to charge the AC properly due to the low airflow. You can "make" it work, but it will be short on charge.

The equipment needs to be installed to manufacturer specs.

If you have a fan rated at 0.5" of water column, then the restrictions imposed by the air filters, evaporator coil, supply ductwork and return ductwork have to add up to 0.5" of water column or to a number that still allows the fan to deliver enough CFM for the system to work.

A room by room load calculation will tell you how much CFM each room needs, and then the ductwork needs to be sized accordingly. This would need to be checked against what you have now. Spot measurements could be taken with a flow hood. Extensive changes could be required.

 

[danski0224]

The real point to ponder here is that the OP is in Texas, and Texas has some pretty stringent requirements in place to become a licensed HVAC contractor.

I wouldn't be surprised to hear that the installing contractor is some sort of "Comfort Dealer" or whatever Trane calls people that sell $XXXXX amount of equipment per month these days.

Too bad that none of that really seemed to help... but it sounds and looks good.

 

[elcom]

The contractor has an active Texas BE HVAC license. Nothing that I saw about "comfort dealer" or anything like that. They list the Trane XV series as a specialty (i.e., presumably that they have specific experience with this line of products).

Prior to hiring them I did some reasonable research. Nothing dramatic, but I checked reviews, spoke to multiple contractors, etc. They seemed like the most knowledgeable and appeared to be highly rated.

Not sure what I could have done better here and what lesson I can learn, other than that Garage Journal folks are more knowledgeable than local contractors...

 

[elcom]

Just thought that I would write a follow up based on a full year living with the system. In part to thank everyone who contributed to this thread. Perhaps some one will find it interesting.

The bottom line is that I am very happy with the systems and my energy bills are 1/3-1/5 of previous! Partly due to more efficient units and partly due to decreased energy cost with Energy Ogre (see below).

So, after a lot of great advice from very knowledgeable people on this forum (Thanks danski0224 and many others!). I have lived with the system, as previously described (two 5 ton Trane XV20i units, ducts as originally installed in home when built + a bit of increased return installed by the contractor for the upstairs system). Basically, the contractor refused to do anything else, except sell me a third system for ~$3,500-$4,000 -- which I declined for the present.

Based on the critical assessment of ducts and airflow on this forum, I was not expecting much improvement. After a full year and in the middle of August (hottest month), I am pleasantly surprised.

The systems are cooling at least as well as my previous systems, I believe better. The downstairs unit meets set temperature quickly and is able to maintain it even on the hottest of days (>100 F). The upstairs comes very close, perhaps 1-2 degrees warmer than set (78 F) when it is very hot outside; when it is below 90-95, no issues at all. The house also seems drier than with old units.

Most impressively, electricity bills are 1/3 to 1/5 of previous! Previously $400-$600/month during the summer, now highest bill was around $130. I expected some improvement, but not that much.

To be fair, I also started using Energy Ogre (https://www.energyogre.com/), a service that optimizes choice of electricity plan. The only downside is that it makes comparison to previous years usage a bit difficult (you have to keep your own usage records since the Energy Ogre switches providers often).

Anyway, thanks for all of the good advice.
Hope that someone else benefits from my experience and the great information/advice on this forum.