Contractor slightly undersized my A/C unit. Demand replacement or no?

Reply

  #1  
Old 06-13-11, 08:44 PM
J
Member
Thread Starter
Join Date: May 2011
Location: USA
Posts: 15
Received 0 Votes on 0 Posts
Contractor slightly undersized my A/C unit. Demand replacement or no?

After getting a new central a/c unit I was somewhat disappointed that it could not hold the set temperature on those hot 90+ days. I had several techs check my unit. All the readings were fine(charge, subcooling, and superheat). I must conclude that the contractor undersized me by 1/2 ton. It is in warranty so I could insist he replace the unit with the proper size with me only paying the difference between sizes. However, I don't know if such a request is unreasonable since I estimate it is only a half a ton off. Should I make him put the right size in or just live with a slightly undersized unit?

My main concern is not the higher temperature, but instead energy costs. Is it cheaper long-term to get the larger size and have it run a normal amount of time or stick with the smaller unit and have it run a bit longer than normal?
 

Last edited by Jim1805; 06-13-11 at 09:35 PM.
Sponsored Links
  #2  
Old 06-13-11, 09:05 PM
hvactechfw's Avatar
Member
Join Date: Jan 2011
Posts: 6,245
Received 2 Votes on 2 Posts
slightly undersized unit will run a bit more, but will actually dehumidify much better. Slightly oversized unit will start and stop excessively and not dehumidify.
 
  #3  
Old 06-16-11, 05:11 PM
E
Member
Join Date: Feb 2010
Location: usa
Posts: 623
Received 0 Votes on 0 Posts
I know you guys are gonna hate me for asking this, but whats the square footage of the area you are trying to cool. What tonnage is your system?
 
  #4  
Old 06-17-11, 06:23 AM
GregH's Avatar
Super Moderator
Join Date: Jan 2001
Location: Manitoba
Posts: 10,128
Received 35 Votes on 33 Posts
Certainly, if the unit is undersized you should get a replacement!
But, you may have unreasonable expectations.
Humidity control is as important if not more important than temperature and you achieve this with run time.

A suggestion is to set the dial to a REASONABLE indoor temperature and for now forget about when the unit is or isn't running.
Will it make you comfortable, keeping in mind a properly sized a/c will run almost continuously during the hotest days of the year?
 
  #5  
Old 06-17-11, 09:08 AM
J
Member
Thread Starter
Join Date: May 2011
Location: USA
Posts: 15
Received 0 Votes on 0 Posts
Originally Posted by ender2272 View Post
I know you guys are gonna hate me for asking this, but whats the square footage of the area you are trying to cool. What tonnage is your system?
I live in a 960 sq ft. mobile home. Insulation is not that good. I have a 2 ton unit.
 
  #6  
Old 06-17-11, 05:03 PM
CasualJoe's Avatar
Member
Join Date: Jan 2010
Location: United States
Posts: 10,605
Received 74 Votes on 65 Posts
After getting a new central a/c unit I was somewhat disappointed that it could not hold the set temperature on those hot 90+ days.
What is the setpoint you are trying to maintain?
 
  #7  
Old 06-17-11, 08:42 PM
J
Member
Thread Starter
Join Date: May 2011
Location: USA
Posts: 15
Received 0 Votes on 0 Posts
Originally Posted by CasualJoe View Post
What is the setpoint you are trying to maintain?
I was trying to maintain setpoint at 74. The temperature went up to 76. I guess that isn't real bad for a hot day but here's the thing: My temperature difference between return and supply is not good. On a 90+ day the split is only like 10-11 degrees. On cooler days the unit can only achieve a 14-15 degree split. I always heard it should be like 18-20. After having a tech here checking everything out and finding nothing faulty with the unit the only conclusion I can reach is that I was undersized.
 
  #8  
Old 06-18-11, 09:01 AM
CasualJoe's Avatar
Member
Join Date: Jan 2010
Location: United States
Posts: 10,605
Received 74 Votes on 65 Posts
I live in a 960 sq ft. mobile home. Insulation is not that good. I have a 2 ton unit.
The lower temperature differential could be because of things beyond the control of the contractor when replacing a unit within an existing mobile home. Are all ducts insulated? The return air temp could be significantly higher by the time it gets back to the unit and the supply temps could be significantly lower when they leave the air handler. Where are you checking the temperature differential, at the vents or at the air handler? IF a load calculation was performed, it's likely that the actual load fell somewhere between 2 and 2 1/2 tons. The decision to go to the 2 ton unit could have been for dehumidification. It's also possible the calculation was based on maintaining a higher setpoint than you are now wanting. What size was the old unit and did it maintain a setpoint of 74 degrees?
I live in a 960 sq ft. mobile home. Insulation is not that good.
The bottom line is, you have to remember this is a poorly insulated mobile home.
 
  #9  
Old 06-18-11, 10:03 AM
J
Member
Thread Starter
Join Date: May 2011
Location: USA
Posts: 15
Received 0 Votes on 0 Posts
Originally Posted by CasualJoe View Post
The lower temperature differential could be because of things beyond the control of the contractor when replacing a unit within an existing mobile home. Are all ducts insulated? The return air temp could be significantly higher by the time it gets back to the unit and the supply temps could be significantly lower when they leave the air handler. Where are you checking the temperature differential, at the vents or at the air handler? IF a load calculation was performed, it's likely that the actual load fell somewhere between 2 and 2 1/2 tons. The decision to go to the 2 ton unit could have been for dehumidification. It's also possible the calculation was based on maintaining a higher setpoint than you are now wanting. What size was the old unit and did it maintain a setpoint of 74 degrees?
I did have a 2 ton unit before. That unit had a temperature difference of 19-20.(supply and return) The measurements were taken at the vents. That unit could hold 74 when the outside temp was 100. The contractor did not do a load calculation. He simply replaced my unit based upon the size of the old one. The vent temp on this one runs about 5 degrees warmer than my old one. I've heard that the newer ac units with a greater SEER do not run quite as cold as the older units but perhaps that is just a myth. As stated in my original post I'm not so concerned about holding set temp as I am about my electric bills. Obviously any unit that blows air 5 degrees warmer has to run longer to achieve the desired results. How much longer I am not sure. I haven't got my first bill yet since using the unit.
 
  #10  
Old 06-18-11, 10:13 AM
GregH's Avatar
Super Moderator
Join Date: Jan 2001
Location: Manitoba
Posts: 10,128
Received 35 Votes on 33 Posts
Is there a possibility that the contractor changed the fan speed when the new unit was installed?
You may be able to reduce the fan speed one step which would reduce the coil temperature and increase humidity removal.
This would reduce the system sensible heat capacity slightly but may make a big difference to your comfort.
 
  #11  
Old 06-18-11, 10:20 AM
J
Member
Thread Starter
Join Date: May 2011
Location: USA
Posts: 15
Received 0 Votes on 0 Posts
Originally Posted by GregH View Post
Is there a possibility that the contractor changed the fan speed when the new unit was installed?
You may be able to reduce the fan speed one step which would reduce the coil temperature and increase humidity removal.
This would reduce the system sensible heat capacity slightly but may make a big difference to your comfort.
The tech that came to check my unit did try adjusting the fan speed to low. After trying it for a few days I did not notice any significant change. I have since turned it back up to the normal high speed for a/c.
 
  #12  
Old 06-18-11, 12:51 PM
R
Member
Join Date: Apr 2011
Location: nj
Posts: 576
Received 0 Votes on 0 Posts
you figured out the problem right there by saying that he didnt do a load calculation. all 2 ton units are not the same. some will do more sensible heat removal while others give you more humidification removal. this is where a manual j load calc coupled with a manual s (system selector) comes into play. the manual j calc will tell you how much sensible and how much latent heat needs to be removed from the space. if a load calc is ever between two sizes your supposed take the next size up.
 
  #13  
Old 06-18-11, 03:26 PM
GregH's Avatar
Super Moderator
Join Date: Jan 2001
Location: Manitoba
Posts: 10,128
Received 35 Votes on 33 Posts
There really isn't a "Normal high speed for a/c" fan speed setting.

High is often used for a/c but the speed setting is normally made when the system is balanced.
The tech should have been able to see a change in operating pressures and hopefully you left it set where he left it to give it a chance.

The discharge air temp should have been lower on the lower speed setting.
 
  #14  
Old 06-18-11, 04:14 PM
J
Member
Thread Starter
Join Date: May 2011
Location: USA
Posts: 15
Received 0 Votes on 0 Posts
Originally Posted by RDSTEAM View Post
you figured out the problem right there by saying that he didnt do a load calculation. all 2 ton units are not the same. some will do more sensible heat removal while others give you more humidification removal. this is where a manual j load calc coupled with a manual s (system selector) comes into play. the manual j calc will tell you how much sensible and how much latent heat needs to be removed from the space. if a load calc is ever between two sizes your supposed take the next size up.
This confirms my feeling also. A 2 1/2 ton probably would have been the proper size.The question of course is what to do now. If you guys think being 1/2 ton off is not very significant then I will simply leave things as they are, but if you guys think it should be made right then I will insist they replace.
 
  #15  
Old 06-19-11, 09:38 AM
HVAC RETIRED's Avatar
Member
Join Date: May 2008
Location: Wisconsin USA
Posts: 715
Received 0 Votes on 0 Posts
Under-sizing a little is easily dealt with...I personally wouldn't up-size

Originally Posted by Jim1805 View Post
I was trying to maintain setpoint at 74. The temperature went up to 76. I guess that isn't real bad for a hot day but here's the thing: My temperature difference between return and supply is not good. On a 90+ day the split is only like 10-11 degrees.

(That low temp-split appears to be a problem - needing a fix.)

On cooler days the unit can only achieve a 14-15 degree split. I always heard it should be like 18-20. After having a tech here checking everything out and finding nothing faulty with the unit the only conclusion I can reach is that I was undersized.
The indoor temp-split depends on a lot of factors; the main one being the percent of relative humidity. Delivered performance is what counts; the performance problem needs correcting!

A high humidity will greatly lower the temp-split; at 50% RH the split should be around 18 -20F.

What is the airflow like; high or low? It normally (if between 350 & 450-CFM per/ton of cooling) has a much less effect on the temp-split than the humidity level.

Then there is a possibility the Return Air Trunk area could be pulling hot air with three times the grains of moisture count, than the indoor air.
The supply air runs may not be well insukatede, etc.

I would rather have a slightly undersized A/C; you can use a floor fan or ceiling fans to circulate the air. The main thing is keeping the humidity as near as possible to 55% Max; 50 to 45% is better.

Even if the A/C were slightly undersized; & I don't think it is; - you can do a lot of things to your mobile home & to the duct system & air flow so it will work effectively to keep you comfortable & control the humidity.

The air flow will be more efficient with a 2-Ton than with a 2.5-ton condenser; also the indoor coil might also have to be changed.

Get a humidity gauge from a hardware store; let us know what the indoor humidity is; everyone ought to have one.
I would be happy to be in your; so called undersized A/C; position!
 
  #16  
Old 06-19-11, 11:07 AM
GregH's Avatar
Super Moderator
Join Date: Jan 2001
Location: Manitoba
Posts: 10,128
Received 35 Votes on 33 Posts
I agree!

You have one of two options:
You could call the company that replaced your system back and insist they do a load calculation or, you could work with what you have.
I too would pick the second option and spend time to try to get it right.

One thing I might add is that if you do as suggested and get a reliable digital hygrometer, make sure that you quote at exactly at what temperature the humidity reading was taken at.
A relative humidity reading without the temperature is totally meaningless.
Outdoor temp/humidity would also help.
 
  #17  
Old 06-19-11, 02:56 PM
CasualJoe's Avatar
Member
Join Date: Jan 2010
Location: United States
Posts: 10,605
Received 74 Votes on 65 Posts
If you guys think being 1/2 ton off is not very significant then I will simply leave things as they are, but if you guys think it should be made right then I will insist they replace.
Something not yet mentioned that you should think about is this. If the contractor gave you a proposal to replace your old 2 ton unit with a new 2 ton unit and you accepted that proposal, you'll have a very hard time convincing the contractor (or a judge should this end up in court ) to GIVE you a bigger unit. I too would work with what you have.
 
  #18  
Old 06-19-11, 05:42 PM
J
Member
Thread Starter
Join Date: May 2011
Location: USA
Posts: 15
Received 0 Votes on 0 Posts
Originally Posted by HVAC RETIRED View Post

Get a humidity gauge from a hardware store; let us know what the indoor humidity is; everyone ought to have one.
I have a gauge. If it's accurate my indoor humidity typically runs 55-58%.
 
  #19  
Old 06-19-11, 05:55 PM
J
Member
Thread Starter
Join Date: May 2011
Location: USA
Posts: 15
Received 0 Votes on 0 Posts
Originally Posted by CasualJoe View Post
Something not yet mentioned that you should think about is this. If the contractor gave you a proposal to replace your old 2 ton unit with a new 2 ton unit and you accepted that proposal, you'll have a very hard time convincing the contractor (or a judge should this end up in court ) to GIVE you a bigger unit. I too would work with what you have.
This is true, although I have service records that show how well my old 2 ton unit performed in the past. I had the unit installed in Oct. last year, so I have a few months to decide what to do. I guess I will just see what my electric bills are with this unit compared to old.
 
  #20  
Old 06-19-11, 06:03 PM
HVAC RETIRED's Avatar
Member
Join Date: May 2008
Location: Wisconsin USA
Posts: 715
Received 0 Votes on 0 Posts
I have a gauge. If it's accurate my indoor humidity typically runs 55-58%.
With that indoor humidity level the indoor temp-split is way too low!

Your 2-Ton is NOT delivering near its Rated BTUH to the rooms.

Get someone that knows how to find the problems!
 
Reply
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: