HVAC Cooling Performance - HVAC - DIY Chatroom Home Improvement Forum
Advertisement


Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > HVAC

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Search this Thread
Display Modes
Old 07-05-2020, 10:03 AM   #1
Newbie
 
Join Date: Jul 2020
Posts: 4
Rewards Points: 8
Default

HVAC Cooling Performance


I have been in my newly constructed home about 18 months. There is about 2115 SqFt downstairs and 985 SqFt upstairs (3100 total). A single Lennox 5 ton model 14ACX-060-230-16 A/C compressor is installed. It is a split system with a bypass. The upstairs is not used much (all bedrooms are downstairs) so I set the thermostat to 81 to save a little energy. Downstairs, the thermostat is set to 78. I have done some data collection and found that in about 3.5 hours (3:00 to 6:30) on a warm (92) sunny afternoon, the upstairs will cycle about 16 times and the downstairs will cycle about 11 times. On average, the upstairs cycle is 4:15 on and 9:45 off (m:ss) and the downstairs is 5:10 on and 15:20 off.

So here are my thoughts:
1. Is seems to me, the "on" times are too short. No sooner than the system reaches peak cooling efficiency the system shuts off.
2. There are many times throughout the day that the system is running solely to cool the upstairs. That is effectively a 5 ton unit unit cooling 985 SqFt. Using the rule of thumb (yeah, I know about rules of thumb) of 600 SqFt/ton, that is 3 times the size needed. Seems to me this could be part of the reason the cycles are so short.
3. Also, there are times throughout the day that the system is running solely to cool the downstairs. Now it is a 5 ton unit cooling 2115 SqFt, which is 1.4 times the size needed.
4. Occasionally, both the upstairs and downstairs are calling for cooling at or near the same time but, during these times, the "on" times are only slightly longer.
5. The humidity in the house feels high. It is running in the mid to upper 50s most of the time (except when the outside humidity is low - not very often here in NW South Carolina)

So I guess my questions are, is my system performing acceptably? If not, what type of corrections/adjustments can be made (by me or an HVAC expert)?

Thanks in advance for your help.

Bill
Coolbreeze510 is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 07-05-2020, 10:42 AM   #2
Member
 
Join Date: Oct 2010
Posts: 322
Rewards Points: 344
Default

Re: HVAC Cooling Performance


While cycle time is important, but to measure the performance of your AC, First you need to get the Delta-T numbers. Delta-T is the room temperature minus the AC vent cool air temperature. You want a number between 16 and 19. if not your AC has problem.
clocert is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 07-05-2020, 11:13 AM   #3
Newbie
 
Join Date: Jul 2020
Posts: 4
Rewards Points: 8
Default

Re: HVAC Cooling Performance


I have that information: The temperature of the air coming out of the ductwork just before the system shuts off is around 58 (and still falling a little). I assume the return air temperature at that time is at or near the thermostat setpoint (78 downstairs and 81 upstairs). So I am looking at a deltaT of 23 upstairs and 20 downstairs. Too high?

Last edited by Coolbreeze510; 07-05-2020 at 11:16 AM.
Coolbreeze510 is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 07-05-2020, 11:28 AM   #4
Member
 
user_12345a's Avatar
 
Join Date: Nov 2014
Location: Canada
Posts: 12,444
Rewards Points: 46
Default

Re: HVAC Cooling Performance


600 sq ft per ton rule of thumb is bs. There are no sizing rules of thumb that actually work universally.

This being said, the 5 ton single stage was a terrible choice by the builder - probably too large for the entire home and way too much for a smaller zone.

They should have put in a smaller 2-stage - like the XC16, or a separate small system for the upstairs.

The 2-stage can be locked on low by the zoning panel until both zones call for cooling.

You should see if the unit is cycling off during a continuous call for upstairs cooling when there's no call for downstairs. It wouldn't surprise me of the zoning panel is cycling it off based on the discharge sensor to prevent the coil from freezing.

Really, when the air duct system is designed correctly to begin with, unless the second floor has a very different load profile (like far less sq ft vs exposed air, wall with hot attic on the other side) zoning/separate system is not required as long as a small temperature difference can be tolerated.
__________________
I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.
user_12345a is online now   Reply With Quote
Old 07-05-2020, 01:19 PM   #5
Member
 
surferdude2's Avatar
 
Join Date: Nov 2019
Location: Southern Illinois, USA
Posts: 2,946
Rewards Points: 898
Default

Re: HVAC Cooling Performance


It's impossible to use a duct bypass system on a 5 ton a/c, or any size system for that matter, and have it perform properly to cool an area less than 50% of it's normal design load. I tried that a time or two and learned enough to never try it again! Your best plan now should be to stop abusing the unit and leave all ducts open, except the bypass. You can flip-flop between which thermostat you want to be in control.

There is no great way to fix what you have. It was a mistake from the start and will require major changes to have it work efficiently and with proper comfort.
__________________
Ah, but I was so much older then...I'm younger than that now
surferdude2 is offline   Reply With Quote
Old 07-05-2020, 01:39 PM   #6
Member
 
user_12345a's Avatar
 
Join Date: Nov 2014
Location: Canada
Posts: 12,444
Rewards Points: 46
Default

Re: HVAC Cooling Performance


^Why people think it's okay to zone single stage equipment and without variable speed ecm to get proper airflow under changing conditions is really beyond me.
__________________
I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.
user_12345a is online now   Reply With Quote
Old 07-05-2020, 02:00 PM   #7
Newbie
 
Join Date: Jul 2020
Posts: 4
Rewards Points: 8
Default

Re: HVAC Cooling Performance


Thanks to all for the replies. I am sad to hear there is no relatively simple fix for my system. Just a couple of general questions to help me understand things:

1. If I were to opt to replace my current system with a 2 stage system, would that require replacing just the outside compressor or would that entail replacing the air handler in the attic also?

2. If this can be answered, how long does it typically take for the system to reach peak cooling and dehumidification efficiency from the time it first starts a cycle?

Thanks again,
Bill
Coolbreeze510 is offline   Reply With Quote
Old 07-05-2020, 02:05 PM   #8
Member
 
user_12345a's Avatar
 
Join Date: Nov 2014
Location: Canada
Posts: 12,444
Rewards Points: 46
Default

Re: HVAC Cooling Performance


Depends on the air handler - what's the full model number.

Would probably need a new zoning panel too if the builder cheaped out.

It can easily take up to 15 minutes of run time to reach full capacity and efficiency.

You may want to try disabling the zone dampers and upstairs or downstairs thermostat for a couple of days. If the dampers are normally open (make sure they are), the system will function effectively as one zone. The bypass damper should be in totally closed when doing this test.

As a single zone it won't short cycle as much.
__________________
I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.
user_12345a is online now   Reply With Quote
Old 07-05-2020, 02:10 PM   #9
Member
 
sktn77a's Avatar
 
Join Date: May 2009
Location: Chapel Hill, NC
Posts: 642
Rewards Points: 1,014
Default

Re: HVAC Cooling Performance


What air handler do you have? Is it variable speed? If so, you may be able to set it to cool based on humidity (which slows down the fan speed in order to increase latent capacity and reduce sensible capacity). Your current system is a compromise but the only area I see it really lacking is in dehumidification. I wouldn't go changing it if you can get the humidity down to 50% or below.
sktn77a is offline   Reply With Quote
Old 07-05-2020, 02:17 PM   #10
Member
 
user_12345a's Avatar
 
Join Date: Nov 2014
Location: Canada
Posts: 12,444
Rewards Points: 46
Default

Re: HVAC Cooling Performance


Slowing the fan when there's already a bypass damper and 5 tons try to cool a <1000 sq ft second floor will more cycling due to shutting down on low discharge air temp or if there's no DATS, coil freezing.

This system needs all the airflow it can get.
__________________
I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.
user_12345a is online now   Reply With Quote
Old 07-05-2020, 03:27 PM   #11
Newbie
 
Join Date: Jul 2020
Posts: 4
Rewards Points: 8
Default

Re: HVAC Cooling Performance


Model number of the air handler is C33-50/60C-2F-6. (Is that the air handler? There is a gas fired furnace below it. I could not find any other model numbers.)

The zone controller is a Honeywell HZ311.

I ran the system to cool the upstairs only by setting the Tstat way down (downstairs way up). After 36 minutes of runtime, the maximum dT between one of the room outlets and return was 24.7 with the outlet temperature of 54.5. The outside compressor never shut off during this time. It looks like to outlet temperature curve started to flatten out after about 12 minutes but was still slowly dropping.

I would be willing to try the system as a single zone for a while to see how the system functions. What is the easiest way to disable the zone dampers? Looks like it might be to disconnect them from the zone controller. The dampers ARE normally open.

Thanks again for all of the feedback. You all are great!

Last edited by Coolbreeze510; 07-05-2020 at 03:55 PM.
Coolbreeze510 is offline   Reply With Quote
Old 07-05-2020, 05:38 PM   #12
Member
 
user_12345a's Avatar
 
Join Date: Nov 2014
Location: Canada
Posts: 12,444
Rewards Points: 46
Default

Re: HVAC Cooling Performance


C33 is an evaporator coil, do you have a furnace?

If you do, post the full model number.

You have to measure the temperature drop between a couple of feet away from the coil outlet and the return going into the air handler/furnace.

You also have to measure the wetbulb return air temperature to determine the target supply temp. The wetbulb takes humidity into account and you get it by wrapping the end of the thermometer in moist thin fabric before inserting it into the duct.

You may not have a discharge air temp sensor and it even if you do, the panel may not be set up to cut off the condenser when the supply air gets too cold or hot.

-----------------
The HZ311 is a single stage panel and would have to be replaced if you upgrade to 2-stage.

If you have a furnace, it's probably single stage too and you'll have short cycling issues in the winter.

May be a good idea to just pull the plug on zoning for now since the builder cheaped out.
__________________
I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.

Last edited by user_12345a; 07-05-2020 at 05:41 PM.
user_12345a is online now   Reply With Quote
Old 07-05-2020, 06:49 PM   #13
Member
 
user_12345a's Avatar
 
Join Date: Nov 2014
Location: Canada
Posts: 12,444
Rewards Points: 46
Default

Re: HVAC Cooling Performance


"
You may not have a discharge air temp sensor and it even if you do, the panel may not be set up to cut off the condenser when the supply air gets too cold or hot."

can't edit - shouldn't say "or hot" in reference to cutting off the condenser.

The DATs can be used to cut the furnace burners or heat strips if the supply air gets too hot in heating mode.
__________________
I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.
user_12345a is online now   Reply With Quote
Old 07-05-2020, 07:23 PM   #14
Member
 
Join Date: Oct 2010
Posts: 322
Rewards Points: 344
Default

Re: HVAC Cooling Performance


The Delta-T is too high which causes the short cycle time and high humidity. Try to get the sub cool or super heat numbers to make sure you don't over charge the system.
clocert is offline   Reply With Quote
Old 07-05-2020, 07:35 PM   #15
Member
 
user_12345a's Avatar
 
Join Date: Nov 2014
Location: Canada
Posts: 12,444
Rewards Points: 46
Default

Re: HVAC Cooling Performance


if the return air is dry a 24f drop can actually be normal, we don't have enough info to jump to conclusions yet.
__________________
I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.
user_12345a is online now   Reply With Quote
Sponsored Links
Advertisement
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

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





Top of Page | View New Posts