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  1. #14
    Join Date
    Jun 2011
    Location
    Virginia
    Posts
    9
    It seems that my A/C is oversized based upon my observations of how the system has been operating. I've been timing the runtimes in the afternoon when I had the opportunity over the past two weeks. I have a Honeywell RTH7500 thermostat set at 75*F. When the outside temperature is at or a few degrees above the outside design temperature of 92*F, I get a runtime of ~15 min. and an off time of ~5min. I did manage to catch one of the +100*F days and the runtime was ~20min with an off time 5min.

    When it comes to humidity, I am lucky if I see anything less than 60% RH. Right now I am blaming my short runtimes for the high humidity. I have seen a number of people on here post their RH at something substantially less than 50%. Is it possible to hit these low RH numbers in areas with hot humid summers without supplemental dehumidification and a properly sized A/C unit?

  2. #15
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,501
    My unit will get under 50% with the blower on low speed provided temps are over 85 outside. My thermostat automatically selects blower speed based on indoor humidity.

  3. #16
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,974
    If I recall from my times in Oklahoma City and nearby Norman, your humidity is not as bad as it is in the Mid-Atlantic area where the OP is located in Virginia.

    A key factor of course is having a heat load, which is why temps need to be higher in order to dehumidify properly.

    In the VA area, the temps need to stay at 85 degrees just to maintain 60% humidity with a standard system.

    This is why it really helps to have a variable speed blower that can be tuned down to below 300 cfm per ton during humid times.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  4. #17
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,501
    Quote Originally Posted by RoBoTeq View Post
    If I recall from my times in Oklahoma City and nearby Norman, your humidity is not as bad as it is in the Mid-Atlantic area where the OP is located in Virginia.

    A key factor of course is having a heat load, which is why temps need to be higher in order to dehumidify properly.

    In the VA area, the temps need to stay at 85 degrees just to maintain 60% humidity with a standard system.

    This is why it really helps to have a variable speed blower that can be tuned down to below 300 cfm per ton during humid times.
    My systems is set for 1000CFM low/1400CFM high on a 3 ton system. 1F95-1291 thermostat switches the SPDT relay as needed depending if humidity is over/under 50%. Not as good as a true VS system but the modification + thermostat works well considering for a low cost alternative. Humidity is definitely not as bad here as the coastal areas, I lived in SC for 10 years.

  5. #18
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,861
    i would go with a 2 zone 2 stage cool and 2stage heat 95 percent furnace and possible duel fuel if propane for sure dual fuel

  6. #19
    Join Date
    Mar 2010
    Posts
    195

    HEY, WE HAVE THE SAME HOUSE

    Quote Originally Posted by Carl_H View Post
    I've been doing some reading here and decided to sign up since

    I am doing early research into replacing my existing system and I am trying to determine what the best balance would be between cost and features. I am trying to get some information to have some idea of what I should be looking for before having somebody come out and give me estimates. At least while I have time as an option when deciding on what to do.

    Current HVAC:
    17 years old
    80k BTU York Diamond 80 Furnace
    3 Ton York Condenser (SEER 11)

    House:
    2000 sq foot
    2 floors + unfinished basement.
    Location:
    Virginia

    Here is what I am looking for in a replacement
    1. Reliability
    2. Improved temperature balance between 1st and 2nd floors.
    3. Less noise from the Furnace. Most of the noise is from the burners and draft inducer.
    4. Same or less noise for A/C.

    I'd like to improve efficiency on the A/C and air handler (ECM blower motor), but I don't think improving the furnace efficiency is on the table since using anything other than the shared b-vent may be problematic.

    Problem with the current system:
    During cooling season the temp difference between 1st and 2nd floor is 3-4 degrees plus one more degree difference for the master bedroom. Placing a fan at the bottom of the stairway during cooling season reduces the difference to 1.5 degrees between the 1st and second floor. The fan probably does not move much is any cold air upstairs and I think it is mostly preventing convection. I have not taken any measurements during the heating season, but the master bedroom is noticeably colder than the rest of the house. The problem with the master bedroom is that it is larger than the other rooms, has more windows, and has probably same amount of supply air as the other bedrooms.

    Possibly zoning with 2 or 3 zones. Maybe Carrier Infinity since it is integrated, right? I'd rather have a one vendor solution if possible. Or is the Infinity system potentially overkill and overspending. I really have no idea what the cost difference is between the different systems. This year we had a very wet spring and although the temp was lower than the A/C set point some dehumidification was need since the humidity was way to high. I had to turn down the thermostat just to make it comfortable, cooler with but still with high humidity. This might be one of those instances where getting something like the Carrier Infinity is probably worth it.

    I may be doing some other improvements before replacing the furnace to see if that helps the improve the situation in the master bedroom, such as low-e windows. My current system is working fine with the exception of the problems noted. I hate to replace something that isn't broken, but with the potential of a tax break that may not always be there, plus potentially lower utility bills may be enough incentive.

    I know there is more information needed to give a fully informed opinion, but what would be a good choice when trying to balance cost vs. features? Zoning adds significantly to the cost? I'm thinking of going with Carrier or maybe Trane. Are things like 2 stage heating and cooling worth it? Variable speed blowers?
    Hi,

    looks like we have the same house. I even had the York Steller and the Dimond 80.

    1. Reliability- Top contractors can re-design your system and replace your equipment with top quality brands that will last for years.

    2a. Improved temperature balance between 1st and 2nd floors - The biggest change in comfort we ever experienced was adding a second zone and T-Stat to our upstairs. This was accomplished by adding a trunk through our basement to hit the upstairs as well as additional returns. We have a TStat that has held our upstairs exactly at 74 since the day it was installed. We also run our fan continuously for a perfect IAQ experience. You will love a second zone with your new system.

    2b. Even though our house is slightly smaller and slightly more northern we had the same furnace and a 2.5 ton AC. Since 2 Stage units come in even sizes a 3 ton two stage was installed in my house. This means you will have 2 distinct capacities roughly 36000 BTU/hr and 22,000 BTU/hr. No need for any type of load calc, you now essentially have two units, a two ton that will run most of the time, and a three ton that will kick in when conditions require. These 2 stage units are great at controlling humidity, you can keep your indoor RH right at 50%.

    2c. We were recently required to install a geo system. The 3 ton was replaced by a 4Ton geo system. This was done to keep the unit from going into 3rd stage heating(electric backup) on all but the coldest days. First stage cooling on this unit is around 29,000 BTU's or about 2.5 tons. We have been very happy to have had that second stage as temps around here have been at/near historic highs.For Comfort, focus on equipment that modulates to changing conditions. Your "load" changes every second, no one can tell you what it is at any given time, much better to focus on modulating your capacities.

    3.Less noise from the Furnace. You will find the new modulating furnaces have much less noise. You will notice the improvement. You can get the sound ratings on the equipment from a good contractor.

    4. Same or less noise for A/C - You will also be quite happy with the much lower sound levels on your new 2 stage condenser (So will your neighbors!)
    Of course a Geo unit has no outside equipment at all, so you can sit outside and listen to the crickets, and your neighbors Air Conditioner!

    Key is high end contractor, offering you the choice of high end systems, and in particular looking at your distribution.

    Good Luck ,

    ACBD

  7. #20
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,744
    Quote Originally Posted by Carl_H View Post
    It seems that my A/C is oversized based upon my observations of how the system has been operating. I've been timing the runtimes in the afternoon when I had the opportunity over the past two weeks. I have a Honeywell RTH7500 thermostat set at 75*F. When the outside temperature is at or a few degrees above the outside design temperature of 92*F, I get a runtime of ~15 min. and an off time of ~5min. I did manage to catch one of the +100*F days and the runtime was ~20min with an off time 5min.

    When it comes to humidity, I am lucky if I see anything less than 60% RH. Right now I am blaming my short runtimes for the high humidity. I have seen a number of people on here post their RH at something substantially less than 50%. Is it possible to hit these low RH numbers in areas with hot humid summers without supplemental dehumidification and a properly sized A/C unit?
    Thank you. You proved my point. Not too bad for a "guess".

  8. #21
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,974
    Quote Originally Posted by 54regcab View Post
    My systems is set for 1000CFM low/1400CFM high on a 3 ton system. 1F95-1291 thermostat switches the SPDT relay as needed depending if humidity is over/under 50%. Not as good as a true VS system but the modification + thermostat works well considering for a low cost alternative. Humidity is definitely not as bad here as the coastal areas, I lived in SC for 10 years.
    1400 cfm of air for a 3 ton system is too much air, unless you live in someplace like Phoenix AZ. For humidity issues, drop that air to about 1020 normal and under 900 cfm for over 50% humidity. If there is a concern for coil freezing up due to higher SP with wetter coil, put a freeze stat in place to force the blower to 1020 cfm under 40 degrees.

    Unless you have some major ducting issues, you should not have freezing issues because you will have lowered the overall SP of the system by lowering the blower speed.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  9. #22
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,501
    Quote Originally Posted by RoBoTeq View Post
    1400 cfm of air for a 3 ton system is too much air, unless you live in someplace like Phoenix AZ. For humidity issues, drop that air to about 1020 normal and under 900 cfm for over 50% humidity. If there is a concern for coil freezing up due to higher SP with wetter coil, put a freeze stat in place to force the blower to 1020 cfm under 40 degrees.

    Unless you have some major ducting issues, you should not have freezing issues because you will have lowered the overall SP of the system by lowering the blower speed.
    I simply have the thermidistat switch the SPDT relay on the blower for high/low. The 1000CFM/1400CFM are the rated amounts for the blower @ 0.5" static pressure, although the actual delivered CFM hasn't been measured. The A/C will keep the humidity level in the house in the mid 40's on high blower speed when it's hot/dry out and running a lot. Once temperatures drop, the humidity increases and the thermidistat switches between high and low as needed. Haven't had any cool/rainy weather to see what they system will do on those days, hopefully it just locks in on low speed. "Poor mans variable speed system"

  10. #23
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,974
    Quote Originally Posted by 54regcab View Post
    I simply have the thermidistat switch the SPDT relay on the blower for high/low. The 1000CFM/1400CFM are the rated amounts for the blower @ 0.5" static pressure, although the actual delivered CFM hasn't been measured. The A/C will keep the humidity level in the house in the mid 40's on high blower speed when it's hot/dry out and running a lot. Once temperatures drop, the humidity increases and the thermidistat switches between high and low as needed. Haven't had any cool/rainy weather to see what they system will do on those days, hopefully it just locks in on low speed. "Poor mans variable speed system"
    Then have it only run at 1000 cfm of air and have someone with expericienc in superheat and subcooling adjust the charge while the indoor blower is operating at 1000 cfm of air. You don't have poor man's variable speed or anyman's variable speed, you don't have variable speed at all. What you have is a system that is operating with too much air volume most of the time.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  11. #24
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,501
    Quote Originally Posted by RoBoTeq View Post
    Then have it only run at 1000 cfm of air and have someone with expericienc in superheat and subcooling adjust the charge while the indoor blower is operating at 1000 cfm of air. You don't have poor man's variable speed or anyman's variable speed, you don't have variable speed at all. What you have is a system that is operating with too much air volume most of the time.
    What are the issues caused by too much air volume? I know 400CFM/ton is industry standard, but some building science people are starting to question the "one size fits all" approach to CFM/ton. Other than high humidity (which I'm not getting) what else should I look for? No CFM test has been done, I'm not even sure that my 3.5ton blower is even delivering 1400CFM on high. From what I've read most contractors don't even check actual delivered CFM at the time of installation...

  12. #25
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,974
    Quote Originally Posted by 54regcab View Post
    What are the issues caused by too much air volume? I know 400CFM/ton is industry standard, but some building science people are starting to question the "one size fits all" approach to CFM/ton. Other than high humidity (which I'm not getting) what else should I look for? No CFM test has been done, I'm not even sure that my 3.5ton blower is even delivering 1400CFM on high. From what I've read most contractors don't even check actual delivered CFM at the time of installation...
    400 cfm per ton is the industry standard for rating system capacity and SEER rating just as 95 degrees outdoor ambient and 67 degree indoor wet bulb temperatures are the industry standard for rating systems. They are simply arbitrary numbers chosen that all equipment manufacturers must go by. That makes all systems "relatively" equivelant in their testing for capacity and SEER ratings.

    67 degrees wetbulb at 50% humidity is about 80 degrees drybulb. So, if your thermostat is set to 80 degrees on a 95 degree day and the humidity in your home is 50% and the blower is delivering 400 cfm of air and the refrigerant charge is exactly right, you may be getting the rated SEER rating and capacity from your system.

    Decreasing air volume below 400 cfm of air decreases capacity and SEER rating slightly, but also decreases the temperature of the indoor coil so that it hits dewpoint faster and stays below dewpoint temperature longer. This is where dehumidification comes in.

    Average comfort level for human beings is 72 degrees F. at 40% humidity, but maintaining 50% humidity in humid regions is a good thing. Over 50% humidity we start getting into comfort and health issues with bacterial and mold growth. So, if you are not maintaining 50% humidity, you should be reducing your blower speed as much as can be done safely without causing the coil to freeze up.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  13. #26
    Join Date
    Feb 2009
    Location
    Portland OR
    Posts
    2,029
    Quote Originally Posted by 54regcab View Post
    High end equipment plus a zoning system may price out close to 2 13SEER systems (one for each floor). You could do a small furnace + A/C for downstairs and a heat pump for upstairs.
    Did you maybe mean to put this the other way around? If I do a 2 system house i put the heat pump downstairs(since heat rises and the downstairs system does most of the heating) and the AC upstairs since there is very little need for heat on the second floor in a well insulated home.

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