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  1. #92
    Join Date
    Aug 2012
    Location
    Florida Space Coast
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    503
    Quote Originally Posted by udarrell View Post
    Maybe it is getting attic air leaked into the return air; then a clean filter would allow an increased evaporator airflow heat-load to possibly trip the head pressure shut-down; it's even more apt to happen, due to hot moisture loaded attic air, if the system is overcharged or has a lot of non-condensibles in the refrigerant system.
    If it's true that "increased evaporator airflow heat-load" from hot, moist attic air could trip the switch, you may be onto something!

    Although I had the duct leaks identified and sealed, including my one return directly through the wall under the ahu stand, one contractor who later did a Manual J and further leak testing (in preparation for his new equipment quote) determined there is still a 17CFM leak at the return that needs to be sealed. This space under the ahu is "unfinished", that is, the wall studs below the top of the stand are not covered by drywall. The contractor who sealed the duct leaks attempted to "seal" the space with ductboard, but apparently his efforts were not 100% effective. I would guess the 17CFM of leakage would allow attic air to leak into the return.

    With a less restrictive MERV 8 filter, the suction from the return would pull more air in from the conditioned space in the hallway where the return filter grill is. With a more restrictive MERV 10 (Honeywell 4") or MERV 11 (1" pleated) filter, suction from the hallway would be restricted and more air would be pulled in through the "leak" from the hot attic.

    Given the poor, but constant, condition of the condenser coil, this added heat load might just be enough to trip the high pressure switch.

    Does that make sense?

  2. #93
    Join Date
    Aug 2012
    Location
    Florida Space Coast
    Posts
    503
    Quote Originally Posted by 54regcab View Post
    If the bill is only $130 on a 3 ton unit either the person has low electric rates or an oversized AC unit. Figure 12+ hrs per day run time in a peak month for a properly sized unit at a 3,000Watt draw. The AC alone would use 1080KWH per month if sized to run 12hrs per day in a peak month. At 10 cents per KWH that's $108 for AC alone. This doesn't include any other use in the house or the base meter charges/taxes/fees.
    Dan has the same rates as I do through FPL, pretty close to the same weather conditions, and about the same size house. I believe I have a higher heat load from 6 sets of sliding glass doors and an open beam ceiling, but I also have an old, decrepit 3.5 Ton Rheem and a $250 electric bill in August. Everyone keeps telling me 3.5 Ton is oversized and I'd have a lower bill if I'd downsize to a 3 Ton. I'm seriously considering taking the plunge to a 3 Ton and I'd like to know what system Dan has that gives him a high bill of only $130.

  3. #94
    Join Date
    Aug 2012
    Location
    Florida Space Coast
    Posts
    503
    Quote Originally Posted by Florida Joy View Post
    Everyone keeps telling me 3.5 Ton is oversized and I'd have a lower bill if I'd downsize to a 3 Ton. I'm seriously considering taking the plunge to a 3 Ton and I'd like to know what system Dan has that gives him a high bill of only $130.
    Another comparison I'm considering is my neighbor's house to the NW. Same basic house, exact same layout with a screened room in the back corner that's been enclosed and converted to conditioned space, just like mine. Only three major differences... (1) instead of 3 sliding glass doors that open directly onto the yard on the NE side, they have french doors that open to a covered and screened porch along the back of the house that provides morning shade, (2) they recently installed solar panels on the roof to power the hot water heater, and (3) they have a 3-ton Comfortmaker heat pump that's about as old as my 3.5-ton Rheem. It's also about as corroded as mine, but at least it's still upright because it's on a full concrete slab. Oh, there is one more important difference... they both work every day... and judging from the compressor runtime during the day, they must be turning the tstat up quite a bit when they're not home.

    I haven't asked what their electric bills are running, but judging from the current reading on their meter, each of their last 4 bills have averaged $95 less than mine. The meters on my street were changed out for Smart Meters, one by one, all on the same day this past June, so my guestimate of their bills is based on the current difference in our two meter readings, representing almost exactly 4 months worth of usage.

    I'd love to save $100 a month or so during the hottest 5 or 6 months of the year!

  4. #95
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,158
    The 3 ton would be more apt to loose a little ground during the hottest afternoons, but would catch up when the sun went down. It would lead to a long run cycle which is optimum for high efficiency/low power bills. As we discussed before it's doubtful your 3.5ton unit is actually delivering 3.5tons, most AC units do NOT deliver their rated capacity. 75% of rated capacity isn't untypical, especially on the larger units.

  5. #96
    Join Date
    Aug 2012
    Location
    Florida Space Coast
    Posts
    503
    Quote Originally Posted by 54regcab View Post
    If the bill is only $130 on a 3 ton unit either the person has low electric rates or an oversized AC unit. Figure 12+ hrs per day run time in a peak month for a properly sized unit at a 3,000Watt draw. The AC alone would use 1080KWH per month if sized to run 12hrs per day in a peak month. At 10 cents per KWH that's $108 for AC alone. This doesn't include any other use in the house or the base meter charges/taxes/fees.
    Given that scenario, my high bill would run around $168. That would be great! Then if I left the house during the day... say, if I got a "job"... I could use setbacks and save more. Probably not gonna happen, but it is something to think about.

    What do you figure the 12 hour daily cost of A/C alone should be for a 3.5 ton?

  6. #97
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,894
    Quote Originally Posted by Florida Joy View Post
    If it's true that "increased evaporator airflow heat-load" from hot, moist attic air could trip the switch, you may be onto something!

    Although I had the duct leaks identified and sealed, including my one return directly through the wall under the ahu stand, one contractor who later did a Manual J and further leak testing (in preparation for his new equipment quote) determined there is still a 17CFM leak at the return that needs to be sealed.

    This space under the ahu is "unfinished", that is, the wall studs below the top of the stand are not covered by drywall. The contractor who sealed the duct leaks attempted to "seal" the space with ductboard, but apparently his efforts were not 100% effective. I would guess the 17CFM of leakage would allow attic air to leak into the return.

    With a less restrictive MERV 8 filter, the suction from the return would pull more air in from the conditioned space in the hallway where the return filter grill is. With a more restrictive MERV 10 (Honeywell 4") or MERV 11 (1" pleated) filter, suction from the hallway would be restricted and more air would be pulled in through the "leak" from the hot attic.

    Given the poor, but constant, condition of the condenser coil, this added heat load might just be enough to trip the high pressure switch.

    Does that make sense?
    Yep, makes absolute sense; your reasoning is on target!

  7. #98
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,702
    Quote Originally Posted by Florida Joy View Post

    With a less restrictive MERV 8 filter, the suction from the return would pull more air in from the conditioned space in the hallway where the return filter grill is. With a more restrictive MERV 10 (Honeywell 4") or MERV 11 (1" pleated) filter, suction from the hallway would be restricted and more air would be pulled in through the "leak" from the hot attic.
    Wow. Another indication of insight at a very high level. Beginning to intuit how variation in static can impact leakage, and that this leakage may not be a linear relationship.

    Now connect that concept to equipment size. It's the same concept from a little different orientation. The 4 ton pump is like installing a permanently more restrictive filter.

    1600 cfm airflow requirement vs 1200 cfm airflow requirement means a whole lot more pressure on your whole system. Your "system" includes rooms, as well as duct, and all these things leak "liquid air" in humid climates. These are leaks that will never be 100% cured.

    Rooms that have more supply than return will be pressurized. Like blowing into a paper bag, they will leak to outside. Blowing harder makes the leakage increase. The opposite effect occurs to areas that are return dominant, air from outside will be sucked in.

    Measuring current ESP will help you understand if that increase represents no, little, or catastrophic design flaw. If the 3.5 ton fan is set at 1400 cfm, and has .5 static or above, don't go up on equipment without duct redesign and replacement.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  8. #99
    Join Date
    Aug 2012
    Location
    Florida Space Coast
    Posts
    503
    Quote Originally Posted by Florida Joy View Post
    With a less restrictive MERV 8 filter, the suction from the return would pull more air in from the conditioned space in the hallway where the return filter grill is. With a more restrictive MERV 10 (Honeywell 4") or MERV 11 (1" pleated) filter, suction from the hallway would be restricted and more air would be pulled in through the "leak" from the hot attic.

    Given the poor, but constant, condition of the condenser coil, this added heat load might just be enough to trip the high pressure switch.

    Does that make sense?
    Quote Originally Posted by udarrell View Post
    Yep, makes absolute sense; your reasoning is on target!
    I just realized my original post has some misinformation and may be misleading. I said I was "using" Filtrete MERV 11/MERV 12 pleated filters. That's not exactly true. Actually, I had "purchased" 3 of the MERV 12's and had a MERV 11 stored in the garage, with the "intent" of using them when I replaced the last of the Flanders MERV 8's that I'd been getting from Home Depot.

    When I picked up the Honeywell fc40r, I returned the MERV 12's for credit, but I couldn't return the MERV 11, which had been purchased over 3 years ago in VA.

    From now on, I'll use MERV 8 or lower until I get a new system with a second return.

  9. #100
    Join Date
    Jul 2007
    Location
    Delaware
    Posts
    3,824
    If you took the filter OUT, you would STILL be pulling air in through a duct leak in the unconditioned space, that difference is not tripping the high pressure switch on the outdoor unit.
    Always here

  10. #101
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,923
    Are we no longer concerned with the high pressure switch tripping?
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  11. #102
    Join Date
    Jul 2007
    Location
    Delaware
    Posts
    3,824
    Evidently? Kinda just fell off the table.....................
    Always here

  12. #103
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,008
    No we seem to be more concerned about merv rating on filters and avoiding actually resolving anything.

  13. #104
    Join Date
    Aug 2012
    Location
    Florida Space Coast
    Posts
    503
    Quote Originally Posted by energy star View Post
    If you took the filter OUT, you would STILL be pulling air in through a duct leak in the unconditioned space, that difference is not tripping the high pressure switch on the outdoor unit.
    Yes, I know that. The 17cfm leak will be fixed when new equipment is installed. No point in fiddling with the ductboard "seal" in the return that's only been there a couple of months when it will all be replaced anyway. It leaks a lot less now with ductboard than it did for the 32 years since the house was built.

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