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Help with AC electricity usage?
Old 05-22-2010, 03:28 PM   #1
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Help with AC electricity usage?

I a hoping that one of the engineers here can help. In the continuing saga of central air estimates, we had an estimate from a contractor on a mini split ductless system. Gross was 16k, net of tax rebates and whatnot is 14k. Uh, not. However, they can install a system that will cool the lower floor of the house (where we spend most of our time in the hottest part of the day) for $3.5k after rebates/incentives. This system would be an 18,000 BTU system with a SEER rating of 19. The alternative is two wall units. One would be a 12,000 BTU unit at about 11 EER and the other would be a 5,000 BTU unit that operates less frequently with an EER of 8.

Electricity is quite expensive here (north of $.20/kwh), so I am trying to figure out if the ductless system is worth doing downstairs if it consumes a lot less power. I cannot figure out how to estimate the power consumption of these two alternatives. Anyone know how to do this or where to look?
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Old 05-22-2010, 05:31 PM   #2
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Far from an engineer, but

Air Conditioner Unit Ratings (AC, A/C) - Air Conditioning Helper

Should be able to get the seer ratings, then use this guide (formula near the bottom of page).

WEll, for $700 you could put in two 13,000BTU costco unitshttp://www.costco.com/Browse/Product.aspx?Prodid=11527877&search=air%20conditio ner&Mo=3&cm_re=1_en-_-Top_Left_Nav-_-Top_search〈=en-US&Nr=P_CatalogName:BC&Sp=S&N=5000043&whse=BC&Dx=m ode+matchallpartial&Ntk=Text_Search&Dr=P_CatalogNa me:BC&Ne=4000000&D=air%20conditioner&Ntt=air%20con ditioner&No=0&Ntx=mode+matchallpartial&Nty=1&topna v=&s=1#reviews

Buy a lot of juice for the price difference....
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Old 05-22-2010, 06:23 PM   #3
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I do not know how much you are currently paying for cooling each season, but the SEER is inversely proportional to the electricity usage. A unit with a SEER of 18 will use 1/2 the energy of one with a SEER of 9.

Looks like calmloki's frugal suggestion is a winner, though I did not see the SEER of that Costco unit. I looked at some of them in stores out of curiosity, and saw that most of them were around 12, if I remember correctly.
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Old 05-22-2010, 06:28 PM   #4
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How about a tree?
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Old 05-22-2010, 07:41 PM   #5
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Thanks, calmloki, exactly what I needed! Looks like best case is that I would save $150 a year. Pass on the single floor system, them. Looks like I will be shopping for a "window shaker" soon.

As for trees, you would not believe it if you saw it. In the front yard is a monstrous oak and a large sycamore that shade half the house (by towering over it). In the back yard is another monstrous oak that towers over the house, supplement by a couple large maples and a tulip poplar (tallest tree that grows in NJ). The house is very well shaded except for on the north side where it is not a big deal.
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Old 05-22-2010, 09:46 PM   #6
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As for trees, you would not believe it if you saw it. In the front yard is a monstrous oak and a large sycamore that shade half the house (by towering over it). In the back yard is another monstrous oak that towers over the house, supplement by a couple large maples and a tulip poplar (tallest tree that grows in NJ). The house is very well shaded except for on the north side where it is not a big deal.
With all that shade, you might need to focus on humidity reduction more than cooling. Not too familiar with NJ climate, but I'm assuming humidity is an issue in the summer, as it is here in N IL.

I read recently that the high SEER units don't remove as much humidity as the lower SEER units. I saw some specs on pints/hour so you might want to check that out.

My Mom's house has a lot of shade, and I know she is turning the A/C down to ridiculous levels (68F) to get the humidity out. But I also think their A/C is over-sized, which doesn't help.

-ERD50
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Old 05-22-2010, 10:13 PM   #7
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ERD50, your mother may want to get a dehumidifier or two.

One of my brothers has them bcause he has a piano and other musical instruments he doesn't want getting soggy and warped.

They work well, and would save a lot of power if he didn't like his house ridiculously cold by my standards

ta,
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Old 05-23-2010, 07:54 AM   #8
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I'm checking out the portable AC units too. BJs Wholesale club has a couple for a similar price. They have a dehumidifying only setting too.

Don't think I want to spring for the central AC or split ductless unit.
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Old 05-23-2010, 09:42 AM   #9
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Originally Posted by ERD50 View Post
...
I read recently that the high SEER units don't remove as much humidity as the lower SEER units. I saw some specs on pints/hour so you might want to check that out.
...
-ERD50
Why would that be? Do the higher SEER units operate with a lower temperature differential at the room air/evaporation coils?
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Old 05-23-2010, 10:35 AM   #10
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FWIW

In air conditioning, smaller is much better. In northern latitudes they should be sized to run nearly constant at the maximum outdoor temperature to maintain whatever indoor temp is desired. The reason is that in a medium/well shaded situations the main objective is to remove humidity.

A large unit (oversized in most homes) cools the air down quickly, but does not remove much if any humidity. So it cycles briefly, often. Starting currents for compressors etc. are fairly substantial. Can have great SEER and throw it all away with compressor short cycling.

So Calmloki's suggestion is great. As for a dehumidifier, bad idea in the summer, unless you really like heat. They generally make about 1.5 times the amount of heat of their electricity consumption. Ie: 1 Kwatt in 1.5 Kw heat generated. Dehumidifiers make pretty efficient electric heaters, get more heat than paid for.

Might consider heat load calculations, then choose a unit SMALLER than what the heat load is. Contractor's like to sell big ones "cools real good". Then you wonder why you feel cold and clammy. And still are miserable.

YMMV.
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Old 05-23-2010, 12:06 PM   #11
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ERD50, your mother may want to get a dehumidifier or two.
She does have one in the basement, might be good to add one upstairs. I think at this point she is just 'dealing with it', I don't think she's too interested in changing anything. It's only a few times a year that it is a big problem.

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I'm checking out the portable AC units too.
Just check that they are quiet enough for your needs. Seems I've read they are fairly loud (makes sense, the entire compressor is inside, rather at least hanging out the window).

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Why would that be? Do the higher SEER units operate with a lower temperature differential at the room air/evaporation coils?
That was the explanation I read. The coils are not as cold, so less condensation on them. Makes sense, getting them colder would waste energy. OTOH, the longer run time (or larger total coil surface) with slightly warmer coils would mean more air exposed to the coils. Again, one should check the specs on the units they are considering, it could vary unit-to-unit. I bet it varies with conditions too.

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FWIW

In air conditioning, smaller is much better.
Agreed, I just don't think she wants to do anything about it at this point - I'd probably be the same at 85 YO.

Quote:
They generally make about 1.5 times the amount of heat of their electricity consumption. Ie: 1 Kwatt in 1.5 Kw heat generated. Dehumidifiers make pretty efficient electric heaters, get more heat than paid for.
Sounds like the answer to the energy problem! A heat pump can move more than their energy consumption, but a de-humidifier is a closed system, the heat generated cannot be more than the energy input.
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Old 05-23-2010, 02:50 PM   #12
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Sounds like the answer to the energy problem! A heat pump can move more than their energy consumption, but a de-humidifier is a closed system, the heat generated cannot be more than the energy input.
Ahem, there is free lunch to be had.

Strictly speaking you are correct, nevertheless a dehumidifier ( a heat pump) running with moisture present will make a nice heater. I was a bit conservative at 1.5x,
I was still thinking of 20 year old dehumidifiers.


......"The most common heat pump efficiency measurement is called the Coefficient of Performance, or COP. COP is the ratio of the heat pumpís BTU heat output to the BTU electrical input, as shown in this equation:
COP = Heat Delivered (BTU/Hour) Divided By Electrical Input(Watt)x3.413 BTU/Watt-Hour

Conventional electric resistance heaters have a COP of 1.0. This means it takes one watt of electricity to deliver the heat equivalent of one watt.

Air-source heat pumps generally have COPs ranging from 2 to 4; they deliver two to four times more energy than they consume. Water and ground source heat pumps normally have COPs somewhere between 3 and 5.
The COP of air-source heat pumps decrease as the outside temperature drops. Therefore, two COP ratings are usually given for a system: one at 47 and the other at 17. When comparing COPs, make sure ratings are based on the same outside air temperature." ....

Cheers.
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Old 05-23-2010, 03:35 PM   #13
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Ahem, there is free lunch to be had.

Strictly speaking you are correct, nevertheless a dehumidifier ( a heat pump) running with moisture present will make a nice heater. I was a bit conservative at 1.5x,
I was still thinking of 20 year old dehumidifiers.


......"The most common heat pump efficiency measurement is called the Coefficient of Performance, or COP. COP is the ratio of the heat pumpís BTU heat output to the BTU electrical input, as shown in this equation:
COP = Heat Delivered (BTU/Hour) Divided By Electrical Input(Watt)x3.413 BTU/Watt-Hour
[B]
Conventional electric resistance heaters have a COP of 1.0.
Sorry, still no free lunch.

You are correct that the dehumidifier will produce heat, but it won't produce more than what you put in.

Those definitions of COP are a bit sloppy. The word 'delivered' is misleading in this case. It refers to the amount of heat energy moved from one area to the other. In the case of those water and ground source heat pumps with a COP of 3 - 5, it means it can move 4 units of energy for every 1 unit consumed. But it moves the energy from the ground to your house (or vice versa).

Put a 1KW dehumidifier (or heat pump) in a closed room, and it will produce the same amount of heat as a 1KW resistor. It can't produce any more or less heat than that. Let a heat pump draw heat from another source (the ground), and it can deliver more energy than it consumes. It is not a free lunch physics wise, as the heat is 'used' from the ground. But to the extent that we don't pay for the heat in the ground, it is a cheap lunch, but there are installation costs.

I like to think of a heat pump as a diesel delivery truck. It can move 10,000 gallons (or whatever) of diesel across town, and only burn maybe 100 gallons in the process. But that doesn't mean it is 10,000% efficient, or that it is producing energy. It is only moving that energy.

It's not a contradiction, just confusing terms.

-ERD50
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Old 05-23-2010, 04:33 PM   #14
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Noise is an issue for me. The cycling of the window units wakes me up. So, I end up turning it all the way up and that freezes me out, and forms condensation on all windows.

Not sure what the best solution is. $8700 for central air is pretty steep. I should get a quote for a split ductless.
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Old 05-23-2010, 06:19 PM   #15
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Noise is an issue for me. The cycling of the window units wakes me up.
Have you tried the low-tech stuff? Maybe install the window unit in another room (where it wouldn't wake you up) and push the air into your bedroom with a box fan in the doorway at night? If you've got a forced-air heating system you could also run the fan to help equalize the temps throughout the house (and run the window unit in one of the rooms near a return). Just running the fan can help a lot with an upstairs bedroom.
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Old 05-24-2010, 10:52 AM   #16
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I have forced hot water heat, so no dice on the ducting. That was one of the things I was considering central AC for. A whole house fan and a way to run it in the winter to equalize temps too.
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Old 05-24-2010, 01:28 PM   #17
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Electricity is quite expensive here (north of $.20/kwh), so I am trying to figure out if the ductless system is worth doing downstairs if it consumes a lot less power. I cannot figure out how to estimate the power consumption of these two alternatives. Anyone know how to do this or where to look?
Can't help with the engineering, but a reaaaallllly low tech approach to saving ELECTRICITY is one I used to use in the midwest which is often hot and sticky in summer. We had central air - it was new so relatively efficient but not up to today's standards, I'm sure. As the outside air cooled at night, we opened the house and used 3 or 4 window fans to draw the cool air into and through the house. We slept under a ceiling fan and had ceiling fans all over the house. In the AM, as it began to warm up, we closed up the house and enjoyed the "stored" cool air and used our ceiling fans (worth about 5 degrees!). If it DID get too hot or humid later in the day, we used the AC to knock down the humidity (and temp). Rinse, repeat. Some summers, we used the AC perhaps a dozen full days (you know it's going to be an AC day when the night temp never gets below 70). Otherwise, we didn't use it more than an hour or two each day. Most days, we didn't use it at all. Our elec. bills (about $.13/kwh) were usually $50 to $70/mo during summer (gas water heater).

The house was 2800 SF 4 BR

Good luck and of course, YMMV.
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Old 05-24-2010, 02:14 PM   #18
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Can't help with the engineering, but a reaaaallllly low tech approach to saving ELECTRICITY is one I used to use in the midwest which is often hot and sticky in summer. We had central air - it was new so relatively efficient but not up to today's standards, I'm sure. As the outside air cooled at night, we opened the house and used 3 or 4 window fans to draw the cool air into and through the house. We slept under a ceiling fan and had ceiling fans all over the house. In the AM, as it began to warm up, we closed up the house and enjoyed the "stored" cool air and used our ceiling fans (worth about 5 degrees!). If it DID get too hot or humid later in the day, we used the AC to knock down the humidity (and temp). Rinse, repeat. Some summers, we used the AC perhaps a dozen full days (you know it's going to be an AC day when the night temp never gets below 70). Otherwise, we didn't use it more than an hour or two each day. Most days, we didn't use it at all. Our elec. bills (about $.13/kwh) were usually $50 to $70/mo during summer (gas water heater).

The house was 2800 SF 4 BR

Good luck and of course, YMMV.

It might work where you were... but not here in Houston... during a heat wave it might not get below 80... and it is VERY humid at night... so what you bring in is very wet 'coolish' air... that feels 'clamy'.... so you have to run your AC to get the humidity out... costing you more...

Now, it does work in the spring and fall...

However, my wife does not like the ceiling fans for some reason.... oh well...
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Old 05-24-2010, 06:24 PM   #19
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I've found the folks at HVAC-Talk: Heating, Air & Refrigeration Discussion - Powered by vBulletin to be pretty helpful, there are quite a few pros on the board - although their stock retort to many questions seems to be "what did the manual J calculation say?"
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Old 05-24-2010, 08:01 PM   #20
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I've never had any kind of air-conditioning. I am visiting in southern Iowa where it is STILL above 90 degrees today and really humid. There is central air. It is amazing. Quiet. Cool. Lovely. I'd pay big bucks for it if I had to be in hot country.
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