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HVAC duty cycle Pages: 12

Mitch on Sun August 31, 2003 6:20 PM User is offline

Year: 1998
Make: Trane
Model: 11 SEER Scroll
Engine Size: 3 tons
Refrigerant Type: R22
Ambient Temp: 90F

We talked about HVAC sizing a couple of weeks ago and I just hooked my recording scope to the compressor relay terminals from the thermostat to record the on and off times.

It's not too hot today, and it is 75F inside the house in the coolest room; 77F in the hottest room. With the low humidity that is plenty cool. The unit is 36,000 BTUs; house is 2100 sq.ft. on one floor; 2x6 frame walls and 12" insulation in the attic; dual pane untinted aluminum frame windows.

Two windows and a sliding door on the western exposure. Attached garage covering the entire southern exposure.

The compressor is running less than half of the total time at just about the hottest part of the day with the full sun. I believe that the average voltage divide by the approx. 26V from the thermostat may be the percentage of on time, but I need to figure that out to be certain.

Ignore the time and date on the scope. I haven't gotten around to resetting it.




Edited: Sun August 31, 2003 at 6:25 PM by Mitch

NickD on Sun August 31, 2003 11:54 PM User is offline

Neat scope, recall when you talked about buying it, ha saves on paper for a chart recorder. How many channels can you track?

Ah, duty cycle, on time verus off time where both times add up to cycle time.

Let's see, if you want to lower your electric bill, you would want to decrease your compressor on time and increase your compressor off time.

Installing a much larger system is debatable as it will cool your house down quicker, but burn a lot more electricity in the process. In theory you have to remove the gained BTU and that can be done at the same rate the BTU's are gain or in other words the compressor could run 100% of the time at a low rate and your electric bill would still be the same. The key here is to get a more efficient system and that is the only thing you can do to decrease this part of your electrical bill.

The off time is when the heat builds up to click on your thermostat, I just got in my electric bill for most of August where we had many 90*F days, it was $35.00 above normal meaning that was my cooling cost for last month, I am cooling 4,000 square feet, but have 18-24" of insulation in the attic. A buck a day is worth it to have a cool dry home, but my July bill was only $13.00. If your attic temperatures are hot, you may want to consider adding an exhaust fan. I am using convection with roof long vents, so it doesn't get that hot. My other trick is that I have 70 foot high trees on the east, south, and west side of my home that offer plenty of shade. In the winter, the leaves are gone so I get plenty of sunlight when I need it, that helps with my heating bill. Ha, I am a tree hugger and proud of it.

Mitch on Mon September 01, 2003 12:23 AM User is offline

Forgot to mention that I have a fake chimney that just goes into the attic and is equipped with a thermostatically controlled exhaust fan. I tried to design it so that it always assists the natural convection heat flow. At 9:15 PM, the outside ambient has dropped to 70F, the attic is still at 86F.

I haven't been able to figure out if the additional electricity cost of the attic fan cuts the AC electricity usage by an equal or greater amount. If the attic insulation is doing its job, maybe cooling the attic is wasting juice, not saving it. I probably need a climate controlled wind tunnel to figure it out, meantime the cooler attic temps are probably doing my stored stuff a favor at the very least.

I think the scope can track two channels as a minimum. I need to read the book a little more. This is only the second time I used it. The first time I tracked the garage temp over a couple of days.

Edited: Mon September 01, 2003 at 12:26 AM by Mitch

NickD on Tue September 02, 2003 12:28 AM User is offline

I recall D Andrews telling me that it's common to spray a fine mist of water on the condenser down in MS, is this done in your neck of the woods?

Course, may be wise to use soft acid free water.

By the way, I am please I installed that water softener last January with the chlorine filters. Now I can clean my sinks with a cloth rather than using an air hammer. Also, not getting those corns any more on the bottom of my feet, dishes are spotless, and clothes much cleaner. The unit has been very low maintenance, even jacked up the regeneration period to 3,500 gallons. May jack it up to 4,000.

Bigchris on Tue September 02, 2003 2:59 AM User is offline

Talk to me Nick. I've got well water with a Rainsoft conditioner that contains magic pellets that get rinsed in brine periodically. It does a good job of softening the water but I'm getting what looks like calcium deposits on the fixtures and glasses and in the showerheads. Is there a magic bullet?

Karl Hofmann on Tue September 02, 2003 4:11 AM User is offlineView users profile

The trouble with domestic air con is that they are caculated to handle the loads for the hottest day of the year. Every other day the system will cycle off more and more as the temperature drops, causing variations in your room temp and humidity (Although from the info on your weather station humidity is not a problem for you) and shortening the life of your contactor set each time it cycles.

I have mentioned these before but in Europe inverter systems are catching on big style, they started with small 2 and 3 Kw units but on browsing through my latest Dean & Wood Catalogue I notice that Mitsubishi Electric have launched single phase units that will vary their cooling capacity to suit the ambient. The largest of these units have a range of 6.2Kw to 15.3Kw with a start current of 5 amps and a running current of 20.73 amps, on R410a. Untill this the largest single phase system on Single Phase that I could find was 10Kw (3Ton) after that we must either go to three phase or install two units.

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Never knock on deaths door... Ring the doorbell and run away, death really hates that!

NickD on Tue September 02, 2003 9:32 AM User is offline

The newer furnances are two stage and some are even continuously variable. For my last two furnace purchases, I selected the largest units that would fit and cut down the jets that gives a lot more heat transfer area, a larger blower, and the only precautions are having the pilot sufficiency ignite the burners and having sufficient updraft through the chimney. I was tipped off by this from an HVAC engineer back in the 60's and have seen substancial reductions in my heating bills. AC is a different problem, you just can't reduce voltage to the compressor motor. I buy an oversized A coil, use an extened plentum of my own manufacture to reduce air restriction and a powerful blower with an outside unit to match. On most of the "professional" installations I have seen, the A coil blocks half the plenum area that not only hurts the AC end but the heating end as well. I designed my own blower circuit, have just a four speed blower, but run it at low speed all the time. I dumped my electronic air filter in lieu of a micron filter, that sucker was just generating ozone in the air besides being a bitch to clean. Good plenum design is very important, I run the blower a 3rd speed for heat and hi speed for AC. My outside unit is well shaded, no sense in having the red hot sun beating down on it.

I crack open a basement window in the winter time, would like to get a pulse furnace, but I would in theory only gain about 5% as opposed to what I have, my furance and AC still look like new, if I find a good bargain, may switch. Ha would have to pay a fortune just to get rid of these old ones now.

You can't do anything nowadays without running into tons of BS, gosh this water softener thing drove me nuts. Also prices drove me nuts, buying the best rated components separately can cost a fortune. The price on the Fleck valve I chose cost just as much as the entire system. The BS comes in on the water flow rate, if the water is passing through the media at 8 gpm, it ain't in the media long enough to remove any of the hardness.

The first step is to have your water analysed to find what it is that you want removed. Then to size the unit for the expected flow rate. I went way overboard on the size of the unit as even recommended by my friendly Mennards and Fleet Farm part time sales clerk, wanted 1" pipes, the best media, and a decent price. First I was after an all in one unit, but learned cleaning this thing can be a bitch so I went with the separate salt tank. Hell that plastic garbage can thing is so cheap, could pitch it and replace it if I wanted to. I am also using Mortons System saver salt, after about nine months of use, my brine tank is still clean. I don't fill it to the top, no need to.

The Fleck is fully programmable and has a real simple valve, some valves are a nightmare. I talked to the engineer at fleck and got some good recommendations as to how to set it up, but it's still playing it by ear. Ebay sucks for buying a water softener, more BS in their ads then what they are worth, plus they want a ton for shipping. Some valves required a 60 buck O ring kit, if you can believe that, and would require a rocket scientist to take the valve apart with all of that gear work inside, and all cheap plastic stuff.

Ohio

Is the best company I found that uses all top line components with free shipping and the guy was very helpful, but you have to tell him what you are trying to do with your water. All the stuff came in the the original sealed factory boxes, two boxes of media, the tank, the salt tank, the Fleck 5600SE, and he also gave me a darn good price on a stainless steel 1" bypass valve that came in it's own box. Kind of like buying a DIY kit. The bypass valve made the installation much easier, and I can pull the whole thing out without killing the water. I think I paid around $430.00 for the whole ball of wax including shipping. I am pleased with it.

Mitch on Tue September 02, 2003 9:44 AM User is offline

Some of the new AC compressors are dual capacity. Maybe we need a unit with a POA or and STV.

NickD on Tue September 02, 2003 11:05 AM User is offline

They are sure simple when compared to some MVAC systems, mine has a LPCO and that's about it.

JJM on Tue September 02, 2003 11:36 PM User is offline

I have to say it drove me nuts when you told me your 3 ton unit for that 2,100 square foot house was off more than half the time; I just couldn't believe it! True, you cited superior building construction and low humidity, but something still seemed awry. Now I know what it is: you only have your thermostat set for 75°F, while the ambient was only 90°F (a 15°F drop) and the RH was only 37%! Somehow I think your test results would be quite different if your thermostat was set at 65°F, the ambient was 100°F and the RH was around 65% or more. I think that sucker would be cranking non-stop. Yes, I like it nice and cold in the the summer, 65°F indoors, though in the winter 75°F is comfortable for me.

Your tests, however, only reaffirm my desire for a two-stage compressor, like the fairly new Carrier 38TBD. I also understand York makes one now. In response to Bohica's brilliant suggestion of getting a three-phase compressor and using a phase inverter, so that it would be able to "soft start" with my 12KW standby generator), I contacted Carrier and the 38TBD isn't available in three-phase. I was also hoping there was a way for the compressor to start and run in low speed or lockout the high speed, but Carrier further informed me the compressor must start up in high speed. I guess during a power failure I'll be relegated to window and wall shakers.

I like what Karl mentioned about the inverter type systems that only need 5A to start – ideal for running with a standby generator.

Thanks so much Mitch for taking the time and sharing your test results with us.

Joe

P.S. Why DON'T they have POA valves on central A/C???

Mitch on Tue September 02, 2003 11:53 PM User is offline

65 degrees!!! Joe, are you a penguin?

Everybody has their own comfort level, but 65 has got to be the lower end for most folks. We usually complain about most restaurants etc. being too cold, and 75 seems downright cold at my house.

I just got the recorder hooked up and I'm waiting for the 100+days. Max I have seen is 109F last year and the AC was cycling off routinely at the hottest part of the day. Like I said, 90F was not too hot of a day.

Over the full 24 hours with 90F max, the total on time yesterday was about 25%. On a 100+F day, I believe it will be on about 50% over 24 hours, but I'll have to wait for the heat.

Of course, the unit has to be sized for the hottest part of the day, and mine still has some to go at my comfort level.

bohica2xo on Wed September 03, 2003 12:28 AM User is offline

Joe:

If you are planning to use an inverter, then just pick a good commercial unit. The inverter will more than make up for the lack of a 2 speed compressor. Or build your own unit, you could have inverters on every motor. The new GS1 series inverters are available all the way down to .25 horsepower models..... think of all the things you could complicate!

Around here, there are HVAC units with evaporative coolers in series with the condenser - somewhat better than misting condensers with water, since the cooler pads get the calcium buildup - instead of the condenser. I have considered installing a cooling tower - my rooftop units see 130f+ inlet air on a bad day.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

NickD on Wed September 03, 2003 1:01 AM User is offline

Granted, it's been 25 years since I have fooled with inverters, single phase in, rectified, filtered, microcontroller controlled back to back SCR's for phase generation, over current protected, and all this times three for the three phases. All three had both amplitude and frequency control precisely matched for three phase motors. Efficiencies were in the 85% range, so what was gained with the efficiency of a three phase over a split phase motor was cancelled out. But the big deal back then was that precise speed control for production machinery. Even with three phase coming out of the wall socket, it was fixed frequency and amplitude that also had to be rectified and filtered first.

So I am at a bit at a lost, where is energy being saved, and how much, and what is the payback period? Here we are talking about an AC system and perhaps just amplitude control would be sufficient. Three phase inverters were very expensive back then and I don't feel the technology has changed that much. I just fail to see the gains.

Karl Hofmann on Wed September 03, 2003 3:42 AM User is offlineView users profile

Admittedly inverters do lose a little in efficiency when running flat out, but over here it is very rare for a/c's to be running flat out, most of the time they cycle on and off all day, causing the room temperature and humidity to fluctuate. Inverters will run constantly but at a reduced capacity, eliminating the all or nothing situation that is found on traditional systems with their massive surges on start up and running flat out untill the set temp is achieved. The added bonus to me is that if the systems running amperage is less than 13 amps then I do not need to run a seperate cable to the distribution board, it will connect in to the domestic ring main.

-------------------------
Never knock on deaths door... Ring the doorbell and run away, death really hates that!

NickD on Wed September 03, 2003 9:24 AM User is offline

My plant had all old machines from the 30's, it took a crane to lift a five HP 3 phase, 240 V motor, that sucker could run all day and be ice cold, it was 95% efficient. Was suggested I buy a new one, on way, they don't make them that way anymore. In the 50's/60's the defacto standard for motors was a 40*C temperature rise. Today with all of the energy BS, and with better enamels for the magnet wire, 60-80*C temperature rises are becoming quite common, but nice for cooking your breakfast on, just don't burn the bacon.

A hot motor is the last thing you want in an already hot compressor, the rules for designing a good motor have not changed over the last hundred or so years, lots of copper, lots of high grade super thin laminated steel. Ha, the alternators of even twenty years ago were running at 60-65% efficiency, today, 50% or even lower is common. Have to save on weight and most important of all, is cost. The full electrical load of the vehicle is also being dissipated in that super tiny alternator. Wear asbestos gloves when testing, wait, asbestos is now illegal.

It just seems if you double the capacity of a central AC unit, the on time would half, if proportional to input power, while the power for the larger unit is doubled, it's use would be halved, so what is saved?

If you want to save on your electric bill, you need an ice cold motor and a condenser.

The bad thing about all AC systems, is that the hotter it gets with this refrigerant pressure temperature stuff, not only is the running time increased, but the efficiency drops off to nothing. Ha, AC systems work great when it's cold outside. Seems like we need all brand new technology.

Mitch on Wed September 03, 2003 9:42 AM User is offline

This is some data from the last 24 hours 6 am to 6 am.

High temp 93F Low RH 35%
Low temp 65F High RH 100%

Compressor run time 24% over 24 hours.

NickD on Wed September 03, 2003 10:26 AM User is offline

Ha, you need some tall trees, my yard always feels about 10 degrees cooler than the barren lands, way much cooler than that huge black topped parking lot at Wal-Mart. The moisture in the leaves evaporates sucking up the heat not to mention the shade.

bohica2xo on Wed September 03, 2003 12:35 PM User is offline

Nick:

You are correct, nothing has changed when it comes to the physics of electric motors. The power control issue becomes more important as you remove iron and copper. Even the most simple inverters now are pretty sophisticated.

3hp Inverter specs

If you look at the cost of 3phase vs single phase motors, a 3hp 3 phase industrial motor is 220 bucks, the single phase motor is 360 bucks! The 3ph motor has a 1.15 service factor, and a NEMA effeciency rating of 89.5% - the single phase motor has a service factor of 1.00, and the manufacturer will not even discuss the effeciency.

ROI for a 2hp inverter on my pool pump, which runs 6 hours a day was about 6 months - based on the cost savings for the motor purchase, and the reduced power use. The reduction is significant, it is visible on the bar graph included with my power bill. I could probably gain more effeciency by adding a flow meter to the system, and adding a feedback loop for constant flow/variable speed operation. The inverter is capable of this, I am just lazy.

If you were to use an inverter, and a feedback loop from the low pressure, you have in effect a variable displacement compressor. This would provide the same benefit as a POA valve - low side pressure control, just like a V5 does it....

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

NickD on Wed September 03, 2003 1:24 PM User is offline

You convinced me bohica2xo, ha, needed another cup of coffee, back in the 60's the manufacturing cost of a good mechanical two coil alternator voltage regulator was $2.90 at back then prices, today our cheapest all solid state version is 75 cents. Every year the cost of electronics is going down even with inflation. Thank goodness we have some competition in the CPU field, now computers are affordable for most people.

Thinking back, inverter technology back then required three huge power transformers, without opening the case of your inverter, I am willing to bet it has three tiny pulse like transformers where switchmode technology is now used with high frequency techniques, plus I bet the unit is made in China. Constantly varying the pulse width at a real high frequency could easily produce an easy to filter chopped sine wave.

I can't seem to get into the specifications page on that inverter site, can you carry yours under one arm? Back then you needed a crane to lift a 5 HP unit. Interesting.

bohica2xo on Wed September 03, 2003 3:27 PM User is offline

Nick:

That spec sheet link is to an Adobe Acrobat document. If you can't open it, then you need to open Acrobat first - it sort of depends on your OS.

The weight of that inverter is 8.5 pounds. You are right, much work is done at 14khz inside that box....
Actually, most of the silicon is in one lump, not very much to look at in the drive.

If you still want iron in your motors, just buy 50 hertz motors....

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

NickD on Wed September 03, 2003 5:28 PM User is offline

Apparently the links in the motor drive site don't work, least for me, I switched over the the technical tab and was able to open it. Of course I have Acrobat.

Now I wonder if I should kick myself, a couple of months ago, I turned down a deal on a 7.5 HP 50 gallon tank air compressor, ha, what am I going to do with it, it's three phase.

Sure would be nice if the power company would run that 3rd wire.

Mitch on Fri September 05, 2003 6:41 PM User is offline

It was a little hotter today and got up to 98F. AC still is only running half the time.

The scope recording is from 1:30 PM to about 3:00 PM.


NickD on Sat September 06, 2003 12:46 AM User is offline

Hooking a receptacle to ground and one of the compressor wires for 120 VAC output and getting one of these old fashion synchronous motor electric clocks would be a poor mans way of learning compressor duty cycle. Set the clock to 12:00 and check it 24 hours later for the compressor run time, then crank it back to 12:00 for the next 24 hour cycle. Go to weather.com to learn the average 24 hour temperature for you zip code. Ha, this would be a poor mans way of gathering data.

Not sure what good it is, the COP changes constantly as does the Btu output with ambient, RH plays a very strong role, measuring KHW would give a far more accurate yardstick of learning what improvements in living style can be made. Tell my kids when they switch on a 100 watt light bulb, that's about another 350 BTU per hour that the AC has to remove kind of thing.

Bigchris on Sat September 06, 2003 2:33 AM User is offline

What I want to know, is with a phone in one hand and a camera in the other, what is he using to type? If it's what I'm thinking, I may order some of those SPAM pills.

Karl Hofmann on Sat September 06, 2003 11:50 AM User is offlineView users profile

LOL Over here SPAM comes in a can and you eat it!

-------------------------
Never knock on deaths door... Ring the doorbell and run away, death really hates that!

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