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Electric cost...2T unit vs 3T unit

Posted by andrelaplume2 (My Page) on
Thu, Sep 3, 09 at 14:47

Well, I have had my new Trane 3T system for the summer now. It replaced a 2.5T, 20 year old unit in the home we have been in for 3 years. That old unit was doing an OK job cooling but for whatever reason, it was having trouble keep the home warm in the winter. There were some discrepancies over staying with a 2.5T unit or going to a 3T unit. Most of my quotes recommended going to a 3T. The one guy who I know did some sort of calc said a 2.5T was borderline. Since we had been having issues keeping the home warm in the winter already and we wanted to drop a couple of ducts to the basement (its being slowly finished and we have hopes it will take the cold edge off) we opted for the 3T system. As far as cooling, the 20 year old unit kept the house fairly except for our bedroom over the garage.

I am happy to report, perhaps due to the 3T unit and some sort of things they put in my main duct line to push more air upstairs....that our house, in general, feels great and the temp difference is not very much going from room to room. Our bedroom is much better too. In fact if we leave the vsb fan set to: ON...our bedroom feels as good as they rest of the home, surprisingly without a nasty draft. I do notice with that blower left in the: ON position, the house humidity appears to rise. Shut it off and it lowers. With the fan: OFF, we find on some cooler nights the bedroom gets stuffy so we end up dropping the temp a degree or so.

All this being said we are happy EXCEPT I have realized no electric savings. My dad is on a budget with the electric company. He got his unit the same time as us and they already lowered his budget. My bills are essentially unchanged which really means this unit is costing me more since the weather has been cooler this year.

I have the rep coming out to discuss this, the humidity issue and the fact that a 15KW heat kit was to be installed and only a 10KW was. I do not want to get adversarial with him over the heating / cooling bill but I did expect it to go down. I now fear my winter bill will actually go up...though perhaps the home will actually be warm for a change.

Was I foolish to expect to save maybe 25% or so on my cooling costs? Is there anything technical I should call into question?

Maybe a programmable stat is they way to go. We pretty much left the stat set at 73/74 and the fan on: ON all summer.

What do you think? Its a 2200 sq ft two story.


Follow-Up Postings:

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RE: Electric cost...2T unit vs 3T unit

Why would you think a larger unit would cost less to run? It's typically the other way around.

In some cases, leaving your fan running can partially rehumidify the air by evaporating the residual moisture on your air handler coil. This is more likely to happen if you have an oversize unit that is short cycling. How often and for how long does your unit run?

If your unit is short cycling, one fix is to increase the swing on your thermostat. The swing is the difference in room temperature between the point that the unit turns on and the point that the unit turns off. It is adjustable on some thermostats.

Check your thermostat's manual for details. The object is to get the unit to run longer and stay off longer between runs. If the unit is not short cycling, this won't help.

FWIW, we use a two degree swing, which we find tolerable. A lot of thermostats default to one degree or less.


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RE: Electric cost...2T unit vs 3T unit

I replaced a 12 seer 9 yr old hp which was probably operating like a 10 with a 15 seer and my electric bill is lower. I'll check the % saved after I have it for 3 months. You might want to check your usage over 2-3 months since it is so new and see the change if any. The savings is based on seer rating not tons, higher seer less kw used


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RE: Electric cost...2T unit vs 3T unit

The new unit has 20% more capacity than the old unit. There are several possibilities.

1. It is short cycling. Short cycling is inefficient. High SEER units do not reach maximum efficiency until they have run awhile after each start.

2. The unit is not short cycling, and is providing a full 3 tons of cooling. The additional tonnage is likely to offset any efficiency gains from the higher SEER rating. 20% more cooling capacity isn't free.

3. The duct system is too small for the higher air requirements of the increased tonnage, resulting in inefficiencies.

4. The system components are mismatched.

5. The system is not operating properly due to a faulty installation.

6. Some combination of the above.


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RE: Electric cost...2T unit vs 3T unit

At the risk of redundancy, higher tonnage = more energy consumption, but higher EER/SEER = lower consumption. Glad your comfort level is good. Put the fan on OFF and you won't reintroduce moisture into the air. Then install a ceiling fan in the bedroom, it will keep your comfort level more even all night.


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RE: Electric cost...2T unit vs 3T unit

I believe the installtion was done properly. The company has a good reputation and though I have no way of knowing for sure, they were methodical on cleaning up, installing things neatly, they took their time etc etc. Everything seams to run fine, no weird noises etc. The house is actually a consistent temp in all room for the first time.

I never timed the cycling. To be honest its so quiet now its hard to tell if its running except for one kitchen vent. Unless we get a REAL hot day, at this point it may be hard to tell what the cycle time is.

So, at this point the dude is coming out today at noon. It there a way to tell if its short cycling? Would short cycling have the same impact in the winter?

As far as being oversized, like I said the one guy said the number shown I could get away with another 2.5T BUT that was bordeline, especially if I was going to drop 3 duct in the basement...which we have dropped 1 so far. Maybe I should add another right away?

Whats the most logical approach to solve the problem.


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RE: Electric cost...2T unit vs 3T unit

andre

your story is not surprising. I went from a 4 ton 10 SEER mismatched system AC/gas furnace, made insulation and window replacement improvements, and down in size to a 3 1/2 ton matched 14 SEER dual fuel system. Savings in AC mode non existent. The rule of thumb from Trane and other top line mfgs are 5% savings for every point increase in SEER. Throw the rule of thumb in the trash. As I recall you did go up from a 2 1/2 ton to a three ton.

I can't recall which stat you installed-the 803 or the HW VP IAQ. You should check the setting for CPH-should be two or three. Of course tech should check the electric draw at both the condenser and air handler plus the refrigerant charge.

what is a normal run time for a cycle with new system?

you may be unhappy on the AC side but I will be surprised if you are not smiling on heat mode side.

I may be wrong but I doubt if the tech finds anything wrong with your system.

if you do change your backup heat strip, I recommend you have two 7.5s wired staged.

IMO


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RE: Electric cost...2T unit vs 3T unit

tiger...long time no speak...thanks for the reply...

re:
andre
your story is not surprising. I went from a 4 ton 10 SEER mismatched system AC/gas furnace, made insulation and window replacement improvements, and down in size to a 3 1/2 ton matched 14 SEER dual fuel system. Savings in AC mode non existent. The rule of thumb from Trane and other top line mfgs are 5% savings for every point increase in SEER. Throw the rule of thumb in the trash. As I recall you did go up from a 2 1/2 ton to a three ton.

***Went from 9 SEER to 14 and from a 2.5T to a 3T. Certainly not seeing 30% electric savings! Based on my prior posts here the system is matched and in fact needed to be for the rebate.

I can't recall which stat you installed-the 803 or the HW VP IAQ. You should check the setting for CPH-should be two or three. Of course tech should check the electric draw at both the condenser and air handler plus the refrigerant charge.

***It was the base model Trane Stat..I am thinking of upgrading to a programmable Trane stat..we'll see. I wish I had know about the 2 degree setting...now I may need to wait until next summer to see how it affects my bill and comfort level...assuming it is not already set at 2...will check at lunch.

what is a normal run time for a cycle with new system?

***I really never timed it. To be honest, except in the kitchen where there is VERY noisy vent (its almost on top the airr handle below) it hard to tell when the sucker is running! It may now be too late to time it unless we get a few 80+ degree days. What cycle time should I be looking for?

you may be unhappy on the AC side but I will be surprised if you are not smiling on heat mode side.

***Actually I am happy on the ac side...its never been hot in any part of the house and everything is more uniform...just concerned that the sucker is cost more $$$$ to run than the 20 year old unit....again, I have seen no decrease this summer and the avg temp was waaaay lower this summer. I hope it heats as nicely but now I fear my bill will go up! Also, for all us lucky folk in PA it will go up 30% next year due to a rate increase. I was hoping there would be savings with this unit to help offset that rate increase.

I may be wrong but I doubt if the tech finds anything wrong with your system.

***I sort of agree..plus its the sales dude coming out. If setting the stat to a 2 degree swing lowers my bill without decreasing comfort I will be quite happy. I just was not sure if they needed to lower the fan speeds or whatever..though it is a variable speed so mybe thats not possible. OR if getting that extra duct in the base might reduce some air movement upstairs and maybe somehow affect the cycle time tom my advantge...again...just scared about heating bills!

if you do change your backup heat strip, I recommend you have two 7.5s wired staged.

**I will mention this! I susoect he will have some bs story now about why we should leave the 10KW strip in place. I want what will best keep me warm at the lowest billed $$$$...not sure which that is....


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RE: Electric cost...2T unit vs 3T unit

andre

first of all, if your bills are roughly the same then I expect you are at least saving close to 20% since you went up in size from a 2 1/2 ton to a three ton system. your dealer would have to compare KWs pulled from old 2 1/2 ton system vs new VS 3 ton system.

and from your earlier posts I feel that a three ton is a better selection for you especially on the heating side.

however as I recall, you received quite a deal with all the incentives including rebates and tax credit. you have not crippled your new system but you have purchased the wrong stat which does not allow you to take full advantage of the capabilities of such a nice premium sgl stg system. that is a mistake. sorry. the HW VP IAQ is the best choice.

a normal run time assuming you are sized correctly would be 10-15 minutes. your new system has never run continuously this summer on the extra hot days?

your dealer should verify that dip switch settings are correct. I would want them at 400 CFMs/ton for AC assuming
humidity control has been acceptable.

staged backup heat strips will definitely pay off in the winter.

changing the stat and the backup heat strips will help your system, your comfort, and operational cost.

IMO


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RE: Electric cost...2T unit vs 3T unit

Ok, here is what the guy said.

My humidistat/temp/moon phase thingy said the humidity in the home was 52%...well in that room anyway. The upstairs one read 49%. He brought something in from his car that read 37%. He said 50% or less was desirable and that my stat may just be off by15 points or sowho knows. My basement read 45% on his thing. I feel a little better that maybe the house is not overly humid..just my imagination and that when it reads 68% maybe its really just 50% or so. I explained that having the fan set to:ON makes it more humid. Still he did not think the unit was running abnormally. He is sending a crew out to double check everything.

He is also thinking that 15KW heat kit should have been installed NOT the 10KW.

As for the bill not going down he was a bit perplexed. I gave him the numbers ($$$) and he said they actually sounded low to begin with. (The electric company website says I use less electricity than expected too) Still I felt it should go down. He did say it was a 20% larger unit (3T vs 2.5T) and I was now adding air to the basement and since I felt the upstairs was now more comfortable all around that I likely was getting more air up their too so it might end up at a wash. Still, I sensed he thought he bill should have gone down some. He wants the techs to come and check everything and the dip (?) switches are set right. We went through an appliance list to see if added anything new. The only thing on his list new we had (and this sounds funny) was an XBOX. He said they actually draw as much power as a refrigerator and my son should unplug not after use. We have a WII too but he said those are fine left plugged in. So, could the extra sizing, the duct in the basement, more air upstairs, the XBOX, maybe my wife running the dishwasher and wash more eat up my savingsI am not too sure.

He did not feel changing the stat setting necessary. I forgot if it was set at 1 or 2. He recommended against a programmable stat because inevitably folks use them with huge temp swings that actually force the unit to run and cost more. He did not like the HD stuff since if it blew out the unit somehow it would not be covered and Id be mad at him. He was none too thrilled with the Trane stats eithersaid he thought they were $300I said that did not sound right. He said a remote might be available for my stat (401?)hed check all this, set up a service call and get back to me. They just called to set up an appt so they are on the ball.


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RE: tiger

tiger...just saw your post....I will need to dig more on the stat and get the exact model...I just know its the one that came with the system. I'll post the number and see what they say...


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RE: Electric cost...2T unit vs 3T unit

HD thermostats hooked up correctly aren't going to blow out the unit. However, they aren't all that good either. Even the Honeywells sold there aren't the best.

Yes DIP switches. DIP stands for Dual Inline Package. It's an integrated circuit term. The switch fits in the same space as certain integrated circuits.


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RE: Electric cost...2T unit vs 3T unit

Well, I thought I'd followup. The guy was nice and took his time though I sort of can see why they may have put the 10KW heat kit in the first time...its less work. He had to run a new wire for the 15KW kit and explained that he'd need to install a stat out side. I asked about him staging it 7.5 / 7.5 and he said they wanted him to do a 10 / 5 with the 5 being outside and kicking on when the temp fell below 25 degrees. He figure that I saw no reduction in my bill die to the upgrade from a 2.5T unit to a 3T unit. He could not speculate on how my winter bills might be affected but felt we should be warm.

He replaced a section of matal duct with plastic insulated duct in an effort to reduce noise in the kitchen. I'll have to lower the stat Sat when it warms up and see if its quieter.

That duct was buy our humidifier (in the main trunk)...funny I swear I feel air coming out the humidifier now...maybe it was always that way...

No info left on a trane programmable stat yet.


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