SEER vs Dual Speed vs energy savings

[bertrand]

Just a consumer here - Northeastern IL.

You experts have been discussing the savings from different SEER ratings - but it seems that many of the higher SEER units also have dual speed or dual compressor systems - doesn't that help energy savings more than SEER?

My own home calcs a little over 2 tons - so a 2.5 ton SEER 13 was initially selected. However, we are looking to add a 4 season sun room in the next few years which would push us up to 3 tons. These sizes are based on worse case design day conditions which we don't have consistently up here.

After discussing with the contractor he recommended going with the 3 ton Infinity SEER 15 AC unit, due to the two stage compressor capability. That way it runs at 1.5 tons mostly - and will only come on full speed under worse case (design day) summer conditions - or if we add the sun room later.

I know going from the SEER 13 to the 15 is not going to save a heck of lot energy for my area - but won't I see some notable savings by having the two stage operation? Specifically - comparing a 2.5 ton unit running vs. a 3 ton unit which ran mostly at 1.5 tons? Is it a direct ratio (60% less current used at 1.5 ton)?

Thoughts?

[Edited by bertrand on 09-27-2005 at 05:35 PM]

[Irascible]

Two stage operation is included in the SEER calculation.

A sun room will obviously have a much different heat load. You'll need a zone system. It's extraordinarily rare that zone systems are done correctly. Two stage equipment is one of the few ways you can do it correctly. That's the real (and only) reason to spend the bucks on 15 SEER in my opinion.

[bertrand]

Originally posted by Irascible
Two stage operation is included in the SEER calculation.



I am not clear on this -

Lets take two systems - each is 3 ton and each has a SEER 15 rating

1) Carrier Performance Series AC - Single Speed

2) Carrier Infinity Series AC - Dual Speed Compresser


Given that the summer the load will vary quite a bit - with the 3 ton load only happening rarely - are you saying there is no significant energy savings with unit 2 which will typically be operating at the lower speed?

[fl1]

Yes there is saving just not nessecarily beyond the SEER.

Think of it this way it runs at 1.5 tons,but may run twice as long as a 3 ton.

Comfort is what you get and it like no other.

[docholiday]

Originally posted by bertrand
Just a consumer here - Northeastern IL.

You experts have been discussing the savings from different SEER ratings - but it seems that many of the higher SEER units also have dual speed or dual compressor systems - doesn't that help energy savings more than SEER?



Two stage is for comfort, it makes the system run longer with fewer off cycles. (no work is done when the unit is off). As far as the load goes, the idea of cycling or shifting stages accomplishes the same thing, that is to produce the required Btu's over the course of time. Heres an lementary example. Lets say you have a 4 ton system producing 48,000 btuhs. That unit is sized to deal with 95 or 100 degree days.
What then happens when you have an 80 degree day and your cooling requirements are 24,000 btu's per hour? Well you cycle that unit on and off running it for say a total of 30 minutes out of an hour. A 2 stage machine with 50% capacity will run the entire hour, maintaining an even temp. More than likley you will have a load somewhere in between. A 70% system would probably be a wiser choice, partyly because it does a better job of dehumidification, and it will not have to cycle to full speed near as much. It may however occasionally shutdown. Short cycling is hard on the equipment but another story alltogether.

I can tell you this, I wouldnt install a system for some future work to be done like a sunroom or any other addition. Why? Because for the time prior, you will have an oversized system and you will not be happy. Two things happen here, 1st, I am stuck with call back after call back on my dime to fix something I knew would be a problem. 2nd, your unhappiness will make you consider whether or not I come back to do the work on the addition.

If you want low life cycle cost, stick with 12 or 13 SEER, if you are interested in comfort, you may have to upgrade to a higher SEER to take advantage of the additional comfort features that generally only come on higher SEER systems.

[Irascible]

Originally posted by docholiday
Two stage is for comfortSo by implication it's not for added SEER not already included in the SEER rating?

Yeah... That sounded silly. I'm sticking to my guns on this one unless Doc slaps me down. System 2 isn't going to save any more money than system one bertrand. The SEER calculation is a convoluted mess PRECISELY because it's trying to be comprehensive. Leaving out whatever energy savings may result from having a two stage unit would be an obvious mistake.

[docholiday]

Well, its really a feature designed to seperate the various models. In the consumer's eye, there is no difference in efficicncy and comfort and this is also true with many contractors.

We all know the cost to go from 13 to 16 SEER is difficult at best to justify regarding efficiency alone. Add additional features available only to higher end systems and it helps with the justification to buy the upper end unit.

GM has altered their business to concentrate on higher profit units (SUV's) The added features of these high end vehicles has many features they simply cannot offer in smaller cars. You will rarely find the heated leather and navigation packeges in the chevy cavalier.

That said, you must remember, ultra high efficiency systems (16+ SEER) only make up 4-5% of the market. They also require alot more in R&D and marketing. The price has to be higher, the justification for the higher price has to be there too.

As for the example its a matter of which is more important. Comfort or cost.

[beenthere]

If you see any savings because of a 2 stage system, its because of better humidity control, allowing you to raise your thermostat setting.

[docholiday]

Good point, I failed to mention that.

[Irascible]

Would the improved humidity control be included in the SEER equation?

[docholiday]

Um no, SEER does not distinguish between latent, sensible or total capcity. Its BTU's out and the setpoints dont change.

[Irascible]

If it's strictly counting BTU's, then would it not account for latent BTU's? Thereby accounting for the improved humidity control?

[beenthere]

Originally posted by docholiday
Um no, SEER does not distinguish between latent, sensible or total capcity. Its BTU's out and the setpoints dont change.

I think what he said.

[Irascible]

Originally posted by bertrand
Lets take two systems - each is 3 ton and each has a SEER 15 rating

1) Carrier Performance Series AC - Single Speed

2) Carrier Infinity Series AC - Dual Speed Compresser

are you saying there is no significant energy savings with unit 2... ? That's the bottom line question. IF the two stage 15 SEER unit is more energy efficient than the 15 SEER single speed by any appreciable amount then the SEER rating system is flawed in that respect. I suppose it's possible. But I'm having a hard time believing they'd leave that aspect of energy savings out of the calculation.

[beenthere]

Without going into slowing down the blower, the latent capacity doesn't increase in first stage per say. The unit runs longer, giving it more time to remove the humidity, instead of just a short 10 minute run time.

[amickracing]

One other thing to throw in the mix...
A 2 stage system will have different SEER for each stage. Stage one might have a seer of 17, since there's a huge evap coil and a huge condensor for only a system that's half that size (since stage one runs at 50%). But stage two might be 15 SEER for what ever reasons. They take the average SEER rating, which would be 16 in this case, and publish this.

I don't have any real numbers on how much seer difference there is between 1st stage and 2nd stage, but there's a difference.

Most likely won't save you much, not enough to count on anyway. But it's another benifit of a 2 stage system, if you need more reason other than the comfort.

Most likely you'll see more savings by going with a variable speed furnace/air handler than with jumping up in seer since it uses less energy.

[amickracing]

Originally posted by beenthere
Without going into slowing down the blower, the latent capacity doesn't increase in first stage per say. The unit runs longer, giving it more time to remove the humidity, instead of just a short 10 minute run time.

True, but you'd have to slow the blower down to run on one stage (50%). Unless you mean deviating from the 400 cfm per ton, in which case I'd believe.

If you don't slow the blower down and run on 1st stage you'll remove very little moisture, and likely not much heat either since the coil will turn into a major wind tunnel lol.

[beenthere]

I was refering to running 400 cfm per ton, on first stage, meaning a 3 ton in first stage, at 50/100 cacity split would have 600 cfm.



I just pulled up the tech cd for York.

The Affinity 3 ton 18 seer, is listed as 18 first or second, but, the ERR is 14.5 first stage, and 13.5 second stage.

So At ari 95od, and 80 id, first stage is more efficient.
Doudt it would run for very long on first stage though

[heatmon]

The SEEER rating is an energy usage efficiency rating that the condensing unit has to achieve to be granted that particular rating. Whatever the manufacturer has to do mechanically, electrically or design wise to achieve that rating is up to that manufacturer. The overall operation of all components achieves the rating, whether it's single speed, dual speed, same coil size, 1/2 ton over-coil rating, 1 ton over-coil rating, built in dehumidifying (adds to SEER rating with the correct controller),single speed evap fan, two speed evap fan, variable speed evap fan, two pass coil, three pass coil, four pass coil, capilliary tube expansion, TXV expansion, orifice expansion. The overall picture is what has to be looked at, especially when comparing different manufacturers systems. And for my money, a jump from 13 to 15 SEER is huge, especially when lobby groups are pushing congress for a 12 SEER national minimum energy efficiency standard.

[bertrand]

Originally posted by docholiday

Originally posted by bertrand
Just a consumer here - Northeastern IL.

You experts have been discussing the savings from different SEER ratings - but it seems that many of the higher SEER units also have dual speed or dual compressor systems - doesn't that help energy savings more than SEER?



I can tell you this, I wouldnt install a system for some future work to be done like a sunroom or any other addition. Why? Because for the time prior, you will have an oversized system and you will not be happy. Two things happen here, 1st, I am stuck with call back after call back on my dime to fix something I knew would be a problem. 2nd, your unhappiness will make you consider whether or not I come back to do the work on the addition.
.


Thank you for all your input.

Please explain more on this issue. If the calcs show I needed a 2.5 ton single stage unit and I am now going to a 3 ton dual stage unit - why would I not be happy? What problems will I experience with the larger dual stage 3 ton unit that I would not with a single stage 2.5ton? I have read not to oversize AC units - but this is just 1/2 ton oversize and has the dual stage....and in any case Carrier does not even make 1/2 size units in the infinity line.

Thanks again.

[beenthere]

You shouldn't have any problems with the infinity system and control, only being half a ton over.