What's the energy consumption of running 5 compressors over 2 if BTU capacity is same

[pingable]

I am debating various configurations. The costs are very similar as the higher SEER units qualify for rebates so they are more or less the same costs for install at the end of the day

Option A:
5 Compressors
Four 9K 26 Seer Units
One 12K 23 Seer Unit

Option B -
2 Compressors
One Tri Zone 9+9+18 16 Seer Unit
One 12k 23 Seer Unit

Both are Mitsubishi lineups...


With that said, on a given scenario of let's say Option A with 3 Compressors on the same given time versus on Option B - with 2 Compressors on at the same time....Bearing that Option A, with me using a single compressor per zone, WILL I be using more energy ~overall~ as a whole due to to SINGLE compressor Setups vs..a scenario with double/tri/quad zones as a possibility with a SEER rating of 16 as a example.

Not sure whether the lesser of the combo (less compressors) OR higher SEER rating will leave a lower energy consumption usage at the end of the day.

[beenthere]

Option A is using higher efficiency compressors. And should operate cheaper.

[GT Jets]

Well...Here goes nothing...


The building load will determine the choice...
Reasons; if the building has a well insulated envelope and does not have a large heat load, the more compressors the better for the part load efficiency.

Another consideration is short cycling (the untimely starting of a compressor to meet the building load) and being able to run at a lower capacity when loads are low...

To add to that, how often will this building have to run balls out?, if sized properly, only a few weeks out of the year....So more information and building analysis is needed, not to mention comparing that info against the heating and cooling days in your area...

GT

[ga-hvac-tech]

Trying to make technical decisions by 'bean-counting' is not a wise thing to do...

Reminds me of the story of the guy that rationalized that one did not need to change the oil in their vehicle but the amount the manual said (7500 miles). Then they were complaining the engine did not last much beyond the warranty.

Bean-counting will give one what they pay for...:payattention:

[pingable]

It's a 80-100 year old house - not very tight..
Some areas of the house that have been gutted, insulation was added
We have a flat roof and I had someone ~blow~ insulation into the roof cavity as well.


It's not a matter of crunching the dollar difference and choosing which is the lower of the latter. I'm just trying to grasp from a tech side of things, will running possibly 3-4 separate compressor costs even more than running 2 compressors even thought the SEER rating in higher...

[beenthere]

NO. Not if the system is sized right, and the compressors/condensers are not short cycling.

[ga-hvac-tech]

It's a 80-100 year old house - not very tight..
Some areas of the house that have been gutted, insulation was added
We have a flat roof and I had someone ~blow~ insulation into the roof cavity as well.


It's not a matter of crunching the dollar difference and choosing which is the lower of the latter. I'm just trying to grasp from a tech side of things, will running possibly 3-4 separate compressor costs even more than running 2 compressors even thought the SEER rating in higher...

OK, I understand what you are dealing with... my apologies for the direct remark in the previous post.

If cost of operation is the issue, I would look at multiple units. Remember that SEER is energy usage... even though it is not really that simple. I am sure some others will come along and explain the details of this.

There is another thing I would consider: Have we ever looked at the warranties of these mini-split units? I did a while back and was surprised to find they are short compared to American made split systems. Where I am going with this is to suggest the old 'KISS' adage (keep it simple). Using more single head mini's might be a better approach than a few multi-head units. Remember that IF (probably when) there is a board (computer that runs the unit) failure on a multi-head unit, someone will need to be able to repair it... Are the techs and parts readily available, and are there multiple sources for parts and techs?

One could easily get backed into a corner by not being careful of the longer term implications of a decision...

[pingable]

Been There - Excellent remark on short cycling. That was on my backburner of my mind as well.
So for example, let's just say a 9K wall mount unit will run at night on a 2.5ton compressor.

Given the load equation, assuming from a mechanical standpoint, I'm thinking that any compressor will want to see a minimum amount % of load on the unit for it versus let's say running at 20-25% of it's rated load.


Based on your remark on short-cycling, any advice on recommendation on what % of the load in respect to the compressor should it ideally be running at ?

[beenthere]

No one ideal % of load. All compressors/condensers are over sized for the lowest load a house/building/room will have.

The idea, is to minimize how over sized for that load. And still be large enough to handle design conditions.

[ga-hvac-tech]

Just a question here: Does an inverter driven compressor (variable refrigerant volume) short cycle?

[beenthere]

Depends on what its min capacity is compared to the min load.

[ga-hvac-tech]

Depends on what its min capacity is compared to the min load.

Do the multi-head mini's have a minimum rating that one can look up?

[beenthere]

I'm sure they have them somewhere. But I can't tell you where.

VRV's capacity reduction is basically limited to the min RPM of the compressor. So you can't have a lower cap then that min RPM allows. Which on some applications will mean the unit will short cycle.

I don't mean it will only run 2 minutes and shut off. But your not going to always get a 15 minute or half hour run time.

[ga-hvac-tech]

I'm sure they have them somewhere. But I can't tell you where.

VRV's capacity reduction is basically limited to the min RPM of the compressor. So you can't have a lower cap then that min RPM allows. Which on some applications will mean the unit will short cycle.

I don't mean it will only run 2 minutes and shut off. But your not going to always get a 15 minute or half hour run time.

THX Been.

I understand what you are saying about minimum speed of the compressor... probably has to do with the design of the VS motor in the compressor.

Just thinking out-loud here; I wonder if the lower SEERS of the munti-head units are because of the lower comp speeds?

Also, I wonder how long a munti-head mini will hold up before it starts acting strange... and if parts (like boards and sensors) will be readily available. Anyone have any experience with this?

[beenthere]

Not sure what the min RPM is.
Probably around 200 RPM. Witch is pretty slow for a compressor.

I wonder how they will hold up over here.

Many keep saying they have been in use in Europe for 10 years now.

Doesn't matter. Here we tend to like comfort more then over there.

So we probably run lower indoor temps then they do.

[chuckcrj]

THX Been.

I understand what you are saying about minimum speed of the compressor... probably has to do with the design of the VS motor in the compressor.

Just thinking out-loud here; I wonder if the lower SEERS of the munti-head units are because of the lower comp speeds?

Also, I wonder how long a munti-head mini will hold up before it starts acting strange... and if parts (like boards and sensors) will be readily available. Anyone have any experience with this?

We have installed quite a few of the multis and they are very very reliable. Some of them have been running 6-7 years now with no problems. Just maintain them and they will last. The only time I have had to change boards is when a snake or mouse or other animal gets is and shorts something out. In this case the boards came next day.

[gary_g]

How about considering life-cycle costs, instead of just operating costs?

What are the life-cycle costs of maintaining 5 units instead of 2?

What is the purchase cost of 5 units versus 2 units?

When will the energy savings of 5 higher SEER units balance the purchase price of 2 lower SEER units?

[cc2cuz]

The minimum the 9K multizone wall unit will go down to is 5,100 btu/h. The 9RLS system can go down to 3,600 btu/h.

The 9RLS systems are crazy efficient. There are lots of variables but at the end of the day, it will cost less to run.

Just compare the current ratings/power use ratings to each one.

I'm an Hvac contractor here in Hawaii, and for some reason we sell ALOT of mini splits, much more than central over here. I have installed literary thousands of mitsu, daikin, fujitsu etc and for the most part the compressors are rock solid. I have never had to replace an inverter compressor yet, nor do I know someone who has, fujitsu/daikin reps will tell you it's very rare as well. The thing we have a problem with is the salt air corroding the condenser coils. In three years, without any cleaning they can look so bad that the most of the aluminum fins are gone, but thats another story.

I personally have a 12RLS in my bedroom. After it had already pulled down the temp and was just maintaining the set temp I put my amp meter on it and it was pulling 0.7 amps with the compressor running. Imagine what the 9RLS can go down to.

[GT Jets]

The thing we have a problem with is the salt air corroding the condenser coils. In three years, without any cleaning they can look so bad that the most of the aluminum fins are gone, but thats another story.


Shameless thread jack....

Have you had the comm. problems on the Daikins at all???


I will recommend a product I use all the time for the condenser coils on the coast here..It is made be Nu Calgon and called Cal Shield, you spray it on a new or recently cleaned coil and repeat once a year with fresh water rinses every few months....Makes the coils last 5 times longer here, I have a 1999 Carrier condensing unit literally 30 feet from the beach and you can poke your pinkie finger through he sheet metal, but the coils are in tip top shape...

http://hvac-talk.com/vbb/attachment.php?


GT