Regular Guest
We are looking to install 2-3 ceiling concealed units in a large house. The key problem we have is dynamic range: the cooling/heating requirement can be a low as 1kW and as high as 10kW.
We are looking at units made by Mitsubishi Electric and Daikin and would like to understand the capacity line in the catalogues. Does "7.1 (3.3-8.1)kW" and "10.0 (4.9-11.4)kW" imply the minimum compressor output is 50% of nominal capacity (ie 4.9kW and 3.3kW)? Is it reasonable to assume this translates into minimum power consumption of 2.6kW*50% and 2.0kW*50%?
How can we estimate real min power consumption of an inverter unit? The manufactures only quote nominal power consumption at 7.1 and 10.0kW (I think with an outside temperature of 35 degrees/ 50% humidity). They do publish fan consumption figure of the concealed units but the compressor must consume something while it is idling once the target internal temperature has been reached.
Professional Member*
There is a factor which cannot be controlled, that being the load the unit is cooling or heating.
This load factor changes with things like humidity inside, humidity outside, the type of heat source inside (in AC mode), things as simple as whether or not the fireplace has a fire in it or not, etc.
IMO this is the reason the manufacturers are a little vague with their power numbers... as one can see; each structure is different, so the loads are different.
GA-HVAC-Tech
Your comfort, Your way, Everyday!
GA's basic rules of home heating and AC upgrades:
*Installation is more important than the brand of equipment
*The duct system keeps the house comfortable; the equipment only heats and cools (and dehumidifies)
*The value of comfort, over the long term; leave economic choices behind!
Choose your contractor wisely!
Regular Guest
I can its not easy but if there a standard for the "nominal" numbers surely they could quote min compressor numbers using that?
Or measure power consumption versus cool air production?
Professional Member*
As with conventional split systems... there is another variable: Installation.
How long is the lineset?
Is the outdoor unit above or below the indoor unit?
How far are the indoor units from the outdoor unit?
How far apart are the indoor units?
Add these to the variables in my previous post...
Conventional split systems are vague on specific energy usage also... due to lots of factors that are building specific.
Hint: Purchasing this kind of equipment 'by the numbers' is not a good idea... rather find a contractor who KNOWS what they are doing and listen to their recommendations.
As with conventional split systems: The equipment is secondary to the installation.
GA-HVAC-Tech
Your comfort, Your way, Everyday!
GA's basic rules of home heating and AC upgrades:
*Installation is more important than the brand of equipment
*The duct system keeps the house comfortable; the equipment only heats and cools (and dehumidifies)
*The value of comfort, over the long term; leave economic choices behind!
Choose your contractor wisely!
Professional Member*
What catalog?Originally Posted by ACPM
.. Link _
Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities
Professional Member*
The kW power consumption numbers are instantaneous, they doesn't always translate nicely into kWh which is what your electricity company bills you for.
The nice thing about the ductless splits is that they can reduce the heating/cooling capacity to match demand. So when only one ceiling unit is on, then you only need 50% capacity and therefore 50% power.
Are you trying to compare these units to conventional split systems to see if these are worth the extra money... or are you just looking to see if they are big enough for your house?
You can call me Sam
It should be a crime to be a mechanical engineer in San Diego
Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)
Regular Guest
MEE: PUHZ-ZRP-VHA/ PEAD-RP-JAQ
http://www.mitsubishi-aircon.co.uk/m....asp?id=191236
http://www.mitsubishielectric.ca/en/..._Concealed.pdf
Daikin: ADEQ-A / RZQSG-L
outdoor unit: http://www.daikineurope.com/minisite...QSG-L%288%29Y1
indoor unit: http://www.daikineurope.com/products....jsp?id=ADEQ-A
Regular Guest
We presently have a similar configuration: outdoor unit (10kW) and concealed internal unit with a fibreglass duct for distribution. There is a gas central heating system so the units are only used for cooling but they are on/off heat pumps and expensive to run. For all the "huha" about SEER and seasonal efficiency, perhaps the EER figure is most relevant in our case.
As to power consumed, I suspect you are right in that you cant take the min cooling output and use the EER to get min power consumption.
But the manufacturers must know the limits: if the unit is oversized for evening use because of daytime needs, what happens? I read inverters save energy buy not starting and stopping but in our case if the min cooling figure quoted is double that needed, there will be a point when the internal thermostat has indicated the target temperature has been reached for some time, even that it has been passed, so would that then result in the unit cycling on /off?
Maybe the datum we really want is the minimum % the compressor can work at. If this was 20-30%, there would be little danger of cycling. Unfortunately, the MEE catalogue seems to indicate 50% and Daikin don't quote anything and the technical support line can only read the catalogue we already have..
Professional Member*
Can you elaborate on this?
What makes this particular system requirements different than other applications?
Regular Guest
principally is the dynamic range. Can you identify HVAC that can work at 10% (ie cool at 10kW or 1kW)? Most seem to reach 50% at best.
Professional Member*
Most inverter units can drop to about 20-25% compressor RPM, which translates to about 30-35% capacity on minimum speed. This is however the compressor. The indoor units I think will only dopr ot mayeb 50% capacity as mentioned. For for 10% capacity, they will cycle on and off. Depending on the structure, it would be probably on for 5 minutes and off for 25 minutes at minimum load.
If you really, really want 10%-100% capacity, you need chilled water cooling with some sort of outdoor reset temperature control.
Professional Member*
You still haven't explained why your load is unique to all other HVAC loads.
Why is it so important that the system perfectly match the heat gain 24-7-365. Inverter units do an incredible job modulating to achieve capacity control. However at a certain point the most efficient and effective thing to do is simply cycle the unit on and off.
What is so special about your setup that that's not an option?
Regular Guest
I spoke with Daikin and Mitsubishi. The Daikin units get to 50% of nominal before stopping. They are inverters but as with any motor, cycling consumes more power than running continuously. If their machines produce 3kW min and the cooling requirement is 1kW, the additional energy consumed in start-stop could be similar that consumed while operating continuously..
Mitsubishi (and Fujitsu) has some units which operate 0,9 to 8kw or 15% nominal. They are a little smaller then what is needed on paper but with zoning they would probably meet intermittent needs.
Fancoil systems are quite a bit more expensive.
Regular Guest
The 1kW min load is a night-time scenario and so the units would need to meet this 8hrs*7 nights or 35% the time.
The concern about cycling is not only higher power consumption but also the potential blast of cold air every 15-30mins which will probably be notable even with inverter internal units.
Professional Member*
Basically, a 3-ton at full load is
38,000 BTU/hr = 3,290 Watts in Heating mode AT 47'F outside and 70'F inside
____________________ _ _ _ _
Inverter can modulate down TO About 30%
Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities
God Speed, Jim. We'll miss you - 2-26-23
When they run at 50% capacity, it won't feel like a blast of cold air when they start.
Professional Member*
You do realize that efficiency is not constant as capacity changes. Your MS will be most efficient at 100% output, and begin to decrease as output decreases. You reach a cross over point where the losses involved with starting and stoping are less than the losses involved with running at really low inputs.
You could always go for multiple smaller units if it was of such a concern
Professional Member*
...and your point is. That's normal. All building through out hte world see low sensible load conditions at nighttime. However in your climate up north, latent load shouldn't be an issue except for a few weeks in winter.
I'll repeat it again. The ONLY way you can precisely control building load with a 10:1 ratio is hydronics using chilled or hot water on smaller scale (below 6 tons). End of story, case closed, no point in digging further. Now, at 6 tons, you could install 2 zoned inverter driven split systems (like Carrier Greenspeed) and each only runs at abotu 30% capacity and you get 15% between 2 systems.
Even with chilled water, the chiller will still only turn down to about 75% capacity and the boiler typcially 5:1, but you can use a storage tank in either case to prevent short cycling.
In you climate, actually, it's possible to do radiant floor and wall cooling. If you have high mass floors, here also a way to prevent short cycling. Then you just need a dehumidifier or HRV for dehumidification.
Finally on the point of a "cold blast". If the equipemnt and ductwork or with mini splits, the indoor units are sized properly or located correctly then there should never be a cold blast from the equipment. In my home even on high stage in heating or cooling, the home never feels drafty. In cooling mode you can feel the cool dry air a little if the system is in a dehumidificaiton on demand mode (lower airflow). But it's actually refreshing and not cold at all.
A properly sized and installed HVAC system is silent, heats and cools evenly, even at low load conditions, and never short cycles, unless it's a small home with out much mass in the building materials. Some homes will short cycle in low load no matter what you do. The space jsut doesn't have enouhg volume or building materials in proportion to it's peak heat loss ang gain. Remember, a smaller home have more surface area to volume and mass ratio. So they have more temperature and load diversity. A larger home with plaster, stucco, tile, hardwood, etc, can take a hour to warm up or cool off even 1F on a fairly hot or cold day. That means when the system comes on, even with no load, it can take almost 10 minutes to raise or lower the temp 1F, which a good controller will maintain if it can stay under 4 cycles per hour maximum.
Professional Member*
You sure about that? In cooling mode and moderate and lower ambinet temps (<90F), I thought they were most efficient at part load, where static pressure is lower and both coils are essentially oversized... unless it's in a dehumidification mode.
Now in heating mode below about 50F and higher ambient, you are correct. That's at least what I've seen on the Greenspeed and even unloading scrolls for the most part. I would expect the VRF systems to be similar. But i agree effceincy is not ocnstant. It can vary widely due to many vairables.
Professional Member*
True, you have oversized ID and OD coils and low ambient air helping you out. But you are also operating a huge compressor at a fraction of its design capacity. That's lots of moving parts for very little cooling. Most motors, compressors etc that I have seen do not get the same efficiency when operating at low load. Also, at a certain point, massively oversized coils begin to work against you not for you do they not?
I don't know this for a definite fact, if somebody can prove me right or wrong with manufacturers data, that would be terrific.
But in my mind, if the unit gets 18 SEER at 100%, I would only expect maybe 14 SEER at 25%
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