Convert BTU to CFM
Can anyone tell me where I can find temp rise data in order to be able to convert BTUh from a load calc to required CFM? I know this can vary quite a bit from unit to unit. I’m looking at two different ducted mini-splits…
Carrier 40MBDQ09 with 38MARB outdoor unit
https://www.shareddocs.com/hvac/docs/1010/Public/08/01-DLS-001-CA-01.pdf
pages 26-28 and 34-35
and
Mitsubishi PEAD-A09 with SUZ-KA09 outdoor unit
I have these spec sheets but I don’t see specific temp rise data. I know I can calculate it from rated BTUh @ specific outdoor temp and CFM, but I’m not certain which CFM speed applies to the rated capacity numbers @ 47F and 17F as both have three fan speeds. Would “Rated Capacity” @ 47F equate to the highest CFM airflow or a lower speed? Same question for the capacity @ 17F. There is both “rated” and “maximum” capacity at 17F which varies SIGNIFICANTLY for the Carrier unit but rated and max is the same for the Mitsubishi @ 17F.
Can anyone help point me in the right direction? Any help would be much appreciated.
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Replies
Most units are rated at max flow. For temp rise you can use:
DetaT= Rated BTU/(1.08*maxCFM)
Somewhere around 30F to 40F for most units.
Akos,
Thank you for the help. I assumed that was the case. That gives the Carrier a temp rise of 35.1 @ 47F ambient. So that's probably about right.
Don't know if you remember, but you gave me advice in a past post (https://www.greenbuildingadvisor.com/question/mini-split-sizing-and-layout-questions). Life stalled my build plans, but I'm back on track hopefully. Design has changed slightly and the block and room loads are slightly lower than in that original post, but fairly close still.
2080 sqft (52' x 40' rectangle)
16,300 BTUh Heating
14,361 BTUh Cooling (2236 latent)
I'm considering using two 9k mini-split ducted units (two indoor and two outdoor...one to one), one to cover each half of the house. The single 9k's appear to have much better efficiency and capacity minimums than a single 18k, especially the Carrier units I mentioned above. Are you familiar with those at all? From the looks of the specs it appears to be similar to Mitsubishi's Hyper Heat model (which I don't need), but they don't seem to have a more standard outdoor unit that's compatible with the ducted unit.
The biggest reason I like the Carrier is the size of the indoor units. They are MUCH smaller than the Mitsubishis and much easier to find a place to install easily.
Using two units bad idea? Benefit to having the higher modulation range of the 9k's over a single 18k?
Jason
Except for the cost, two units would work fine. It would simplify your runs a bit as you no longer have to run across the whole house.
When it comes to brand names, I would pick one you can get local installer and support for. The nice part about the Midea units is they also come branded as Carrier (some unit but different sticker) so you might have a better luck with an installer.
For the money and performance you get, Midea is a great deal. I don't know why the modulation range on your 18k unit is so much worse than either 12k or 9k. Keep in mind that the 12k and 9k units are low static, so you have to watch your ducting. They can also be mounted vertically which might make installer easier.
There is nothing wrong with using a hyper heat in any climate. In some ways it is a better fit as the heating capacity tends to be higher than cooling capacity on most. Hyper heat units are a bit more costly, you can save a bit by selecting a different product line.
Another budget option is the Mrcool Universal. This is much closer to a standard air handler, trades would be much more comfortable installing it.
The 9K Carrier (40MBDQ09) has a cooling capacity range of 2,400 - 12,500 and a heating range of 1,700 - 15,600. So the two running simultaneously could in theory run as low as 4,800 BTUh cooling and 3,400 BTUh heating.
The 18k unit in the same model has a cooling range of 6,500 - 18,800 and a heating range of 8,900 - 22,000.
That seems like a really significant difference, especially with the loads I have. So it looks like the the two 9k units would do better during the shoulder seasons and days with much lower heating or cooling loads, unless I'm looking at it wrong.
The 9k also has a higher SEER (23 vs 19.6), EER (14 vs 12.5), HSPF (12 vs 11), and COP (3.62 vs 2.93). Strangely enough, at least to me, the max airflow on the 9k (264 cfm) is below the MINIMUM airflow on the 18k (300 cfm).
The 9k speeds are 147 / 211 / 264. The 18k speeds are 300 / 400 / 480.
Your thoughts?
I would worry too much about it, I think either option would work well. The nice benefit of two units is that you can run only one during the shoulder season at a higher load which will help with humidity removal.
If you sharpen your design pencil and bring your cooling load down a bit, a single 12k unit might also carry the house:
https://ashp.neep.org/#!/product/25349
Akos,
You mentioned in your first reply that the rated capacity was at max CFM. Is this correct? If that's the case then where does the max capacity that is significantly above the rated capacity come from? For example, the 9k Carrier ducted mini-split has a rated cap of 10k BTU/h @ 47F but a heating cap range of 1700 - 15600 BTU/h. I assume this max of 15600 refers to 47F as well. Rated cap at 17F is 6450 BTU/h and max cap at 17F is 11600.
It would seem to me that the max capacity would have to coincide with max CFM. Am I thinking about this incorrectly? Assuming this is true, the temp rise @ 47F with max CFM would be 54.7F and @ 17F would be 40.7F. Are these possibly correct? I just don't see how you can accurately calculate needed CFM without knowing for certain.
Also, here's full data on the Carrier 38MARBQ outdoor unit. Look at pages 19 and 20. Can the TC data ("total net capacity") at a given indoor and outdoor temp be used to calculate temperature rise at assuming max CFM?
https://www.shareddocs.com/hvac/docs/1009/Public/00/38MARB-01PD.pdf
Also, here's a link to an older older post where Marc Rosenbaum checked with a engineering contact at Mitsubishi and he stated...
"Rated capacity is the capacity as the equipment was tested in a lab setting to obtain the published efficiency values, maximum capacity is the actual capacity that the unit is capable of producing as it operates in the field, you should always use maximum capacity when selecting/sizing equipment for an application."
Max capacity will also be at max CFM. Since maximum fan speed is fix, to get the extra capacity, the unit must provide a larger delta T across the coil.
This extra temperature increase reduces the efficiency of the unit, if you look at the COP numbers at max vs rated, the COP drops by a bit.
This is also the reason they don't test at max as the unit will look less efficient. One way to look at it, the 9k unit is really a 15k unit that is run at lower load to make the HSPF numbers look better.