# Thread: 9k + 9k + 24k = 36k??

1. How long have you been doing HVAC work?

2. Originally Posted by pecmsg
How long have you been doing HVAC work?
2 years.

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3. I appreciate the honesty.

I replaced a similer system in a summer house. Went from 2) 7 & 1) 12 to a 12K Ducted. Problem solved.

This system is seriously oversized and doomed for premature failure

Is there a senior tech that can help with sizing and layout?

4. Originally Posted by pecmsg
I appreciate the honesty.

I replaced a similer system in a summer house. Went from 2) 7 & 1) 12 to a 12K Ducted. Problem solved.

This system is seriously oversized and doomed for premature failure

Is there a senior tech that can help with sizing and layout?
Yes. I’ll be consulting with guys that have mentored me and won’t move without their approval. I just wanted to do some of the “footwork”, as it helps me to learn. Thanks as well for the honesty. That’s why I come here

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5. We have members here who can work the numbers.

6. Professional Member*
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Originally Posted by Sabr7
I’m not the best at math but I know 9 + 9 + 24 = 42.

I’m tagging along for an install of a multi zone mini split. Two bedrooms having 9k BTU head each and an open space having 24k (and I’m fairly new to the trade) but I just want to understand this... why is the condenser at a rating of 36k BTU...?

Also, after doing the BTU calculations it says we will need 21,600 BTU for the open space. That’s how we got at the 24k number.. is it generally better to scale up or down in choosing the tonnage?

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most multi-head mini splits are installed on the assumption that all heads will not be running at capacity at all times you have to look at the literature for the brand they're selling to see if that condenser matched those 3 heads

7. Originally Posted by dkalasz
most multi-head mini splits are installed on the assumption that all heads will not be running at capacity at all times you have to look at the literature for the brand they're selling to see if that condenser matched those 3 heads
YUP...

And if someone has done lots of conventional split system zoning...
They know that it is very rare, for the system to be running at full capacity.

There is more to this... than the math...

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Some manufacturers allow you to over size the total indoor unit rated capacity above the outdoor units rated capacity by a certain percentage. You need to read the product literature to know how much allowance you have if any is allowed at all.

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9. Originally Posted by thatguy
Some manufacturers allow you to over size the total indoor unit rated capacity above the outdoor units rated capacity by a certain percentage. You need to read the product literature to know how much allowance you have if any is allowed at all.

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Most combinations you can "apply" 150% of the OD unit capacity.

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