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Mini split for multiple rooms with small loads

I asked an earlier question about how to deal with my existing central heat and no a/c system. Based on the responses, I'm planning to repair the ducting system some. However, I still need a cooling solution and doubt that I can effectively centrally cool the house due to the vertical split level nature of it. I already know that 1/2 of my cooling load is in rooms that effectively don't have central ducting.

So I'm looking at using either a multi head mini split to cool the house or multiple single head units. I'll probably get at least one heat pump unit so I have that option.

Several questions/issues come up in researching options:
1. How to cool low cooling load rooms?
I have some rooms where cooling is important (bedrooms) but the loads are very small (~1500 btus cooling, perhaps 3000 heating). While a multi head unit could address multiple rooms, generally the smallest blower units seem to be for 6000 btus. That's overkill for these rooms. I understand the inverter technology allows efficient cooling at a low load but that doesn't address install and blower head cost efficiency. Another option is to get a 'concealed duct' head and do a mini duct system between these rooms. I like that option and may have enough room in the service cavity to make that happen however two questions arise.

2. Why are the concealed duct units only available in a significantly lower SEER? There are now several choices at 25 to 30 SEER for single head wall mounted blower units. However, the most efficient concealed duct head I'm seeing is 21 SEER. What's the diff? Surely if the compressor can work at SEER 30 with a wall mount, it can do the same for the concealed duct unit?

3. Is there a way to customize the balance of btus between two rooms connected with such a system? In one case, I have two adjacent rooms with similar cooling loads (1500, 3000) but in another case, I have two adjacent rooms with very disparate loads (7200, 2100).

4. Why are larger BTU mini splits lower in SSER?
A final possibility is to not supply air directly to some of these rooms with very small loads. I could get a single large head and let the central furnace (ECM variable speed blower) move air. It wouldn't address the unducted areas with large loads but it might work for the smaller rooms. Two issues arise: first, is this is a stupid idea (my intuition says yes). second, the cooling load in the main space is ~13k on its own. Once you go over 15k btus, you are back to having more limited SEER choices (~20 max).

5. Why are multi head mini splits lower in SEER?
Of course, I could address the issue with those oversized 6k blower units in each space, but again, each blower seems to lower the SEER of the unit (as well as the HSPF). Why?

Ok, so this is partially a discussion of using mini splits to address a larger houses load and also a question about why the SEER varies so much based on what you attach to it.

I'd especially love to hear from anyone who has tried using mini splits for anything other than the single head cool one room situation. I know they work great for that.

I'd also love to hear about a low BTU high efficiency 'window' unit that can be sealed well. It's not an elegant solution in my opinion but it is an option.


Asked by Keith H
Posted Tue, 05/06/2014 - 23:19


1 Answer

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Mini-ducted cassettes take an efficiency hit over the wall-coil counterparts primarily due to the duct impedances. The shorter/fatter you can make those duct runs, the higher the SEER/HSPF numbers will be.

You can balance the air volumes to better match the loads using duct vanes to throttle back the air flow, or by using proportional sizing of the ducts to the two rooms to achieve the same effect. This will increase the duct impedance cutting into net efficiency a bit, but that's how it's done.

Using a central air hander to mix the air is not very effective at distributing heat or cooling to where it's needed- the temperature differences between the air entering & leaving the space is too low to provide much heating or cooling (except at ridiculously high air volumes.)

Multi-splits have lower efficeincy numbers than single head units due to the complexity of the algorithms when a single compressor unit is serving multiple masters. The compressor speed & blower speed of the outdoor unit is much easier to optimize with a single interior head blower's speed & refrigerant volumes.

For low cooling load rooms a ceiling fan under occupancy sensor control usually works just fine.

Answered by Dana Dorsett
Posted Wed, 05/07/2014 - 13:46

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