HVAC folks - multi split AC system ?

[JHZR2]

Hi,

Toying with the idea of a ductless multi-split AC system for some rooms in my home. There are some questions that I have that I'm just not seeing answers to online.

For a multi-split (single outdoor unit to multiple indoor units), what is the minimum BTU rating achievable given the outdoor unit? I think this relates to turndown.

For example, Say I installed a 36k btu outdoor unit to service four indoor systems. As I understand it, one can install greater BTU rated units inside than the outdoor handler can support (eg 4x 12k BTU indoor units on a 36k BTU outdoor). That seems sensible. What confuses me is turndown for very small units.

Let's say instead I have a real big indoor unit in one spot, and then some 6k BTU units elsewhere. As I understand it, the system modulates to optimize efficiency, so a 6k BTU system for example may not always be flowing enough refrigerant for 6k of cooling. But a single 6k BTU indoor unit operating would already be an 84% turndown on the outdoor unit (6k of 36k is 1/6th or 16%) from its rating. If the indoor unit modulates more, say it only requires 3k BTU, can the system handle this?

Similarly, say I build a system up over time. Say I buy a 36k outdoor unit rated for four indoor split units, but only installed one small (6k) indoor unit to begin with. Would this be an issue?

The last thing I want is this great system, which over cools the air and thus doesn't dehumidify, because it is being used in small rooms and cannot turn down properly.

Insight into designing split ductless units would be appreciated.

Thanks!

 

[SteveSRT8]

I owned a Daikin system in my last home (http://www.daikinac.com/residential/home.asp)
and while it cooled well it did not dehumidify the air properly.

Very energy efficient and used a tricky variable displacement compressor that can be an issue as it ages.

With the newest central AC's I see no advantage unless you don't have ducting options.

 

[JHZR2]

I don't have ducting in my old home, and don't want central air blowing dust all over the place.

 

[henni]

I've setup a few Fujitsu and Mitsubishi minisplits and would always recommend to go with the manufacturer recommended matched pair (or triples or quads if you multizone). Personally, I prefer independent systems that connect to no more than 2 indoor units, gives greater redundancy and the cost is only a little more for the extra outdoor variable speed compressor. Make sure all systems are 240v and at least 23 SEER or higher. Have seen 3500sqft houses cooled down to 74F with a single indoor unit and 1ton(12000btu) Mitsubishi compressor using just 400-650watts of power. The house(s) have original central A/C but it's now rendered functionally obsolete or into a backup role. Now that is impressive!

I've also seen Daikin retrofit systems that connect to an existing central A/C system supply (attic, crawlspace) and the only component taken out was the old conventional outdoor unit and replaced with a "suitcase unit".

 

[JHZR2]

Thanks. The thing with having fewer indoor units per outdoor one is that then I need to mount more unsightly outdoor units.

The basis of going with manufacturer kits is good, but they will let you have, say, a 6k btu indoor unit on a 36k btu outdoor one, and since they are smart and throttle refrigerant and all, I have to wonder if you get full capability of such things because turndown on the compressor is too high.

 

[henni]

One recent installation has a single outdoor inverter variable speed compressor and 56 indoor units, all connected to the central system board and it's working very well, all done according to manufacturer specifications. The unit owner is very impressed over the silent operation and loved his first post-installation power utility invoice.

 

[SteveSRT8]

Henni is so correct about new variable displacement compressors. They are quiet. Super quiet, really.

They throttle down nice and low so they don't just 'snap cool' and not dehumidify properly. Too big of a system is always a big no-no, you want an efficient system that can run a while without spinning the meter. Our new system has infinitely variable inside fan and soft starts so there is no clunk when it turns on as well.

And you can get really efficient monster filters that do better than HEPA quality filtration. That means no dust!