[Greenbuilding] One more question on AC

John Straube jfstraube at gmail.com
Mon Jul 8 08:59:30 MDT 2013

It takes about 400 cubic feet per minute of airflow to deliver a ton (12 000 Btu/hr 3.5 kW) of cooling in normal systems.  You can reduce this to maybe 350 but at this flow rate you risk frost building up on the coldest part of the coil, which spreads until the coil is a block of ice.
It is difficult to find commodity AC systems under 1.5 tons, so minimum ducting is usually for 600 cfm, and 2 tons with 800 cfm is far more common.  600 cfm can also deliver about 30 kBtu/hr of heating BTW, so a nice normal minimum.
Given than an HRV is unlikely to be designed for much more than 100 cfm, the short answer to your question is "NO, not with normal systems"
Minisplits are available that are smaller and have variable output that drops to only 3000 or 4000 Btu/hr, but most are not compatible with ducts.  Even the ones that are available for ducts are assuming ducts that are larger than normal and thus have less pressure drop than normal systems.
I have a very similar situation that you do.  I dont need much cooling, if any at all, but the humidity is a huge issue. I have a dehumidifier running in the part of my basement that is not insulated under the slab (built 1960) and that helps but does not solve the above grade part of the house.

For your house , I think it really is best to find a central spot near the open area where you can install a ductless mini-split.  I am installing a Fujitsu ASU12RLS2 system based on experience with several projects and hundreds of other home installation.  This unit can run down to about 3000 Btu/hr and should therefore provide little cooling and a lot of dehumidification.  Will see.  Other houses, not as efficient as mine, have used this and very similar units, and found it worked very well.  It will cool your house however unlike a dehumidifier, so you will have to "live with" temperatures that are a few degrees cooler than now if you want the RH down to 50% (where you should be targeting).

On 2013-07-08, at 10:47 AM, Lynelle Hamilton <lynelle at kos.net> wrote:

> Can I run AC through the HRV ducting I already have?  I don't like the look or noise of mini splits, so am looking for any other option.  My HVAC guy says that an AC unit will freeze up, as the HRV ducts are too small.  I did have one run through similar HRV ducts in my old home without problem, however.
> Does anyone have any experience with such a configuration?  I've googled, but came up empty.
> Should add that I don't need much cooling....it's the humidity that's the problem.  House is well insulated and sealed. However, I'm on the water so humidity gets to 80% in the house unless I run a dehumidifier. Running it raises the heat in the house 8F and doubles my hydro bill--hardly green.
> Many thanks!
> Lynelle
> -- 
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John Straube

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