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I have a small shed, 8x7, near the house that I use as a pump/filter house for a koi pond. This building has a 7 foot deep basement made of cinder block which houses an open top settling tank, so humidity will be high. The basement and main floor have a grate between them. I was planning on having an air intake near the ceiling on the main floor and an inline fan in the basement to vent outside. Is this backwards? Do I need the intake coming into the basement and the fan exhausting from the main floor? This building has R-19 insulated walls and R-30 ceiling.

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Can you draw a diagram? I'm having trouble piecing all this info together. Why r-30 insulation on a shed you are planning to put an air in take into? Also, if it's storing water, there's no real way to get rid of moisture. – DA01 Jan 14 '14 at 5:53
Put a (removable) cover on the settling tank? – Ecnerwal Jan 14 '14 at 13:37
It is insulated to help reduce temp in summer and keep heat in during winter. Pumps will be running in winter, but tank will not have water during winter. Ventilation is for summer only to reduce temp and humidity. Small wall mounted heater for winter to keep it from getting below 35 or so. – Ryan Jan 15 '14 at 1:26
Cover on tank would be good, would have to fabricate something to fit. The tank is 48" diameter polyethelyne. – Ryan Jan 15 '14 at 1:28
Spray foam insulation although expensive would be ideal. For a small job like this i seen some DIY setups. Fiberglass insulation is a bad idea. – Justin K Jan 15 '14 at 3:46

Unless I am missing something, this seems a perfect application for an air-to-air heat exchanger. Fully insulate the building, seal it up, and install the heat exchanger in a convenient location. A blower through it could be controlled by a humidistat.

If winters are very cold, some supplemental heat might be needed inside the building unless there is quite a large mass of water.

A reasonably effective plate heat exchanger could be built by anyone with basic sheet metal experience.

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Ventilation for summer only makes it easy. In at the bottom, out at the top - or else close off the grate (use thick glass if you need to see through there) and run an in and an out on the bottom.

A "hula hoop" (not really, I don't think they are that big - but same thing made from ~12'8" of heavy black poly well pipe and a coupling) plus a sheet of plastic would be one lightweight, non-rotting approach to a cover. If you handled it with care, you might be able to use the shrink-wrap plastic "winterizing film" for patio doors and have it be quite clear, if looking through the grate into the tank is important for monitoring. It will probably still get water droplets condensing on the bottom of it, so it might not be all that see-through.

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