Fan-coil unit

I think the 9"x9" coil I bought cheap on eBay would have been sufficient for this project, and I liked the nice fat copper tubes in it (which would have provided low resistance to the flow of water). But my friend Matt had a spare in-wall fan-coil heater unit with a nice efficient variable-speed blower conveniently mounted in a complete housing. It was just too nice to pass up! There will be a couple of challenges with this unit, but overall it seems like it will make some things much simpler.

First of all, this is a heater unit, so it's not designed to collect condensation from the coils. I don't expect MUCH condensation in this system since the temperatures I'm dealing with will be much higher than those in an air conditioner. But on humid days, opening the server cabinet will let moisture into the system, and it will be very possible that the coil will be below the dew point. So I needed to modify the fan-coil unit to collect and drain condensation.

This wasn't difficult at all. It looks like this unit might come in a version that handles condensation, because it was really only missing a couple of "gutters" and a drain pipe. I split a piece of 2" PVC pipe on the table saw, capped the ends, used a dremel tool to split off the tops of the end-caps, forming a PVC trough. I drilled a hole in one of the caps to add a 1/2" exit drain (I didn't use a reducer because that would have centered the 1/2" drain pipe in the middle of the 2" pipe, whereas I needed the drain pipe to be aligned with the bottom of the 2" pipe).

Here's a close-up of the back-side of the coil after removing the back of the unit (click for a full-size image):



Here's the same view after adding some gutters and a temperature sensor:



The other potential issue with this fan-coil unit is the small-diameter copper tubing, which adds a lot of resistance to the flow of water. I want to keep energy consumption to a minimum, so I really want to stick with the 20-Watt pond pump I purchased for the project and avoid upgrading to a more powerful circulator pump like the 70-Watt Taco 007 pump (about $60) used in many radiant-floor heating and solar hot water systems. So I'm being very careful not to add any unnecessary resistance elsewhere in the system.

With the efficiency of the blower in this unit, I think I'll be able to run it at the lowest setting, which only consumes about 22 Watts.

Eventually, this fan-coil unit will be ducted to the server cabinet and everything will be thoroughly insulated. But for initial testing, I have ducted its exhaust to its intake with 25 feet of 6" insulated flexible duct.

This gives a closed-loop air system with some extra exposed duct to approximate the heat load of the metal server cabinet.

In this photo, you can see the insulation board front that I made for the unit, the flexible duct, and you might be able to make out the wires leading to the temperature sensors that I added to the water-in and water-out ports.

You can also see the 1/2" PVC drain mentioned earlier for the condensate. Note that I formed a trap at the bottom so water will collect there, blocking the flow of air. We don't want the condensation drain to be an air leak in the system!