Should I use small electric water heater as drainback tank?

Drainback tank size?

Could you post a schematic of your system?
I too have my system set up to drain back (I pulled the discharge check valve out of the circulating pump, the lines are all sloped towards the tank). I am using an 80 gallon tank, it provides a little bit of storage. The tank is vented and I left enough space to contain the drainback volume. All I do is shut the power off to the pump and it all drains back (5 Alberta winters and no problems). I have a split system, when the tank gets up to temperature I switch the flow to dump heat into the floor slab of my work shop. I have a second loop which circulates from that tank of heated water and exchanges the energy to a pre-heat tank (which is actually my old hot water tank with the burner & vent stack removed) of water that supplies a conventional demand hot water heater.

I don't know what is special about a "drain back" tank. My tank is a repurposed small pressure vessel.

Why can't you just drain back to your storage tank?

My system, when it is built, will use a 120 gallon schuco tank that I got cheap as the reservoir for pre-heating water for my conventional water heater. The schuco tank has a heat exchanger that I'm planning to circulate the solar loop through. There isn't spare volume for the panels to drain back into, unless I use another small tank for that particular purpose. That's the plan anyway... Other than a sight glass I, too, don't know what is special about a drain back tank.
Doug

Drainback tanks are usually stainless steel with tappings top and bottom.
Water heaters are glass lined steel.
In a drain back tank there is always a large amount of air (oxygen etc) that will eventually rust out the steel tank and allow rust to circulate through the system.
The stainless tank should last for the life of the system.

Ah, that makes sense. Is that a general property of drainback systems (more dissolved or entrained oxygen) than glycol systems? Do drainback systems have a tougher time with corrosion? I have a bronze pump, and I'm plumbing with copper. I was anticipating filling the solar loop with distilled water. Is there an an anti-corrosion additive that people use with success too?

Doug

As long as you use a bronze or stainless pump corrosion is not a problem with copper piping. Just flush out the system thoroughly to remove any flux or solder that may be in the lines. If your soldering technigue is sloppy you can use some Rhomar Hydro solv 9100 to flush the system for a few days then flush and fill with distilled water.
No anti corrosion additive required.

In a glass lined tank its only the ceramic which is exposed, there is not even a spec of steel which gets in touch with water, so i feel that even glass lining is fine so far as corrosion is concerned. Rest venting could be taken care of with simple venting option on cold inlet.

I don't feel that the glasslining in tanks can handle the extreme temps encountered in the drainback tank for an extended period of time. I have seen many glass lined water heaters fail after 5 years without running more than 120 degree water through them. Corrosive water maybe. The glass lining is not 100 % impervious.

Yes, even we have noticed twice in last 8 years amongst 150 odd that we have installed. But since the question was considering the economical consideration as well, so it would not be such a bad choice if the duration of usage is going to be limited.