DIY Solar hot water heat exchangers
 

I'll be working on my solar hot water setup quite soon and am thinking about heat exchangers. It seems like data to help you size a heat exchanger is next to non-existant online, so lets put some together.

I love Gary's 300' of 1" pex that he uses for his hot water setup. Its simple, and works great. However, mine will be used for more than just DHW. It'll also be used for space heating. In Gary's testing, he shows that at higher flow rates, the PEX can not keep up. So, I'm looking for something a bit beefier. Some thing that can exchange heat a bit faster. I don't have my house all on hydronic heat yet, but that is the plan so I will design with that in mind.

The initial thought was a spool of copper tubing. Copper conducts heat faster than pex. So, run a few lines in parallel to reduce pressure drop and you have a nice heat exchanger. Make the lines relatively small to ensure good heat transfer. The problem here is copper is expensive. However, I think I'm going to at least need to use some copper to make this work.

Anyway, I'm looking for any help here. Any tips to make a good heat exchanger that can handle moderate flow.

I also just had an idea to increase the efficiency of the heat transfer of copper tubing. I've seen 'turbulators' sold to increase heat transfer in piping. Its essentially a bent piece of metal that is inserted into the pipe that mixes the fluid as it goes through. This reduces the boundry layer and eliminates laminar flow. It also increases pressure loss, but as long as it is accounted for, you should be fine. Here is an illustration.

http://www.thoroughbreddiesel.com/pe...ator-small.jpg


Now, I don't want to have to buy or even these turbulators. However, I like the effect they have on heat transfer. What if you were to simply crimp the tubing here and there every few inches or so just to stop laminar flow? I would think you could get roughly the same effect.

why do you need an exchanger for the heating? Just run 1 line for the domestic hot water in the pex. Then run a different loop to the space heating system. unless I'm missing something there is no need to run that through an exchanger

Thats a good point. I think most hydronic heating systems are pressurized at a lower pressure than domestic water. This helps reduce the amount of power needed to move the water. Still, it will need a decent heat exchanger to pull heat out of the tank all day long. But you're right, I shouldn't have to deal with super high flow rates as long as the DHW is seperated.

The DHW will be on a seperate loop at a higher pressure and would probably be just fine with the 300' of 1" pex.

my thoughts on using solar to space heat and DHW is treat them as completely seperate systems that happen to share the same storage tank. My "plan" is to just pump heated water from the tank to a radiator to begin with. eventually I hope to add radiant tubing under the bedrooms then I'll just pump straight through them when additional heat is needed. No need to do an exchanger on that part of the system since you don't drink the water.

This is exactly what I am planning on doing. I plan on submerge the pex coil into the tank for my DHW and have a separate system to then pump the tanks water through a radiator of some type for space heating.