thermosiphon question

[uppsalawayland]

I am trying to instal a thermosiphon water heater, but find that due to increases in copper prices recently, copper pipe is prohibtively expensive. Will Black plastic pipe work, or does it not conduct heat well enough?

[alancorey]

I wouldn't think conductivity of heat would matter as much as how well it can stand hot water.  If you mean the cheap black plastic polyethylene that comes in rolls I'd have my doubts.  We used some 30 years ago and ended up replacing it with copper.  PVC or ABS rigid plastic meant for indoor plumbing (there's also another type, can't think of it) would be better.  Even copper gets pinholes due to acid rain in my area.  I'm in a house now with 30 year old PVC and no problems.  Also bear in mind that this water may get hotter than normal domestic hot water, especially if something malfunctions in circulation so the water can't move.  The black polyethylene just keeps getting softer as it gets hotter, and eventually splits open.  Having low pressure might help.

  Alan

[stephent]

CVPVC is the high temp type plastic pipe and is rated for somewhat higher temps.

[wlcoldiron]

Hi

 what temp are you trying to achieve? SDR 11 poly pipe is rated for psi and temp.    Thermosyphon systems require large pipe sizes for the water to move. I have never seen cpvc in sizes large enough to thermosyphon. You might look at PEX tubing.  Weldon

[wdyasq]

"I have never seen cpvc in sizes large enough to thermosyphon. You might look at PEX tubing."

I'm looking at some 4" CPVC right now - how large do I need to go?

Ron

[stephent]

Wouldn't want to go too big with any pipe for thermosiphon, the process will break down and just circulate up/down in the same pipe if given enough room (like going up and down at the same time in the same pipe, loss of flow in/out, etc.).

Might want to see how big an inlet and outlet vs square footage some of the commercial units use and judge accordingly for your home built unit.

[harrie]

Yes, PEX tubing is what to use!!

[jimovonz]

I wouldn't recomend 'black plastic pipe'. Also, plastic pipe rated for standard hot water will probably not be suitable either. Domestic hot water is typically ~60degC where as solar heated water (thermosyphon systems especially) can reach 90degC+. Personally I would only ever consider copper for pipe work where the fluid carried is subject to an uncontrolled heat source.

The thermosyphon effect does not depend on the thermal conductivity of the pipes carrying the fluid between the hot and cold sources.

[hydrosun]

I'm not clear what your heat source is for this thermosyphon. Are you making your own solar water heater from these pipes?  For heating swimming pools at lower temps it would be okay to use plastic tubing for the heat gathering unit(ie the roll of pipe thrown on the dark colored roof).  But I wouldn't  try using plastic under glass where the temps are alot higher.

 If the plastic pipe is used to transport the heated water up to a water tank, I've used 3/4 inch cpvc pipe for years to thermosyhon from a solar water heater to the tank. And from copper coils on the wood stove. I used 1/2 copper in a zig zag pattern on the outside of the stove and plaster of paris with chicken wire to bridge the gaps to the stove. I recycled old copper pipe from someone elses  solar  collector to make the wood stove water heater.  The hot water tank is 7 feet higher and 20 feet away and the heat is moved in 1 inch cpvc pipe. I did use 3/4 pipe before but though the bigger pipe would work better with the higher heat of the stove. I transistion from the copper pipe to the cpvc a foot from  the stove with a threaded coupler.

Chris

[robl]

I'm not sure if this will be helpful but...I've had a successful thermosyphon system for more than 20 years. The collector is home-made (4 foot by 6 foot) using 1/2" copper pipe. The  30 foot lines to the tank are still the original 3/4" Butylflex (spelling?) type (wrapped in high-density foam liners) despite their bad rep. I've had to replace a couple of fittings but that's all. There are several vacuum/pressure/temperature reliefs in the system but it rarely reaches boiling temp at the hot spots (top of the tank and top of the collector). If was going to upgrade/change the system today, I would switch over to 1" or 3/4" Pex as the Butylflex fittings do make me nervous.

Regards

Rob