View Full Version : using a condensing coil as a solar heater for hot tub

26-07-2012, 11:51 PM
so i thought about laying out a conderser coil, building a frame with a glass opening and painting everything black. then using a submergible water pump to circulate water through it to heat the hot tub. i calculated i have about 400 feet of copper in this coil.

my biggest concern is the brazing material and how it will leach into the water.

any inputs? will it be a toxic bath or are the levels so low that it will not matter?



27-07-2012, 12:16 AM
Can't see it being an issue if standard brazing materials used...until the advent of plastic piping for plumbing we all used copper and brazing systems for a our drinking water...

27-07-2012, 12:38 PM
i wasn't around back then=] my largest concern is that i don't know what they use at the factory to braze the copper end bends and distributors. id figure if it ran for 10 years with high pressures most of it should have come out, but then again when you put chlorinated water through it that might be different story.

so you think that it would be a safe choice?

The Viking
27-07-2012, 01:34 PM
In swimming pool applications, where chlorine and other chemicals are frequently used in high concentrations, copper are best avoided. It tend to react with the chemicals and leave a green tinge on stuff.

For your application, why wouldn't you use a long length of black plastic piping? Seen that done successfully even in situations where I thought it wouldn't work. (like 200m of black plastic piping, all overgrown but still supplying enough hot water for a constantly used shower. This at ambient temps just around 20C)


29-07-2012, 02:23 AM
i just have coils, lumber and shower doors laying around. plus the idea of being able to get boiling hot water even the cold florida winter days=] kinda excites me.

the green folks around here are doing all kinds of stuff from plastic hose to beer can solar heaters and if this is safe to use then hell with all the aluminum fins painted black it should work miracles.

29-07-2012, 04:33 AM
Hi Arttu.
as the viking indicated the chlorinated water will eventually corrode holes in copper, but give it a go anyway if you have the bits lying around, put some reflective aluminium foil underneath condenser coil to get a double wammy effect. Long term look at the black plastic piping, here it is called alcathene (cheap) I did a freebee for a school swimming pool, summer the pool was too hot, had to add covers to collectors.

Rob White
29-07-2012, 11:01 AM

If your condenser was brazed with cadmium bearing rods then
it is recomended that it is not used in any way, where the thing
can come into contact with food or food stuffs, so that may mean
liquids that come into contact with the joints and then you..........

Cadmium is used to make the filler rod flow at lower temperatures
and there is a possibility that it has been used on the condenser?????



Read section

A6.2.2 Indirect Exposure of the Public to Cadmium from Brazed Joints
In November 2003, a high concentration of cadmium was detected in water in the island
of Rhodes, Greece. Following investigation, it was found that water originated from a
nearby water cooler and the source was cadmium-containing fillers in fittings at the inlet
and outlet point of the cooler tank. Extraction from this source was considerable when
water was stagnant for extended period. A preliminary survey indicated that cadmium
concentrations differed significantly according to water cooler brand; consequently all
drinking water coolers were considered to be “imminently hazardous consumer products”
and examined in the years 2003-2005. 30% of the examined water coolers found to
produce water with cadmium content exceeding the legislative limit of 5 ppb Cd and a
program was carried out to ensure that all water coolers tested and found to contribute
cadmium to drinking water, were repaired, permanently removed or replaced (Matsis &
Nikolaou, 2006).
In a similar case, in 2004, cadmium concentrations above the limit of 5 ppb were detected
in drinking water distributed by cooling devices that were positioned in public buildings in