The goal of the project was to heat my hot tub in an economical way. The existing built-in electric heater used almost 6000 watts when running and at the new electric rates, keeping the tub up to 104 F made the utility bill very high. I attempted for a while to keep the temperature at a lower setting and turn it up only in anticipation of using the tub. This was unsatisfactory since it makes spontaneous use of the tub impossible and still used expensive electricity for heat.
The idea began when my 21 year old household gas water heater appeared to develop a leak. The leak seemed to grow so I bought and installed a new tank gas water heater. I then had a somewhat viable gas water heater on hand. Why not use it to heat water for the hot tub? The hot tub is located in the back yard lower patio, and the only apparent place for the extra gas water heater would near the gas source next to its replacement in the basement, about 40 feet distant from the tub.
Step 1: Adapting the Hot Tub
The hot tub has provision for an ozone generator, but this option was never installed. What this means though is the tub has an unused jet with stubbed off 3/4" hose connected, and a stubbed off hose on the manifold which comes after the hot tub pump and electric heater. The manifold has several tubes which direct water from the pump to the various jets which shoot water into the tub. My plan was to connect the stubbed ozone tubes to the tank heater in the basement and disconnect the electric heater.
The hot tub pump runs at two speeds. It comes on slow when it comes on automatically twice a day for an hour of filtering, and slow also when the thermostat tells it it needs heat and the heater circuit is turned on. It comes on fast only when manually directed by push button and then only stays on fast for 20 minutes. I wondered if the hot tub pump at slow would have enough pressure to move the water over the relatively long path to and from the gas water heater.
Step 2: Using an Extra Hot Water Tank - Skip This Step
My initial approach used the extra hot water tank heater (HWT). I stood it next to the household HWT, put in the extra gas line and exhaust gas vent and ran Pex tubing 40 feet back and forth between the HWT in the basement and the hot tub in the back yard.
Step 3: Heat Exchanger
Giving up on the Hot Water Tank
Using the old tank hot water heater (HWT) worked for a while. But it didn't fill to the top, allowing air to enter the system, and it generated lots of rust that came into the hot tub. After fighting with the hot water tank for too long, I finally decied to go with heat exchanger (HEX) approach. A heat exchanger would use the heat from my household hot water tank and transfer it to the hot tub. Most of the plumbing was already in place. And based on my struggles with the dedicated (now dead) HWT approach, I had a pair of one way valves and a extra Taco 007 motor.
I looked online and there are great stainless steel heat exchanges you can buy. Online I also saw the sidearm style of HEX, which sometimes use convection to move the water. I priced them online, found out about sidearm style, which parallels the HWT and sometimes use convection to move the hot water through.
Heat Transfer Fins
A neighbor suggested this approach for heat transfer fins. 4 inch lengths of 3/4 copper, slit lengthwise but leaving 1/2 inch uncut to make 4 bendable fins. One slit is cut through so the circle can be stretched to fit over the inside tube. I have them all facing the same direction and soldered in place at even intervals.
I used 5 feet of 1/1/2 inch heavy copper tubing for the outside. The end pieces are 1 1/2 reducers that happen to be bushing style, which means they fit into the tee. If I had used regular non-bushing reducers, I would have had to cut some 1 1/2 nipples from the 5 foot piece. Notice that the bottom of the HEX has a street 90 soldered in instead of having the threaded fitting come straight out. I should have done the top this way too. I had two temperature gauges left over from another project, and I put these into the top and bottom of the hot tub circuit so I could see the temp. differential. Also they look cool.
After running for a few days the new system appears to be working well. Due to heat loss at the hot tub, the system calls for heat about every 1/2 hour and the system runs for about 10 minutes before reaching 104F. It would have been "unfortunate" if the HEX was too small to bring the tub up to temp. But 5 feet of length seems sufficient. Probably larger would be better.
Initially, due to plumbing confusion, I had the flows through the HEX both running the same direction. Published lore says they should be flowing opposite directions, so I changed it. Based on my log of electric energy used to maintain temperature during the night, it only works marginally better flowing opposite. But in sum, the HEX is superior to the HWT approach. There is NO MORE RUST in the tub. I suspect the natural gas usage will be less since I'm not maintaining a second large tank. And space in the basement has been reclaimed. Cost not including pumps was about $200.
Line below the Bottom Line - Addendum several years later
I noticed one day that the hot tub had become more full. Hmm. The only explanation was that the HEX had failed and that pressurized household water was coming into the hot tub pipe. Avoiding the big box store, I used a local plumbing supply and bought all new copper for about $85. Less than before. So I rebuilt the HEX, this time using type L copper which is thicker than the type M I had used before. Also, I'm resolving to watch the Ph more carefully and not let it become too acid.