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The following, in brackets, is the old way, I have changed it on 3/15/18-
(I am using transformer oil, I was lucky to make a trade for 3 Gallons of new oil. The first 4 or so inches of the tubing connected to the tube cooling structure used to pump the oil through the tube, is Teflon tubing to allow it to withstand the higher heat encountered as the metal structure will be hotter then the oil flowing through the cooling system.
The HVAC Air Sensing Switch was acquired on eBay, there are a lot of these, and not expensive, some of them are rated in less then a pound of vacuum to operate the switch. To make sure you do not let oil in the switch, I added a burp tank to provide a way to let the air and oil be separated while maintaining system oil pressure. The burp tank is mounted so that the top of the burp tank is way above the top of the oil reservoir. This makes a air pillow at the top of the burp tank which will be under pressure once the system is on and the pump is running. The burp tank must be closed so no outside air can get in or out. The Tee I used was a standard 3/8" galvanized pipe Tee with a 3/8" to 1/8" reducing barb on the side orifice, found this in the nylon pipe fittings also got the 3/8" to 3/8" pipe to barb fittings there also. There is probably a more HI-TECH way to do this, but this was LO-TECH and cheap.
The fact of the 3/8" vs the 1/8" hose forces the smaller one to have a higher PSI so the burp tank will actuality have a small pressure rise from the pump, less then a pound but it is all that is needed.)-

I had the pump go bad and decided since I had to change the pump to change some other things as well. The pressure sensor worked but would fail sometimes and turn the amp off. Then would have to bleed the system. I got a FLOWMETER ROTAMETER Inline Flow Meter 0.5-5 GPM, from eBay for a low price, it has 1/2" NPT female threads so all I had to do was to put in a 1/2" to 3/8" barb adapter in both ends (if you go to find one of these careful most have metric threads). This does not have a switch to turn off the amp but you can read it very well. The new pump I bought is a Grundfos UPS 26-99 FC, 3-Speed 1/6 Horsepower Circulator Pump, I got from eBay for $152.00 shipping included, this is a much better pump and at that price I ordered 2 of them to have a spare . The old pump was 1/25 HP and this one is 1/6 HP and puts out 1.5 GPM, the tube runs a lot cooler than it did before.

When the pump went bad I lost most of my oil, luckily the leak was in the crawl space so did not make a mess. I needed more oil, so went to the power company to see if I get some more, but they don't sell it any more. I got online, and check to see where I could buy it. Well it is now so expensive I could not do that, I talked to a person that I knew at the power company and he told me the oil I had originally was the new oil they use, they do not sell it, but he told me what it was, they changed to Canola oil and I went to the store and bought some Wesson pure Canola oil, for less then $10.00 a gallon much cheaper then the transformer oil at $100.00 a gallons. I was always curious when this oil was yellow when I got it years ago because it did not look like the transformer oil I had seen before, the old pcb stuff. I had it checked out and canola oil was the oil I have been using all the time. That ment that I was able to just add Wesson to my system with out cleaning it all out. This is really good for us hams to build a dummy load or what you we can get Wesson pure canola oil and cost is minimal.

The heat exchanger is made from two automotive transmission coolers, again cheap from eBay, the cooling fan is just a square 3 speed box fan found in Wal-Mart or similar, use it on high speed with the coolers fastened to the exhaust side of the fan with ty-wraps. The oil reservoir is a container I found in the house wares section of a cardinal hardware store, needed a container that allowed for three things, hold enough volume, be clear enough to see the oil level, and have smooth enough sides to get a good liquid seal with connecting fittings. I looked around a bit but did not really find any connectors that were very liquid tight, being an electrician I remembered the fittings they make for the new 1/2" plastic flex conduit connectors and you can get them in straight or 90's. They come with O rings already, but I wanted to be sure I had some good compression when installed into my Oil Reservoir. They come with plastic 1/2" conduit nuts which I did not trust to be strong enough so I took a standard 1/2 inch galvanized coupling and cut it in half, could have used two actually but only found one in my junk box HI HI. I also did not like the outside diameter of the coupling for pressure against that plastic as it could in time begin to migrate through the plastic, so put a 3/4" to 1/2" reducing washer on each fitting on the inside of the reservoir under that coupling used for a nut, also the rubber O ring goes on the outside and fits snuggly against a smooth molded nut, I tightened it is much as felt and did get a good compression on the O ring and it has never leaked a drop. I found that 1/2 ID poly tubing fit snuggly onto the flex end of these connecters so put about 6" on each connector with a hose clamp, then made a barb adapter using a 3/8" to 1/2" bell adapter and a 3/8" and 1/2" nylon Barbs. The GS35B was perfect for this project as the fin structure is removable, I was able to remove the fins and then just have a cylinder that attaches to the tube. I purchased a section of Copper tubing 3 3/16" OD and the same length as the 1 15/16 OD cylinder with the cooling fins having been removed, the length of this structure is 3". This means there are about 2 cups of oil contained in the cooling chamber at any one time. I had two doughnut shaped pieces made to seal the top and the bottom of the cylinder, I then brazed the whole unit together. I tapped the top and bottom of one side for 1/8" pipe threads and put the brass 3/8" pipe to 3/8" barbs. This was a great idea but there was just not enough threads and the oil would weep, so did the obvious, removed the fittings, cleaned the male and female threads with lacquer thinner and mixed up a batch of JB weld and used it as you would pipe sealing compound, never has leaked a drop since HI HI. And again I used about 4" of 3/8" ID Teflon tubing to connect between the metal barbs on the tube and the 3/8" ID Poly tubing. I have a IR temperature gun and have measured the outside temperature during times of long use like in a contest, the highest so far was about 110 Degrees Fahrenheit, a long way from the maximum temperature ratings for the tube, (Spec Ratings  grid=120C(248F) Anode=200C(392F)). So 110 Degrees F on the anode is running very cool.

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