Is the leak in a horizontal line? If so, he probably he cannot get it to seal because of residual water. Standard advice is to put a piece of soft white bread in the line. The idea is that the bread keeps water away from the joint, allowing a good seal. Afterwards, the bread can dissolve to not cause any problems from being in the line. I have what I think is a better (but closely related) approach. I spoon a bunch of flour into the line. I then shove a pile of it an inch or so up the line with a dowel close to the same diameter of the pipe. The flour has the same effect as the bread, but it does not have to "dissolve," as it is already a fine powder.

I doubt very seriously if the failure of the humidifier supply tube is a result of heat. First of all, let us consider the temperature of your incoming cold water supply. The mean average of the earths crust is 55degF (geothermal temperature of the crust). Soil is a rather poor insulator so the surface soil then cools proportional to the ambient air temperature. While the crust is 55degF the actual surface of the soil will freeze for a short depth which we define as the "frost depth". While the frost depth is the deepest point at which freezing commonly occurs it must be understood that from that depth the temperatures increase gradually from freezing temp(32degF) to the natural geothermal temp(55degF). Typically geothermal temp occurs at approximately twice the frost depth. In order to protect our water lines from freezing the codes require that we bury the line 6" below the frost line thus when the dirt is frozen to the frost depth the average temperate at the water line may be only slightly above freezing (34degF+). Fresh water reaches a maximum density of 231cu.in per gallon at 39degF. There is a very coplex formula to determine the rate of linear expansion but for the sake of illustration let us use the maximum here. When water is expanded from its maximum density of 39degF to the boiling point it expands by approximately 40:1 thus we could say the physical volume of a 40gal. water heater expands by one gallon. To put this in perspective consider that a 1/2" pipe has a volume of one gallon for each 98 linear feet. We must now keep in mind that water is not compressible therefore as the volume expands it must go somewhere. Normally the increase in volume is compensated by physically pushing some of the water back to the municipal main or to your well pump pressure tank however if we install a check valve in the line there is no place for the additional volume to go. In this case the excess volume would by necessity be relieved by discharging through the water heater Temperature & Pressure Relief Valve (T&P valve) BUT let us consider another potential problem here. Imagine for the moment that your T&P discharge line discharges out through the wall of your house. The discharge end of the line is then exposed to atmospheric temperature and in a worst case scenario the outside end of that line could freeze shut. At this point there would be no place for the additional volume to be released and it could result in blowing out fittings or valves or rupturing a pipe or the water heater vessel. Now keep in mind that the T&P valve is a spring loaded valve that is set to release when the pressure reaches 150psi and most likely the plastic line to your humidifier is failing slightly before the internal pressure is sufficient to release the T&P valve. On the other hand, if you did not have that plastic line there is a very strong likelyhood that the increased volume could be relieved by blowing out a compression fitting at a fixture angle stop, a pipe joint might fail or premature failure of Bibb Washers in your faucetts. The proper solution is to install an expansion tank on the water heater side of the check valve to absorb the increased volume. Installing an expansion tank is quite simple. You simply install a Tee on the cold water line with a male thread adapter on the Tee stub out, then screw the expansion tank onto the thread adapter.

Before you begin any work on the project you are required by law to call to have all existing utility line locations marked. This is a free service but they generally require a minimum of a 48hr notice. The actual name of the organization that makes the survey varies from one location to another by example in my region one are calls it "Dig Safe" while in another area it is known as "One Call". You can call your local code enforcement office or any local contractor that does any excavation work, I.E. excavaton contractor, gas company, water company, plumber or electrician, and they will gladly give you the local phone number. They will be able to mark the location of any buried public utility lines which are on your property such as your electric line, gas lines, municipal water, sewer or steam lines, cable tv lines or commercial gas or oil pipe lines. Generally the public lines are only in the easements adjacent to the roadway but such is not always the case. By example, my sister has a 60 acre farm and there is a 16" diameter commercial fuel pipeline running across her property from end to end. Once again, this is a free service but if you fail to do it and you happen to damage one of those lines you can and will be held both civilly and criminally responsible for any damage that results from damaging one of those lines so I caution you to take this step very serious. The line to the barn must originate on the discharge side of the pressure tank which if I understand your description correctly is in the house. You should install a full bore type valve (ball valve or gate valve) where the line ties into the pressure tank or at any convenient point inside the house before the line goes through your footer wall. The line must be buried a minimum of 6" below the frost line. Your frost depth is 24" so your minimum line depth is 30". The line must pass through your footer wall and into the structure at the required depth and you must sleeve the pipe where it passes through the wall. To make a sleeve you first install a short length of pipe at least two nominal pipe sizes larger than the desired pipe through the wall and extending out 2" to 4" on either side. The sleeve is then sealed to the masonary wall by any convient means, mortar, tar pitch, expansion foam etc. After the desired pipe is passed through the sleeve the space between the OD of the desired pipe and the ID of the sleeve pipe is then filled with expansion foam. Operating a ditch witch is fairly easy but they tend to jump around a lot and even an experienced operator will refrain from operating within 3 or 4 feet of the foundation wall or within 5'either side of any buried utility line. For safety these areas should be carefully hand dug. Whether you go over or under the electrical line would be dependant upon the depth of the electrical line in reference to the required depth of the water line. This would be a judgement call once you have hand dug that area and located the electrical line or confirmed that it is below the depth you need. Also, in some jurisdictions buried electrical lines are encased in concrete. When encasing an electrial line in concrete they add a red dye to the concrete so if you happen upon any red concrete try to avoid disturbing it. You could install a Curb box type main shutoff valve at the barn end of your line in the same manner as a residential water service curb side shutoff but these valves are rather expensive and if you have the main shutoff at the house you really won't need it. Check the installation specs on your hydrants carefully. The actual valve of a hydrant is below grade at the bottom of the valve and when you turn the water off any standing water in the riser then backflows and discharges below grade. Genearlly they require a pea gravel bed at the base of the valve to absorb the water in the same fashion as a french drain. Typcially we use roll polyethelene pipe for this type of run. At the pressures permitted on residential water service a 3/4" roll poly line has a friction head loss of one foot of vertical static head per 100 linear feet. (-.434psi per 100ft). Your run is 200' so the friction loss would be approximately 1psi (not enough to be a problem). The change in pressure from elevation is determined by the total difference in vertical elevation from the source at the house to the demand at the barn. The change is .434psi for every foot elevation change. In this case the barn is lower than the house so the pressure at the barn end is actaully higher than the pressure at the house. When the line goes down then back up the pressure loss to the rise is offset by the pressure gain on the fall so that portion is self cancelling and the overall pressure remains the same at the load end.

I just finished tying in a sprinkler and yard hydrant into the 3/4" copper lines in my crawlspace. I used a combination of solder and Sharkbite connections. I would have liked to use all solder connections, but the Sharkbite fittings offered a significant advantage in some locations. Being a crawlspace in a 60 year old house, there is limited space to work (about 32") and a bunch of pre-existing structure (gas/electric/low voltage lines, insulation, plastic water lines). I soldered as much of the new piping as possible before bringing it into the crawlspace (ball valve, male fitting to connect to backflow preventer, some elbows/T's). I used the Sharkbites in areas where solder joints were impractical. This included a joint that had multiple network lines running in the joist bay directly above the joint, and in areas where the joints were in close proximity to insulation or joists. I could have made a heat shield from a piece of HVAC duct or something, but I've seen more than one house burn after a quick soldering job sparked a fire.... In summary, I think they're great for here and there use, but too expensive for widespread use. I'm not worried about them freezing, cause I plan to blow that section of piping out in the winter anyway.