I don't like two-line oil systems since they're constantly circulating oil through the filter(s), pump strainer and the oil is cold. In situations where the return line is out of plain view like outdoor tanks, underground storage tanks or under mobile homes, the pressurized return line could leak and the system would still run so the homeowners wouldn't be alerted of an issue. In the field many filters really are really undersized considering the amount of oil they're circulating with two line oil systems. We generally convert most two line oil systems to single line systems or install a TigerLoop and remove compression fittings, check valves bad fittings, bad flares or any other sources of vacuum leaks and potential future problems.

First of all, if the DWV(drain,waste & vent) piping is installed correctly there should be an adequate pitch to insure there is never any standing water in the pipes so freezing is basically a non-issue. The exceptions to this would be P-traps which may be below the sub floor such as under the bathtub, shower stall or laundry standpipe. In this case the P-traps will generally be well up in the joist bay and very near the underside of the sub floor. General purpose insulation can be installed in the stud bay on the underside of the trap but do not insulate the top of the trap. Keep the top area open so that residual heat from the living space above can come through the sub floor and maintain a minimal temperature in the joist bay in the proximity of the P-trap. Water supply lines are a whole different problem. We must keep in mind that insulation does not generate heat, it only retards the rate of heat loss. Even well insulated supply pipes will freeze if exposed to sub freezing to sub-zero temperature for a prolonged period of time. One method of protecting the pipe is to run it as high as possible through the joist bays and insulate the underside in the same manner as suggested for P-traps. Another method is to run the hot & cold lines very close together, then install a circulation loop on the hot water line and cover both the hot & cold lines in a common insulation. In this manner the slight amount of heat radiating from the hot water line would be retained in the common insulation and thereby protect the adjacent cold water line. The next option would be to install thermostatic controlled electric heating tapes on the pipes, then cover that with insulation to keep the heat in. In regards to what type of insulation. It really doesn't make a lot of difference. While the preformed poly-foam insulation is what the homeowner is usually most familiar with, they make both poly-foam and rigid preformed sections of fiberglass pipe insulation with either a paper or plastic covering in sizes ranging from 1/2" diameter up to 20" diameter and they also make matching preformed covers for Tee's and elbows but you would have to get those types of insulation from a local HVAC or Plumbing Supply.

Two scenarios: 1) Basement: From unfinished area pipes run from furnace to a finished room in the basement. There's a very small opening for the copper pipes to run through to that room, I want to insulate that without spray foam. 2) From basement to living room, there's a small opening in the floor for the baseboard hot water pipes. I want to insulate that as well.

For a gap that small, silicone caulk is probably the easiest. Spray foam is a better choice for larger gaps, and even then it can be hard to maintain control of the expansion - foam goes everywhere! Bruce