Q: We’ve found another property we are interested in.  It is bank owned so we are not able to get much background information.  We know that the property had an oil storage tank, and that they switched from oil to gas in 1960.  However, this was before Mercer Island was incorporated so the city has no record of the tank being removed or decommissioned.  Although, there is a record of a permit for switching from oil to gas.  Do you know how tanks that were no longer used were dealt with in 1960?  We were unable to find the filler cap when we were looking at the property so we don’t know if the tank has been removed or decommissioned.  Any insight you can offer would be appreciated.


A: Typically they were just abandoned. Usually with some oil left in them, it wasn’t worth the salvage a cost to remove it. It is common to find them this way on houses that were converted from oil to gas, prior to 1995.

Now we have them retired to code, requiring them to be pumped out, rinsed and then the filler cap broken off. The pump out typically includes any remaining oil and the water that may have found its way in.

So it maybe a good sign if the filler cap was possibly removed. It’s also good news if it wasn’t available for old oil and paint to find its way into it either.

You can try to locate it yourself or hire an oil tank company to come out and locate it with a strong metal detector and soil probes. When located, they can determine if has been properly retired and if not they will. If they don’t find it, we usually assume it’s been removed.

I always look for the obvious locations to install a tank, as the house was being built. There is an important rule of construction to always remember: if there is an easier way to do it, it usually was the primary option. Trenches and ditches are always the straightest possible line, to avoid extra digging; oil tanks are close to the foundation, requiring less excavation and shorter oil lines.

So look in the areas that required backfill first.

On the inside, or in this case in the crawlspace, look under the house for the oil lines. These small copper tubing, always a pair, will run from the furnace location to the exterior foundation wall closest to the exterior tank. Easy to trace when doing the dirty deed of exploring the crawlspace.

With a basement, it is typically in the area of back fill, closest to the furnace and chimney. Oil and older gas furnaces, as well as gas water heaters, were always vented through chimney or metal gas exhaust pipe, through the roof.

In a basement of a 1950’s and older home, look on the floor by the furnace for copper tubing breaching the concrete slab. The direction to the exterior location is usually discernable, often times with patched path of concrete, where they buried the tubing. It is common to find the oil / air separator mounted on the basement wall, it looks like a smaller copper tube running into a larger copper pipe and then back to tubing.

On the exterior, look for the filler cap in the yard, next to the house as well as the tank vent pipe. The filler cap is usually at grade, but sometimes just below. It will be within 5 feet of the house and within reach of the oil trucks long hose.

The vent pipe is typically a 3/4″ galvanized steel pipe, leaving the ground, next to the foundation. It usually runs up the siding about 5 feet with a ∩ (inverted U) shaped fitting on the top. The tank filler cap is at one end of the tank and the vent pipe exits the other. This vent pipe allows the air pressure to equalize in the tank and runs from the tank to the foundation wall and up the siding. If this vent pipe has been removed, you can look for built-up paint, on the siding, in its shape.

If you don’t find it, we can try to spot it prior to calling a service.

It is very common to find a tank not retired to the newer code, if it hasn’t been on the market in the last 15 years or the oil furnace was removed over 15 years ago. If it has been sold within the last 15 years, or the gas conversion was done since then, odds are good that it has been properly retired. So check the age of the furnace too.

It will cost about $350 to $400 to have it properly retired in place including all of the required documentation and $100 to $150 to have it located.

Removal typically makes sense if you expect to build an addition in that area, otherwise people usually let sleeping dogs lie and do the proper retirement.