Old 12-03-2002, 01:16 PM
Join Date: Jun 2000
Location: Last house on the right.
Posts: 2,365
Fuel Oil Tank Problem

I have two fuel oil tanks. They are connected in series, and each has a valve for the outflow. Physically they are next to each other, at the same level, with a pipe running from the valve on tank A, past tank B, connecting to tank B's valve by a 'tee', then on thru a filter to a pump near the furnace. This pump runs when the furnace is on, feeds the furnace; and sends excess fuel back to the tanks thru a return line. The return line can be positioned to enter either tank, and goes in at the filler cap. Thus the filler cap is loose on the tank with the return line, so that tank is vented.

Currently the valve on tank A is on, tank B is off, and the return line is in tank A. The elderly gentleman whom I purchased the place from instructed me to use one tank, then switch everything over to the other tank when necessary. He pointed out that if both valves are open, the level in the tanks will even out.

My question is: So What?

Suppose I open both valves. The fuel oil in tank B (which is fuller) will slowly drain into tank A, until the levels in each are the same. The system will become one big tank. Everything will work just the same as when the valve on tank B was closed. Right?

Or am I missing something? Do I need to vent tank B, too? What about the return line?

I figure I've got a couple months before I need to make a switch, and I'd like to figure this out before then.
Old 12-03-2002, 01:32 PM
Join Date: Sep 2002
Location: Houston, TX, USA
Posts: 18
It sounds as if the tanks are actually parallel rather than in series if they share a pump via two lines connected by a tee junction.

If I am understanding your set-up correctly, you should be able to leave both valves open, and leave the return line as-is... as long as you vent both tanks.

You can probably call a business that sells these tanks and just ask about it. People will usually help out on things like that for free, in hopes that you will remember them in the future if you need service (consider it advertising).
Old 12-03-2002, 01:36 PM
Join Date: Sep 2002
Location: Houston, TX, USA
Posts: 18
oooo.... clarification needed....

you say that tank A connects to tank B's valve by a tee...

On which side of tank B's valve is the tee? Is it upstream of the valve, or downstream?
Old 12-03-2002, 05:41 PM
Join Date: Jun 2000
Location: Last house on the right.
Posts: 2,365
The valves are on the outlets of the tanks. The pipe leading to the filter is fitted with a 'tee' at tank B, and an 'ell' at tank A. These connect to the output of the valves. I guess that means the tee is downstream of the valve. The valves can isolate the individual tanks from the feed portion of the system.
Old 12-03-2002, 06:14 PM
Join Date: May 1999
Posts: 18,416
I would question why you would want to drain both together. If I were you I would work on one tank. when empty (or close) switch to tank B. Refill tank A when the price is right.
Old 12-03-2002, 06:47 PM
Join Date: Sep 2002
Location: Houston, TX, USA
Posts: 18
Keep in mind that I’m not an engineer. I do however design, do hazard assessments for, physically construct, and run continuous flow chemical reactors that are much more complicated than what is being described here (although on a much smaller scale (pilot plants), but the principles remain the same). Take that for whatever it’s worth. Remember… I’m sure that anyone who sells this type of equipment would be glad to confirm any of this at no cost to you.l

If the tanks:
1. are the same height,
2. are on the same level (their bases and tops are at the same elevation)
3. are both vented to atmosphere (or at least have the same headspace pressure),

I see no harm in opening both valves. You are right. You will essentially be turning two smaller tanks into one large tank.

I am also assuming that the linear flow rate through the tee at any given time is relatively slow (as in… not 10 mph or more). If, when the pump is pumping the fuel oil, the linear flow through the tee is rapid, you could run into slight level differences in the two tanks that could cause a spill out of the top vent if the tanks are ever full to the brim.

I do agree with k2dave about the practicality of doing this though. If you are surprised by running out of feed in one of two separate tanks, no biggie… You can switch tanks, and order more fuel oil. If you are running essentially one tank, and are surprised by running out of feed, you are SOL until you can get more fuel.

Good luck in whichever route you take.
Old 12-04-2002, 07:05 PM
Join Date: Jun 2000
Location: Last house on the right.
Posts: 2,365
Thanks for confirming my own theory of how this will work. I will call some local pro to confirm. I don't expect I'll run out of oil unexpectedly -- the two tanks together are probably close to 700 gallons.

k2dave touched on the reason for having two tanks in the first place: one can get fuel at a discount if one orders it in bulk. Also, I should be able to wait until the price is low before ordering. I expect I can save as much as .15 to .20 cents per gallon, maybe more. Well worth some trouble in checking tank levels.

I hope to make the switch soon, and will monitor tank levels both before and after for some time. Look for the results in this thread sometime in February, if all goes well.
Old 01-11-2005, 06:20 PM
Join Date: Jun 2000
Location: Last house on the right.
Posts: 2,365
Took me two years to remember this thread, but here are the results, as promised.

I opened the valve to both tanks, and sure enough everything worked great -- the tanks acted like one big system. At the end of the heating season, both tanks were less than 1/4 full.

I waited 6 or 8 months for a good price point, then ordered both tanks filled. I forgot to close the valve, which was a problem. Seems that the tanks are not exactly the same height -- so when tank B was full, the level of oil in it was higher than tank A. So I had a mess to deal with, and wasted a few gallons that seeped out of the return line port in tank A.

So now I'm running tank A in isolation until the level goes down enough to support the potential overflow. I'm still messing with the valve, but not the return line.

Another problem is corrosion and humidity in the system. Seems that it is bad to have a tank less than 1/2 full -- it tends to collect condensate and cause rust. Very bad in that it can plug up the filter, and potentially cause problems with the furnace. So having two tanks exacerbates this problem, and might actually obviate any potential benefit. My leaving both tanks less than 1/4 full for over 6 months made my furnace guy twitch.

In the meantime, I've replaced my furnace (not because of fuel issues -- it was at least 30 years old), so I'm using less fuel anyway. I'm thinking I don't need the second tank at all.

Anybody want to buy an oil tank?
Old 03-23-2017, 09:38 PM
Join Date: Mar 2017
Posts: 1
Just bought a place with two tanks

Not sure if anyone is paying attention to this thread, but:

We bought a place last fall wit two 330 gallon oil tanks, each with its own fill, each with its own gauge. Tanks were full in November when we bought. I filled them sometime in late December. The gauges were reading slightly different. Took just under 300 gallons total, and I have no idea how much each tank took.

Right now, one gauge reads dead empty and the other reads about half. When I screw off the plastic thing over the gauges, the "empty" does in fact seem to be "bottomed out", and the one reading half does in fact bob up and down appropriately.

Anyone have any thoughts about why one tank would be empty, and the other not, if that is in fact the case? Am I at risk for anything "bad" happening?

Thanks for any suggestions.
Old 03-24-2017, 12:20 AM
Join Date: Feb 2012
Location: Colorado
Posts: 2,683
Originally Posted by Danalan View Post
k2dave touched on the reason for having two tanks in the first place: one can get fuel at a discount if one orders it in bulk. Also, I should be able to wait until the price is low before ordering. I expect I can save as much as .15 to .20 cents per gallon, maybe more. Well worth some trouble in checking tank levels.
You must be planning to buy a LOT of fuel, if saving one-fifth of a penny per gallon is important to you.
Old 03-24-2017, 12:38 AM
Join Date: Mar 2013
Posts: 4,194
Another issue is that when one tank is nearly empty, it may well suck air instead of oil.
This may confuse the heck out of you, or even cause bursts of larger flame in the burner, which might be a hazard.

Another issue is that the tanks are prone to rust, as the ligher oil is not particularly hydrophobic,
and one benefit of having two tanks is that you can empty one and get it maintained, while still having oil in the other. Of couse the service guy could just pump one tank to the other, but it seems easier to just service one.

One benefit of having oil in both is that if one gets blocked, you have the other.

I suspect that if you are in a place where you need the oil to live there, ie its saving your house from destruction in extremely cold weather, you really want to be sure you have a tank of oil ready to go.

Therefore you would use only one tank, and if it gets blocked, eg by rust or paint flakes from the corroding top part, you would be able switch to the other tank. Even if it was empty you could manually supply it with oil from the other tank somehow. If you use both tanks, you would only find out that both tanks are blocked, and then be stuck with neither tank working and requiring work.

Also its possible that the oil may leak out of a tank, eg if a line is damaged by some means, and so its safer to keep one tank off, so that not all the oil is leaked.

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