Tankless coil and direct tank in series?

steamnoob

A few years ago I replaced my tankless coil domestic h/w from my older oil boiler with a gas fired tanked. I considered putting an indirect reserve tank on, but since I am planning to go to natural gas for everything eventually, figured this was a good cost effective first step.

I cut the copper input/output lines from the tankless coil, and hooked them up to the water heater. But now I'm wondering if I could/should have hooked it up in series? Besides some efficiency benefits by using some residual heat from the boiler when its running, I'm also thinking that I'd like the option of running my h/w from the boiler to use up all the #2 in my tank if I decide to get around to ripping it out this or next summer. I hear horror stories about letting oil boilers sit over the summer, so this would allow me to get some use from it now and again.

Is there any downside to doing this? I have an expansion tank on the hot water line. The tankless coil system "worked", but temp was inconsistent. Probably should have tried a new tempering valve first... but what's done is done.

EBEBRATT-Ed

Bring the cold water in to the tankless and the hot water out of the tankless to the cold water inlet to the gas water heater. Put a 150psi relief valve on the cold water pipe to the tankless. The tankless will preheat the hot water

EdTheHeaterMan

I would rewire your oil boiler control for "cold start" operation as if it did not have a tankless coil. this will reduce the amount of oil you use in the fringe seasons. Most plumbers just tell you to shut the switch off in the summer. This is how I find most Tankless Coil boilers with Gas water heaters next to them. It is wasteful in the spring and fall to maintain boiler temperature at 150° to 160°.

Well, li looks like you are already past that point. If you want to preheat the cold water before it enters the Gas Water heater. Then you will reduce your gas usage somewhat at the cost of using more oil. So there is really no savings on energy cost. But your logic of using up the oil in the tank before abandoning the oil boiler and tank has merit. If this is a DYI project, then go for it. If you need to pay a pro to do it, then weigh the cost of the re-pipe with the cost of the oil that will be lost at the final disconnect, if you plan it just right, You can end the winter season with very little oil left in the tank.

If you are on automatic Delivery from your fuel dealer, then you must remember to discontinue that service before you end up with a Gas heater and a full tank of oil... or worse, a basement full of oil where the old tank used to be.

Mr.Ed

steamnoob

So I would put a relief on the pipe into the tankless coil... not after?

I disconnected the "lo" feature when I installed the tank (wiring instructions were inside the aquastat cover), so I think I'm good there.

Ctoilman

No need for another relief valve, doesn't the tank already have one?

EdTheHeaterMan

steamnoob said:

So I would put a relief on the pipe into the tankless coil... not after?

I disconnected the "lo" feature when I installed the tank (wiring instructions were inside the aquastat cover), so I think I'm good there.

Yea @EBEBRATT-Ed why the additional relief valve? The one on the water heater should handle any overpressure. Unless you believe the one on the cold inlet to the coil is needed in case a valve between the water heater and the coil may get closed. That makes sense!

And this has always been a point of interpretation on this website. After the water leaves the tank;ess coil it is hot. now when it goes into the Gas water heater already HOT would that make the gas water heater a "Hot Water" HEATER?

Mr Ed

EBEBRATT-Ed

@steamnoob

The gas water tank should have a T & P relief valve sized for the BTUs of the gas input. I would stick a 1/2" pressure relief near the tankless coil. It can go on the hot or cold water line....just to be safe.

When you oil tank is empty make sure you completely remove the fill pipe from the tank and put pipe plug in the tank. I would leave the vent pipe until you remove the tank

EBEBRATT-Ed

Just in case there is a valve between the tankless and the HW tank, probably not needed but cheap insurance