View Full Version : Converting from On Demand Coil (Oil Boiler) to Electric Hot Water Tank.

06-10-2008, 07:45 AM

Like many other people out there that heat with oil, I am ready to shut that boiler down. I have found the OnDemand coil to be unpredictable, causing scalding at times when the water that was in the coil reaches the faucet, changing water temperature depending on where the boiler is in its heat cycle, etc. At almost $5 a gallon, I see no reason to keep that box of steel hot all summer just for hot water. I have just enough space to put a 40-50 gallon electric tank right next to the boiler.

OK, now on to my actual questions:

-I am good to go with installing the new hot water tank...but how do I disconnect the coil from the water supply? I know that I still need cold water supply to enter the boiler for heating system's water, but I don't think I can just cap the coil's input and output, right? Could expansion cause something to burst? Can I simply remove the coil and run the boiler in the winter only without it?

Thanks in advance for any advice. You guys have helped me a lot in the past and I appreciate it.

Mike Angelini

06-10-2008, 09:47 AM
I'd use the boiler as a preheater when the boiler is on...feed its output to the new tank. If full electric heat for the water really is cheaper than the oil, proceed as shown below.

If you decide to bypass it entirely I think you could just disconnect the input and output and reroute to the new tank. The problem comes in the control for the boiler. It may try to keep the standby temperature of the boiler (when you turn it back on for the winter) higher than a similar boiler's control would if it didn't have the coil, wasting oil keeping things hot when the house isn't calling for the heat. that might require a new control box, or if you are lucky, just flip a switch or adjust something to tell it to ignore the potable hot water section and control. For that, you'd need to read the manual carefully.

Another option would be to add an indirect WH, and use the boiler to heat that tank. This typically has the best recovery rates, and normally, you can get by with a smaller tank than used for an electric or gas-fired one. You can also get them in SS, which should last for a very long time as opposed to the typical glass lined ones. This would have the same control problem as noted above...you don't need the boiler on keeping the coil hot all the time. An indirect often is set up to look like a separate zone to the boiler.

Sandpiper Plumbing
06-10-2008, 08:36 PM
I agree...an indirect water heater is the way to go here. It's as simple as adding another zone to your existing heating system and runs off an aquastat instead of a thermostat. Easy, reliable, fast recovery...what's not to like!

Bob NH
06-11-2008, 07:35 AM
There are always heat losses from an oil-fired boiler when it is not running, and it is necessary to keep it hot if you are using it for water heating. That applies if you are using the tankless coil or an indirect water heater heated by the boiler.

Adding an electric water heater is a lot less expensive than adding an indirect water heater and with $4 to $5 oil the electric unit saves money in the non-heating season.

With the current price of fuel oil it saves money to use electric heating in the non-heating season. I installed a 40 gallon electric water heater connected into my hot water line after it leaves the tankless coil.

There are 3 ways you can plumb it:
1. So the electric is always in the line after the tankless coil, just cut the pipe and connect to the inlet and outlet of the electric water heater. This is the simplest.

2. If you use Tees off the hot water line and put a valve between the tees the electric can be bypassed.

3. You can connect it so either the tankless coil or the electric water heater can be bypassed.

Mine is connected so that the water always passes through the tankless coil, but there are bypass and supply connections so that either can be taken out of service if necessary.

There is enough mixing in the electric water heater so the temperature variations from the tankless coil have completely disappeared.

There is usually a temperature modulating valve on a tankless coil system and that should remain in the system if you are using the tankless.

1. The electric water heater is set at a temperature below the normal output of the tankless coil.

2. During the heating season the tankless coil heats the water going into the electric water heater and no electricity is used, unless you are away and don't use any hot water for a very long time.

3. When the heating season ends, turn off the boiler. There is no requirement to open or close valves. The water still flows through the tankless coil but the electric heater provides the heat.

Installing and electric water heater is much simpler and much less expensive than installing an indirect water heater and additional zone off your boiler, with the priority controls that are usually used with such installations. With the price of oil you will reduce energy costs because you will eliminate heat losses from the boiler during the summer. If you are using air conditioning you will also save on A/C because some of those boiler losses contribute to the heat load in the house.

06-11-2008, 10:31 AM
Most boilers maintain some minimum temp whenever they are on. The newer ones do not. My boiler may only run once a day to reheat the indirect. I've seen it sit with the internal temp at room temperature, so it does not have a lot of standby loss. But, on yours, since the controller is designed to keep the thing hot for the coil for DHW, unless you can reprogram it so it doesn't there would be a fair amount of standby loss. An electric is close to 100% efficient, your burner isn't, so you have an immediate loss depending on the energy costs differences. An indirect can use the entire heat capacity of the boiler to produce DHW, so recovery and continuous use will likely be better with an indirect and you wouldn't need as big a tank, which can reduce the standby losses from that tank.

06-20-2008, 06:00 PM
I just received my solar water heater package and I currently have a tankless coil. Am I correct to say, after reading this thread, that I should put my solar tank in series after my coil and somehow turn the aquastat off on my coil and turn my boiler off in non heating seasons? I think this will work, so long as I can turn off the aquastat. I'm not sure if it matters but I am in Vermont.

06-21-2008, 01:08 PM
If you physically turn the boiler off, then you don't have to deal with the aquastat in the boiler for the DHW. As was noted in previoius posts, a boiler with DHW coils keeps the (small) quantity of water in the internal tank hot all year so that your ability to make potable hot water doesn't have a delay. This can be inefficient. In the midst of winter, it isn't a big deal since the thing would be firing fairly often anyways. But, it may be set to a higher temperature than needed, so if the system design allows for lower water for heating, then you'd be wasting energy to keep it hot so you have a chance of making DHW when needed. Changing or adjusting the controls in the boiler may or may not be easy to disable the DHW function for the internal coil. Your boiler is probably made both with and without that coil, and they may use the same controller, so if you are lucky, it may just be flipping a switch to turn off that function. It might be even labeled as a vacation switch.

If you keep the boiler in series, then it can augment the new tank, which may be needed in the winter.

10-28-2011, 03:59 PM
I too have a tankless coil oil burner but I also have a gas line which runs the Clothes dryer.

Like everyone has stated, it's costly to run the boiler 8 months out of the year just to heat the hot water so I was looking for a separate hot water heater. But do I go with gas or electric?

The problem I see with gas is the chimney issue. I'd have to line it I would guess and somehow hook two different appliances (oil burner and gas water heater) into one.

Bob NH had what I think would solve my problem, just get an electric hot water heater.

There is also this GE hybrid model for 1399 but it has an federal credit of 300 dollars and a local electric credit of 150 but I don't know if it could be hooked up like Bob NH detailed.

I am having a plumber come in. Would the average plumber know the best solution?


10-28-2011, 05:00 PM
Some plumbers are up on what's new and can give you intelligent options, others will lead you towards the easiest path with the best profit.

The simplest is probably an electric WH. Natural gas almost always has a better recovery and a lower cost to run. If you go with a power-vented unit, you can run the exhaust out the sidewall, rather than up the chimney. The GE hybrid model will be sucking heat from the room to heat the water. This may or may not be useful. It is essentially a heat pump (an a/c unit running backwards).

10-29-2011, 05:36 AM
Thanks for the input Jad. I received a call from a plumber who said he had 30+ years experience and he's a 3rd generation plumber. When I mentioned the scenario he was familiar with the problem but he said that electric is the least efficient way and that the electric costs of running an electric hot water heater would be the same as what I am spending on oil for that purpose. He did mention a gas WH vented through the side because of my chimney issue or a hybrid but not GE's.