Oil Heat furnace add Electric Water heater

Old 01-17-17, 11:09 AM
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Oil Heat furnace add Electric Water heater

I hope someone can offer some insight. I purchased a home with have Oil heat which also heats the domestic hot water. I would like to add a 40-50 gallon electric hot water heater (gas is not an option as I do not have gas service to the house).
Replacing the current furnace is not an option financially, but the current system does not provide enough hot water. My wife and daughter take baths and the hot water runs out too quickly.
I've heard of two different options.
1) In Series: Adding the hot water after the furnace by hooking it the Hot water OUT from the furnace to the Cold water IN to the Water heater. This would allow me to still turn off the furnace in the warmer months
2) In Parallel: By branching off the Cold water going into the furnace to the Water heater instead.

So the questions are..is on more desirable than the other?

For the In Series option wouldn't the furnace still fire (in the winter months) as the water is passing through it to fill the Hot Water tank, wasting oil?

If the Parallel option is used, what would i do with the Domestic hot water line coming out of the furnace? I've read not to cap it because of expansion. The furnace has 2 hot water lines coming out. One goes to domestic hot water and the other to the heaters in the rooms.

I definitely decided I want to add the Hot Water heater just looking to see which way i should connect it.
Old 01-17-17, 12:04 PM
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First of all, you have a boiler, not a furnace. A furnace heats air that is conveyed to the various parts of the house through ducts.

Second, despite what one member states, a "tankless coil" is the second worst method ever invented for heating domestic hot water. The absolute worst is heating a kettle of water on a wood-burning kitchen range.

Third, while it is a popular opinion, the tankless coil could never generate enough internal pressure to be a problem if the inlet and outlet were capped. Solidly capping the inlet/outlet prevents leakage if the coil were to develop an internal leak, not an unheard of circumstance.

I would disconnect the domestic water from the tankless coil and route it to the electric heater. However, if you choose to use the tankless coil as a booster then connect the output of the coil to the inlet of the electric heater and re-route the domestic water from the coil outlet to the electric heater outlet.

There is also the matter of disabling the low temperature control that maintains a minimum boiler temperature for the hot water. This is generally set at about 140 degrees and should be set as low as possible.
Old 01-17-17, 04:49 PM
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If you connect the stand alone water heater in parallel it is only necessary to have inlet shutoff valves for each.

Except to futureproof the system in case one leaks you would need both inlet and outlet shutoffs for each but the outlet shutoffs can be left open all of the time that nothing is in need of replacement due to leaking.

For connection in series it is better to have the stand alone heater second. The second of the two has to remain "on" at all times to prevent cooldown of the water as it exits the first of the two.

With the series connection the tankless coil as a preheater will mean that the stand alone heater does not have to raise the water temperature too much which in turn means it will keep up with the demand more easily.

I would say that at no time in winter are you wasting oil due to domestic water passing through the tankless coil. Whatever extra burner run time results from the domestic water, less electric element run time in the stand alone heater is needed.

The parallel connection is designed for using the boiler and tankless coil only in winter and using the stand alone water heater only in summer.

It is possible that the tankless coil has lime deposits inside that reduce the amount of hot water per minute. It is possible that your boiler maker offers a higher capacity tankless coil.

If you do not have two or more people taking a bath at the same time you may find that the tankless coil is adequate.

Last edited by AllanJ; 01-17-17 at 05:14 PM.
Old 01-18-17, 11:59 AM
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Thank you for the replies.

Furd, I would have to agree with you that this method of heating domestic water is horrible. We have 1 standard side tub and you can nut fill it enough for a bath without running out of hot water. I dont know if the boiler is too old or we just like our baths really hot. And since replacing the system is not financially possible right now, I see adding the stand alone hot water heater as the best option.

AllanJ, thanks for your input as well, like i said above the tankless coil is not adequate to provide the hot water we need...maybe the system is too old...Ideally I want to replace it with a new HVAC system down the road (hoping I can get natural gas into the home)

I think I will hook it up in series and just shut off the boiler in the warmer months. Thanks for your help all

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