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steveinpa 08-09-2010 08:47 AM

Oil fired furnace and hot water heater question
Hi, My son recently bought a house here in Pa. which is heated by an oil fired furnace and a baseboard hot water system. The oil furnace also doubles as a hot water heater. There is no external water tank or heater.The hot water output capability of hot water per minute is not really up to the new demands on the system. The previous owner of the house had purchased a new 50gal electric heater. But, never had it installed. The house does not have natural gas supply. So, electric was her choice. So, we have an oil fired furnace and a brand new electric water heater sitting right next to it.

My son would like to install the electric water heater to be able to shut his oil furnace off in the summer and also to have a larger supply of hot water. Now for my question. Would it be possible to plumb the current hot water outlet from the oil furnace into the cold water inlet side of the electric water heater tank? Rather than disconnecting the furnace hot water coil and plumbing a cold water supply into the water heater.

I understand that in the summer there would be no energy savings. But, during the cold weather months, the hot water from the furnace coil would feed into the electric water heater to raise the water temp and save some electric. Also, the two systems would together and further increase hot water output. At least that's the idea.

So, what do you think?


secutanudu 08-09-2010 09:38 AM

That is exactly how my house is set up. He doesnt drink his boiler water....right? On my boiler, there is the send and return for my radiators. There is also a drinking water that runs into the boiler through a coil then out of the boiler. This water pipe simply runs through the boiler, the water is not mixed. That then feeds into my 50gal electric water heater.

I have two valves near the water heater. One from the boiler and one right from the meter. Each leg has a valve on it. I just swap them in the summer. One is on, one is off.

I have no idea if this is a good or recommended setup - just how it was when I moved in.

steveinpa 08-10-2010 08:35 AM

Yes, The furnace has a separate internal coil used to just to heat the hot water for faucets, etc. I guess my concern is the effect of hot water from the furnace coil used to fill the electric water heater. I'm not sure of the exact temp of the hot water. But, I do know that the hot water coming from the taps is very hot. The gallons per minute of hot water is not up to the new demands on this type of hot water system. But, while the hot water lasts, it is hot. I'm thinking that in the winter time, when the furnace is cycling more often, that the water temp may be a bit higher. The temp of the water entering the water heater would be higher than actual water temp setting of the electric water heater. If it is ok to plumb the water supply in this way, then it would save electricity of the water heater.

Having said that, would it be a good idea to add a mixing valve to mix hot water from the furnace heater with cold water to reduce the water temp inlet going into the electric water heater? The inlet water would be warm, but not as hot as directly from the furnace. Also, we don't want to use the system of shutting off the water supply to the electric heater from the furnace in the summer time or shutting off a cold water inlet directly from the main in the winter. We would just turn off the furnace in the summer and allow the water to pass through the furnace coil and feed into the water heater.


secutanudu 08-10-2010 08:42 AM

Have you actually tested the temperature of the hot water before it turns cold? Are you sure it's above what you would want in an electric heater?

I have no idea if this is true - but the reason a plumber told me it was good to switch off the coil in the summer was that more sediment builds up in the coil (the one in the boiler) when water runs through it and that you cannot replace them. So why "use it up" when you don't need to. Seems ridiculous to me...why would water sitting in the pipe be much better than water flowing through it?

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