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[Roger2561]

Hello all, Okay, I'm getting the items together to plumb my domestic hot water to be heated by the OWB.  Here's my plan; Hot from OWB through the heat exchanger then through oil furnace (In the event I lose the fire in the OWB, I still able to have a hot shower until I find and rectify the problem with the OWB) then to the hot water tank.  My question is, do I need to install a backflow preventer, if yes, where and will I need an expansion tank on the domestic hot water tank? 

Thanks in advance for your help.

Roger

[gmviso]

Not sure where you are thinking of installing the backflow preventer, but I will tell you what I did versus what I had planned to and probably will do in the near future.

First, I have a 50 gallon electric water heater and a 20 plate brazed heat exchanger for my hot water. As I was considering the plumbing of the cold water thru the heat exchanger, I too wanted to install a back flow preventer so that the 180 degree water did not heat the cold water supply that feeds the rest of the house.

I had previously removed a check valve (the body of which is made of brass), from my older water heater that the previous owner had installed to keep the 120 degree water from heating the cold water supply. I will tell you that prior to removing it I noted that it really was not doing a very good job of isolating the hot and cold piping as evidence by placing my had on the pipe both above and below the check valve.

I will also add that those heat traps they sell you that are galvanized pipe with plastic flow restrictors in them don't work all that well  either (IMHO any way).

So, since I could not come up with another solution I just plumbed the cold water straight through the heat exchanger.

I had previously installed a Honeywell tempering valve in anticipation of plumbing in the heat exchanger. You will definitely need one of those as the hot water that is generated by you heat exchanger is going to be REALLY HOT.

Now, when I turn on a cold water tap, if a significant quantity of how water has been used just previously, for example by showering, washing dishes, etc., the cold water comes out pretty warm until it is flushed from the pipes.

I am thinking about reworking the cold water feed to the heat exchanger to create a "trap" by plumbing the supply a foot or two below the heat exchanger and then back up to the cold water feed.

I am hopeful this would work, since I believe the problem I have is due to the fact that heat rises, but I am not sure.

[RSI]

If you put any sort of back flow prevention in then you need an expansion tank. You are better off just adding more pipe before it so the water can back up as it expands but not make it to the other cold water lines.