Making a side arm heat ex-changer. Need some sizing help

[Biglurr54]

I am going to make a copper side arm heat ex-changer for my domestic hot water. I run a Woodmaster OWB all winter long. I have a plate ex-changer for my boiler heat. I am going to tap into the return line after the boiler. This will flow through the side arm to create the hot water. This water will be between 150-170. The side arm will be on an electric hot water tank and it will be set up to thermosiphon. My question is how should i size the copper tubing for the side arm. My cold in water line is 3/4 copper and my hot out line is 3/4 copper. For the side arm I was thinking of making it 4 feet long and have 3/4 copper pipe as the ex-change surface. The outer shell is what I'm not sure how to size. would 1 inch pipe allow enough hot water around the 3/4 or should i go with 1 1/4 or 1 1/2? The houses current needs aren't great as it is only two of us in the house right now but that should be growing very soon and i would like to build the system to handle a house hold of 4 people so i won't have to rebuild the system in a few years. Any help would be great! Thanks everyone.

 

[Butch(OH)]

I dont think that you will get enough heating from a small diameter shell plus you dont want to restrict the loop.
Mine is 1" on the domestic and 2" on the shell. There is a fitting made just for this purpose available from Woodmaster but pricey, link to part 2" x 1" x 1" Copper Tees

With 5 in the house and a couple kids who had to be drug out of the shower we ran out of hot water quite often with the side arm and had to turn the water heater back on to keep up with demand. After the first winter we installed a plate exchanger before the tank but also kept the side arm since it was already there. Now we have the best of both, all the hot we can run plus the tank stays hot with the side arm.

 

[Biglurr54]

My next door neighbor did a plate exchanger for domestic hot water and plug it up beach year. After three years and three exchangers he did away with them and is back on electric He didnt know anything about side arms.

 

[Stihl310]

For starters you should calculate the flow area of the 3/4 lines your using (ID) then size your outer jacket to have at minimum the same flow area as the 3/4 your dumping into it while taking into account the OD of the 3/4 line that will carry your homes hot water.

 

[dave_dj1]

I built one years ago that had 3/4" line inside a 3" copper pipe, it worked ok but it was kind of difficult to regulate. Water would be so hot in the morning that you had to be careful or you would get burned. Then it would cool down quite quickly. It worked but not to my satisfaction. I used it 6 winters. I had the power shut off to my electric hot water heater so I don't know if that was the problem or not.

 

[stihly dan]

Why not just use a hydronic water heater, zoned thru boiler? boiler summer, wood winter.

 

[Biglurr54]

Ok I'm still not too clear on how to properly size this. The boiler is getting fired up soon so I'm looking to order this stuff today or tomorrow. The cold water in and out of my electric hot water is 3/4 copper. The hot water supply and return from the OWB is 1 inch PEX.

1. Should I use 3/4 inch pipe for the line inside the shell. I figured 3/4 because that is the feed and supply line size. Or should I go up to 1 inch with 3/4 reducers on either end where it enters the hw tank?

2. Should I use 1 3/4 inch pipe for the shell. My supply lines are 1 inch so there shouldnt be any restriction here. Should I move up to 2 inch or bigger to allow more water in to heat the water? Should I stick with the mantra of Bigger is better as is the case with all plumbing?


I'm just trying to piece this all together and get it right before I order all the parts.

Thanks in advance

 

[dave_dj1]

I would use 3/4" on the center line, I would go with 2" if available for the outer shell with 1" ports for your pex.
I made mine the height of the water heater.
check this one out or google image side arm heat exchangers
DSAGBST17 GB Sidearm Tight Applications Single Walled Heat Exchanger - eComfort.com