Homemade shell & tube HX for DHW...?

[sorethumbs]

Hi, I'm new here. I've read quite a bit and wow, this site is a great source of info. I have a OWB, a CB, and I'm really happy with it, but I'd like to use it to heat my DWH. I'm looking at adding a homemade cu pipe-in-pipe HX to the side of my hotwater heater to accomplish this. It would be a thermosyphon system. I'm looking for advice on how to best accomplish this.

What I really need is advice on sizing of the two pipes. My plan is to have the boiler water be in the "shell" and the potable be in the inner pipe. I don't want to add any extra flow resistance (head) to my main boiler loop. What would be the best?.....1/2 inner pipe and 3/4" shell? or something more like 3/4" inner pipe and 1.25" shell? My main boiler loop is a combo of 1" pex and 3/4" cu.

besides the diameter of the pipes, there is the length. What is best there?

I hope this is not an issue that has been discussed to death. I did a search and didn't find anything. Thanks!

 

[Eric Johnson]

You're talking about what's called a "sidearm" heat exchanger. There are a number of threads on the topic here, along with diagrams, photos and descriptions. They work really well. The last one I built, which I'm still using, is about 4 feet long. It's made of a 1.5-inch outer copper tube with a 3/4-inch copper tube running down the middle. You can use black iron for the outer shell if you prefer. Most everyone uses 3/4-inch copper for the inner tube, but the outer shell size varies depending on available materials and desired capacity. You can buy them on Ebay for less than $200. You pretty much get an endless supply of hot water as long as you're pumping hot boiler water through the outer shell.

 

[steam man]

I think you'll find it has been discussed to death and the penalty is.....................kidding.

Try searching "sidearm" heat exchanger as they are pretty common and guys here have built them. I would think you would want the larger pipe sizes and make the length about the height of your hot water tank lined up with the inlet and outlets. The guys will chime in with the specifics.

Mike

 

[sorethumbs]

Thanks eric, that was a really fast reply, and good info. I'd really like to read those discussions you mentioned. I did run quite a few searches including "side arm." (I think that one got me 39 pages of info on all sorts of subjects) Can you give me some tips on trying to find those posts? Thanks again.

 

[Eric Johnson]

Here's one:

https://www.hearth.com/talk/threads/8168/

 

[sorethumbs]

Thanks for the link. Just what I was looking for!

I see how you tee'd off of the T&P;port, does your T&P;valve leek at all because of that? I guess I'm not sure what temp those are set to open, but if I did that I'd have nearly 180 *F at that point. Is that near the opening temp of the valve?

I also see the bleeder el at the top of the HX, is that used only for initial filling of the system, or do you find that air gets trapped in that high spot occasionally? My well, at times, does shoot a bit of air into my water.

 

[Eric Johnson]

The T/P valve has never let go due to heat, but I'm probably pretty close to the edge on that. My water temps get up to 195, so you ought to be OK. If it was giving me problems, I'd get a pressure-only relief valve. In my previous house I ran boiler temps in excess of 200, and it would occasionally pop off. Makes a steamy mess in your basement when that happens. If you have an electric water heater, however, I can guarantee that the circuit breaker on your thermostat will trip from the heat. Make a note of that next time you try to turn the water heater back on. If you forget, you'll probably wind up scratching your head and possibly even replacing an element or two (been there & done that). The breaker resets when you poke it back in with a screwdriver.

I like to pipe bleeders into high points, and that one seemed like a good candidate. I haven't bled it since the system was last filled.

Finally, if you have hard water, it's not a bad idea to drain off 5 gallons from the bottom of the tank every couple of weeks. I have a water softener but I still get scale buildup there, and it will eventually stop the gravity flow if you don't keep it clear. You'll know that's happened when you hear the dreaded "Honey, there's no hot water!" Sends chills down my spine just thinking about it.