Going to an in-tank heat exchanger.. Advice needed.

[Gooserider]

The search didn't do much for me, I know we discussed this last year with no definite answer on the numbers. I had an engineer friend over tonight and we did some of the numbers and the closest we got was that if copper has a 70 Btu/hr/ Ft2/ *F (as per Nofossil's spreadsheet), the Pex has 23 Btu/hr/ Ft2/ *F. This than looks like I will need 3X the Pex and in order to get the 135,000 Btu I think I am looking for, I would need 900 ft 1/2" PEX :bug: or 295 ft 1/2" copper.
Hmmm, copper does not look that bad maybe, I just know I have no time for endless soldering or brazing...

Doesn't sound to bad, and there wouldn't be that many joints to make if you did the soft copper tube - I believe it comes normally in 50' coils, so that would only be 12 joints for 300' of coil... I would probably look at trying to do a pair of manifolds to do several short parallel loops rather than one big loop just to keep the head losses down.

Gooserider

 

[hkobus]

Just checking pricing again. I can actually get this in 100 ft rolls on special order and even then I will need 4 or 5 paralell coils to reduce head loss. The 50 ft may not be bad.
I will make some headers to go to 1" copper and live goes on.
Still wondering if anyone has used Pex in this way and with what results?

Henk

 

[kabbott]

Just checking pricing again. I can actually get this in 100 ft rolls on special order and even then I will need 4 or 5 paralell coils to reduce head loss. The 50 ft may not be bad.
I will make some headers to go to 1" copper and live goes on.
Still wondering if anyone has used Pex in this way and with what results?

Henk

I use pex for my heat exchanger and it works... well ...OK.
I have 4 x 175' x 1/2 pex. It will sink the total output of my boiler when the storage is 185 and lower(never had it hotter).
The boiler was built by me so i cant really give you good info on the btu output, but assume close to an EKO 25.
I made a frame out of cpvc and made 4 big spiral coils of pex. If i were going to do it again with pex I would just slap 4 300' coils
of 3/4 inch in. The thing I dont like is the temp drop between the tank and pex output when heating from the tank added to the
fact i have baseboard and i can only draw down to ~136-138 degrees.

This may be just as bad with copper though, I can't say. If I were to start over I think I would just use a flat plate. I am in so deep
now what is one more pump. :vampire:

 

[deerefanatic]

My update? Well...... Chasing leaks....... I just truly hope that this copper doesn't turn into a very expensive pile of scrap...... Still working on it.... It's all down in the tank now which is making life really interesting.... grrrr......... I'm almost beginning to wish that I'd stayed with my flat plate.......

 

[mwk1000]

Some jobs go like clockwork others you just have to BEAT INTO SUBMISSION! Sound like you caught number two so take a deep breath and start beating.

I am curios since I also have an open tank and a HX. But I may just wait for someone to invent the miracle plastic that conducts like copper , is dirt cheap, and plays like pex. I would have though the geothermal folks would be all over that by now.

Good luck

 

[hkobus]

Some jobs go like clockwork others you just have to BEAT INTO SUBMISSION! Sound like you caught number two so take a deep breath and start beating.

I am curios since I also have an open tank and a HX. But I may just wait for someone to invent the miracle plastic that conducts like copper , is dirt cheap, and plays like pex. I would have though the geothermal folks would be all over that by now.

Good luck

I' m sure this product is out there, just not yet on sale at home depot. :lol:

 

[deerefanatic]

Well guys... the copper is coming out.. GRRRR...........

WORD OF CAUTION: DO NOT BRAZE UNLESS YOU ARE REALLY GOOD AT IT!! My heat exchangers started springing leaks left and right as soon as the joints got hit by water... My theory is that the joints had many tiny pinhole leaks that were filled with flux, which dissolved into the water......

I'm thinking I'm gonna run 5 - 500 foot loops of 1/2" pex in the tank..... At least that will be leak free.. :|

 

[kabbott]

Ouch.... That is frustrating.... maybe step back and take a deep breath.
If the copper is still usable i would try to make it work even if i had to resort to a few compression/flare/shark bite etc.....or can you not get it
in/out of the tank now?
I went with pex for one reason--MONEY-- copper will be better in almost every other way.

What were you using silfos (or similar) or soft solder?

 

[deerefanatic]

Well, you got me thinking now.... I made the heat exchanger in 16" wide sections (4 per grid, 6 grids total) that were joined together with pex. I could very easily take them back out of the tank, cut out all the brazed joints and put it back together with sweat solder fittings...... And, those fittings will cost me much less than buying a bunch of pex.....

So OK... I'll try it.

Oh, I didn't use solder at all. I used brass brazing rod with flux coating... I think that was my downfall........

 

[kabbott]

If you want to try again without fittings you may want to try sil-fos or something similar. I am no expert with it but
I have found it very easy to work with.No flux needed for copper to copper joints.

Should be available at welding supply house or maybe plumbing supply/refrigeration supply house.
I think I used sf-1 by turbotorch but I think sil-fos sf-15 would be fine, maybe a refrigeration pro will chime in.

Here is an older post of mine with a close up of the joint it produces. These were done with a TIG torch for the heat
source but can be done just fine with mapp or oxy/acct.
https://www.hearth.com/talk/threads/29488/

If you get it to hot it will soften the copper but i suspect the brazing did too.
I don't know if you have ever tig welded before but it is very similar, flows nice and there is no flux to hide any pinholes and
such, makes it easy to get leak free joints the first time.

With that said it is no small job take your time with the fit up and it will go smoother.

Fittings and soft solder would be OK too, just another expense but would be worth it to savage the copper.
You will wind up like me..........most I have ever paid to heat my home.......and my wood is FREE.. :-S

 

[deerefanatic]

Man! Nice work...... Alas, my fittings are already on the way.... :| But, with the luck I've been having, maybe it's just as well.......