Sidearm Heat Exchanger Not Working

[smaxwell]

Hello Everyone!
Here's my situation. I've just installed an outdoor wood boiler and it's delivering 175ºF water to a stainless steel, double-walled sidearm heat exchanger installed immediately alongside my electric water heater. The bottom of the sidearm is plumbed into the drain outlet of the water heater, while the top end of the sidearm connects to the pressure relief outlet on the side of the water tank. The hookup is just what's needed for proper thermosiphon action, but to boost water circulation through the inner part of the sidearm I have a Grundfos 15-58 circulator on top of the sidearm, drawing water from the bottom of the tank and moving it up to the top connection. A Grundfos 26-99 circulates boiler water through the outer passage of the sidearm. Water flows are opposite to each other – downward flow of the 175ºF boiler water, and upward flow of the DHW from the the bottom of the tank to the top.

The sidearm gets hot to the touch on the outside, but it doesn't transfer heat to the water flowing through the inner pipe. I know for sure that water is flowing through the inner part of the sidearm (I can hear it flowing if I partially close a shut-off valve at the bottom of the sidearm), but even with the system running all night, and no DHW being used, the tank only gets to about 100ºF with the electric elements turned off.

I thought at first that the problem might be too much mixing of the DHW in the tank because of the 15-58 circulator, but that's not the case. The tank heats up fine with the circulator running and the electric elements turned on. The more I look at things, the more it seems like that sidearm simply isn't transferring heat. I had originally planned to go with a flat plate heat exchanger in a similar set up to what I have, but opted for a pump-assisted sidearm because of the hard water I've got.

The sidearm I've got is double-walled, and perhaps theres a design flaw in this approach. If "double-walled" means there's some kind of an air space between the inner water passage and the outer one, that sounds like the problem to me.

I'm at a loss on how to proceed. Can anyone offer some insights? I know sidearms work because I've seen them work. Mine is plumbed for thermosiphon action, but it's simply not heating the water flowing through it, even when pump-assisted. Is there anything I can do other than scrap a brand new, $250 sidearm heat exchanger and plumb in a flat plate exchanger?

Thanks a million!

Steve

 

[John D]

Do you know the surface area exposed to the dhw water in the electric hot water tank? I put a 10 plate hx and I can tell you that it is amazing ..if you were to put a $150 one in place of the sidearm , you would have to put the 58-15 on a stat to keep the water from getting too hot ... I put tees with threads and garden hose adapters, shut offs, and quick disconnects on mine so I can easily cleaning with vinegar occasionally.. I can shut off valves drain the water through the bottom tee, fill with vinegar and flush into the slop sink all very easily....

Sent from my DROID RAZR using Tapatalk

 

[Fifelaker]

Mine does not have a pump on it, maybe the water does not have enough "contact time" in the exchanger. Mine gets to the boiler temp fairley quick 1-1.5 hrs. then the tempering valve brings it down to a useable temp. I would try to bypass your pump.

 

[smaxwell]

Thanks for your suggestions, everyone. I thought of bypassing the pump, too, except that I know of people with pumps on sidearms that work really well. I suppose it's worth a try, though. My initial thought was that the short contact time between water and heat exchanger would be more than made up for my many more contact opportunities, as the water passed through the sidearm more often.

My water isn't crazy hard (it's about 15 or 16 grains), and I'm wondering how often you have to give your flat plate the vinegar treatment. Can you fill me in?

Thanks very much, again!

Steve

 

[Iska3]

Thanks for your suggestions, everyone. I thought of bypassing the pump, too, except that I know of people with pumps on sidearms that work really well. I suppose it's worth a try, though. My initial thought was that the short contact time between water and heat exchanger would be more than made up for my many more contact opportunities, as the water passed through the sidearm more often.

My water isn't crazy hard (it's about 15 or 16 grains), and I'm wondering how often you have to give your flat plate the vinegar treatment. Can you fill me in?

Thanks very much, again!

Steve

Steve, I'm on my third year with my 10 plate exchanger. I have well water but I soften the water. I might clean mine this summer but still keeping the OWB warm for hot water. Two small chunks of wood or scraps from the splitter keeps the OWB at 160 degrees and more than hot enough. I run my exchanger directly off the hot water heater to take the chill off of the water. Is your side arm below the top of your tank? Lot of the members use them. You might want to take picture so they can help you.

 

[oneoldbanjo]

I wonder if you have air trapped in the outer shell of the sidearm? If you can hear the water flowing inside - it could be that there is air trapped in the system and the water is dropping down to the bottom of the sidearm with little contact.

When I fill my system the water fill is at the bottom of the sidearm which is the lowest point in the system, and all the air is bled out as the system is filled.

Perhaps you need to do something that will allow you to bleed the air out of the siderm.

 

[smaxwell]

Hello Again Everyone,
Your latest suggestions all make good sense, but I've checked on all these before. I know for sure that there's no air in the outer shell of the sidearm (the whole thing gets very hot), and there's no air in the inner chamber since I've got a purge valve at the top of the sidearm to get rid of air.

Does anybody know exactly how a "double-walled" sidearm is built? I expect there are two walls inside, not just the one pipe wall that separates boiler water from DHW. If this is true, and if by chance my sidearm was built with an unintentional air gap between the double walls, that would short circuit the system.

I've consulted the place where I bought the sidearm, and they can't see that I've done anything wrong. They're going to send me a single wall sidearm and I'll swap it out and see if that helps. If not, then I'll plumb in a flat plate heat exchanger, configured for easy cleaning with vinegar.

What length, width and thickness of flat plate makes sense to supply DHW for my family of 7?

Thanks again to everyone! What a resource you all are.

Bye,

Steve

 

[Arbonaut]

I'd get rid of that booster pump and the tempering valve if code allows there. Also, if your sidearm exchanger is downstream from the water to air or radiators heating your house, there isn't enough heat left to bring up that domestic water heat. I wouldn't be suprised at all if you needed to run your electric heating element in tandem with that sidearm exchanger, especially since it isn't copper. Or put another sidearm exchanger parallel to the one you've got, I think they are only rated at about 35,000 BTU. I'm no expert but ran a system like this for ten years and have seen things I didn't expect.

 

[Fifelaker]

I built my side arm 1" copper inside of 1.5" copper with reducing tee's on each end. Boiler water enters from bottom exits the top then onto the water to air exchanger then the dryer then return to boiler. The side arm percolates dhw for lack of a better term in the water heater no pumps on it at all. I will try and draw a picture if any one wants.

 

[LaurenceCly]

I put a 10 plate hx and I can tell you that it is amazing