Heat Exchanger Gunking Up

Swisher

What can I do to stop heat exchangers from getting clogged in a hydronic infloor heating system?

Two flat plate exchangers have completely plugged over the last three years. The plugging always happens on the heat source side of the exchanger (an outdoor wood boiler running a 20% mixture of glycol and soft water), not the building side. This same boiler also heats another building and its heat exchangers do not clog. The gunk is a kind of soft, white goop that is more or less impervious to anything I use to dissolve it. Supply piping to the exchanger is PEX all the way, pumped by a Grundfos cast iron circulator. The only cause I can think of is two bushings on each side of the pump. They are galvanized steel (not black iron) that adapts from the 3/4" diameter of the supply PEX to the 1" holes in the pump. Could the tiny bit of exposed zinc of these fittings really cause enough gunking to plug two exchangers? It doesn't seem likely to me, but what else could it be? Also, can anyone suggest a solvent I can use to clean out the exchanger?

Any advice would be much appreciated.

Thank you!

Steve

marsmen79

Is the system an open system.

Swisher

Thanks for asking. The side of the exchanger that guns up is connected to an outdoor wood boiler. It's not pressurized. The boiler has a cap that has a tiny vent space. One other thing to remember. This same boiler feeds three other heat exchangers in different systems. None have ever gunked up. Thanks again.

Firecontrol933

No idea what it could be or how to clean it. The difference between the one that is clogging and the others might be the flow (speed) of the fluid. If the plugging one has a very slow flow the restriction and multiple paths for the fluid to take (even slowing down each path's flow more) could be allowing whatever the material is to settle out and plug the heat exchanger.

If this is the case, then I would think either the other loops are shorter, larger tubing size allowing for more flow or have larger circulators on them.

Just a wild guess.

kcopp

You need to test the water. Companies like Fernox and Rhomar can help to figure out what's in there and give you solutions.

Swisher

Good Morning and thanks for the suggestions. I guess a water test is really the only logical next step. Before I do that, what are your thoughts about those two 3/4" to 1" galvanized bushings. I've heard that zinc can react with glycol and form a sludge. The two bushings I mentioned are the only zinc in the entire system, and are found only on both sides of the circulator that's immediately before the gunking exchanger. I'll be changing these bushing in the hopes that it will help, but it doesn't seem reasonable that such a small amount of zinc could have gunked up two heat exchangers over 4 years or so. The gunk is not a sediment, but rather a sort of greyish brown slime. When you blow through the exchanger after taking it off it sounds like someone trying to breath through their nose when they have a bad cold.

Any other thoughts?

Thanks again,

Steve

SWEI

Lose the bushings. Try something like these in their place.

Swisher

Yes, I was thinking the same thing. I've got two stainless on order. I cleaned the exchanger using various solvents and hot water flushed through. At least the exchanger has quick couplings. It's not perfectly clean, but pretty good. I'll swap out the bushings, then wait a year or two to see what happens.

warno

I'll say I doubt the bushings could have enough coating flaking off to gum up your HX. I what look into something like lime scale or some other hard mineral build up. I know you said it was soft water but it's still a possibility. I only say this because my whole boiler insides and all my fittings were coated with a white scum from this seasons running. I couldn't clean it off worth a crap.

Maybe add in a wye strainer to catch some stuff.

SWEI

I'm not saying the galvanized bushings were the cause.

I am saying that the cost and labor (now that the HX is off for cleaning) to replace them with stainless is trivial.

Swisher

Thanks again, everyone. As far as I can see, the only place the gunk builds up is in the HX. Just the same, I'll put in a filter when I change the bushing.

hot_rod

With an open system like that you have a constant, and large ingress of O2. Any and all ferrous components will breakdown and form a sludge. Same thing happens in non barrier tube systems even when they are closed..

At the very least you need an ongoing O2 scavenger chemical added to the system. Most of the manufacturers of that equipment as well as the dealers have that inhibitor available.

Strainers and dirt separators will help, and they will need an ongoing maintenance program.

Add some isolation valves to flush the HX occasionally, add some O2 scavengers and check it often.

Swisher

Thanks Bob. I appreciate you writing. What do you make of the fact that only 1 of the 4 exchangers fed by this same outdoor boiler gunks up?
Thanks again,
Steve

hot_rod

Gunk usually settles out at the lowest velocity areas, bottom of cast iron boilers for example. I think Firecontrol may be on to the reason.

It could also be a bacteria of some sort? Waters and even oils can harbor bacteria. A water analysis would be the only way to determine the makeup and best treatment. Or a lot of trial and error

I had an open concrete tank in my shop for a season, covered with foam sheets, but not sealed to atmosphere. It was multiple copper coils in a tank lined with EPDM. Over the summer some slime started growing on the surface. I tried every household product to make it and the smell disappear, without any luck.

Mark Eatherton

Try flushing the HXer backwards. Scale or fouling of any sort lays down in the direction of flow. Running fluid through the HXer backwards breaks this fouling off, providing for a clearer fluid passage way.

As for why, I suspect it has to do with water quality and velocity being lower. Velocity is a good thing with heat exchangers. It negates fouling and provides for a higher log mean temperature across the plates to provide better heat transfer.

Check with the HXer manufacturer and make sure the pumps you are using are adequate for the application. Don't forget that in order for the pumps to put out their rated capacity, that they need at least 5' of head on their inlet side.

Once it's all cleaned up, check fluid temperature differentials on both sides of the HXer Under load., and if the delta is too large on either side, there is a problem.

ME

Swisher

Good Morning Mark! Thanks for your note. I wish I could show everyone what this gunk looks like. It's not a sediment and it doesn't flush, even with a high flow rate of hot flushing water running through the exchanger both ways. The gunk is more like the stuff you blow out of your nose when you've had a bad cold that's getting better. Think of it like sticky, greenish brown yogurt and you'll get the picture.
All this said, I will check temperature delta. There isn't 5 feet of head on the pump feeding the HX, but judging by the short time it takes for the cold water in the buried line to be replaced by hot, I estimate the flow rate is 2 to 4 gal/minute.
Thanks again for your suggestions!
Steve

hot_rod

Does the system have automotive antifreeze in it by chance?
Automotive fluids are blended with silicates and will not work well with hydronic systems. the gunk could be a combination of the wrong fluid and zinc coated components in the system.

Any way to eliminate the glycol altogether, it will need a lot of maintenance in an open system.

If you MUST use glycol:

Flush and clean the system with a hydronic detergent.
Fill with pre-blended hydronic glycol fluid or use DI water to blend the fluid on site.

You need at least a 30- 35% mix of hydronic glycol to provide adequate inhibitor. If you blend below 30% you need to increase the inhibitor package in the fluid.

Then you will need to check and maintain the fluid yearly. With an open system the O2 scavengers in the glycol will be rapidly consumed, so you will need to test and boost that component from time to time.

Considering the expense and ongoing time and $$, consider converting to a closed, pressurized system In addition to eliminating the fluid maintenance issue you may double the efficiency by going to a sealed gasification boiler. Most OWF run in the low 40% efficiencies, and that is with dry hardwood as the fuel source.