Heat Exchanger Performance Decline

Swisher · January 2018

Hello and Thanks for your help,
I've got a 3"x8" Bell and Gossett 20-plate heat exchanger that use to work really well for three years and now it doesn't. Can you help me figure out why?

This heat exchanger heats water before it enters my electric water heater tank. One side of the exchanger gets heat from my outdoor wood boiler, the other side heats domestic water before it does into the tank. There's a 3 to 5 gallon per minute flow on the boiler side (80% water, 20% propylene glycol) as measured by the screen on the Alpha circulator pump in that loop.

I understand the need to descale the domestic side of the exchanger regularly and I do this every year by running vinegar through the domestic side with a portable circ pump.. The vinegar comes out grey at first, then clear. This winter I noticed that although there's plenty of hot water going into the boiler side of the exchanger, when domestic hot water is being used, the outlet of the boiler side of the exchanger is just lukewarm. Water temp going into the hot water tank isn't nearly what it used to be, either. It's just a little warmer than lukewarm, not the scalding hot it used to be.

"Well, the boiler side of the exchanger must be clogged or scummed over somehow", I thought. It's not delivering enough heat to the flow of domestic water. This struck me as strange, since the Alpha pump registers a 3 to 5 gallon per minute flow on the boiler side. This is what it has always said, even when the exchanger worked well. But just the same, I hooked up the portable circ pump to the boiler side of the exchanger and ran vinegar through it for an hour. While I was at it I ran vinegar through the domestic side just to be sure, even though I'd done this maintenance at the beginning of the season.

I expected water flow through the boiler side of the exchanger to be slow with the portable circ pump, but it wasn't. Going into the bucket the flow looked quite full and the usual thing I see with this pump. So what's going on?

I have 165ºF water entering the boiler side of the exchanger and it's flowing more than fast enough to do the job according to the Alpha pump and by eyeballing the flow through from a portable circ pump. Water coming out of the exchanger on the boiler side is about 110ºF – way lower than it used to be. The domestic side of this exchanger is now squeaky clean, too. So how come this exchanger doesn't heat water like it used to for the last three years? Everything flows well and is clear as far as I can tell.

Thanks for your insights!

Steve

Jamie Hall · January 2018

The high delta T on the boiler side suggests just one thing: you aren't getting the flow you want. How does the Alpha measure flow rate? Is it a real flow rate measurement, or is it something else?

Swisher · January 2018

Thanks Jamie,
I don't know for sure how the Alpha measures flow rate, but I suspect it's an actual rate rather than an estimation. The reason I think this is because the flow rate varies on the LED screen on the pump. It goes up to 5 or 6 rpm when no other parts of the system are calling for heat, then drops to 2 or 3 gpm when heat is being called for elsewhere. Also, the exchanger flows plenty of water when I'm running the portable circ pump for cleaning. It seems clear. Puzzling.
Bye for now,
Steve

Gordy · January 2018

Could be a couple things. Fouled plates. The cleaning solution may come out clean, but are all the plates clean? cross contamination. Cracked plate, or leaking gasket. Which would explain high delta on boiler side. But that would be seen through increased system pressure on boiler side.

Is this a double wall HX. If not it should be. So cross contamination to domestic side can’t happen.

Zman · January 2018

The Alpha is estimating flow based on feedback from the ECM motor A bit above my pay grade.
I suspect it is accurate to 20% or so. I would not use it to troubleshoot.

Paul48 · January 2018

What mode are you running the Alpha in?

Jamie Hall · January 2018

Zman said:
The Alpha is estimating flow based on feedback from the ECM motor A bit above my pay grade.
I suspect it is accurate to 20% or so. I would not use it to troubleshoot.

Kind of what I suspected. Thank you, @Zman ! In which case, it is not actually measuring flow at all, but the input to the ECM. If the head loss is significantly higher than expected, the actual flow will be significantly less.

Gordy · January 2018

The op claims it's always read the same however.

Paul48 · January 2018

"If it's in "Constant Pressure Control" it maintains a constant differential pressure across the pump, irrespective of the flow".

That's from the manual.......and wouldn't that imply, it doesn't care what the flow is?

hot_rod · January 2018

I have seen the gpm readout up to 80% off, It can read a GPM even with the pump cavitating.

Are both sides of the HX pumped? Or single pass on the potable side? Has the incoming water temperature dropped? What is inlet and outlet on the B side?

3X8 is not a large HX for instantaneous, single pass heating

Be sure both sides of the HX are clean, comes a time when vinegar is not strong enough if it is heavily fouled. Get some of the acid sold with the tankless water heater cleaning kits. Most plumbing wholesalers carry pints of it. Hercules Sizzle is another good cleaner for hard water scale. Circulate for at least 30 minutes on both sides.

If flow rates are in fact the same as always on both sides you have a fouled HX. B&G is typically a good quality, US made, brazed style HX.

Heat Exchanger Performance Decline

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