Cavitation in Mod-Con Combi Boiler DHW Hydro-block.

mrhemi · March 2023

I have searched and have not found a discussion on this specific problem. Approx 2 years ago my original combi boiler (Triangle Tube Challenger) was replaced with a new combi boiler (Triangle Tube Instinct). The application utilizes a DHW storage tank as per the attached drawing. The new boiler emits a rather pronounced how when in the dhw mode. This has be traced (with a stethoscope) to be the flat plate exchanger (hydro-block) or the passages in the discharge manifold. The plate exchanger has a much higher pressure drop than the original installation exchanger, so potential cavitation is believable. Any insight or experience with this type of occurance?
Of note, the original installer is of little help. Still waiting for them to return (not likely) to optimize the boiler setup.

GGross · March 2023

Generally, when storage is requested it is wise to use a boiler mate and boiler, this is kind of a Frankenstein setup. What pump are you using to flow through the DHW side of the flat plate? My guess is that you were flowing the tankless heat exchanger on the old challenger and now you are getting noise from flow velocity, the flat plate should be less restrictive than the old challenger heat exchanger

hot_rod · March 2023

If you have the spec on the HX and the pump spec you can calculate the velocity. Any other place that there could be a restriction, strainer valve partially opened?
Do you have a valve that you could restrict flow through the HX to see if it changes the noise.

Properly applied plate heat exchangers are an efficient way to generate DHW and easier to service compared to coil in tank DHW. Energy Kinetics has perfected the external HX method of DHW, and post purge.

Jamie Hall · March 2023

I note that if you are piped as the drawing suggests the heat exchanger is on the suction side of the circulating pump. This is just asking for trouble. I doubt that you are getting true cavitation in the heat exchanger -- though you may well be in the pump, which won't last long if so -- but you may well be running a low enough pressure in the heat exchanger to cause some flashing in there, which would sound much the same.

mrhemi · March 2023

I wouldn't say this is a Frankenstein setup, uncommon maybe. The included schematic is actually from Triangle Tube's installation manual. It serves its purpose well and since the storage tank is actually an electric hot water tank (wired but not normally energized), it allows for redundancy (for DHW) should the boiler be non operational. Which was exactly the case 2 years ago in the dead of winter when the original boiler failed and I heated my home with the fireplace while Triangle Tube took two weeks to decide what they were going to do. I had hot water through this by simply swinging a couple of valves and energizing a breaker. I will give Triangle Tube credit, they replaced the original failed Challenger combi at no charge, other than the installation labour. The DHW exchanger in a Challenger boiler had very low delta p as compared to a plate exchanger, so this phenomena was not present. Of note Triangle Tube has discontinued the Challenger line.

Hot Rod, I was able to get the exchanger curve (attached) from Triangle Tube. The original Challenger install included a UPS 26-99 circ pump on the DHW as per Triangle Tube's recommendation and performed fine. When the boiler was changed out to the Instinct, the cavitation was immediately present when in DHW mode and the DHW flow rate was only 2.4gpm on the highest setting. The Instinct 155C will support 4.0gpm. With the exchanger curve I came up with a UPS 26-150 pump for sufficient head pressure. That pump has been installed for more than a year with no issues and maintains DHW throughput at 4.0gpm on the lowest setting. Cavitation volume is about the same for either pump by the Mark 1 Eardrum. My main concern in addition to the noise, is cavitation damage to the equipment. I can eliminate the cavitation by pinching down a downstream valve, but this naturally reduces flow too much. Wondering if anyone has utilized any type of restriction orifice or flow restrictor to control cavitation.

Thanks for any feedback.

mrhemi · March 2023

Jamie Hall,
The installation is installed as per the drawing. The pump is on the upstream side of the exchanger, which is located inside the boiler enclosure, not in the storage tank.

hot_rod · March 2023

If it has been in for a while and worked, I would suspect the HX is getting fouled. Hows you water? hard water will scale heat exchangers quickly and reduce flow.

You want a good velocity on both sides to scour the plates clean, they call it channel velocity.

A contractor I know had a similar issue with a Viessmann combi about a year after install, production dropping off. After replacing a bunch of parts that the factory recommended he ripped the HX out and found the boiler side was fouled, not the domestic side.

It would be nice to install iso/ purge valves on both A and B side of plate HX to descale them quickly.

Cavitation is more of a crackling sound, like Rice Krispies, or a sound like gravel in the circ. It's the vapor pockets imploding.

Does the circuit have an internal check? It could be a cause of restriction, if it is jammed.

What is the boiler pressure? Low pressure and high temperatures are an invitation for cavitation.

Make sure the air purger is not plugged, if it is an air issue.

mrhemi · March 2023

Hot Rod, I appreciate your suggestions, I am on the same page with you. Interesting about the Viessmann. I am at the same point of changing out the hydro block. If the exchanger is fouled, it was fouled to begin with. The noise has been there since the day it was commissioned. I have descaled the DHW side once per year since installed to no effect. CH side does not have purge valves as it is integral to the boiler. The water here is quite soft. Municipal, and the source is Lake Huron. The boiler pressure is 18-20# on the primary loop, 29# at the discharge of the integral primary pump when running. The CH temp is 154 deg. to provide 146 deg. DHW during a call. The domestic water pressure is usually 50#.

At the end of the day, I have not experienced any degradation of performance. It is the annoying noise that can be heard in the house during a DHW call. This is with the door closed on the basement boiler room. If the noise is actually cavitation, I am concerned about future equipment damage. If the noise is something I have to accept I may place sound deadener mat on the boiler cover.

Thanks for all your suggestions. I will post if I find a smoking gun.

GGross · March 2023

@mrhemi

I would remove the flat plate heat exchanger for cleaning the next time you are due to flush it, they come out pretty easily but you would need to drain the boiler. Possibly something is not seated correctly, possibly you can clean something out of the boiler side. We had one that made some noise but more importantly was spraying water out as it was misaligned presumably due to rough shipping.

hot_rod · March 2023

https://forum.heatinghelp.com/discussion/comment/1743712#Comment_1743712

I agree, maybe one of the assembly gaskets of seal has patrtially blocked the FPHX opening. I think a breakdown would be telling. That is what solved this one.

The debris shaking out is from the boiler side on a year old install, filled with city water, purged and flushed. Once cleaned on both

Screenshot 2023-03-21 at 2.15.12 PM.png

A and B side the system operated as expected.

Cavitation in Mod-Con Combi Boiler DHW Hydro-block.

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