Cleaning heat exchanger coils

The troubleshooting of our busted boiler (a Trinity LX150 by NTI), led me to the fact that mineral deposits inside the heat exchanger led to partial or complete blockage.

I knew that I had to clean out those mineral deposits, but I wasn’t able to get my hands on the recommended Fernox DS-40 descaler and cleanser – at least not quickly.

My next best option was to use widely available cleaning vinegar. But for this to work I had to isolate the boiler. In other words, I needed to just flush the boiler with the cleaning vinegar, but not the whole hydronic heating system, because that would take gallons upon gallons of cleaning vinegar.

Isolating and flushing the boiler should have been a cake walk, if our boiler had been installed with the drain and isolation valves as shown in the nifty manufacturer-issued plumbing diagram.

The reality of our boiler plumbing required a little creativity, because I only had one isolation valve and did not have drain valves as shown in the diagram.

I isolated the boiler by turning off the isolation valve on the outlet side, and turning off the isolation valves for the boiler pump on the inlet side. Removing the boiler pump was the best substitute available for the missing drain valve.

I then proceeded to remove the pressure relief valve on the outlet side and replace it with a simple ½ inch riser.

By pouring about one gallon of cleaning vinegar into the boiler through the riser, I was able to displace the water in the boiler and heat exchanger.

I added about another gallon or two into the bucket below the boiler pump, set my small sump pump into the bucket, and connected it with a vinyl tube to the riser on the outlet side. With this setup I was able to start flushing the boiler in reverse (outlet to inlet) with almost pure cleaning vinegar.

I also added a small wire tray to monitor the debris discharge. It took me three days of boiler flushing to get the mineral deposits out of the heat exchanger. I actually flushed for four days, but had no more debris discharge on day four.

And don’t be fooled by the rapid flow rate in the video, which I took on day four. On day one, I had a fraction of that flow and the sump pump was cranking pretty hard.

How did we get by without a boiler for three days?

That was thanks to the resilience of our system. We didn’t use any hydronic heating during the three days of boiler maintenance, but instead relied on our minisplits for heating.

And what about domestic hot water?

After each day of flushing, I reconnected the boiler and turned it back on to recharge our buffer tank and domestic hot water storage tank. With both tanks recharged, we had enough domestic hot water for a day.

I sat with the boiler during the flushing process and manually turned it off to let it cool down once there was a hint of hammering. It was time consuming but it worked.

I also noticed during the three days that the boiler operation got increasingly more quiet. Not only did the hammering and banging cease, but so did the hissing of flowing water. With the obstructions in the heat exchanger removed, our boiler ran almost silently and very efficiently once again.

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About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

4 thoughts on “Cleaning heat exchanger coils

  1. It seems likely that you have new water being introduced. Perhaps there’s a leak, and the auto-feed (if you have one) is keeping the system filled?

  2. Smart to put a strainer into the loop, by that you save quite a portion of the “chemical potential” of the acetic acid which otherwise would be lost by dissolving particulates already removed from your system.
    As a rule of thumb to increase the temperaure by 10 ° doubles the speed of a chemical reaction. That means that by operating at elevated temperature you can speed up the process, higher temperature even increases solubility.
    However it is import not to overheat your Chemistry. When using volatile organic acids as acetic acid there is even a potential odor issue to consider.
    In the end ambient temperature seemed to work for your system.

  3. Oliver, thank you for the additional insight and excellent tips. I will keep all those aspects in mind for the next cleaning – which hopefully won’t be until a few years down the road…

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