5 Responses to “What Is An “Immersion Vent”?”


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  1. Steve E.

    Hi Dr. Ben,

    Like Alan, I’ve found your work interesting and fascinating. I’ve also read the web page for Holocene and am encourage by the successful you’re involved with. It seems you’ve been able to make a career of a passion I’ve had but only dreamed about.

    As I get older (I’m now 53) I dream about a career diversification but it’s not easy to start over. Actually, I’m certified as a DSHW installer, but there just isn’t the scale of economy to justify the investment in MOST applications, and money is tight.

    My dream is to be an ESCOM for high consumers of DSHW. Design, own, install and maintain medium to large capacity systems. The recurring revenue stream would be more profitable than a one time install. The problem is starting a business is not cheap, easy, or of great comfort to my wife.

    I’ve really enjoyed your blog and everything you’ve posted. I’m impressed with your concept of self contain storage. It appears that you’re using a tube and shell heat exchanger without the shell in the storage tank to preheat DHW. That’s a little different concept than what I’ve seen in Wisconsin, but it makes complete sense for large capacity projects. Considering it’s prepackaged, if you have the space for it it’s a great idea.

    I’m a member of a local school board and I’ve volunteered my time to install a 2 panel system in a new charter school we have that’s a S.T.E.M. school. The school doesn’t consume much HW because all dishes are sent to the H.S. as part of a centralized system. That’s where the low hanging fruit is, but little steps first. This is purely an educational project with side benefits. Everyone is excited about it including me.

    I’m a commercial HVAC tech with 30 years in the trade, hence my understanding of everything in your videos. I really enjoyed all the photos you’ve posted. I picked up on some of the applications like the series pump set-up for high lift and the multiple heat exchangers for higher capacity or staging.

    I also completely agree with you on the need to verify the end user consumption as the very first step to design and economy of a project. In fact, our city water department gave us a water meter to install on the incoming CW to the HWH and the students will be logging the consumption as part of the experience. I just received the pricing from a local wholesaler for the components and now my employer will put the scope together.

    We’re looking for local corporate sponsorship(s) for the project and on my list is Badger Meter. Are you familiar with their metering and data collection software? What we really want is a web based graphics program so the data is more visual than static raw numbers. Our district can host everything…I’m a little out of my league here.
    I do have 2 more questions for you.

    1. Being an open system with a steel tank fabricated from hot rolled steel and a make-up water system, what keeps the tank from corroding and how do you keep the mill scale from collecting in the water in the form of iron oxide?
    2. During non-solar operation the pump is off. How is the design flow rate of the heat exchanger accomplished to heat the domestic water without flow in the tank around the tube bundle?

    Thanks for sharing everything with me (us). The pleasure has been all mine (ours).

  2. Dr. Ben


    Thanks for the kind words. You ask about corrosion in the tank. We use a corrosion inhibitor that includes sodium nitrite to prevent galvanic corrosion between copper and steel, borax as a cleaner, and phenylthalene, which is an oxygen scavenger. The solution is diluted 250:1 in the tank and is a basic solution.

    Does the tank rust? The answer is yes, a little bit. We recommend not installing the corrosion inhibitor until all the piping is done and the system is filled and running. We want everything tested and leak free before putting the inhibitor into the tank. If there is a leak, we don’t want to have to dump the inhibitor along with the water. So, during this time the tank rusts some. Actually, tiny leaks will rust shut. The rusting also removes some of the oxygen from the water. This amount of rust is trivial and of no real consequence.

    When the inhibitor is put into the water, it stops the progression and stabilizes the system. The phelylthalene is a pink color when fresh. Over time, as it scavenges the little oxygen that gets into the system, the color fades to clear and it is time to add a new charge of inhibitor.

    There have been many systems where the first charge of inhibitor was all that was ever put into the tank. Some have lasted more that 20 years with no additional inhibitor. We don’t recommend this, but with a captive charge of water, little fresh corrosion chemicals come into the tank and they generally last a long time.

    Your second question is about the operation of the heat exchangers. Heat exchangers are operated is two ways. In one method drinking water flows inside the exchanger and tank water is pumped over the outside. In other words, there is forced flow on both sides of the exchanger pipe. This is called double pumping.

    Other heat exchangers have forced flow only inside the exchanger. This is a single pumped system. Here is how it works. Let’s say the tank is hot, and cold water flows into the exchanger tubes. Heat is transferred from the hot surround water through the tube wall and into the cold water. The chilled tank water then falls toward the bottom of the tank because it is denser than the hot water. This falling water is a flow that causes all the water in the tank to stir. In a 1000 gallon tank, a big heat exchanger can cause a 17 gpm circulation of water through thermosyphoning.

    Forcing flow over the inside and the outside of an exchanger is more efficient than forced flow only inside, but eliminating the pump and controls makes the system much simpler and maintenance free. Oversizing the exchanger can compensate. I used to make double pumped systems, but decided that single pumped system reliability was worth the trade offs.

  3. Steve E.

    Thanks, Dr. Ben.

    I noticed that in a prior post you mentioned that tank design tmperature is 140 deg. F. Why is the system design not 180 deg. F to take full advantage of every solar BTU possible? I’ve seen smaller systems that provide anti-scalding protection with a 3 way thermostatic mixing valve added to the domestic water supply line entering the base building system set at the main tank temperature.

    • Dr. Ben

      Actually, we use 160F as the high limit. Then the tank must fall 15F to 145F before it can start again. This prevents short cycling.

      Why not raise the limit as high as possible? Two reasons:

      1) If the tank is 160F, that means no one is using the energy. To continue running the system can just waste horsepower. If the system is being used, then the tank temperature will not go that high.

      2) Normally, if the tank is 160F, the water temperature coming out of the heat exchanger is in the 140F range. For those systems with no tempering valve, this affords a measure of scald protection. Since we do not control whether there is an anti-scald valve (tempering valve) installed down the line, shutting the tank off at 160F fits the overall safety requirements of hot water systems.

      Finally, the owner of the system can open the control cabinet and adjust the high limit temperature anywhere they want. We just ship them out set at 160F.

      In the late ’70s I got a phone call from a builder who had installed 16 collectors and a 1000 gallon tank on his house. He told me the tank was up to 220F. I explained that if that were true, the tank would be boiling. He said the tank WAS boiling. I told him the high limit must not be working. He said he had disabled it. “Why did you do that?”, I asked. “I’m just having fun” was the reply.

  4. You forgot to mention that the immersion vent doubles as the tank overflow, ensuring the proper water level!

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