It’s that time of the year, when the weather turns cold and people start to think about winterizing their home to reduce heating costs. Usually it takes the first winter heating bill to provide the motivation to undertake this task. With this in mind, I would like to talk about pipe insulation. Specifically, the foam wrap insulation you can find at any hardware store (http://www.homedepot.com/h_d1/N-5yc1v/R-202318552/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053)
In my case, I started to look into pipe insulation for an entirely different reason. During our home inspection, we were told that much of our copper pipe was run alongside the HVAC ductwork. In some spots, the metal strapping for the duct was touching the copper, which would cause some galvanic corrosion. The simple fix would be to wrap the pipe with something to prevent this contact. When I stumbled across this foam pipe insulation for about a buck for 6 feet, I was sure I had found my answer.
Since I was going to the trouble of going into the crawlspace, I figured I should buy a bunch of these and wrap as much copper pipe as possible, because my pipe is exposed to the cold outside air in the crawlspace, so I thought there could be quite a bit of energy efficiency gained. Plus, as the master bathroom is at the opposite end of the house, I had noticed it takes a minute or two in the morning to get hot water. My new hope was to be able to shower or wash my hands in the morning without having to waste water and time waiting for it to get hot.
During the installation of these foam pipe covers, I found it difficult to get the foam insulation in different areas, whether because of T-joints, strapping or bends. I also didn’t buy enough foam, so there was still some exposed copper. This is when I started to think about running a CFD analysis on this problem. If there is some exposed copper, because of its excellent conductivity, will the heat just move to this opening, rendering all my insulation efforts moot? Did I need to make the long crawl through my crawlspace to put more pipe wrap on, or were these little portions of exposed pipe inconsequential compared to the many feet of newly insulated pipe.
For this analysis, I used our general purpose CFD tool, FloEFD. I needed some baseline numbers, so I modeled a 1 meter length of copper pipe, then I would analyze that copper pipe completely covered with the insulation (best case), then introduce a representative “gap” in that insulation for my current setup. From some research I found that hot water comes from the tank at about 50 degC, and a shower can draw about 2.5 gal/min. For the air, I wanted to simulate the worst case air temperature, which I think in the winter in my crawl space would likely be about 5 degC (above freezing for sure, though preventing pipe freezing is an added benefit of these pipe insulations).
Now we all know convection heat transfer improves with air velocity, so I wasn’t sure where to go here. From my previous crawlspace blog, where I looked at my soil water issue, I found the air speed down there was sluggish to say the least. Yet, the copper pipes running to my shower run within 1 ft of the crawl space vents, and I could feel a breeze at that location. So I decided I would need to run a no wind and a 1 m/s wind case. Below are my results.
The other main result was that the heat transfer rate was basically the same. In fact it’s slightly worse with insulation compared to without, due to the insulation having a larger surface area.
Now, looking at the results for the with wind case, where we see a bit of a reversal on the heat loss trend. Now that forced convection is dominant, the increase in surface area for the insulated pipe doesn’t seem to be a factor. At the end of the day though, the water temperature is still pretty much the same value.
It’s at this point when I started to look at this problem in a different light, as I didn’t want to have wasted money putting on insulation that isn’t effective. My new thinking is that a steady state analysis of this problem is not ideal. We will never be running the water for hours on end. My goal is to have the hot water that is in the pipe to stay hot for as long as possible so that it doesn’t take 5 minutes of running the tap to get hot water at the sink/shower. This wastes water and energy and my money, and that’s what I’m hoping gives me my ROI for my foam insulation investment.
With that, I decided I needed to simulate this as a transient analysis, starting from when the water has stopped running and timing how long it takes for the water to sufficiently cool. I figure that would be somewhere around room temperature when you would think the water isn’t “hot”.
At this point I only simulated the wind case and full/no foam, as I’m more interested in the worst case scenario (and justifying my insulation purchase).
Now this is what I’m talking about. Without insulation, the water in that pipe gets to a chilly 10 degC in about 11-12 minutes, whereas with insulation, the water doesn’t get that cold till 100 minutes. That’s a 9 times improvement for $1 per 6 feet of pipe. It’s hard to argue with that return on investment. Now, I usually shower first thing in the morning, so the water won’t stay hot throughout the night, so I’m out of luck there. For everyday washing of hands and kitchen stuff, we will definitely be wasting less water because of these pipe wraps, and that’s what matters.