Wednesday, December 30, 2009

Solar Hot Water Part V: Insulation

A much underappreciated  part of making a solar hot water system efficient is properly insulating it to reduce parasitic losses. There are two aspects to this:

  1. Properly insulating the pipes carrying the hot water and the heat transfer fluid from the solar collector.
  2. Ensuring that the solar tank and any backup storage (the gas fired tank in my case) are sufficiently insulated to reduce loss during water storage.

The contractor did a fine job on the pipes carrying the heat transfer fluid outside leading from the solar collector to the inside of the house. This is a particularly crucial area, because if the air temperature is low during the day the losses in this section can be really high. But for the pipes on the inside, if you take a look at my previous post on installation (which you'll find here), you can see that the contractor sort of insulated the pipes on the inside. The black rubber you see around the pipes in some of the pictures is pipe insulation. The contractor did an OK job with most of the pipes except around the gas fired hot water heater. The pressure relief valve pipe and the hot water pipe leading into the house were not insulated and radiating heat. Also, at places where the pipes had a bend, the black rubber pipe insulation stopped and left small areas of copper exposed that could radiate heat. Getting pipe insulation right takes a lot of time and effort, something most contractors don't want to do (otherwise they would have to charge you for it).

So I spent some time doing the pipe insulation myself. I installed the black rubber insulation around all the pipes which the contractor forgot, and sprayed some high temperature foam insulation around the pipe corners, and around the connections between the pumping station and the feed-in lines. Here's a picture of how the pipes look in the ground floor gas-fired hot water heater closet:

 The blobs of pink matter you see hanging down from the pipes where a corner is are the high temperature foam. Note that this is not the standard polyurethane foam that you can get for insulation at the hardware store, this is foam which is rated for installing, for example, between floors in apartment buildings to help retard the progress of a fire. It won't stop a fire, since it is organic and will ultimately burn, but it is rated to a much higher temperature than the standard, white polyurethane foam that's used for sealing around doors and windows. It's available at Home Depot, and the important distinguishing factor is, as you can see, that it is pink. I used high temperature foam because the temperatures around the pump station (which you can't see here) and around the furnace flue which leads up through the closet can get pretty high, maybe around 180F, which might cause the normal foam to get soft, if not melt. Finally, the white blobs you see are tags indicating what each of the pipes do (sorry, the picture  is a  little out of focus because the shutter was open for a long time and my camera doesn't have image stabilization).

The second aspect is the tank. The Superstor comes with 1.5" of closed cell foam insulation, and they claim that it loses around 0.5F per hour (as we will see in a later post, this is optimistic at best). To my mind, that is simply not enough insulation. My Hot Springs hot tub has around 6" of closed cell foam, and it loses maybe 2F overnight if I turn the temperature down to 80F after having it up around 102F, and that is outside where the ambient temperature in winter is a lot lower than inside the house. I think the reason they don't add more insulation to the Superstor is that it would make the tank's diameter bigger, and, of course it would make the tank even more expensive (the Superstor is pretty expensive as is).

My first plan was to spray the tank with closed cell foam before the contractor installed it, but a quick check with the city building inspector indicated that they would probably not approve the project if the tank was coated with closed cell foam. I then came up with the idea to take radiant barrier insulation made from aluminum foil and plastic bubble wrap and make a form the size of the tank, then spray it with closed cell foam to make a kind of jacket. So I tried that (sorry, no pictures) but the result was too rigid once it had dried and I couldn't get it into the narrow space between the tank and the wall.

Since I had some radiant barrier left, I decided to wrap both tanks in radiant barrier insulation as a start. I did this work in summer right after they installed the system, and the solar tank was radiating heat strongly, a bad sign. I did the wrapping in two phases, first the left side then the more complicated right side around all the connections. Here's a picture of what the Superstor looks like when it is completely wrapped in radiant barrier insulation:

One issue was what kind of tape to use. Standard duct tape probably won't work because the glue will melt from the high temperatures in summer. I first tried an aluminum tape from 3M that is specifically for furnace flues and other high temperature applications. It wasn't ideal, it was stiff and didn't adhere unless you really pressed it, also, it had sharp edges, but it sort of worked, although some places that weren't pressed hard enough came loose. I then found a similar tape from Nashua which sticks a lot better and doesn't require strong pressing to bind.

After I finished the radiant barrier job, I discovered that a new line of high temperature duct tape had been introduced on the market. I used some that on the R-13 fiberglass blanket which I wrapped around both tanks on top of the radiant barrier. You can see that on the bottom of the picture above of the gas fired tank. Here's what the Superstor looks like nicely clothed in its new fiberglass blanket:

The gray stripes around the tank are where I taped together the R-13 batts.

One question that might come up is: why not used one of the premade vinyl insulation jackets that are commercially available? I thought about this, but there are a number of problems. First, I couldn't get a jacket big enough for the Superstor. It is 60" tall, much larger than a standard 40 gallon gas or electric domestic hot water heater. Second, they only provide something like R-7 of insulation. I now have R-13 with maybe R-7 or so of radiant barrier. Finally, vinyl is not a good material to have around the house, especially around hot areas where chemicals can outgas. I looked around for a jacket that didn't have a vinyl skin but could not find any.

In the next post, I'll talk a bit about how the system is performing, and provide some data about how well my insulating treatments have worked.


  1. Anyone who is thinking about solar water heating will really appreciate this series - even I found it interesting to dip in and out of the topics. I'm not too worried about outgassing from our vinyl replacement windows (which by the way I hardly have the strength to open and close they are so sticky and stiff) because our house is extremely draughty. How's that for qualifying a negative with another negative!

  2. I mean I just read through the entire post and it has been really great but since I'm more of a visual learner, some more vids and charts would help a lot. Nevertheless, the topic here is really worth pondering about.
    hot water heater price

  3. "...with maybe R-7 or so of radiant barrier..."

    Firstly, this is a misuse of nomenclature, R-value applies _only_ to conductive losses, not radiative losses. Even if everybody does this (and they do), it is best to keep in mind the proper terms.

    Secondly, radiant insulation only works when there is at least an inch air space on the warm side of the radiant insulation, see any manufacturer's instructions. The insulation value of that foil lined bubble wrap is thus approximately the value of just the bubble wrap (i.e. R-0.5).