I've received a couple emails from people with vacation homes or cabins in cold or mountain climates wondering about a solar (or other) solution to keeping the pipes from freezing without having to run the furnace all the time.
The same situation applies to people in cold climates when they go on long vacation or trips away from home.
The usual approach is to leave the heat on enough to keep the house/cabin from going below freezing. This is expensive and results in greenhouse gas emissions even when you aren't home!.
Here are 3 possibilities -- if you have others, please let me know.
Insulating Thermosyphon Solar Collectors in Windows
Drainable Plumbing System
For this solution, you need some windows that get sun at least part of the day. While solar collectors work best when facing south, this solution could also work for east or west facing windows that get good sun.
The idea is to build removable thermosyphon solar collectors for the windows. That's a fancy name, but they are very simple, and should take less than half an hour a window to do. The collectors are made from rigid insulation board, so that they not only provide some heating, but also greatly reduce the heat loss from the windows they are used in. The combination of solar heating plus reduced heat loss should significantly reduce the heat fossil fuel heat needed to prevent pipe freezing.
One thing I want to make clear -- windows by themselves are very good solar collectors. Building these insulation board collectors will not make the window collect any more heat than it would by itself. The problem with using just the window as a collector is that it will lose lots of heat at night.
The benefit of these insulation board collectors are that: 1) they allow the window to gain solar heat during the day, and 2) they dramatically reduce heat loss from the window at all times.
Sketch of Collector: The light blue panel is rigid insulation board that you cut to fit the window opening. The brown openings are holes that you cut through the insulation board -- the lower two openings are the inlet vents and allow room air to flow into the collector area to be heated. The upper two openings are outlet vents that allow heated air to flow out of the collector. The two outlet vents are fitted with poly film backdraft dampers that prevent reverse flow at night that would cool the room.
It is best to use polyisocyanurate insulation board, as it will hold up well to the high temperatures between the window and the insulation board. Most lumber yards carry this insulation board, but they may not know it by this name. If in doubt, ask to see the insulation boards they carry -- if its the right stuff, it will say "Polyisocyanurate" on the board face.
In operation, the sun heats the dark surface of the insulation board, this heats the air next to the board, which causes it to become less dense and rise, exiting the top vent. As hot air exits the top vent, cooler room air is pulled into the bottom vent.
Here is how to make a collector:
Cut the insulation board panel to
fit the window opening. The fit should be reasonably snug, but small
gaps won't effect performance much. Leave as much space between the
window and the insulation board as the window opening allows.
See picture -- the insulation board cuts easily with a razor blade knife.
Cut the upper and lower vents in
the insulation board. For a typical window, the vents should be about
3 inches high, and should cover most of the width of the insulation board
(the official guideline is to make the vent area at least half of the cross
section area between the window and the absorber). Mark them off with
a felt tip marker and straight edge, and cut with a long razor blade knife.
Paint the window side of the
insulation board black. If you prefer the look of a dark blue, green,
or brown rather than black, the performance will be just slightly diminished
compared to using black. It should be a dark color for good solar
Cut a piece of hardware cloth to
fit over the vent opening and secure in place. Hardware cloth is a
coarse screen or wire mesh material that is sold (surprise) at hardware
stores. A good quality duct tape is probably adequate to hold the
screen in place. The purpose of the screen is to prevent the poly
backdraft damper film from being sucked through the vent opening.
Cut a piece of thin poly film that is an inch or so larger than the vent opening for each of the top vents. I used Costco kitchen plastic garbage bags for this, but any thin poly will do. The poly has to be light weight enough so that the rising hot air from the collector will push it out of the way. Tape this to the insulation board on the top edge only. That is, use one piece of tape along the top edge of the poly to tape it to the insulation board. The poly must be free to swing open when the air flow from the window cavity and out through the top vent.
Pat yourself on the back -- you have just made a solar collector for $1 per sqft -- commercial solar collectors cost $30 per sqft!
Making a collector panel -- click pictures for full size:
This shows the test collector installed in a west facing window with part sun. The temperature gage only shows a 23F temperature rise with 70 ft/min airflow through the collector under these conditions. This is worthwhile, but it will do much better under good sun conditions.
From the outside. If you don't
like the black square look, just paint the
collector any dark color.
The backdraft vents will allow the air heated in the collector during the day to flow freely out the top vents, but at night when the air wants to flow the opposite way (which would cool the room) the poly gets sucked up against the hardware cloth to stop the flow. It is surprisingly effective.
The insulation panel should fit the window fairly well, but does not have to be perfect -- gaps of the order of 1/8 inch will not effect performance significantly. If the window is too larger for one 4X8 insulation panel to fill, just use two panels.
One style of insulation board that works pretty well for this application is Atlas R-Board. It has a high R value per inch (6.7), and it comes with fiber face sheets on both sides that will take paint well, hold up to some abuse, and look better than bare insulation. I understand that Thermax comes in a similar style.
You could use 1 inch insulation -- this will cut the heat loss through a low-e double glazed window by 2/3rds, or you could go for 1.5 or 2 inch for even more reduction in heat loss.
For windows with not solar gain, just use the insulation board without any vents.
The collector shown above is for a small window, but you can do the same thing for windows of any size. It will, or course, be even more effective for large windows. Some vacation homes have very large view windows that are huge sources of heat loss, so adding these panels will make a large difference.
Another variation on this type of collector that can be left permanently in place is described here...
This arrangement uses a regular cellular shade with collector vents above and below the shade. When the shade is down, it acts as the absorber for a thermosyphon collector.
The backdraft damper on this collector is located on the inlet vents rather than the outlet vents.
This achieves pretty good window R values and some solar heat collection with an arrangement that looks good and can be left permanently in place.
Like the other collector, this will only be effective on a window that gets some sun during the day. Its probably not worth the effort for windows that get less than a couple hours of sun on a sunny day.
For windows that don't have sun exposure, you can use the same type of rigid insulation panels described above for the collector, but just leave out the vents and backdraft dampers.
A set of these panels that are applied before you leave will dramatically reduce fuel consumption, especially for the cabins/vaction homes with the big expanses of window. Even a 1 inch thick rigid foam panel will cut the window heat loss by about 2/3rds.
All of the other methods that cut heat loss, such as more insulation, better sealing, ... will also help both when you are there and when you are gone.
Closing off areas that don't have plumbing to protect will also reduce heat loss.
I used to be involved in a volunteer labor ski area that was only open on weekends. At shutdown time on Sunday afternoon, we shut off the water supply and opened a valve that drained all the ski lodge plumbing to an outside drain. We then went around and opened all the faucets to allow air in so the water in the pipes would drain. In addition, the toilet tanks and traps were drained, and the hot water tank was drained. This system allowed the ski lodge to be left all week in weather well below freezing without any damage. The lodge has been operating since the 1920's, so its a pretty well proven system :)
In this ski lodge, the plumbing runs were sloped for drainage. In a home or cabin without sloped plumbing it might be necessary to use some compressed air to blow the water out of the lines, but it should be possible to work out a system to do that fairly easily. For example, at same point where you add the plumbing drain, add a fitting for compressed air.
One caution is that when you return to the house, you must heat the house up to above freezing BEFORE you turn the water back on!
This is probably the best all around way to go in that no heat at all is needed, and you don't have to worry about a furnace failure, or thermostat failure, or running out of propane, or your furnace burning the cabin down. Its a nice simple way to go with zero energy use and zero CO2 emissions.
Any other ideas ?? Please let me know Gary...
Gary October 28, 2008