The lessons that I learned from the computer model of our solar hot water system are as follows:
- insulate the pipes leading from the basement to the collectors to at least R6, preferably R10
- insulate the storage tank to R50
- install a 1000 litre storage tank
- install 3 collectors
- there is extra heat - install a system to harvest it
We bought the collectors, drainback tank, pipe insulation and other knickknacks from Trimline Design Centre just down the road from the Mill Creek NetZero Home. My builder Peter was very impressed with the clever, simple design of the flat plat collectors that Trimline manufactures. Yes, that’s right, they manufacture solar hot water collectors right here in Edmonton!
(schematic of the MCNZH solar hot water system)
The three collectors will heat water in an R50-insulated 1500 litre tank that I'll call the "pre-heat" tank (the schematic incorrectly indicates that the pre-heat tank is only 1000 L). The municipal water will first pass through a drain water heat recovery coil, and then enter a 200 foot coil of copper tubing that will be inside the hot water tank. The water in the pre-heat tank (the 1500 litre one) will be for storing heat only – it won’t be potable. Anyway, the water from the city will first go through the 200 feet of copper tubing before it reaches the 50 gallon electric hot water tank, the "top up" tank.
We are also adding a means by which to use extra solar heat for space heating, but I’ll talk about that in another post.
We installed the collectors on the roof, put a drainback tank in the loft (the water drains back to the tank when the sun isn’t shining, preventing it from freezing), and ran copper piping to the basement mechanical room.
(a solar hot water collector in the living room – at 8’x4’, they’re bigger than they look from the ground!)
(Tristan soldering fittings onto a collector)
(installing a solar hot water collector. I’m the guy being useful – that’s right, hands in the pockets)
(Peter Amerongen, president, Habitat Studio & Workshop)
My modelling informed me that lack of pipe insulation might be the single biggest mistake that people make when they build these systems – in the MCNZH the pipes run up over 8 metres, so their surface area is significant. We insulated them with Armaflex’s Armacell pipe insulation. It’s a closed cell foam insulation that can withstand the high temperatures of a solar hot water system.
(Armaflex pipe insulation)
We got a bunch of 1” thick stuff, and also some 3/4” thick insulation because they didn’t have enough of the thicker stuff. That brought the insulation value to somewhere between R3-R5.
We then supplemented that with foil-backed insulation:
(the pipes that run hot water from the basement tank to the collectors – we couldn’t fit any more insulation in the wall cavity)
That gets the insulation value of the pipes up to a ballpark of R6-R10, right where we want it.
Originally we had planned to use a tankless hot water heater to top up the heat of the water, but we decided against it to reduce complexity and cost. Some tankless hot water heaters don’t work well if the water coming in is preheated, you need to have special electrical wiring to handle the surges, and the tankless models cost 2-3 times more than a regular hot water tank.
Standard electric hot water tanks come off the shelf quite poorly insulated – R10 or so I believe. We’ll add R40 insulation to the top up tank so that our standby losses are reduced to 10-15 Watts. By doing this we'll be making an ordinary $300 to $400 electric water tank almost as efficient as a demand heater that would cost considerably more to buy and to install. Our annual standy losses will only be about 40 to 50 kWh per year if you consider that in the middle of summer the standby losses will be mostly be replaced with solar (that water in the top up tank will be so hot that the losses won't matter) and in the heating season they will heat the house.
We built the pre-heat tank ourselves, and had a hitch or two before we achieved success:
(Tristan lining the 1500 litre hot water tank for water tightness)
We built a box with 1500 litres of volume, then decided to line it with a roofing material for flat roofs:
In retrospect, we were doomed from the beginning – water will find a way through an seam no matter what. We filled the tank partway with water to test it:
and we soon found a tiny trickle of water leaking out. The next step was to line it with a pond liner that landscapers use:
Without seams, this solution seems to be working for us. We’ve had it filled with water for about a week now. The seamless membrane is 45 mil EPDM material that is sold by Canar Rock Products as a pond liner.
The top up hot water tank will be placed beside the bigger tank (where the ladder is in the picture above) and surrounded by insulation.
One of the last components that we needed to build was the heat transfer coil. Cold water from the city will pass through 200 feet of this before reaching the top up tank.Tristan built a frame out of PVC pipe, and created the coil by wrapping soft copper pipe like so:
So we have the collectors, insulated pipes to the basement, an insulated heat storage tank, and a coil to transfer the heat to the potable water. We’re almost ready to hook everything up and see how it runs.
(cross posted at raisingspaces.com)