Solar Hot Water

**EDIT, March 2011 **

The modelling that we did with WATSUN (and upon which this blog post was based) was completely inaccurate. It hasn't been near 100%. In January our tank sits at 20 degrees, and it fluctuates between 25 and 40 during February. Not even close to what the software promised us.

I'm not sure what happened, but modelling with WATSUN was a complete waste of time for me.

Our solar fraction is probably in the range of 60%-70%.

The Mill Creek NetZero Home (MCNZH) will collect solar energy in three different ways: using passive solar design, using photovoltaic (PV) modules, and using solar hot water (SHW) collectors. The SHW collectors are the ones that heat water - they are the black ones at the top of the above picture.

According to a recent article in Home Power magazine (Oct/Nov 2008, p.40), SHW collector efficiency is 50%-70%. That's pretty good when you consider that the best PV module is about 17% efficient.

I've been contemplating the design of the MCNZH's SHW system for months now. The pieces started to fall into place once the federal government released a crucial tool: The  WATSUN 2008 SHW System Simulator.

Computer Modeling - WATSUN 2008

The ability to model real-life systems with computer software has widespread application in every area of technology. In residential home building, though, I suspect that it's given short thrift. In my experience, rules of thumb and "what worked last time" are what steer the boat when designing residential homes. I prefer to trust in reliable, time-tested computer code more than gut feelings.

WATSUN 2008 is available from the Canadian Federal Government as a free download right here. It performs hourly simulations of SHW systems, and it's based on algorithms that have been in use since the 1970s. One of its great features is that it contains weather and sunlight data for Edmonton, Alberta. Once I downloaded and learned about the software, I contacted Didier Thevenard. He is the primary mover and shaker behind WATSUN 2008, and he generously helped me figure out some of the collector data numbers that I needed.

WATSUN 2008 - Results

I had some pretty important questions to answer when I began the computer modeling:

  • How much of my hot water can be produced from solar?
  • Should we install 2 collectors or 3?
  • How big should our storage tank be?
  • How important is pipe insulation (the pipes that run from the collectors to the storage tanks)?
  • Is there any energy left over for space heating?

So I set up some simulations. The collectors will be of the flat plate variety, purchased from Trimline Design Centre. They manufacture the collectors right here in Edmonton.

Pipe Insulation

In the first set of simulations, I was testing the effect of three different variables: pipe insulation (R3 vs R6), tank size (316 litres vs 1000 litres) and number of collectors (2 vs. 3). Here are the results:

The bars represent the energy, in kWh, that it will take to heat our domestic hot water.

This first modeling run really impressed on me how important the pipe insulation is. Two Collectors, with a 316 litre tank and R6-insulated pipe (the red dotted line) is virtually as performant as 3 Collectors with a 316 litre tank and R3-insulated pipe (solid blue line). That is amazing, and another testament to the absolute necessity to spend on insulation first.

My simulations got more refined. For one thing, I settled on some hot water usage assumptions:

  • There will be 2 adults and 2 children/teens living on the main and second floors, and one adult living in the basement suite (if zoning allows), for a total of 3 adults and 2 children/teens in the home.

Other Assumptions (thanks to Gordon Howell for some of these numbers):

Usage assumptions are very tricky to decide on, but I think these are reasonable and conservative. So the hot water load has increased from the first modeling run to about 149 litres of hot water per day. Here are the results of the second run:

This simulation focused on tank size and number of collectors. I should point out that the 316 litre tank is insulated to R28, and the 1000 litre one is insulated to R50. That's because we would be purchasing the 316 litre one off the shelf with an R8 rating. I am assuming that we would only be able to add a (fairly arbitrary) R28 insulation value. The 1000 litre tank would be home-made, so we would have full control to insulate to R50.

Too Good To Believe?

The best system, with 3 Collectors, a 1000 litre, R50 hot water tank, and R10 pipes, is projected to generate 94% of my annual hot water from the sun. That is an amazing amount of hot water production for a system that is done right. While it does seem quite difficult to believe that more than 90% of my hot water could be produced by the sun in February, for example, I am going to trust my modeling on this one.  After all, I believe that the average system is very poorly done, with R2 pipe insulation and R8 tank insulation being the norm. If that's the case, then maybe the rule of thumb that we in Edmonton can only get 50%-60% of our hot water from solar is incorrect. Even if WATSUN is off by 10%-20%, the results would still be impressive.

Space Heating?

The final modeling run is trying to answer the question: Is there any energy left during the cold months to heat the home with?. The one place in the MCNZH that may have a problem staying warm using passive solar energy is the basement. In the basement, the windows are partially blocked, so the tenant may be using a lot of electric baseboard energy to stay warm. Could the solar hot water system dump some heat into the basement?

I told WATSUN 2008 that the hot water load would include 62 litres of 40-degree water per hour, for four hours, every afternoon. This extra hot water load represents kWh of heat that we would use (in winter) if they were available.

A lot is going on in this graph (sorry it's so small). The green bars are the energy load without the extra heating (domestic hot water only), and the lighter bars are with extra heating load (248 litres per day, at 40 degrees Celsius).

The last three lines in the legend demonstrate that the 1000 litre tank is perfectly sized to three collectors. You can see in the graph that 2000 and 3000 litre tanks make almost no difference in the final amount of heat delivered. In fact, when we need the heat most in January, the bigger tanks are less performant than the smaller tanks. Also, 316 and 500 litre tanks (at R50) aren't quite big enough, although they are competitive with the 1000 litre tank.

Space Heat!

The most important thing that this graph tells me is that there is indeed useful space heat to be harvested from a SHW system in Edmonton. The MCNZH as a whole only needs heat from November to March, but the basement may need extra heat even in October and April due to its lack of windows and the tendency for air to rise. The numbers behind this graph say that the SHW system can provide the following amounts of energy:

  • January: 110 kWh
  • February: 155 kWh
  • March: 238 kWh
  • April: 220 kWh
  • (summer doesn't matter)
  • November:  125 kWh
  • December 78 kWh
  • Total: 927 kWh

Obviously, the results need to be taken with a grain of salt. Even if the SHW system produces half of this heat, though, it might be worth spending, say, $1000 to include a way to dump heat into the basement when there's a surplus.


My WATSUN modeling results have enabled me to make the following decisions about my SHW system:

  • insulate the pipes to at least R6, preferably R10
  • insulate the storage tank to R50
  • install a 1000 litre storage tank
  • 3 collectors
  • there is extra heat - install a system to harvest it

 Note: I've attached the excel spreadsheet with the last two graphs in it for easier reading. It's here.

(cross posted at

MCNZH-Solar_Hot_Water_Analysis.xls73.5 KB

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first time viewer of your site. very cool. I am wondering though, why you choose to use flat plate collectors instead of vaccum tube collectors for your home? Everything I have read says that flat plate collectors are not good in cooler climates(ie. climates that get below freezing).

Thanks Cameron! To answer your question, I emailed Peter Amerongen (the MCNZH builder). He had this to say:

We went to flat plate collectors for a couple of reasons:

  • My understanding is that they are less expensive per unit of energy collected.
  • They are also reputed to be more robust. Gordon Howell observed some breakage of evacuated tubes installed in a project in Okotoks. The evacuated tubes need a dissipator to dump excess heat when it is not being used. This requires a pump, a 3 way valve, and a controller. If any one of these items fails the collectors can be damaged. I don't think you can use them with a drain back system.

Rob Dumont hasn't used evacuated tubes on his house or Factor 9.

It would be worth asking Harold at Trimline. He sells both. They are appealing for being able to collect more energy with less area.- I'd like to try them on a simple project where they would be easy to get to if something went wrong. Or better yet wait until a bunch of other people have tried them with good results.

I haven't found a mention of mechanical ventilation yet on your site. You would be using an HRV I'm sure since it would be a code requirement in your well-insulated house. I was wondering if you know of the system used in the Drake Landing community in Okotoks? Of course the storage system is to die for but the Nu-Air HRV is a combo HRV and heat exchanger utilizing domestic hot water at a temperature of 55 to 60 degrees Celsius. Made in Canada ( always nice ) it looks like an interesting alternative for a retrofit to a house with forced air ducting in place and a good solar hot water system. Would have to scale your system for it of course.

That does sound interesting. I haven't gotten around to mentioning the HRV - it seems so basic to me now, but in actuality most new houses still don't have them. Our HRV manufacturer is Canadian, but I don't know any more details.


I love what you are doing. Thanks for taking the risks and being a ground breaker for the rest of us. I have long dreamed of building a house like this. One day...

About the solar hot water. It looks like the company helping you is pretty experianced. They seem to have a lot of products. I think the drain-back system is the way to go. No messing with coolant. Water is also a better heat transfer fluid than coolant so the system will be more efficient. I am trying to get into the solar DHW business. We have done a few systems so far. I will describe for you what I see as the Holy Grail of SDHW systems.
Flat plate, drain back system. No controller, only a 12v or 24v solar panel powering a DC pump. When the sun shines, it runs. A cheap simple flat plate heat exchanger on the side of an off the shelf 80 gallon electric hot water tank. The final heat exchange works via thermosyphon which puts the hot water exactly where you need it at the top of the tank, like an Enerworks system. You would disable the bottom element and the top element could be on a timer to come on in the evening or just put on a switch that you turned on 20 min before a shower. If you use the timer, it would turn on the top element after sundown, hopefully the tank is hot enough that the element doesnt come on.

This could be done much cheaper than most systems as you dont need a controller or pump station. Pretty much everything is off the shelf.

Good luck


Good work Conrad!

I recently installed Watsun and found the learning curve really steep. I can appreciate the amount of work that went in to the estimates.

What were the cost of the panels and the 1000litre tank?
I've been using Thermo-Dynamics G32 collectors at a cost of ~$800ea. A standard 80USG electric hot water tank costs around $330 (I got mine from Home Hardware). At $800 per panel, I've found the best ROI to be a single (4'x8') collector.


Interesting analysis. If the WATSUN software covers evacuated tubes then I really would run that comparison. They _are_ much better when it's cold outside and that's when you care about efficiency. It's easy to get enough heat in summer. And the cost complaint is simply not true: certainly in the UK ETs are now cheaper than flat panels due to (huge) volume production in China.

I'm surpised by how much difference the pipe insulation made. It is a factor, but other analses I've seen in the UK sugest that so long as pipe volume is kept low (10mm pipe now standard here) it's not that big a deal.

Thanks Wookey. We'll be monitoring the system closely to see how it matches up to the WATSUN modelling.

I was very surprised by the size of the improvement that thicker pipe insulation brought. I haven't found it on your site but are there construction details available such as pipe diameter, pipe run length, whether the run is inside or outside the house, pump control strategy etc?

I found your site while searching for info about WATSUN. I tried to run it on my Linux box but so far without success. Do you know if that is possible, or any information sources where I might find out?

Thanks, Dave

Hi Dave,

I'll contact you privately about who to contact regarding WATSUN on Linux. As for the system details, I don't have them project details handy, but I could send you the WATSUN file if you ever get it set up :)


Hi Conrad,

In your Conclusion
My WATSUN modeling results have enabled me to make the following decisions about my SHW system:

* insulate the pipes to at least R6, preferably R10
* insulate the storage tank to R50
* install a 1000 litre storage tank
* 3 collectors
* there is extra heat - install a system to harvest it

What should I install to harvest the extra heat from my SHW system? As I did install a SHW system recently.

I am thinking to use up this equipment and get the most benefit from the sun when we are not using that much hot water in summer.

Do you have any suggestion?


Hi Conrad,

Great site, I think you have done a tremendous job on taking the initiative to build this house and done an even greater job of getting all the details to the public. I see you have inspired several others (myself included!) to build NetZero houses in the future which is far more important than the impact of building your single house; great work!

I have been following your NetZero year energy usage closely with anticipation and I was wondering if you have any detail on the solar fraction you are getting on your hot water use? Has it been as close to 100% as your model estimates?

Thanks and really do appreciate all the great information you are passing on here!!


Thanks for the question. I would love to hear more about your project.

The modelling that we did with WATSUN was completely inaccurate. It hasn't been near 100%. In January our tank sits at 20 degrees, and it fluctuates between 25 and 40 during February. Not even close to what the software promised us.

I'm not sure what happened, but modelling with WATSUN was a complete waste of time for me.

Our solar fraction is probably in the range of 60%-70%.


P.S. I'm going to update both posts that I did about WATSUN right now.


Sorry to hear that, as an engineer it is always nice when models are accurate so it must be a bit frustrating. However I think an important finding will be what your annual solar fraction turns out like (ie do the majority of the other months have 100% fraction). Did you ever consider other forms of backup heat such as using your wood stove? I' m considering making a brick surround for my wood stove which has imbedded tubing and acts in parallel with the solar heater. The idea being that it would add heat to the tank at times when the stove is used most, which likely will be when the tank will need the most supplemental heat. I like the idea of being capable of being completely independent in the case of long electrical power outings.


I've thought of it many times Andrew but we never implemented it.

Will you be able to harvest enough heat, given that many (all?) wood stoves are meant to not radiate much heat backwards in order to reduce the risk of accidental combustion?

We are watching our hot water energy use carefully, but we won't have exact data because we don't have our hot water separately metered.


First off Conrad - Great blog! Thanks for sharing so much valuable information.

I have a solar hot water system for my triplex in Anchorage, Alaska for the last 2 winters. We used Polysun 4 software and Btu production is about 20% below what the software suggested. It performs very poorly within a month of solstice, so I would not recommend it for space heating through the dead of winter in a northern climate. With a lot of storage one might get farther into the shoulder season, but you will still need a main source of heat. The "Sunrise House" in Fairbanks is trying some interesting ideas.


Thanks Seth, and agreed: solar hot water is not a viable option for space heating. At least not in Edmonton. It is great for providing hot water for 8-10 months a year.



Thank you for the great inspiration over all the years I have been following your progress.
Finally we are going to build a Netzero house with strawbales in Edmonton.
After talking with Peter Amerongen, Rob Harlen,Andy Smith and many more in the netzero
design world we have a way of making a strawbale wall close to R50.
We use 6 inch Poured on Papercrete as a plaster, 3 inch on each side adding 2.16 per inch, gives R-12.96 to the strawbale of around R-36 total R-49.
The total wall thickness is 2 ft on a narrow lot 33 ft a building with 25x50 ft foot print and
a loft style upstairs has around 2000sq ft of living area.
Due to the lower cost of strawbales and prefabrication of the wall sections 10x10 ft there is funding available for the solar system.We are making the basement with hempcrete prefab 10x10ftand a papercrete interior finish and dimpleboard outside.I am planning to use 60 used 45 gal plastic drums in a hempcrete insulated trench under the basement floor for heat storage and 4 evacuated tube panels for heat.As a backup heat source i have ordered a prefab masonry stove with a heat pick up coil in between the primary and secondary wall.
Supplemental heat from a standard electric hot water tank.The netzero producing pv array can handle the short time use of the supplemental hot water tank.

Would you think a 12000 ltr back up hotwater system for dhw and space heating would work?


First of all, whoa! Wait a second! You can't just tease us with this very cool sounding house and then disappear! Are you blogging about it? If not, would you like to do so on Green Edmonton? If not, can you send me updates or something that I could post? Sounds like an exciting project.

As for the tank, I'm not a big believer in hot water space heating. Complex, expensive, and the heat is simply not available when you need it most. Also, are you confident that the panels have the capacity to heat the water during the swing seasons (which is when you would need it for space heating)? That's a lot of thermal mass.

I would abandon the space heating idea and go with 2 evacuated tube panels for domestic hot water alone. But that's just me. I would love to see an argument for the space heating.

My conclusion to the "no solar energy available when we need it most" problem was, after efficiency and passive solar design, to use stored solar energy - wood. Since you will have the best wood burner available, the incremental gains from the solar hot water space heating will be minimal. Do you want the added complexity/cost to save 1/6 cord of wood per year?

Anyway, I'd love to hear more about your project in the future.


Saskatoon has a net Zero Home Exhibit on Display at the Western Development Museum. The Website is Dr. Dumont worked with Ronn Lepage to construct the home which uses solar energy. The home has managed to reduce its energy costs by over 75% before you even take solar into account though. I give tours at the home on weekends.

** I would love to see an argument for the space heating. **

Heating a building with solar is proven, the first known success was by MIT in 1936. In the last few decades Austria has refined underground annualized solar storage - so the summer surplus is more than enough for the winter load. There are plenty of scientic papers that studied solar heated homes, with several in AB. and BC.

A solar heating system with about 20m2 collectors and 15m3 seasonal solar storage underground will work on a high effeciency 200m2 home, like a Passive House or Near Zero, even in Edmonton. Currently this system would be less than $15k cost, and will cover 70% of the total home energy, that is clearly economically viable and on par with comparable heating system capital costs.

Annualized solar heating is compelling for Near Zero and super-insulated new home construction, when the design incorporates the seasonal solar storage tank below the slab, before the footings are poured.

Hi Conrad,

I have a solar drainback system where the water drains back into a cistern and have to replace the pump. I was looking for the detail on your pump installation because I thought I'd seen it before, and you used a gooseneck for priming. I can't find it now. Am I thinking of someone else's project?

Great blog. Inspiring. Sarah

Hi Conrad,

You mentioned that you would like to have some updates on what we are doing with the Prefabricated Strawbale and Hempcrete basement house. I think you are right to say that we should keep the heating system as simple as possible and that in floor hydronic heating is complicated and not needed. My neighbor Don Curry from Airaid heating recommends a CRV with a heating coil. We have a building near Elk Island park to install first before we can build in Edmonton. The rough in has a solid air tube above the weeping tile going around the building to have tampered air temperature before it enters the CRV(cooler in summer and warmer in winter).We also have access to 3 x 2000 gallons underground concrete tanks for septic ,water cistern and rainwater collection that we can use for warm water storage of a roof mounted hot water collection system delivering heat through a heat exchanger coil in each tank. The DHW is supplied by oversize tanks heated by solar evacuated tubes, masonry stove coil and backed up by an electric coil. We are working on keeping the production cost down by using the straw and hemp and aiming to produce NetZero for a cost near or equal of conventional construction in a super green way.

I would like to keep you updated with our progress and I would like to bug you with questions that come up as we go on, Peter.

Cool! Please do (both). And send pictures if you can :)

Hi Conrad,

We wanted to know if the hempcrete basement would perform as an energy efficient and sustainable build basement would in an application as typical Edmonton infill housing on a narrow lot.The first thing investigated was the structural capacity of the hempcrete to handle the load of the first and second story loft(remember the steep roof at 53 degrees needed for the solar array gives the opportunity for a loft space).The hempcrete test cylinders we tested came out at 28 and 56 days with 3.2 and 4.8 MPa double what was needed for a wall with no 2x6 studs as structural support just a monolithic 20 inch thick hempcrete wall 10' by 10'prefab. The papercrete plaster with recycled card board and local clay loam and lime attached very well to the hempcrete and formed a smooth natural pigment pre-colored base finish. The final finish has milk casein and clay in it.
We are increasing the amount of fly-ash above the commonly used 20 to 30% to well over 50% including clay additives to the binder and achieving still very good mix strength and at less cost to produce. The outside basement wall will have water proofing and dimple board protection running over the joint with the footing in to the weeping tile.we are a little concerned to use hempcrete as a under floor insulation and we are thinking it would be good to pour a thin layer of Poraver globes as a moisture barrier over or under the radon barrier. (see org/2endeavourcentre.012/06/prefabricated-straw-bale-walls-for-canadas-greenest-home/)
What do you think of that idea?


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