It’s done! Today I tested my wood powered water heater and I’m glad to say that it works. In this post I’ll outline the build, the results of the performance test and what improvements could be made.
If you haven’t read Part 1, the aim of this project is to make a water heater that is connected to the tiny house plumbing system but is actually located outside the house.
This has the benefits of not overheating the house in summer and not requiring an (expensive) double wall flue installation. Furthermore, having it outside will safe space and the heater can be ‘quick and dirty’ (e.g. no need for a fancy sealing door).
It has recently occurred to me that this heater could also be useful for ‘topping up’ hot water produced by solar energy. This would be handy in the winter or on cloudy days.
The Build
The water heater was made from 2 gas bottles which originally contained 13kg of butane.
After emptying the bottles of gas and purging them with water, the top was cut from both cylinders and the metal support ring was also removed from one of the cylinders.
One of the cylinders became the ‘firebox’ (bottom), the other became the ‘thermal store’ (top).
The thermal store had a hole cut in the bottom and a piece of 75mm tube welded in place. I had barely used my welder before this, so my welding was terrible. It took a lot of grinding, testing and re-welding before the join was watertight.
By the way, the welder I used was a Sealey Mighty MIG 100. This uses flux cored welding wire (no gas required). It’s quite basic but seems to be fairly decent quality for a budget machine.
The rectangular hole in the firebox was cut big enough so that it could be filled with fairly large pieces of wood.
Removing the original blue paint was achieved using a Bosch abrasive wheel fitted to an angle grinder. This took a little time but the wheel was very effective and lasted for the whole job (still going strong).
Three bolt-on legs were fitted to the firebox and some brackets welded onto each half of the heater so they could be bolted together. This was a safety issue as I didn’t want the thermal store (scalding water) to be able to be easily detached from the base. Prior to this, the thermal store was just sitting in place.
The final step was to put a heat exchanger coil inside the thermal store. This was made from a 10m length of soft copper ‘micro bore’ pipe with a diameter of 10mm. I thought this might be too small but I gave it a try anyway.
Test Procedure
I added 20L of cold water to the the thermal store and connected one end of the heat exchanger to the mains via a garden hose. The other end of the heat exchanger discharged into a bucket via a short length of hose.
Digital thermometers were put inside the thermal store and the discharge bucket.
The firebox was filled with scrap wood from a pallet and the fire lit.
Performance Results
The video below shows the water heater being tested.
The initial temperature of the water in the thermal store was 17 deg C.
After 15 minutes of heating the temperature was raised to 47.5 deg C.
After 35 minutes, the temperature in the thermal store was 70.9 deg C.
Test 1
At this stage I drew off 2 buckets of hot water (10L per bucket). The first bucket had a water temperature of 37 deg C and the second was 34.8 deg C, giving an average of just under 36 deg C. The temperature in the thermal store had dropped to 51.6 deg C.
Next, I added more wood to try to get the water in the thermal store as hot as possible.
Test 2
After a further 20 minutes of heating, the water in the thermal store was 87.8 deg C.
I drew off 3 buckets of hot water. Their temperatures were 51.4, 42 and 36 deg C. So the heater had given 30L of water at an average temperature of 43.1 deg C.
The water in the thermal store had decreased to 60.4 deg C.
By this time the flames had died down and firebox consisted mainly of embers.
Test 3
As a final test I waited for 10 mins, without adding further firewood and drew another 3 buckets of hot water from the heater.
The average temperature of this final 30L of water was 31 deg C.
Regarding wood consumption, after the initial fill, I topped the firebox up once. That equates to, perhaps, 3 planks from a pallet and one or two chunkier blocks.
The heater burned quite cleanly i.e. there was not a great amount of smoke visible from the top of the flue.
During the course of the tests, the water level in the thermal store dropped by 1-2 cm, presumably from evaporation.
Conclusions
I was more than happy with the results of this test.
Firstly, the water heated up remarkably quickly. With a full fire the temperature increased by about 10 deg C every 5 minutes.
The heater performed best when the water in the thermal store was very hot (almost 90 deg C). This temperature could be easily reached in 45 minutes from cold.
With a thermal store temperature of around 90 deg C, the heater could provide 30L of water at an ideal bathing temperature (40 – 45 deg C). If used to fill my Hozelock portashower, this would be enough for 6 showers (plenty for my 3 person tiny house).
I see no reason why it would not be an effective way of topping up the temperature of solar heated water on cloudy days.
The small capacity heat exchanger coil worked surprisingly well at removing heat from the thermal store. I had considered doubling it’s length or replacing it with a larger diameter coil but I’ve decided to leave it as it is.
Improvements
Heat is almost certainly being lost through the open hole in the firebox. Adding a door may help reduce these losses. Of even greater benefit would be insulating the firebox. It should be possible to do this by adding an insulation blanket (such as ceramic fibre) to the inside walls and base. It these two improvements are made, then more heat would be transferred to the water above, increasing the performance and/or reducing the wood consumption.
External insulation of the thermal store might also improve performance. At the very least the water would stay hotter for longer after it had been heated. I will try to cast a thin layer of hempcrete around the thermal store for this purpose. It will cost me nothing to try it (I have plenty left over from the tiny house build).
Another possible improvement is the fabrication of a lid to cover most of the top of the thermal store. This might reduce heat loss as well as water loss from evaporation. My idea is to make it hinged so that it could be swung back out the way for easy filling of the thermal store by bucket.
Besides the above, I will give the heater a coat of black high temperature paint and cut the flue into 2 sections that can be re-assembled. This will enable easier transport of the heater by car.
FELICITARE