Month: September 2023

In this post I’ll describe how I chose and installed a new wood burning stove and flue system in the tiny house for less than £700. I’ll give a breakdown of the costs and share my experience of the stove’s performance.

Why choose a wood burner?

As my tiny house neared completion it was time to install a heating system so that I could (hopefully!) use the house during the colder months.

A wood burning stove was the obvious choice, for the following reasons:

1. Simplicity and reliability

A wood burning stove is as simple as it gets. No need for electricity, circuit boards, pumps, burners, sensors or any of that stuff.

2. Availability of fuel

I have a fair amount of scrap timber left over from the build that could be used as firewood. It is only softwood (Douglas Fir) but is dry and free from chemical treatment. As I don’t live in the house full time, I’m sure it would last for quite a while.

I’m also lucky in that the house is located in an area which is wooded and good quality firewood (seasoned hardwood logs) are readily available.

3. Renewability

While it may not be perfect, wood is at least a renewable resource. That’s assuming we can consume wood fuel at an equal (or slower) rate than the rate at which it grows. I’ve no idea if this is true in my region, country or at international level. But I think it should be possible. It’s certainly more achievable than waiting 50+ million years for some more oil to be formed.

4. Comfort

I admit to a hankering for the cosy comfort of a wood burning stove. One of my dreams is to escape the madness of the festive season by retreating to the tiny house at Christmas. There I could spend my evenings with a glass of whisky, a book and a few logs crackling in the stove. Bliss!

5. Cost

The absolute cheapest method of heating the tiny house, in the short term at least, would have been to continue with a paraffin heater. It’s not very pleasant though, as it does give off some odours and a bit of smoke. Longer term, I suspect that the cost of buying drums of paraffin would be greater than that of using firewood.

Buying and fittng a wood burning stove requires some up front investment. However, the ongoing heating costs using firewood in a stove are said to be lower than using gas, electricity or oil.

To be honest, I didn’t investigate the initial costs of a gas or oil powered heating system. I didn’t want the complications of these systems or the fact that they are non-renewable energy sources.

As for electricity, this could be quite a cheap install (one or two panel heaters) but, as I am off grid, it was never an option anyway.

To summarise, I haven’t done any detailed calculations regarding the overall cost of heating with a wood burner vs the other options. That’s partly because the other options have been ruled out for other reasons. In any case, the accepted wisdom is that using wood in a modern wood stove is an economic method of heating. In a small space such as the tiny house, I’m confident the annual costs will be low.

6. Heating/Cooking

It’s possible to heat water or even cook on a wood burning stove, depending on the model. Very handy!

Choosing a stove – A Rocket Mass Heater?

My initial plan was to build what is known as a rocket mass heater (RMH), an example of which is shown below.

This is a hybrid of a rocket stove and a masonry heater. Fascinating though they are, a full explanation of these systems, and the various types of RMH, is beyond the scope of this post. If you would like to know more about them, the Wikipedia page provides a good introduction.

Without going into the technical details, the advantages of a RMH are:

1. Much more efficient than a normal wood burning stove (uses less wood for the same heat output)
2. Burns very cleanly
3. Only has to be fired a few times per day – after which a comfortable heat is radiated over a period of several hours by a large thermal mass.

I did a lot of research into RMH’s and spent some time designing one for the tiny house. I even bought some materials for its construction, such as high temperature glass, insulation and steel. However, I decided to abandon the idea of a RMH for the tiny house for the following reasons:

Weight Concerns

Mass heaters are, by nature, heavy things and I was worried about the effects of constantly increasing the weight of the house. Firstly, I didn’t want to overload the timber floor and secondly I didn’t want to add significantly to the load carried by the foundations.

Lack of space

I had already managed to squeeze quite a lot into my 17m2 floor area. Trying to add a bulky mass heater seemed like a step too far.

Lack of time

The whole house had been something on an experiment (a fairly successful one, so far) and I was not averse to a novel heating system. However, I wanted to install the heater in my usual 5 week summer holiday period. As I also had a bathroom and hot water system to install, I simply didn’t have the time to build an experimental RMH too.

So my focus then turned to finding a ‘good enough’ conventional small wood burning stove.

Criteria for a normal wood burning stove

Price and availability

As ever, I was on a tight budget. I couldn’t justify a super fancy stove for my rustic tiny house. So I was looking for something simple and inexpensive. Getting a second hand stove could have been a good option but I didn’t think I’d find one locally that was small enough. I also needed one quickly and couldn’t depend on one becoming available during the few weeks that I was in France.

Delivery

Ideally, I wanted the stove to be located in mainland Europe so that I could have it delivered to France at a reasonable cost.

Performance

I didn’t need the stove to have stellar performance, but I didn’t want a load of junk either. I hoped to find a model with some good feedback from existing buyers.

Result? This is what I bought …

After some deliberation, I decided to go for this Bulgarian made wonder, the Mini, made by a company called Prity. At about 300 Euros (£260), including delivery to France, it offered much cheapness. It was also in stock and had generally favourable reviews.

The 5kw output is OK for surfaces up to 30m2 (perfect) and it’s compact and not too heavy (48kg). It has a cast iron top with one heating plate and is lined with fire bricks. These provide some thermal mass and prevent damage to the steel sides. The full specification is shown below.

Other stove options

I did consider other stoves before opting for the Prity Mini. As far as buying one in France goes, I couldn’t find anything small, cheap and available that I could buy ‘off the shelf’ and install myself.

The choice of small (5kw) stoves available in the UK seems to be quite good although they are, unsurprisingly, more expensive than the Prity. One stove that caught my attention was the famous “Hobbit” by Salamander Stoves. With a 4kw output, this nice little stove is almost as powerful as the Prity and is more efficient (81.4%). However, it does cost more than twice as much (£625) and has a 14 week delivery time. There would also have been the cost and hassle of delivery to France, especially now that the UK has left the EU.

Prity Mini: Order, delivery and first impressions

I ordered the stove from the the French eBay site and got it delivered to a friend’s house. It was sturdily packed in a wooden crate and, at 50kg, I could lift it into my car without too much difficulty.

On opening the crate, everything was found to be in good condition. The only assembly required was to place the fire bricks into the rear and sides of the stove.

As for the quality, I was pleasantly surprised. It looked quite nice and the wood and metal door catch worked well. The door didn’t have a glass fibre rope seal and I tried to ‘improve’ it by fitting one. This made the door harder to close and, in practice, it fell off anyway (despite using the proper glue). After using the stove, I concluded that the seal was unnecessary, for reasons I will go into later.

Flue components

These various bits and pieces are essential to the safe operation of the stove but the costs can quickly mount up. I tried to get the best deal by buying them in the UK and taking them with me. All of the parts were sourced from eBay but I should point out that the sellers were established UK specialist retailers. I never had any problems with the quality of the parts. (UPDATE: Since first writing this post, some parts have increased in price and others are out of stock. This might be due to Brexit. In any case, I have left the prices as they were in 2021)

The flue components (5” diameter) consisted of:

• Single wall flue pipe, 1m lengths, qty 2, £60
• Twin wall flue pipe, 1m lengths, qty 2, £136
• Single wall to twin wall adapter, £25
• Vented fire stop spacer, £42
• Joist support, £27
• Flashing kit, £29.99
• Rain cap cowl, £49
• External air kit, £16.40

Total cost £385.39

Interestingly, the Hobbit stove tiny home flue installation kit costs £485 (4” system). So my eBay penny pinching paid off!

Had I tried to buy the flue components in France, I would probably have been scratching around trying to find them and would almost certainly have had to order some bits. The total cost would have been higher too. As it was, the only extras I had to buy were a couple of 45 degree bends and a can of spray paint. I can’t remember the exact price for these items but it would have been under €50.

Fitting the new stove

And so on to the exciting bit – fitting the stove! Unfortunately, this also meant doing some boring research on how to do it safely.

Somewhat lazily, I consulted the UK regulations rather than the French ones. No-one is going to come and inspect the installation anyway. Obviously I have a strong personal interest in not burning my house down or getting carbon monoxide poisoning, so wanted to reduce these risks as much as possible.

The basic idea is to keep combustible materials a safe distance away from the stove and the flue pipe. There are also rules about the layout of the flue pipe, the materials used, the height of the flue cap above the roof ridge and the supply of combustion air to the stove.

Stove Location

This was the first thing I had to consider and it was a bit of a headache.

One constraint was that the flue pipe obviously had to pass between the roof joists before passing through the roof tiles and this affected where the stove could be placed on the ground. Another constraint was that I wanted the stove to be close to the gable end wall (but I couldn’t have it too close, for safety reasons).

According to stovefitterswarehouse.co.uk the distance from the stove to the wall for small stoves (less than 7kw) can be as little as 120mm, if a metal heat shield is used. Otherwise the distance depends on the manufacturer’s specifications, which could be around 400mm or more.

I decided to make a heat shield from a sheet of aluminium I found at a local DIY shop. It was already the perfect size so no cutting was required. Interestingly, the rules say it should be made from Galvanized steel. I couldn’t think of a good reason as to why this would be preferable over all other metals.

The sheet was spaced from the wall with 25mm lengths of 14mm dia copper pipe and fixed using screws with large washers under the screw heads to avoid damaging the shield. The theory is that the sheet becomes hot and that the air around it rises, to be replaced with cooler air and so on. In this way there is a constant current of cooling air behind the stove. It’s likely that the shield helps reflect some heat back into the room although I can’t say for sure if it actually does.

The heat shield can be seen in the picture below.

Hearth

It’s necessary to sit the stove on some kind of protective hearth. This is especially important in my tiny house with its timber floor.

According to directstoves.com, the hearth for a free standing stove should :

• Extend at least 300mm to the front and 150mm either side
• Be at least 12mm thick
• Be made from non combustible materials
• Cover a minimum area of 840mm x 840mm

Alas! A minimum area of 840 x 840mm was impractical in the tiny house. It would have taken up so much floor space that it would have been a trip hazard.

Instead I made a hearth that extended a ‘sensible’ distance past the rectangular form of the stove. I didn’t see a problem with this, unless I was going to make a habit of letting burning logs spill out of the stove, which I wasn’t.

I chose to cast the hearth from hempcrete (see picture above). This cost me nothing as I had the material left over from the build. It’s also non combustible and looks ‘good enough’. You could tile on top of it if you wanted something more aesthetic. Casting the hempcrete hearth was easy, I just screwed a wooden frame to some scrap OSB and filled it with a hempcrete mix that was heavily dosed in lime. After a couple of days it was rigid enough to be removed from the mould and installed (it would harden further and become very solid with time). The thickness, from memory, was about 40mm.

The hempcrete hearth was simply placed on the timber floor and the stove put on top.

Flue Pipes

Two types of flue pipe were used, single and double skin. The single skin pipe is not insulated and gets very hot. The double skin pipe has a layer of insulation between the layers, and this reduces the temperature of the outside skin significantly.

The double skin flue pipe is used where the flue passes through the ceiling (and relatively close to the combustible wooden joists). Another benefit in using it outside is that it helps keep the exhaust gases warm – improving the draught and reducing the chance of condensation.

The single skin pipe has the advantage of being cheaper and helps radiate a bit more heat into the room. As it gets very hot, it has to be kept well away from combustible materials. In my installation it runs from the stove for a distance of about 1.5m before connecting to the twin wall pipe.

There is a point where the single wall pipe runs fairly close to the wall which isn’t ideal. Although the hempcrete is fire resistant, the potential danger concerns the timber stud frame within the wall. I think the risk is pretty low, but I could install a smaller metal shield in this area at a later date.

As far as connecting the flue pipes goes, the single skin pipes are just a push fit. I bought a tube of high temperature silicone to seal the joints if necessary but I haven’t needed to use it. The draught of the chimney seems to create enough negative pressure to stop smoke leaking into the room.

It’s perhaps worth pointing out that the pipes should be connected as shown below.

This means that any possible condensation or other gunk running down the inside of the flue pipe will not leak out through the joints.

The pipe I bought came in enamelled black and was very sturdy. Unfortunately the bends I used were a bit more flimsy and came in an aluminium colour. I sprayed them with high temperature matt black paint, which looked OK but was quite easily scratched.

In order to cut the black single walled pipe, I used an angle grinder with a metal cutting disk. I guess you could use a hacksaw instead although it would take a lot longer.

The twin walled pipes were connected by slipping the pipes inside each other and closing a latch, which tightens a metal clamping band around the pipe joint. It’s a very neat and secure system.

Ceiling flashing

Taking the flue through the roof was something that I was not at all looking forward to. I knew it would be messy and awkward and wasn’t confident about removing and re-installing the roof tiles. Furthermore, I don’t really like working on roofs and I didn’t know how I’d manage to install the last bit of heavy and awkward twin wall pipe from the roof up to the flue cap. It all seemed very easy to mess up.

At least the weather was dry and sunny on the day of the installation. I wasn’t quite so happy about the wasps that were buzzing around me on the roof (they must have had a nest nearby) but I just ignored them and they didn’t give me any trouble.

Working from the inside, I removed an area of the timber ceiling and then the lime and hemp insulation. This was the messy bit as it all fell down onto the floor. Next, I removed some tiles from the roof and cut away an area of the roof membrane. Now I had a square hole through the ceiling and roof.

The next fiddly bit was to install the joist support and the section of pipe that went through the roof. The joist support clamped around the pipe and onto the roof joists and took the weight of the pipe as it went through the roof. It took a bit of trial and error to get a solid fixing and also ensure that the pipe was reasonably vertical.

I tried to have the twin wall pipe passing between the rafters such that there was at least 60mm of space around the pipe, as recommended on this website.

Now the flue was through the roof, I had to add the final section of flue and I hoped this would be long enough to satisfy the requirements about the height of the flue cap. These regulations are described here. In my case the flue exited the roof less than 600mm from the ridge and so had to extend vertically above the ridge by more than 600mm. The final flue section comfortably exceeded this, so I had high hopes that the flue would give a good draught.

With all of the flue sections in place, I replaced as many tiles as I could and I think I only had to cut one of of them. Then I put the flashing over the flue (lubricating the silicone rubber ring with water) and dressed the lead flashing over the tiles with a rubber mallet. The shape of the tiles were such that I couldn’t get the lead to follow the shape of the tiles completely. A small bead of clear silicone was applied between the rubber sealing ring and the flue pipe.

Being somewhat paranoid about water leaks, I added some bitumen/aluminium tape around the rubber seal and where the sides of the flashing met the tiles. The tape didn’t actually stick too well to the seal, so I later removed these pieces but left the other bits of tape in place.

The final job (on the roof side) was to clip on the flue cap (chimney hat). Very satisfying.

All that remained was to make good the ceiling inside the house. This involved the fitting of a ventilated fire stop spacer and some timber framing around it. As the spacer was fitted horizontally and the ceiling was sloped, this required quite a bit of fiddling around. I also had to make it fairly rigid as the spacer also helped keep the flue pipe vertical. The end result was acceptably neat (see below).

Does it work?

Now for the performance – is it any good?

Yes, I’m pleased to say that it works pretty well. The air intake is controlled by sliding the ash drawer out slightly. This really allows the fire to light easily and burn hot (the draught is excellent). Closing the door reduces the airflow and subdues the flames somewhat and there is also a damper in the stove where the flue gases exit, for further control.

In practice, I preferred to run the stove quite hot so that it would burn cleanly. I don’t like the idea of leaving the stove smouldering overnight (poor combustion, pollution, deposits inside the flue). The hot burn requires a good intake of air and the previously mentioned lack of door seal is of no consequence.

The stove was used for a period of one week in October. During this time it was quite mild in the afternoons (about 15 deg C) but chilly in the mornings (0-5 deg C).

With the stove about half full and the ash drawer open slightly, it would burn hot for about 20-30 mins. There was little visible smoke leaving the top of the flue. During this time the fire bricks would heat up (storing some heat) and I could also heat a kettle of water for washing. If I ran the stove for long enough, the kettle could be made to boil.

I found that the inside temperature of the house dropped from 20 to 15 deg C overnight. Running the stove for a 30 minute blast in the morning was enough to make it cosy again. Typically I ran the stove once or twice in the evening too.

Obviously in the winter I’d need to run the stove for a greater amount of time. Unfortunately the current Covid travel restrictions mean that I won’t be testing it this Christmas! Consequently, I can’t comment on how stove performs in the coldest months.

I don’t have a stove pipe thermometer, but I guess that a significant amount of heat is being lost up the flue pipe. In future I may try a ‘heat exchanger’ to increase the surface area of the the single wall flue pipe and extract a little bit more heat (see below).

Another possibility is to stack some bricks around the sides of the stove to add a little bit more mass. This might allow the house to stay warmer for longer after the fire goes out. It’s probably worth a try for the cost of some common house bricks.

I bought a fresh air intake kit to allow the stove to be fed with cold air from outside. This consists of a 100mm diameter flexible pipe to which I will attach two end plates (covered with wire mesh to protect against bugs and other debris). I haven’t fitted the kit yet. Until this is done, I simply open the window nearest the stove to allow the combustion air in. The window’s external shutter can be partially closed at the same time to protect against the wind and rain.

I also fitted a battery powered carbon monoxide and smoke detector for safety’s sake. These are so cheap and easy to fit that there is really no excuse for not having them. The carbon monoxide detector did actually go off when I was using the paraffin heater (I obviously hadn’t provided enough ventilation). This gave me some reassurance that the detector does work well and could even save my life.

Conclusion

Overall, I’m very satisfied with the wood burning stove. For around 800 Euros, I have a complete new system that burns well and fits quite tidily into a small living space.

According to this website, the typical cost of a cast iron wood burner, supplied and installed by a French artisan, is between 1,000 and 4,000 Euros. I suspect that in my case it would be at least 1,500 Euros (stove, flue installation, custom hearth and commissioning). So I have saved at least 200 Euros and probably a lot more.

The installation, although a little bit messy and fiddly, was not as difficult as I feared. Happily, there has been no evidence of water leaks from the flue flashing and it all seems quite safe.

So far I have only burned small pieces of well seasoned softwood and the performance has been good enough to heat the tiny house quickly. It would be interesting to see how the stove performs with hardwood logs such as Oak.

It remains to be seen how much the wood consumption would be over a year (and how much it would cost) were I to live in the tiny house full time. I suspect that the generally mild climate in South West France, coupled with the reasonably good insulation, would mean that the annual firewood bill would be low.

Update – September 2023

It’s now around 2 years since the stove was installed and, to be honest, it hasn’t seen an awful lot of use. I still haven’t been able to liberate myself from the UK during the Christmas holidays in order to give it a winter test. I have used it on chilly days in Spring and Autumn though.

The stove still works as it did when first installed. The only slight problem is a ring of rust at the top of the stove where the flue is attached. Some moisture must have travelled down the inside of the flue (despite the chimney hat). This probably happened in windy conditions. The rust is only cosmetic – I could clean it and get some stove polish to make it black again.

It’s also surprising how much wood is used when the stove is used daily for only a week! I think that one winter would be enough to finally consume the rest of my scrap timber from the house build. Admittedly, most of the wood is Douglas fir, so it does get burned quickly.

If I were able to spend more time in the house I would either buy some hardwood logs (oak, most likely) or experiment with the use of wood pellets. I recently saw a video on YouTube that give some clues as to how this could be achieved. My idea would to create some sort of cartridge or insert that could be loaded with pellets then placed inside the stove and burned in place of normal logs. Hopefully the pellets could burn cleanly and steadily for a decent length of time.

Another advantage of pellets is their convenience. It would be easier to keep a couple of bags of pellets in the house than to have a big pile of firewood outside. Furthermore, even the nearby supermarkets stock pellets these days, so it would be easy to re-stock when needed. With no physical modifications to the stove, it would also be easy to go back to burning logs or timber offcuts whenever I wanted.