Home-made strawcrete and stone storage heater Part 2 - building the wall

Looking backwards to go forward, our introduction to storage heaters, came about because we moved down to England from Scotland and bought a wonderful ruin of a 1930’s semi, which had been subject to a whole host of abandoned heating systems, spanning several decades, one of which was an electric storage heater. The start of this project, creating the strawcrete pad can be found here

Previous experience with storage heaters and why we decided to make our own

 

In the front downstairs room of the house, which was to become our dining room, there was was an electric storage heater. This was powered with a low tarif supply but as far as we were concerned it was ugly and inefficient, being placed on the wall between us and the neighbour. We had already decided to remove the sixties era fireplace in this room and replace it with a  cast iron Art Nouveau one we had bought from a builder and open up the chimney.

At the same time the storage heater would be moved upstairs  to a spacious front bedroom, with fine views of Warwick castle’s firework displays, great acoustics for its concerts and which we intended to use as a sitting room. Its function would be to supplement the superb original 30's Art Deco bedroom fireplace. To do this, the heater was isolated from the power supply and dismantled.

If you’ve never seen  what's inside one of these heaters then you ain’t missing much. Basically, they comprise an electrical resistance, running between heavy bricks. The heat output being regulated by a moveable flap to control the warmed air leaving the unit.

The inside rear wall is covered with a pad of insulation. A word of caution: Asbestos was often used as the insulating material in storage heaters and although you will see on many professional websites, dates of 1970 and 1980 quoted as marking the end of its use, you really need to find out exactly if the model you have contains Asbestos before you start taking it apart. Be warned that further to the obvious dangers of Asbestos, this form of insulation, when old can become powdery and thus ever more hazardous to the lungs. It is also of tantamount importance to ascertain if and when Asbestos was banned for use in household appliances/building materials in your particular country of residence. For example, when we moved to our present ruin, here in France, we found that the roofing material on an old abandoned rabbit hutch, attached to a Bread Oven in the garden, contained Asbestos but thankfully,  there were disposal procedures in place, which we could follow to rid ourselves of it.

Far left showing rabbit hutches roof cover, thus prior to Asbestos removal and right a couple of years later on.

 

The key point of our old storage heater’s relocation was that it was to be placed against the internal brick wall of the upstairs room which incidentally led directly onto the stair well. Over a period of days after reconnecting the heater in its new room, we began to notice that this wall had started to store heat as well, such that as one ascended the stairs from the entrance hall, one could feel that there was a significant amount of heat emitted. So although we had known that locating it on an inner wall made sense, we got a much greater benefit from the heater than we’d originally expected. This was a valuable lesson to us of how useful it was to be able to store heat. The means of supply (electricity) was certainly not ideal but the way in which a house could benefit from the simple method of charging up a mass with heat and letting convection do its stuff, was.

Fast forward 20 years and back to our non-toxic storage heater, by appointment to the genius loci

It took about four or five days for the strawcrete to start to feel firm, I was still able to dig into the surface with my fingernail and the surface still felt damp. Nevertheless, I decided to remove the shuttering from around the pad so as to expose more faces to the drier air in the kitchen. 

I also decided to turn the pad upside down exposing the smooth bottom face to the air and I laid the pad onto an old wire shelf from an oven so that air could circulate freely around it.

 
 

After a further seven days, the strawcrete felt firm and resisted my finger nail, it also felt considerably lighter. If you compare the colour with the little pure lime sample you are immediately struck by the warm golden colour the straw has imparted to the lime, something we exploited when we used strawcrete as the foundation layer for our hemp and lime insulated walls in our kitchen.

Work could then progress on making the wall. 

The first thing I did was move the stove off its plinth and sweep the chimney, I’ll describe this exercise in another post. 

 

Next I put the strawcrete into position on the tiled surface, the smooth, flat side down. I had decided to fit shuttering on three sides of the plinth to act as a guide for laying the stones and to prevent excess mortar from the joints from dropping into the gap between the rear wall of the heatsink and the lounge wall. I used chipboard sheets recuperated from pallets for this. The rear wall was a coated sheet but the side walls were bare of any such coating and thus I covered these with a layer of polythene so as not to  draw moisture from the mortar. Across the front elevation I screwed two narrow strips of pallet wood so as to allow me access for laying the stones. 

 

We have plenty of stones in the garden, some are from the house where we have removed parts of walls for windows and doors and some have been dug up over the years of our gardening. I needed stones that had ‘flat’ faces and were fairly regular in shape. The selected stones were cleaned of any soil or clay using a scrubbing brush and water. 

 

Prior to laying the stones in mortar, I first tried various selected ones, altering their orientation until I felt I had achieved the best fit, then removing them and laying them to one side in the same configuration. I’d decided that any large gaps between the stones would be filled with mortar and small stones. The other point in the construction was to lay longer stones front to back so as to conduct heat into the body of the wall, thus certain ones were selected to go onto the next layer. This also would make a strong bond within the wall i.e. there would be little or no mortar seam running continuously vertically down the wall. 

P.S. Extra repurposing tip - see the natty overall - we found a laundress who has a market stall, on it she sells old company overalls when they are 'past their sell-by-date'. They are really good quality and very cheap. We even once got a life-boat man's suit, which was great for heavy building work - and really good in wet weather!'

The mixing of the lime mortar and the laying of the stones and testing of the new storage heater will be dealt with in a separate blog.

Left -  another great find for our old house, this time at a yard sale, it replaced a sadly defunct Rayburn anthracite heater with a burnt-out back boiler.

The continuation of this project can be found here

If you've enjoyed this article and found it useful please feel free to share it or to comment and/or make observations. All the very best and until next time,

Andy

© Andy Colley 2014

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Home-made low-cost stove night storage heater from strawcrete and stone

For some time we have been toying with the idea of changing the heating system in our sitting room and even considered starting over from scratch and making a rocket mass heater. However, we actually have a fully functioning and useful Godin in there, which it would be a shame to part with. To this end we have decided that it would be quicker and more economical to actually work with what we already have and build what is known as a 'night storage heater'. thermal wall or heat sink at the back of our stove.

Introduction

Our house is heated  entirely by wood-fired appliances: in our kitchen we have a clean-burn wood-burning cooker which by convection heats the rooms above.

The walls, as you can see we insulated, in the kitchen with chopped straw and lime and then on top a layer of hemp and lime. Paradoxically, if we had left the space behind the cooker and the Godin bare we would have had a natural heat sink ready made. However, this is a 300 year old vernacular farm house and the bare walls, unlike the section over the fireplace were not particularly decorative and were never meant to be exposed to view. In the past they would have been whitewashed with lime, probably on a yearly basis.

The cooker is set partly into a huge granite fireplace which itself acts as a heat sink, thus providing the room with some heat even when the cooker isn’t lit. Thus, I have been thinking how I could create something similar in the sitting room.

In our sitting room we have a small Godin stove, which we bought second hand some 16 years ago now and it is on this

that I need to focus my attention before the arrival of Winter. The stove works really well, even though it is primarily set to burn coal and even in the coldest periods will rapidly heat the room. The only real problem with it is the room gets too hot resulting in us letting it burn itself out and then having to relight it as the temperature drops.

What I want to do is store some of the heat emitted by the stove and to let it  transfer its stored heat as the room temperature falls – exactly as a night storage radiator works.

To this end I have decided to build a heat-retaining wall behind the stove. I am planning to make it very economically entirely of stone from the garden and using a lime mortar in the construction. I estimate the material cost to be somewhere in the region of between 5-10 Euros, which at the present rate of exchange is approximately  £4-8  or $6-13 US

Design

The stove stands on a tiled plinth, the plinth is laid on a  foundation on the ground beneath the suspended wooden floor.

To avoid removing the tiles I chose to build the wall on a strawcrete pad , this to furnish a ‘softer’ interface between the tiles and the walls and to ensure minimum heat loss from the wall by conduction into the plinth.

I had initially thought of building the wall in an adjoining stable and transporting it, still in its’ shuttering, into the lounge. I rejected this idea when I realised that the completed wall would weigh in the region of 180-200 kg.

The 'soft' lime mortar to be used between the stones will withstand the elevated temperatures.

Materials

Strawcrete: made from chopped straw mixed with a lime mortar. I had used strawcrete before as an insulation material on the interior walls of our kitchen. One of our outbuildings had a loft full of straw, this I chopped using a modified lawnmower. The strawcrete made from this material made a significant difference to the warmth of the kitchen and the following summer we plastered the entire kitchen walls with hempcrete on top of the straw.

We now buy the straw from a local organic dairy farmer to use in our hen houses. The long stalks were unsuitable for the strawcrete and needed to be chopped into shorter lengths. Some time ago I made a guillotine from some steel scrap to chop up old fruit crates for kindling. I found this could chop the straw after I had ground the blade to a more acute angle

You can find the film on how I made this guillotine from scrap metal here

Stone: The stones used are those which have already been used in the walls of the house and have been removed in the construction of doorways and windows.

Lime: I use hydrated lime NHL 3.5 

Safety gear: Although hydrated lime is much less aggressive than aerated, it will sting if you get it on your skin or worse still, in your eyes. You should therefore provide yourself with a pair of thick rubber gloves and a pair of eye goggles. It is also a good idea to keep a bottle of clean water on hand just in case of any slip ups, so as to wash off any splashes of lime.

Quantities

When working with natural building materials one of the first things you notice is that they are not uniform. This was something we were very much aware of when insulating our house walls and floors with linen, hemp and straw, on no day were either the mix quantities nor the elasticity of the mortars the same. They were dependent on humidity levels in the air, ambient temperatures out and inside and the different batches of the raw materials. There is obviously certain quantities one can work to but be aware that you need to get a feel for these materials as you go along and that, in particular the quantities of water used will not always be the same. However, take your time, work slowly and enjoy being in tune with the organic qualities of natural building.

We found in general when we worked on our house that the quantities of linen, hemp and straw to water and lime were pretty much the same. They were as follows:

1 bucket of lime (in a 35kg bag or 77 lbs of lime, there are just over 5½ buckets)
1 kilo (2.2 lbs) of straw, hemp or linen
6 litres (8¾ Imperial pints) (1.32 Gallons US) of water

Construction

The first stage was to make the strawcrete pad.

I constructed a wooden mould to the required size and lined it with a thin polythene sheet.

Next I mixed a mortar of lime and water.

I kept slowly adding water to the lime until the mix had the consistency of a batter, you will see this more clearly in the film.

Once this was mixed to a smooth consistency, I added the chopped straw.

As with the water, I added the straw little by little, mixing it into the liquid with each addition.

I was seeking the stage where there was no longer any liquid lime left in the mixing receptacle and all the pieces of  straw were coated with the lime mortar.



The mix could then be transferred into the mould, ensuring that strawcrete was tamped into the corners and along the bottom edges of the mould.

In between the addition of strawcrete the mix in the mould was tamped down using a wooden block.

Once full the excess strawcrete was scraped off using a straight piece of wood and the pad was left to dry.

Thus stage one of the storage heater is completed, the rest will be built in situ, follow this link for: Part 2 Building the Wall Now if you'd like to, sit back and watch the film.

If you have enjoyed this post and found it useful, feel free to share it, comment, ask questions or relate your own experience of using strawcrete. Hope to see you for part two and in the meantime all the best from Normandie,
Andy

© Andy Colley 2014

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