Insulation products became very popular in the 1970s and have diversified significantly ever since. There is therefore the belief that we have exhausted all opportunities to benefit from this approach to saving energy, and we should therefore be looking at newer technologies for producing heat in a sustainable, environmentally-friend manner. Whilst this is certainly true and technologies like solar thermal and various types of heat-pumps are new and exciting, there may still be more to do on insulation.
Consider this fact:- if a room is perfectly insulated, once you have raised the temperature of that room to a desired 20 degrees centigrade, you would never need to apply any more heat to it to maintain that temperature. Obviously what this means is that the quantity of heating required to maintain a pleasant temperature, is equivalent to the quantity of heat leaking out of that room. So perhaps there’s still mileage in looking at insulation.
Where are those leaks?
The UK housing stock is generally old and not very thermally efficient. Much of it was built at a time when the most common heating fuel was coal and the lack of damp-proofing required a certain amount of air to be drawn (by a coil fire) over the floors and across the walls to keep them dry. In effect, the house was designed to be leaky. We have now reached a time when near all houses are damp-proofed and those gaps around the doors and windows have been mostly sealed.
Many people have now replaced coal with renewable wood, using a log-burning stove. But there are two big issues with this setup:- Firstly, when the stove (or an open log fire) is not being used, a vast amount of heat is lost up the flue. The flue is designed to suck air out the house. Secondly, very few people build a dedicated ventilator (air supply) for the stove or open fire. Without this, and fire or stove will suck air from the room/house which will have to be replaced with some cold air from outside the house via any remaining gaps in the doors and windows. If the house is effectively sealed so as to be free from drafts, the fire/stove will struggle to burn and may produce toxic fumes into the house.
We are all now aware of the benefits and importance of double-glazed windows, loft insulation and more recently, cavity wall insulation. The oldest insulation products, draft excluders, are less of a precise science due to imprecise door and door frame sizes, which often leads to the greatest heat losses. Warm indoor air is far lighter than outdoor air causing a pressure difference which will force warm air out if a house through upstairs leaks, being replaced by colder outdoor air through leaks downstairs.
Due to doors and windows being opened/closed frequently, the draft excluders around them become leaky over time. The objective of achieving maximum insulation requires the house to be air-tight. Once this objective has been achieved, unlike double-glazing, cavity wall and loft insulation, draft excluders should be considered as a ‘regular maintenance’ item/task.
Most UK homes have a chimney and the use of a log-burning fire or stove obviously requires one. The problem is that most chimneys do not have a mechanism for closing them. This means that the chimney becomes a major heat leak from the house when the fire/stove is not running. An energy-efficient home must have a method of block air flowing up the chimney.
Ideally some kind of butterfly valve would be best, but experience has shown that seasonal changes can be implemented very effective with nothing more than the use of an inflatable beach ball within the chimney.
In order to happily exist in our world, human beings need to consume oxygen and will generate carbon dioxide and water. The perfectly insulated, air-tight house will not allow this, so another solution is required.
In the vast majority of homes this solution comes in the form of compromise. An air-tight seal is not created and instead we have a compromise of a certain amount of leakage in order to maintain the required levels of oxygen, carbon dioxide and moisture. Almost everybody will open a window when the house/room feels stuffy and/or humid.
Actively managing this compromise can result in major heat savings. For example, a window should never need to be opened due to the house being too warm. That scenario should obviously have been avoided by using less energy to warm the house in the first place.
If we break the ventilation problem down into two factors:- 1) human needs, and 2) human behaviours, we can make progress on optimising the ventilation compromise. A certain amount of ventilation is required to provide human life with a supply of oxygen and a removal of carbon dioxide. There is also a ventilation requirement to remove moisture that the human body generates.
Take a look at our Heat Recovery Ventilator Systems
Understanding wood-burning stoves
Wood burning stoves come with an efficiency rating (well, the ones that sell in any quantity) expressed as a percentage. The obvious assumption of how this would work would be for the efficiency rating to indicate how much heat would be generated as a proportion of how much calorific value was in the logs being burned. So a score of 100% would indicate that all of the calorific value of the wood fuel was given out in heat.
Unfortunately, this isn’t how it works. Wood-burning stove efficiency rating is a measure of the proportion of unburned hydrocarbons are released into the flue. A 100% efficiency rating on a wood-burning stove means that there are zero hydrocarbons in the flue gases – only CO2 and H2O are present in the flue gases.
At first sight, this may seem as being insignificant but what is done to achieve a high efficiency rating is very important. There are two factors effecting wood combustion efficiency in a stove:-
- Air flow
In order to achieve any where near complete combustion, the combustion process must be very hot. This is why all of the top contending efficiency rated stoves have insulation blocks around the fire hearth. We often assume that the insulation is all about safety, but it isn’t. It is a method of ensuring that as much of the fuel is burned as possible.
It is well known that the height/length of a stove’s flue is critical to its effectiveness to burn the fuel. The reason for this is that the length of the flue is proportional to the stoves ability to suck air in which is used for combustion. The longer the flue, the greater the air in suction and the better the combustion.
The mechanism behind this air suction is also critically important. The way it works is that heated flue gases are less dense than ambient air. This means that they are lighter and therefore rise and are being replaced by heavier cooler air. The longer the tube of rising flue gases, the greater the speed of accent and therefore the greater the speed of replacement. However, if the lighter flues gases lose their temperature, they will also lose their lightness, become more density and therefore slow down.
So what this means is that if any heat is removed from the combustion chamber, the efficiency of combustion will reduce and not all the fuel will be burned. It also means that if heat is removed from the flue gases the draw of air into the stove will slow, thus also reducing the combustion efficiency.
This means that all thermal benefit from any stove is a compromise against the effectiveness of burning the fuel.
It may sound like a great idea to have a back boiler on a stove but all it means is that some of the fuel will not be burned effectively, and it may sound like a great idea to have some form of heat recovery around the hot flue gases, but that will result in the same.
The architecture of the perfect stove
There are stoves on the market that offer efficiency ratings in the higher 90%. These stoves have mechanisms within them to provide secondary, tertiary and quaternary combustion stages. These are nothing more technical than methods of introducing more air at higher positions in the main combustion zone. These are the stoves that should be used.
Relatively speaking these stoves will not give out much heat into the room in which they are situated, but they make up for this by burning the fuel better and usually look great. UFL will soon be offering a method to recover significantly more of that heat.
The next thing to do is to ensure that the flue gases achieve a good rising speed. In order to do this, the flue needs to be long and very well insulated. This will ensure that the flue gases rise quickly and pull plenty of fresh replacement air into the combustion chamber.
Next, the air feeding the stove MUST be from a dedicated supply from outside the house. Air from outside is cooler and more dense so there is more in it per unit of volume than in internal warmed air. Also, there is no point heating the air in a room if you are just going to suck it out into the stove and up the flue.
There are a couple of company’s providing stoves with what is sometimes termed ‘air/room sealed ducting kits’. These are dedicated ducts that supply the stove with air from outside.
This is all very fine but how do we get any thermal benefit from burning wood if recovering heat compromises the burning process. The answer is to recover the heat from the top of the insulated flue. A heat exchanger/boiler can be fitted at the top of the insulated flue that can effectively recover heat from the hot flue gases into water (which can be transported to anywhere in the house) without compromising combustion efficiency.
There are currently no such devices on the market but there are a few in development. Watch our ‘products’ web pages for new devices because UFL will be bringing such a device to the market soon. But in the meantime, be aware of the compromises that are being made when using wood-burning stoves:- don’t try to extract too much heat from the stove/flue, and ensure that adequate dedicated air supply and flue length is available.