#6 Heating Systems

In a cold climate, effective and efficient heating systems are paramount. Because I'm off-grid, I also want a heating system that is fail safe, that is, at least one of the heating options has to be chosen so that it works even if there is no power at all. I wanted at least a couple of systems that work well together, and provide a level of redundancy. We have a couple of options to work with; I will discuss both the advantages and disadvantages. 

Before I get to that though, we need to have some idea of how much heating we're going to need. This will determine the size of the heating system, how long they can work between firing/charging sessions. This is going to involve some math - math is good!

Heating losses

What determines the size (in terms of power and stored energy) of the heating system is essentially one thing: heating losses. If there would be no losses at all, you'd heat up the place once and you're done. This is of course not realistic. Heating losses are impacted by the amount of insulation or lack thereof, and temperature difference between inside and outside. The colder it is outside, the more energy you need to keep the inside at a certain temperature because the losses increase at higher temperature differences - one of the laws of thermodynamics. The better the insulation you have, the slower this loss of energy from the warm inside to the cold outside.

Let's do some calculations. I'm going to use metric for this: in part because that's what I'm used to, and in part because I don't want to deal with a bunch of seemingly random conversion factors. This is also why I use Watts and Watt-Hours (Wh) when talking about energy, and not e.g. BTUs. This also means I will use the metric U values, unit: W/(m² K), and not the imperial R or U values when talking about insulation properties. Don't worry, it's all pretty straight forward.

The first thing to calculate are the losses of the house. This depends on the wall/floor/roof insulation, window types, doors, and amount, etc. Over here this is part of the so called Energy Performance Certificate that all houses need to have. My guess is you have something similar. If you know all your surface areas and U values you can calculate this: what I need are all the losses in Watt/Kelvin. This number tells you how big your energy losses are per degree Kelvin difference between inside and outside - called the specific heat loss. You get it by adding up the U values for every square meter of the house (outside walls, windows, doors, floor and ceiling). Let me use my place as an example and put some numbers on this.

My ceiling area is 114.51m² with a U value of 0.09W/(m² K). This means that the total heat loss for the ceiling is 10.3W/K. With this number we can already figure out what the heat losses are through the ceiling at any given temperature difference. For instance, suppose it's 20C inside, and 0C outside (a delta of 20K), the losses through the roof equate to 206W, or about 206W x 24h = 4.95kWh per day. If the temperature drops to -10C, this 30K delta will lead to a loss of 309W, or around 7.5kWh a day.

We can do this for all the windows (U value of 0.70), doors (U value of 0.75), the floor (U value of 0.16) and the walls (U value of 0.5). As I mentioned before, I emphasized insulation in the floor and ceiling. The windows are triple glazed and filled with Argon gas, and of course the walls are log. If we add everything together, we get our overall specific heat loss number of 130.41W/K.

Now that I have that number I can figure out how many kWh I need to keep the building at a certain temperature. Suppose for example you start again with the building at 20C and outside it's 0C, a delta of 20K. Therefor, if you have a loss of 130.41W/K, you need to put in 2.6kW (20 x 130.41) to keep the temperature at 20C. If the temperature drops to -10C (a delta of 30K), this a loss of 4kW. This again means that in order to maintain 20C in the house, you now have to put 4kW in.

Taking the above, I can now turn those power numbers into energy requirements. Suppose you want to have the house at 20C for 24 hours, and the temperature outside is 0C. I end up with an energy requirement of 24 x 2.6kW = 62.4kWh. At -10C, this becomes an energy requirement of 24 x 4kW = 96kWh. In other words: not a Passivhaus, but a result of large windows, high ceilings, and building with log, and yet, keeping the place comfortable isn't difficult nor very time consuming. The key property to make this work is thermal mass.

One important note: the above numbers are pretty much worst case scenario. In reality, less energy is required. For one, curtains have a rather big impact. In addition, the numbers above don't take sunlight entering the building into account - and with large windows, and southern orientation, you get a lot of sun even in winter months (except during the darkest winter days).

Masonry Fireplace

Everyone who has ever heated a house with a wood stove knows this: it puts out a lot of heat, too much sometimes which means opening a window. When you go to bed it's too hot, when you wake up it's too cold - and keeping a fire going all night long requires a bit of finesse. Keeping the fire going thought the day takes time and effort (aka 'a slave to the stove'). When you look at the efficiency of a typical wood stove, you're around 60% to 80%, but overnight when you dampen the fire to keep it going, you also get worse efficiency. The hotter you burn, the higher the efficiency since you also burn the stuff that creates creosote and smoke - however doing that also leads to a lot of the heat getting blown out of the chimney. 

I don't like wood stoves for the above reasons. Luckily, there is a better method to use wood as a fuel inside a fireplace in the home: one that always does an optimal burn, one that doesn't require baby sitting, one that you don't have to tend, nor have to keep going all night long to be warm in the morning. That method is the masonry fireplace. A masonry fireplace is nothing new - the operating principle has been used for millennia, and it has kept houses warm here in the north for centuries. The rocket mass heater borrows from it as well. How does it work? Thermal mass. Instead of relying on a constant fire to keep the place at temperature, you store the heat of the fire (which you burn as hot, and thus as efficiently, as you can) in a large mass (for example stone), which in turn will radiate this heat back in into the house at a steady pace over a long period of time. You make one or two fires a day, and that's it: the mass will radiate heat for the next 24 to 36 hours or more, keeping the place at temperature, also at night. Because of the same law of thermodynamics I mentioned before, the system is self regulating: the colder the room, the faster the fireplace radiates the heat. The warmer the room, the slower this process. Result: the room stays at constant temperature. 

My masonry fireplace, pictured above, has a thermal mass of over 2000kg made out of soapstone, and can store 60kWh of energy, at a burn efficiency of 88%. I can burn just 16kg of dry, seasoned birch wood to charge this fireplace (1kg of birch contains about 4.2kWh of energy). As we've seen from the calculations before, this means I can keep the place at 20C for 24 hours when it's 0C outside. When it gets to -10C, two burns per day (usually morning and evening, both less than 16kg) will do the same. Because of the efficiency of the burn (very hot, 800C-1200C), very little creosote is formed (its components are just considered fuel), and no smoke is produced. The exhaust gas temperature sits under 150C, which is much lower than a traditional wood stove. In addition, the air intake comes directly from outside, so that warm air that is inside the house is not used in the burning process. This also removes the 'issue' that airtight buildings have where the fireplace doesn't ignite properly because of bad draft since air isn't sucked in the house through cracks and other leaks.

The disadvantage of a masonry heater is that it takes quite a lot of time to heat up the house in the first place. It's a great tool to keep the temperature once you reach it, and doesn't require any electricity to do so, but getting there is slow. While it has an oven built in at the top which is great for cooking stews and pizza, it doesn't make hot water. This is also the reason why this fireplace isn't my main heating system.

Wood Gasification with Accumulator

A typical wood fired central heating system that is used for both room heating and domestic hot water typically looks like this:

All the way on the left, there is the wood burner. This wood burner heats a large water storage tank, and in turn, water is taken from there to circulate for room heating or heat is taken out through a heat exchanger coil for shower and other hot water needs. It's a closed system whereby the water in the tank and that for room heating stays within the system at all times. Cold water goes through the coil and heats up for domestic use.

That's the theory - simple. Let me tell you what it all means in practice... The first question I asked myself is: how big does the tank need to be? This boils down (pun intended) to  thermal mass again. In this case, the thermal energy that can be stored in the water. I won't go over the details of the formula to calculate the stored energy in water. There is a great calculator and graphs for that on the Engineering Toolbox site for that. For example, if you take a 1000L tank at 30C and heat it up to 80C, this represents around 58kWh of energy stored. Going back to the calculations we did for the heating losses, I could just about keep the place at 20C for 24 hours with this example when it's 0C outside. Doubling the tank size doubles the amount of energy stored, so now I could do two days. 

Of course, there are losses from the (very well insulated) tank itself as well. The hotter the water, the more the losses - for the same thermodynamic reasons I mentioned before. So in principle, a large tank kept at lower temperatures is better than a small tank at higher temperatures. This also implies that the room heating system is more efficient when it can be used at lower temperatures. Traditional radiators need a pretty high temperature, but hydronic floor heating does not. That decision was easy: floor heating embedded in the concrete floor. The floor itself is also a thermal mass again, so the same advantages we have with the masonry fireplace apply. 

In the end, I ended up going with a 3000L tank. It was the most practical to get in the allotted space, without going too crazy. It sits in a separate building, together with the wood burner. The same building also houses the batteries, inverters and charge controllers for the solar system, so noise from any of those systems or any problems that could arise from them is separate from the house.

The other thing I had to decide is what wood burner to get for this. The old types of wood burner were pretty inefficient, with smoke development and high chimney temperatures. Things have moved on, but there are still a surprising amount of these types in use here in Finland. In part, because of sparse populated areas relying mostly on wood and thus causing less problems with neighbors. It's also because other heating systems are popular and effective in more densely populated areas - methods such as district heating and heat pumps, not wood burning boilers. While in other EU countries (especially Germany) these old types are not allowed to be sold anymore, they're still available here, and rather cheaply so. 

Modern wood burners are so called wood gasification burners, whereby the wood is turned into gas in one chamber, and then burned into the other chamber. This happens at high temperatures allowing for a very complete combustion of the fuel - more than 90% efficient. Heat is extracted from the hot exhaust gasses with heat exchangers and stored in the tank. The image below shows the gasses burning in the second chamber.

Of course I went with the gasification burner and not the traditional burner. The low emissions and high efficiency are part of this decision, but also the typically larger volume of wood that can be placed at once and a high level of automation means that to heat the tank from cold to 80C takes one burn, which takes about 10 minutes of supervision to get the burner started and loaded with wood. The automation, which also guarantees an optimal burn throughout the burn cycle is done with a Lambda sensor: an oxygen sensor - something that is also used in e.g. cars to make sure the fuel/oxygen mixture is optimal. While there are gasification burners available without Lambda sensor, these tend to require much more manual tweaking, especially when changing the type of wood. With a Lambda probe, I can burn soft or hard wood, or mixed, without having to tune anything. 

There are quite a few manufacturers available that make these burners. On the high end list of equipment we find manufacturers such as Fröling, ETA, Windhager, and Hargassner to name a few. These come with a pretty hefty price tag, usually starting at around 10k€ for a burner in the 40kW range. In places like Germany, you can actually get substantial subsidies for this type of heating, but not here. I didn't really want to spend that amount of money on the burner considering I also needed the storage tank, floor heating manifold and pump, etc. all of which are not exactly cheap. 

Luckily, one doesn't have to look only at the high end of these burners. There are a lot of lower end manufacturers, mostly located in Eastern Europe, that make these burners - and quality is pretty good. Some of these manufactures include Vigas, Centrometal, Atmos, and Attack. I went with one of the Attack burners, an SLX 40 Lambda Touch.

Especially with a price well under half that of the top tier ones... Yes, I don't have the same service and if something breaks it's up to me to fix it. As with my battery pack, I wouldn't want it any other way. Spare parts are readily available, some of which I already have here, just in case. 

Even on the coldest winter day at -30C and below, this system alone keeps the house warm and provides all the domestic hot water for 24 hours on a single burn. This single burn heats up the water in the 3000L storage tank all the way to 85C, storing some 175kWh of energy. On milder days, I only have to burn every two to three days. Together with the masonry fireplace, this is a very efficient and effective way to keep things warm.

The disadvantage of the system is that it needs some maintenance. Ash only needs to be removed once a week or so, but there are some larger cleaning efforts that need to be done regularly such as cleaning the heat exchangers and other parts of the burner. In principle not a big deal, but it's definitely more involved than a wood stove or masonry fireplace. The upfront cost, especially with the top tier products can be off-putting. There are a lot of individual components (floor heating pump and controller, storage tank, burner itself, connections, etc.) that all have their individual cost. It adds up quickly.

Heat Pump

While the masonry fireplace and gasification burner are excellent, I want to reduce the reliance on wood for the early spring and early autumn period. These times of the year are still cold enough to require heating, with temperatures regularly dropping (or staying) below freezing, but (especially in spring) do tend to have a lot of sunny days. In spring, days notably get longer again, and power production from solar reaches the point where I have excess. I don't have enough to just be able to dump this into the tank though a resistive heater, but with a small air to water heat pump with a coefficient of performance (CoP) of 4 or so, I should be able to offset the amount of wood I burn: every kWh of excess electricity can be turned into 4kWh of heat. 

Of course, we can take this one step further: since a heat pump can work both ways (see air conditioner), it can also cool the water (typically called a water chiller). For those hot summer days, I can pump cold water in the floor heating and use that to cool the house - radiant cooling. The floor heating controller is already set up for this purpose: since you have to be able to monitor the dew point (to prevent condensation) it takes a bit more control logic. This is currently work in progress, but should be finished somewhere this year, time permitting. 

The advantage should be obvious: less wood burning, more comfort also in the summer. The main disadvantage is the cost of the air to water heat pump, especially one that is reversible. It's not been a priority, more a 'nice to have' feature. By the way, in summer I already produce all the domestic hot water with a smaller, electric (resistive heating element), hot water boiler. Efficiency is not important at those times, because of the huge excess of power I generate. I can't wait to get an EV...

Propane Heater

This is the back-up in case all the rest stops working. Since I cook on propane (at least in winter), I always have at least one full propane tank. If something goes horribly wrong: alone, snowed in, no power, broken arm, or a combination of those things or all those things combined that make it impossible to operate the fireplace or wood burner, this propane heater can at least keep me warm until help can get there. 

I put this section in here in a kind of a more light-hearted way, but when you live off grid and have no neighbors and are far away from the nearest village, you need to be a little prepared and have alternatives for your basic survival needs. A small heater like this, in combination with a decently stocked medical kit, a source of clean accessible water and food, heat and dry clothing, a communication device like a spare mobile phone, and things like a fire extinguisher and fire blanket are better to have near. There are all kinds of possible scenarios (falling through the ice while fishing for example, or indeed breaking a leg or other accident) that can get very bad real quick if you're not prepared. 

Ventilation

While not strictly heating related, the ventilation system forms an integral part of the heating energy use of the building, so I will briefly discuss it here. One of the advantages of building with engineered log, is the automatic air tightness you get. This means I have to provide a way of replenishment the air inside without just getting cold air in the building. For this, I installed a heat recovering ventilation system, also known as an HRV. This mechanical ventilation system contains a heat exchanger to warm the incoming air with the exhaust air. Fresh air is guided to each room through a series of ducts installed in the ceiling. This type of ventilation seems to become more common in other places in the world nowadays, and forms an integral part of zero energy or passive house standards. Here in Finland, it's been in common use for decades. In warm climates, you usually go for an ERV (energy recovery ventilation) instead, which pre-cools and dehumidifies in the warm season.

Final thoughts

Off grid, reliable heating in a cold climate demands an investment. Not just financially, but also in effort, time and design. In northern climates, wood is usually the main, abundant, source of energy. Processing of this energy source (I'll come back to this in a later post) is time consuming but well worth it. Even if I would have to buy the wood from a local guy, cut, split and seasoned, the energy cost would still be lower compared to any other source, on grid or off. For comfort reasons, a typical wood stove was not considered: inefficiencies left aside, the constant need for tending the fire, the challenge of keeping the fire going all night, the cold mornings and other aspects that impact comfort were my main objections. Thermal mass is key, both for the masonry fireplace as well as the gasification burner. This leads to a comfortable, warm home at all times without being a slave to the fire.

I would like to thank you, the reader, for making it this far. I know, a blog... in 2022... I really appreciate it! I hope you enjoyed reading this, even though I know many of you came here for the solar system in the previous entry. Let me know in the comments if you have any questions, suggestions, or just to say hello!