What size boiler do I need?

You have a home, workshop or office building and you’d like to get away from oil heating and move to cheaper and renewable wood options.  How much heat do you need?  What size of boiler will provide you with enough heat?

You’ll never find on our site an equation the links boiler size directly to the square footage of the area to be heated because it’s simply not possible to achieve accuracy and efficiency without doing a bit more homework.

Every building is different.  Turn of the century farm houses can still have sphagnum moss insulation and leak heat like a sieve.  Newly built homes will lose a fraction of the heat because of new building codes and greater insulation thickness.    If you size a boiler for each of these examples based on square footage, the farmhouse won’t have enough heat and the new house will have a short cycling, inefficient system.

Would a professional electrician install a wire without knowing the amperage it will carry?  Would a professional plumber install a pipe without knowing the flow it must carry?  Heating professionals should not design a heating system without knowing how much heat is needed!

With consistent fuel, a Modern Wood Boiler will provide an accurate amount of heat to match a known heat demand.

Heat loss through a wall or a window is entirely calculable. We can estimate the total heat loss of a building by an exercise looking at R-Values of all the walls, windows, doorways etc. of a building and considering domestic hot water demand and some other factors. It can’t be more than an estimate but it needs to be WAY more accurate than a simple equation based on square footage. This process is called a Heat Loss Assessment and we provide it with every system we sell.

With gas or oil boilers, it isn’t as important to get the boiler size right. Fossil fuel appliances can more easily start up and shut down (short cycling) so these boilers are often oversized.

Wood boilers need to avoid short cycling (see the blog about Thermal Storage Tanks!) to reach peak efficiency.  A short cycling wood boiler (regardless of chips, log or pellet) will emit more smoke and cause unnecessary wear on parts.  We make sure to design all our wood pellet, firewood and wood chip heating systems to match the required heat demand so that peak efficiency is met.

We use the term “Modern Wood Heating” to describe our systems but more importantly to differentiate them from conventional wood stoves and outdoor wood boilers. 

The design of a modern wood boiler will result in the control of primary and secondary air flow, fuel feed and several temperature points.  This means that we will know precisely what the heat output is of the boiler at any time.  For eample, a 38KW pellet boiler operating at a 100% setpoint will provide 38 KW/h (129,661 BTU/hr) of heat.  If the heat demand drops and the boiler lowers its output, it will give 30.4 KW/hr (103,729 BTU/hr) at an 80% setpoint.  Modern Wood Heating appliances will control this on their own.  In contrast, an outdoor wood boiler will output an unknown quantity of heat within a wide range, not manageable for comfort heating.

When we know what the heat output of a boiler will be and we know what the heat demand of a building will be, we can design a system to optimize efficiency and minimize fuel use.  This is the improvement with Modern Wood Heating and why we can’t simply provide a boiler for X amount of square footage.  A bit of homework and planning while selecting a system will result in high efficiency, low emissions and big savings. 

If you’re choosing to use a Fröling system, we can do this homework for you.  If you are a do it yourselfer type, there are some good resources out there to help you learn about modern wood heating and hydronics.  John Sigenthaler’s book titled “Heating With Renewable Energy” is a great resource and is available here.  A more structured learning process by the same author can be found at Heatspring.

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