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5 top tips for choosing a biomass boiler for your home

Posted by Gabriel Wondrausch on 12 August 2013 at 9:23 am

Q: I am going nuts about installing a modern biomass boiler. We are committed to renewables for our stable conversion and have planning permission for a boiler house especially built for a biomass pellet boiler. However, I am struggling to find quotes and advice (eg. how the pellets will be delivered and loaded into  hopper to go in to the new boiler house when it is built; what boiler to have etc). Your help will be welcome.

A: There are a lot of variants in the world of biomass boilers and a lot more to discuss than we could ever cover here. In this blog we have tried to cover the most common commonly asked questions about domestic biomass.

1. What type of biomass boiler should I look at for my property? (medium sized domestic household)

There are a few different types of biomass boiler available. Broadly they can be separated by fuel type: log gasification, wood chip and wood pellet fuelled boilers. Each fuel and boiler comes with their own pros and cons relating to installation cost as well as ease of operation and ongoing costs (fuel as well as maintenance).  

In most households the heat load is 10kW to 45kW and the short answer will be a pellet boiler.  Good quality pellet boilers are compact, automated and as close to a fossil fuel system from an end-user point of view as biomass systems get.  On top of this they offer lower running costs than oil or LPG and nearly zero net carbon footprint. With the added benefit of the domestic renewable heat incentive (RHI) (starting in spring 2014) they are an excellent investment.

2. Do I need a purpose built plant room? 

As you have successfully gained planning permission for a dedicated building to house the boiler and associated equipment, this is a great start.

There are options for pellet boilers that can be installed inside your home, providing the aesthetic benefits of a real fire.  Generally however, you will need a dedicated space such as a large utility room, part of a garage or a purpose-built space.

3.  What features should I look for in a pellet boiler?

As mentioned above, a quality boiler will prove a pretty hands-off, automated heat source. There are a lot of units out there with varying levels of quality so it is worth doing your research.  Here are a few key features to look out for:

a. Manufacturer: Look for a manufacturer who has been designing biomass boilers for some time.  Is the unit the manufacturers’ own or a rebadged unit from another manufacturer?  If it is rebadged this is not necessarily a terrible thing depending on who it is licensed from.

b. Automatic features: Almost all pellet boilers have automatic ignition (by heating element or hot air blower).  There are a few other features to look out for as described below.

c. Combustion chamber ash:  Biomass is a solid fuel and even though pellets are the most refined of the woody biomass fuels, there is still ash produced (~2% by mass of fuel). For continued efficiency this needs to be removed from the combustion chamber. Some units will require the end user to periodically sweep or vacuum the combustion ash out. This means high efficiency levels just after it has been cleaned out, but these levels of efficiency will drop off as ash builds up and until it is next cleaned out.  A good quality unit will have an automated mechanism to remove the ash periodically, negating user involvement and maintaining efficiency.

d. Heat exchanger cleaning:  The heat exchanger surfaces can become coated with a layer of fine fly ash. Over time this reduces heat transfer and so reduces efficiency. A basic unit will have a manually operated lever used to clean the surfaces. A quality unit will have motorised cleaning controlled activated automatically which ensures continued high levels of operational efficiency without any need for you to touch the boiler.

e.  Automatic ash removal: Ash removed from the combustion chamber and heat exchanger will generally drop into an ash bin in the bottom of the unit.  High quality boilers will incorporate an auger screw to extract the ash into a larger ash container, which may only need emptying once or twice a year.

4. How often does a pellet boiler need re-fuelling & how does this work?

Again, this comes down partly to the boiler you choose, but also to your preferences and budget. A standard small pellet boiler will be fully self-contained with the boiler and fuel store in one unit. On low output boilers this could be a 40kg capacity hopper that would last a couple of days (depending on weather and what the thermostat is set to).  A larger boiler might have a hopper with 150kg capacity, enough to last a week or so.

With this type of system you would buy your wood pellets in 10-15kg plastic sacks, often in the form of a 1 tonne pallet. Then it is up to you to keep an eye on fuel levels and empty sacks of fuel into the hopper as and when required. This isn’t particularly hard work or messy but it is important to consider who will be doing this and whether there will be someone around to do this when needed.

With larger capacity boilers that will use a lot more fuel, or in situations where it isn’t convenient to be manually refilling the hopper, a bulk store can be specified. The boiler will still have its integrated hopper but this will now include a level sensor which activates a vacuum feed mechanism when the fuel level drops below a set level. The boiler hopper is connected to the bulk store (a pre-fabricated steel or fabric silo, or a section of a building constructed to house pellets) via two flexible hoses. When the vacuum operates pellets are sucked down one of these hoses to fill the local hopper, the second hose is to balance the pressure differential.

The bulk store is filled in much the same way: two solid pipes with specialist connectors are built into the store and a pellet delivery truck will connect onto these and blow the fuel into the store. In principle, the delivery method is very similar to oil delivery. It is always important to check a delivery truck can get to within 10-15m of the fuel store as this is the top end of distance that the pellet can be blown.

A bulk store is ideally sized to the boiler and anticipated fuel use.  A good rule of thumb is to multiply the heat demand of the building (in kW) by 0.9 to get the volume of store required.  

e.g. 15kW x 0.9m3/kW = 13.5m3 storage room volume (including dead space below sloping sections and above pellet storage level).  Fuel storage space can be limited by available space or budget. It is always important to check how long the fuel store is expected to last!

5. Are the controls easy to use? Can I keep my existing controls?

There are a number of options here, but in short you can usually keep your existing controls to avoid re-plumbing a set-up that works well or has been recently installed. If you wish to get maximum efficiency out of your system, specialist controls can be installed which incorporate features such as weather compensation. Again your installer should be able to advise you on these and whether they are necessary or appropriate to your property and budget.

by Mark Howard

Photo: SunGift Energy

More information about biomass boilers and the RHI

YouGen guide to biomass boilers

YouGen guide to the renewable heat incentive

14 things to ask your biomass boiler installer

Why you should service your biomass boiler

12 fascinating facts about wood pellet

About the author: Gabriel Wondrausch is founder and director of SunGift Solar, which installs solar thermal and other renewable energy systems in the South West of England.

If you have a question about anything in the above blog, please ask it in the comments section below.

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