Calculate the size of your Biomeiler

Passivhaus_thermogram_gedaemmt_ungedaemmtHow much heat do you need?

You may use one of the many calculators online, use a simple formula or work it out from your last gas bills. However, you have to be aware that these calculations are very general and can have large margins of error.

There is one more aspect to keep in mind - over-sizing of boilers. Have a look below where we explain this in more detail.

From your Gas bill:
Add up or check the summary on your gas bill for see how much gas you used in the last year in kWh.
You need to read the explanation on your gas bill for this calculation or look for it online at your suppliers website.

How much heat do you really need?

What you really need is the heat you loose once the room has been brought up to its comfort level. This is a lot less than the calculations above are suggesting when sizing up your boiler.
Currently most homes in the UK are running a start-stop cycle and in many cases similar to someone racing around an F1 track chasing the world champion.
It all starts with warming up the house. If your house is cold in winter and you need to bring it up to comfort level your system has to work hard to give you the 20°C quickly. It goes full power and runs until - well, the return in the heating system is up to cut off temperature. The boiler stops and cools down. If your radiators are too small, the circulation is not working well or you did not open enough radiators this start stop cycle will be quite annoying and might extend the time it takes to warm up the house. In any case, the boiler is not working at its optimal range - starting up - revving up - braking - cooling down and off again.
We all know if we keep driving at a constant speed of 50 mph we will save a lot of energy. All our cars consume more in town driving then cruising down the motorway. Sure, cruising at 50 mph might be really boring when driving, saving energy is not.
So, you need to run your system at cruising speed and avoid braking and accelerating. A biomeiler does just that. It runs and runs its combustion process of decomposition - day and night, 24/7.

Now, calculate your real energy need. You could do that for the whole house or room by room. Room by room will be more accurate as it takes into account how you are actually using the room.
You should work this out in two ways:Warmwasserheizung.jpg

  1. based on the standard temperatures for designing central heating systems - BS 5449  -1°C and -3°C or -5°C in more exposed areas (North of England / Scotland) for external and the recommended table for internal temperatures. - This should give you the higher figure.
  2. take the real conditions you have around your house for the outside temperature. for most of the heating period. Take your own readings or you can get them from the Met Office - here. For the internal temperatures you might use the recommended ones or decide what you prefer - higher or lower.

You can use the calculator from the Plumb Center and add the parameters you found or decided on for one and then the other method. While you do that run one more check:

  • take one of the settings and now change the internal room temperature by just 1°C up or down. You will see the effect this has on the required energy, yes each degree Celsius more or less equals around 4-10% of energy output requirement. This equates on average to around 3% on the fuel bill per 1°C (1.8°F). If you can live with lower temperatures by one degree Celsius you can save 3% on your fuel bill; if 19-20°C (66-68°F) is enough rather than 22-23°C (72-74°F) you can save around 10% on your fuel bill.

On extremely cold days you should consider to either reduce the temperature in your house in general or just to bring up some rooms to the full comfort levels and leave less important areas a little cooler, you will not only save energy, you will also be able to buy a smaller boiler. Ventilation heat loss is around 20 - 25% of the Fabric Heat Loss and you need to make sure your calculation includes for this.

Typ. Design Temperatures
Living 21
Dining 21
Hall 21
Study 21
Bathroom 22
Toilet 18
Bedroom 18
Kitchen 18

For combination or support systems you decide how much of your total heat requirements you would like to provide through the biomeiler.

What size of Biomeiler do you need?

The energy output of a biomeiler depends on a number of factors including shape, volume, location, external temperature, type of wood chips, materials used and construction. The following tables are based on averages calculated for garden conditions without extreme exposure.

Round Base - these are just some examples
Diameter   Height Area Layers Volume Energy output
m   m   kW
4   2.1 13 2 20 1.1
5   2.8 20 3 40 2.6
6   2.8 28 3 60 4.2
7   3.5 37 4 100 7.9
8   3.5 50 4 145 11.3
Rectangular Base - these are just some examples
Length Width Area Height Layers Volume Energy output  
m m m   kW
4 3 12 2.1 2 22 0.9
5 4 20 2.8 3 50 2.5
6 5 30 2.8 3 77 4.5
7 5 35 3.5 4 110 6.5
8 5 40 3.5 4 125 7.8

These are just examples and we can calculate the size of biomeiler for your requirements and your space available. We can tailor a biomeiler for your energy requirements and space.

Example Calculation:

We take a semi-detached, 4 bedroom, 2 bathroom house, Victorian (1899), 120 m² floor area on two levels, pitched roof with loft insulation between rafters to second floor, roof is cold, brick walls with cavity. Window areas to front and back, limited to side, double glazing 12mm, timber frames, timber floors, insulated ground floor,

Calculation 1:

Domestic heating sizing wizard - Energy Savings Trust (until they removed their calculator in the summer of 2011):

Take the internal measurements of the dwelling
Length: 9m Width: 7m Height: 2.8m Number of floors: 2
Calculate boiler output (in kW)  =      8478  +      2381  +      2000
   =     Total fabric heat loss  +     Total ventilation heat loss  +     Water heating
 Required boiler output  =     12859 (W)  =  ~ 13 (kW)  

The combi boiler we would have to order for this house would be a 13 kW. If we used a hot water tank, we could save a little on the boiler, but have to add for the hot water storage tank and the installation. Combi boilers may be reliable, but the constant switching when we require hot water does put some strain on the system. And having a hot water tank makes life a little more comfortable.

Alternative Calculation - Institute of Domestic Heating & Environmental Engineers:

The required boiler output is 14.11 kW.

Calculation 2:

Calculators for radiator sizing (per room) - Plumb Center -
Base Data used as above with the following options:

Inside Temperature Outside Temperature Heat Loss in Dif.
in °C in °C W/h kW/h in %
21 8 1800 1.8 8%
20 8 1650 1.65
 21  10  1500 1.5 10%
 20  10  1350 1.35
 21  -5  3600 3.6 4%
 20  -5  3450 3.45
 21  -10  4350 4.3 4%
 20  -10  4100 4.1

Now we know how much heat we actually loose per hour at the given internal and external temperatures. This is the amount of heat we would have to add to our house constantly to keep it at the required internal temperature. By not using a start-stop system of the traditional central heating boiler the amount we would need is for most of the year very modest. We can also see how much less energy we require be turning our thermostat down just by 1°C - the difference ranges between 4% - 10% adding up to around 3% average on the fuel bill for every degree we turn our system up or down.
We should also keep in mind that this calculation is based on heating all the rooms in the house to 21°C. We kept this at a constant 20/21°C for all rooms to simplify this exercise. But would we want and have to heat all rooms to 20/21°C. We would have a deep, sound sleep at 16-18°C in our bedrooms and we may not use them during the day. Our home office might only be used for a few hours a day or per week, so, no need to keep it always at 20/21°C.
A combi boiler needs to produce hot water in is for that reason often oversized as all the heat has to be produced instantly. A hot water storage tank would need to be heated up as a priority and with clever timer settings this does not really require additional boiler output. 


For this house the system chosen is a biomeiler to replace an old inefficient combi boiler from the 1990s with a new hot water storage tank (300l) with electric immersion heater to ensure regular (daily) heating up of the hot water to the safety limit of above 60°C against Legionnaires’ Disease (bacterium - legionella pneumophila).

The biomeiler will have the following dimensions:

Biomeiler  specification: units details
base    rectangular
dimension radius / l x w m  5 x 4
layers    3
height m  2.8
volume compressed  45
volume Wood chips  50
Wood chips fresh, green (70% M - or more)  tons 15
Heat output
 kW 2.5
Investment  £  2,000
Investment - DIY   yes
Fuel cost old system - annual  £  1,400
Fuel cost - DIY    yes
Fuel cost net - annual  £  6-700
Saving to old system on net  £  7-800
Savings to old system in %    50-57%
Return of Investment from Savings years 2-3
Fuel plus depreciation & maintenance - annual  £  8-900
Savings to old system in % including depr. & maint.    36-43%
Return of Investment from Savings years 4

Cost - investment £ 2,000 plus DIY labour, annual fuel bill is estimated at £6-700 plus DIY labour. The total annual fuel bill should add up to around £8-900 plus DIY labour including depreciation compared to a current fuell bill of £1,400 (without depreciation as the boiler was written off 2 years ago). A saving of around £5-600 p.a. equal to 36-43%.
It is also interesting to note that this system will be repaid entirely in 3-4 years just from the savings.

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