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How do we know what size of biomass boiler to buy?

Posted by Cathy Debenham on 6 June 2014 at 4:20 pm

Q: We have had 8 assessments and quotes from local companies to install a biomass boiler. Our barn was converted in 2002 is 346m2 and has 30 radiators and 6 bedrooms. We are confused because we have quotes ranging from 18Kw to 45 Kw wood pellet boilers and 700ltrs to 1500ltrs buffer tanks. We have had a EPC/green deal which assess [our heat demand] at 34,906kWh per year. We are now confused as to what size wood pellet boiler we need for our house - could you point me in the direction of where to find this info, so that we can make a decision on what size boiler to go for.

A: There is a significant difference between these quotes, and I'm not surprised to hear that you are confused. While I wouldn't expect them all to be identical, the extremity of difference is odd because there is under the Microgeneration Certification Scheme there are strict standards that installers must meet and methods they must use to make the calculations. 

I had a chat with a technical expert at accrediting body Hetas to see if they could shed light on your problem. It seems that there are a couple of possible reasons why installers may come up with different sized boilers - although he said the range of yours seems excessive.

Firstly, the key figure that the boiler size is based on is the heat loss calculation. The MCS requires installers to do thorough calculations of this. However, for a property of your size it could take up to a day to assess it properly, so it is likely that they have used a rule of thumb measure to do the heat loss for the quotes. Once they got the job, then would then do a thorough assessment.

The second factor is likely to relate to the range of boilers that each installer supplies. They often tend to work with just one or two manufacturers, and will specify what they've got available, with a tendency to err towards a bigger size. 

Put these two factors together and you could, as you've discovered, get quite a range. For example, if installer A estimated the heat loss to be 22kW, he might offer a 30kW boiler which is the smallest he has that is greater than 22kW. Installer B estimates it at 18kW, and the manufacture he uses has an 18kW boiler so he specifies that. So a 4kW difference in heat loss calculation has lead to a 12kW difference in boiler specified.

So how to you assess who is giving you good advice?

Here are a few suggestions:

1. I'd check that the installers are all MCS accredited. If they aren't you won't be eligible for the RHI, and they won't necessarily be working to the same standards.

2. Ask them what heat loss calculation they have based the quote on and how they got to that figure. This will give you an insight into why the differences have occurred, which ones have the biggest discrepancy and which ones seem most credible.

3. Ask them why they chose the product and manufacturer that they supply. I'd hope to hear replies that focused on quality, value for money, good support and training, good warranties, customer service and reputation, and would discount those that focus more on price or don't seem to have a good reason. You want to know that the product is unlikely to go wrong, and if it does the manufacturer will still be in business to put it right.

4. The best way to find the right sizing is to use a reputable installer who you are confident will do a good job. Given that you've already engaged with eight, I wouldn't add any more into the mix. You'd probably end up more confused. But do ask the installers who give credible answers to the questions above to put you in touch with three previous customers who have had their biomass boiler running for at least one winter season. People just starting out on a similar journey could  who don't know people who can recommend an installer, can search for one local to them in our member directory. We encourage installers to ask their customers to recommend them if they are happy with the work, which helps give you a flavour of their approach. 

Picture by Travelling.Steve


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1 comments - read them below or add one


Boilerdoctor247Comment left on: 9 November 2014 at 6:28 pm

As mentioned above installer giving you qoutes shouldn't be geussing if theve been on site and taken into account things such as windows ,materials of build bedrooms ,size of property and rooms in property etc from this they should be able to do heat loss calculation and give you a exact figure of what kw your boiler should out put .if you feel that they are geussing you should ask the contractor to supply you with heat loss calculations feel free to take a look at my website maybe able to guide you either on videos or boiler section 

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