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Published by NickSted on 30 April 2013


Part 5 The Performance


No - I hadn't forgotten Part 5, I just wanted a year's worth of experience and data.


Executive Summary – excellent boiler, big carbon saving but not a great investment opportunity

unless DECC remove the uncertainty surrounding the RHI scheme and improve the incentive payments.




How much fuel was it expected to use?

  • The quote thought I would use 17,550 kWh of heat, split as to 3,300 for hot water and the rest for space heating.
  • The house has an EPC B rating with an estimated heating requirement of 24,000 kWh.
  • The quote used the official SAP (standard assessment procedure) method which looks at each room, the number of windows, level of insulation etc. etc. The SAP method under estimates local environmental factors just using a general south of England factor. Marlborough is much colder. Lockeridge is a frost pocket on the Downs and night time winter temperatures are often below -10C. This year was particularly cold according to Met Office (and Marlborough's own Windrush station) data:


  • I have detailed monthly fuel consumption going back 15 years and this house needs about 27,000 kWh thermal (more if a family of 4 lived in it). About 5,000 kWh comes from other heaters such as a wood stove, a calor gas radiant fire and a small air-to-air source heat pump. So the boiler needed to produce 22,000 kWhth.

No problem as the boiler size was indicated as 19.1 kW in the quote and the next size up in the range was 25 kW which was the size selected. Now a 25 kW boiler is capable of around 35,000 – 40,000 kWhth pa if you fit a heat reservoir tank in the loop. We did not fit such a tank and opted for a slightly lower efficiency with direct demand heating.

So the quote said 17,550 kWhr which at 89% efficiency and 4,800 kWhr per tonne of wood pellets = 4.1 tonnes


How much fuel did it use?

  • 5.2 tonnes or 22,200 kWh. It should have been more but the boiler was off during the summer and was also off for 3 weeks during the winter because of a faulty thermostat.
  • The CO2e saving over heating oil (including processing and transport costs) was 7.1 tonnes.


Conclusion: it produced exactly the amount of heat expected (by me). It saved CO2 (and it doesn't smell). It works
more or less like any other boiler and, by turning it down a little, we can go away for a week or more in the winter without worrying.


Running costs

Wood pellets can be delivered in bulk but we opted for bagged deliveries. These come as 10kg bags on one tonne (96 to 100 bags) pallets. This method adds 20 to 25% onto the cost as against bulk 'blown' deliveries. There is some discount for ordering two or more pallets at a time.

My average cost of heat delivered into the house has been 5.7p per kWh (about 1p per kWh less than oil). So the annual fuel saving over heating oil was £220.

The boiler has not been serviced yet but this is expected to cost about £200. This is about £100 more than an oil boiler. Also the chimney needs to be swept once a year at a cost of about £35.

The overall saving is not huge but at least I'm partly sheltered from oil price variations as wood prices are expected to go up steadily and slightly more than inflation over the next 8 years.

Converting to bulk storage would cost around £1500+ or a 6 to 8 year payback.


Routine maintenance

Once a week the hoppers need to be refilled with about 20 bags. Very quick as you dump the bag on the hopper grill, slit it open length and width with a Stanley knife then roll it over:





Also once a week you waggle each of the 4 tube springs up and down for a minute:



There are three access points that have to be checked and/or cleaned:



The top one is the burner brazier which just needs to be vacuumed:



The middle one is the main ash pan which needs to be emptied once every 5 weeks or so (onto the garden) but vacuumed behind once a week:



The lower one collects fly ash and needs to be emptied and vacuumed once a week:




In all about 20 to 30 minutes per week.



Fuel Deliveries

2 pallets about 3 times per year. They are heavy and you need plenty of flat hard space to allow the large truck to offload them. This is what 960 kgs looks like:



Although they are well waterproofed it's a good idea to get the bags under cover as soon as possible. This is good exercise and with a garden trolley takes about 60 to 80 minutes per pallet.

Beside the boiler there is 1.7 tonnes of storage space – 7ft x 2ft x 6ft high plus 4ft x 2ft x 4ft high.

Beside the house is an alleyway with a new metal shed 3 ft x 8ft x 6ft high which can hold 1.3 tonnes:





The Cost

  1. The MCZ (Italian) boiler (+ chimney + plumbing work) had a fitted cost of £9800. The additional automated hopper system added another £2200. If I had opted for a fully automated Austrian or German boiler it would have cost about £4000 to £5000 more but I would have saved myself the weekly routine maintenance.

  2. I received the Renewable Heat Premium Payment (RHPP) of £950.

  3. I am waiting for the domestic Renewable Heat Incentive for which I will qualify. It has been delayed at least 3 times and is now scheduled for Spring 2014.

Here is how the domestic RHI is supposed to work:

It is called a boiler replacement scheme (ie to compensate for the extra cost of replacing your boiler with a renewable heat one instead of a fossil fuel one) with additional hassle factor payments added. Payments will be estimated from a Green Deal EPC assessment where the house must reach a certain fairly basic standard of insulation. The annual heat estimate will be multiplied by a figure, possibly about 8p per kWhr and paid for 7 seven years. The final year will have the RHPP knocked off.

I can expect 24,000 x .08 x 7 = £13,440 - £950 = £12,490.

However, there is quite a degree of uncertainty with a worst case of £5,447 to a best case (DECC's sample calculation) of £15,871.


I doubt a large number of people will be persuaded to have a wood pellet boiler. It is expensive to install, requires careful watching and needs quite a bit of space and time. A great deal will depend on DECC's clarification of the RHI scheme this coming summer. Watch this space.

The Governments target of increasing domestic biomass heating from under 2% to 12% by 2020 looks completely unrealistic unless DECC remove all the uncertainty around the RHI scheme and improve the incentives. If oil prices accelerate well ahead of wood pellet prices, then the fuel savings will make it more worthwhile.

Next Blog:

Is there enough wood? (or more technically: The Future of UK and Global Biomass Supplies and Prices)






















Published by NickSted on 26 April 2013

A blog on the DECC website today summarises a recent UNICEF/Mori poll:

74% of children in the UK want the Government to do more to tackle climate change to protect their future.

63% of children in the UK are worried about how climate change will affect children and families in other countries.  

Nick Stedman


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