I am confused about the application of a wood burning/condensing boiler combination, as discussed in Heating technology (BSJ 08/06). Would the case not be for the space heating in general to be the greater heat load of a building rather than the domestic water demand?

I would reverse the roles, asking the wood burner to meet the greater load, incorporating hot water mixing to meet lower output supply water temperatures, also heating the domestic load via a suitable heat exchanger. I would keep my gas-fired boilers offline as long as is practical, maybe peak load only or on standby in the event that the wood-fired boilers fail.

As far as wood fuel (biomass) is concerned, pellets are currently sold at an artificially high price, keeping their costs just under those of oil to justify their use. Yes, they are more efficient in terms of moisture, burning and loading techniques. However, woodchip is by far the most cost-effective raw fuel supply, ranging from £5-10/tonne self-supplied or anything from £30-60/tonne delivered – and having and average moisture content of 25-30%. Pellets range from £95-150/tonne delivered!

Ideally, biomass should be available locally – if it has to be delivered over long distances this can spoil the overall environmental benefit. In recent years, we have found just how local biomass can be sourced if investigated.

My intention is simply to develop the use of this alternative technology. Woodchip boilers are now very sophisticated in the main and able to meet the demands of modern buildings.

Bruce Boucher MInstR, The Bruce Boucher Consultancy

Source

Building Sustainable Design