Boilers meet renewable energy

MHS Boilers, biomass boilers
Solar-thermal systems, such as this one at Rayleigh Fire Station, can be integrated with boilers to deliver domestic hot water or to support space heating.

The world's gone mad for renewables. And not so many years ago, it went nuts about condensing boilers. But what about combining the two? Ian Bradley looks at their integration.

Once again the H&V industry awaits a change in the Building Regulations — even though it seems like only yesterday when they were last revised last time. We should not be too surprised though. As more and more pressure is put on builders and developers to build sustainable and energy-efficient buildings, there has to be some kind of legislative framework to introduce and monitor the changes. For our industry that’s the Building Regulations and the good old Code for Sustainable Homes.

 

And as the regulations develop and change, it is inevitable that some technology and systems will remain popular throughout the transition phase and others will fall by the wayside. For example, when the regulations changed in 2002, standard-efficiency boilers were phased out (except in exceptional circumstances). However, high-efficiency or condensing boilers are with us for some time yet. Which does beg the question, how can they best be integrated with the latest new kids on the block? But as I am often heard to say, it is very much a case of horses for courses — some systems integrate very well and some are somewhat incompatible.

So what are the choices with renewables?

 

MHS Boilers, biomass boilers
One of the most popular renewable-energy technologies is biomass.

One of the most popular technologies to come to the fore is biomass. Many boiler companies now offer a full range of biomass systems. Biomass boilers are generally not too difficult to integrate with condensing boilers — they are all boilers after all! However, biomass boilers generally need to operate at return temperatures that are typically too high to allow condensing boilers to operate at their higher efficiency levels, so while you may be reducing your dependence on fossil fuels, you aren't getting the best efficiencies from those fuels. To overcome that problems, clever system design that allows separation of low-temperature return-water circuits (such as underfloor coils or variable-temperature radiator circuits) can be utilised with boilers like the MHS Ultramax R600 condensing boiler, which has two return connections. This type of setup will enable some condensing operation to occur while operating the condensing boiler at a flow temperature in keeping with the demands of the biomass unit.

Another renewable system gaining in popularity is heat pumps, both ground source and air source, but these are a little more difficult to combine with condensing boilers because of the differing ∆T between flow and return temperatures (heat pumps have a ∆T of 7 to 8 K and condensing boilers 20 K). The best way to combine these appliances is to interface the boilers via a plate heat exchanger. That way the ∆T of the boiler can be modified to synchronise with that of the heat pump.

Finally, solar thermal systems can be integrated in number of ways — either as conventional DHW application or to support space heating. These types of integration are familiar to most consultants and contractors, and the usual way to design the system is to support the production of domestic hot water. However, if it is intended to support a system that is producing both heating and domestic hot water with solar thermal energy, which is primarily heated from the boiler plant, the system would need to be a reasonable size to make it viable.

In conclusion, heat pumps and condensing boilers are probably the best partnership, provided the system is well designed, because the operational temperatures involved with a heat pump are such that the condensing boiler is in condensing mode all the time.

Ian Bradley is managing director of MHS Boilers.

MHS Boilers, biomass boilers
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