Boiler Management Control

"Back in the day" as they say - I used to sell something called the IFC FuelSaver - a clever wee box of electronics designed to cut fuel consumption of indirect fired warm air heaters. 

I persuaded the makers (BBC Industries) to build a boiler control unit using similar control algorithms and return flow sensing and we sold masses of them - a fantastic piece of kit that basically inhibited the burner circuit until it was actually needed - as determined by suitable drop in circulating water temperatures. 

Sadly BBC Industries took a very different direction a few years ago (Bob now sells very good cheese and wine so if you are interested email me) and so for years I have been waiting for someone else to take over in this key area of energy efficiency.  That time has arrived - but what am I talking about - why do we need boiler controls?

Lets take a typical example of something I come across all the time - but this is an actual quantified case study if you like.  Three boilers were surveyed as part of an audit of a major secondary school for one of the Scottish Councils.  The boilers were observed to fire up when Return Flow reached 165^oF or 74^oC, which represented a very small drop from the output temperature of 170^oF and is indicative of a phenomenon known as ‘dry cycling’.

Space heating energy use constitutes a major proportion of total UK primary energy consumption - at least 60% for example in the domestic sector.

Despite improvements in energy efficiency (better insulation etc) energy use in our homes is increasing.  The number of homes is increasing too and we keep our home warmer than we ever used to (average 16^oC in 1990 rising to 18^oC in 2004).  So how do we maintain this drive for more and more comfort whilst reducing our energy burden - is UFH the answer?

UFH comes in two forms - wet and electric.

Lets look at wet systems first - fed by the circulation of hot water through pipes under floors.  These systems warm the floor structure by conduction causing the surface to radiate heat into the space above and the primary source of heat is typically a boiler - although many other options now apply which i will deal with later.  The boiler heats water to 40-50^oC and this is distributed in plastic pipes to one or more manifolds each of which comprises a flow and return header from which loops are taken to serve areas of the building to be heated.  In a simple two-storey office for example there would be one manifold serving the ground floor and one for the first each feeding UFH loops in the individual rooms on that floor.

Getting good conduction between the UFH loops and the floor structure is the key to success. Different methods of transferring heat from the pipes are applied for different floor structures.  The pipes are usually fixed to the floor insulation and a concrete screed poured over them further to which floor tiles, carpets or other surface finishes are applied.  Where floor tiles are used heat radiates into the "treated space" at roughly 100W/m².  The design output achievable with suspended timber floors averages slightly lower at 70W/m².  Note the use of the word radiate - UFH is more akin to radiant heating than is for example the use of radiators which are mostly convection.  Radiant heat means less rising heat lost to the roof void as convection - indeed with UFH the reverse is probably true - that is temperature inversion where the floor is hotter than the ceiling.    All of this means that the boiler operates at a lower temperature which promotes good combustion increasing efficiency.  Typically a condensing boiler serving UFH would be operating at 90% compared to 87% for the same boiler feeding radiators.

These low operating temperatures also means that UFH is the ideal partner for "renewables" such as ground source heat pumps and air source heat pumps (where the heat exchanger carried out an air to water transfer).  Why ideal - well simply these systems tend only to be capable of heating water to 40-50^oC and although some people like to boost this with a small boiler or an electric immersion element it seems a bit daft to me so I prefer to use the water at the temperature the heat pumps produces it in UFH systems - but heck what do I know?

Conclusions:  if you are building a new property then potentially great in terms of using new renewable technology.  Initially expensive in terms of material costs and requires specialist labour (not just any old plumber can fit this stuff) but long term your running costs are likely to be substantially lower than with traditional boiler fed systems.