Effective underfloor heating depends on good design

The hard facts of good design — Keith Westcott.
Keith Westcott argues that the current boom in demand for underfloor heating makes good engineering based design even more crucial to maximise comfort and efficiency.As the market for underfloor heating expands at an ever-increasing rate, is it possible that commonsense and good engineering are being swept away in the excitement? We need to stop and reconsider the hard facts, re-examine some the practices and how they stand up in a fast-changing world. First, even though underfloor heating may now be accepted as a cost-efficient solution for a whole range of applications — schools, hospitals, churches and large, open high-ceilinged areas of all kinds — design is no less important. Each project must still be considered individually, with a system designed and developed to match its unique characteristics. This may seem self-evident, but increasing market pressure means that some companies cannot provide the right kind of project-matched technical support and concentrate on ‘standard solutions’. Nowhere is design more important than in the commercial and public sector, where the larger scale of project magnifies the fallout from any problem. Three key areas require careful consideration. They are insulation, flow temperatures and boiler efficiency. Insulation Building Regulations now require either thicker standard-grade floor insulation such as expanded polystyrene, or more often switch to other materials such as extruded polystyrene or polyurethane. This is obviously good news for overall energy efficiency, but creates a potential problem for the large number of underfloor heating (UFH) systems that depend on insulation-based fixing systems developed to satisfy the previous regulations. It is crucial that products such as pre-routed insulation material, castellated system boards (with ‘egg box appearance) and roll-out insulation designed to accept pipes held in place by staples should all comply with the current regulations. If this is not so, an extra layer of insulation must be introduced to the floor construction to make up the shortfall. Any heating engineer knows that heat takes the path of least resistance, so it is important to consider the thickness of insulation from the bottom of the insulation to the underside of the pipe, rather than the overall thickness of insulation. Flow temperatures As insulation levels increase, the need for close control of flow temperatures is vital to achieve the lower floor-surface temperatures and ultimate outputs required. If too high a flow temperature is put into a screeded or solid concrete floor, the air temperature thermostat will turn off the relevant pipe loop(s) once it is satisfied. However, the energy will have been forced into the mass at an accelerated rate, which will not stop emitting heat just because the flow of water in the pipes has been halted! The result is overshoot, as the inertia of the floor construction continues to raise the floor-surface temperature — increasing the system output and air temperature beyond the required set point. The consequences may be more detrimental than just perspiration! Floor temperatures may exceed recommended maximum surface temperatures — possibly even resulting in damage to vinyl and wood finishes. Good system design with proper control of flow temperatures will protect the floor and save energy into the bargain. Condensing boilers Low flow temperatures sit very well with new heat sources such as solar power, ground and air-source heat pumps, but there is another heat source that is most efficient at low temperatures — the condensing boiler. It is all too common to hear that underfloor heating and condensing boilers are perfect partners. Unfortunately, many systems are working inefficiently due to the misconception that all UFH systems work well with condensing boilers. We regularly see condensing boilers set at high flow temperatures to serve appliances off the same primaries as those feeding the UFH. The problem lies with the belief that the low return temperature from the UFH will mix the primary return down beneath the point where condensing takes place. In reality, in its usual ‘top up’ condition, the mixing control built into the UFH system to reduce the high primary temperature to a level suitable for supply to the loops in the floor will re-circulate the majority of its flow from the UFH return and require only a small primary injection to raise its return by 5 to 10 K. That ‘small’ injection from the primary flow in turn means a ‘small’ return injection to the primaries; in many cases there will not be a sufficient percentage of cooler water to return the return enough for the boiler will condense. It is worth remembering that exactly this method has been successfully used over the last 20 years to protect conventional boilers against the risk of low return temperatures causing condensation and corrosion. In general the most efficient way to run a condensing boiler with UFH is to connect it direct to the manifold with a flow temperature suitable for supplying the floor loops. The resultant low return temperature will allow the boiler to run in a deep condensing mode virtually all the time. It is always wise to check with the boiler manufacturer for specific advice on best practice. Some boilers now require 20 K differential. Although the mean temperature remains the same, the reality is that the flow temperature into the floor can be unusually high, resulting in the last section of floor heating loop (return) being only a few degrees higher than the desired floor surface temperature. Bear in mind that flow temperature has a direct relationship to surface temperature and, ultimately, output — so mean temperature alone cannot be the defining calculation on the suitability of such a large ∆T. Underfloor heating has so much to offer in terms of space-saving comfort and increased fuel efficiency that its future is assured. For the consultant and contractor, however, that future will be all the happier if they insist on suppliers who can deliver sound technical advice as well as sound materials. Keith Westcott is with Even-Heat.
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