BSEE HEAT PUMPS
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FAN CONVECTORS & HEAT PUMPS Here’s everything you need to know
Q A
Which kind of heat pump would you select?
Although there are some great air source heat pumps available, my own preference, purely from a technical perspective, is for ground source heat pumps. Ground source heat pumps use a much more stable energy source than air source heat pumps. Typically, you want the most heat when it’s coldest and relying on an air source heat pump can have its disadvantages in changeable climates.
The technical case for pairing fan convectors with air or ground source heat pumps is convincing and here, Simon Butcher, Technical Services Manager, Smith’s EP, answers some of the questions that are raised when heat pumps are being considered as a solution for heating projects.
Q A
For what reasons would a heat pump be specified and which heat emitters would be potentially selected?
Heat pumps are specified for many reasons, for meeting clean air requirements in the heart of a city, to being the best way of managing energy costs over a number of years, to meeting ErP obligations. There are effectively three categories of heat emitter that may be specified, the radiator, the fan convector or underfloor heating (UFH). The MCS Temperature Star Rating awards four stars to radiators, five stars to fan convectors and six stars to UFH. The challenge for radiators is that they typically need a water flow and return rate of 75°C and to achieve that with heat pumps, which will usually operate at around 40°C, requires much larger radiators than you may really wish to specify for the room spaces you have available. Even with heat pumps that can achieve efficiencies at 60°C, the size of the radiator is much larger than it would be with an ordinary gas or oil boiler. The revised MCS 021 heat emitter guide from May 2015
http://www.microgenerationcertification .org/images/MCS_021_Issue_2.1.pdf states that ‘A fan-assisted radiator (fan convector) will have a higher heat output than a standard radiator the same size. You can therefore achieve a higher Temperature Star Rating without the heat emitter becoming too large for a room with a fixed specific heat loss’. Fan convectors do not need to be oversized to operate effectively and efficiently with a heat pump. Finally, UFH is the optimum solution but even here, UFH works best in a constant climate than where there is changeable cold to mild weather and it is usually considered prohibitively expensive to use them in anything other than new build.
Q A
How does a fan convector work?
Commercial fan convectors are usually hydronic and the technology that they employ is extremely uncomplicated. They distribute heat using forced convection driven by a small electric fan. Behind the convector’s casing is this small fan and an aluminium-fin heat exchanger with copper pipes. These pipes simply connect to a standard two-pipe central heating system in much the same way that a panel radiator does.
The heat pump will heat the hot water and this passes through the heat exchanger, the heat transferring to the aluminium fins. Cooler air drawn in by the fan is heated as it passes over the heat exchanger is gently expelled back into the room.
The circulation of the heat around the room from when the heat source – here the heat pump – fires up is almost instantaneous and it is very easy to control fan convectors’ output, unlike other heat emitters. The heat is available almost as soon as the water flows through the heat exchanger. Most significantly, fan convectors function using just 5% of the water content of an equivalent output radiator, and heat using forced-air convection, via a small and energy- efficient electric fan, which means they circulate heat almost instantaneously and are easily controlled. Significantly, this means fan convectors function very effectively at water temperatures below 45°C, ideal for many heat pumps to work at optimal efficiency.
Q A
What are the advantages of pairing a heat pump with a fan convector?
Heat pumps are most efficient when the system temperature is as close as possible to the source temperature (air or ground). Fan convectors in all their various forms and guises, from natural convectors, to standard fan convectors to trench heating, all lend themselves very obviously to working with this technology.
Apart from the operational advantage and the sizing advantage, the major advantage that fan convectors offer the specifier when planning his heat pump heating is the versatility that they provide. They are physically versatile because fan convectors can be sited low on the wall, high, in the ceiling, concealed; in short they can be located entirely for the overall building’s convenience, aesthetics, and technical requirements. They can also be controlled extremely flexibly, depending on your overall requirements and building controls.
uMNIB by Smith’s EP trench heating.
30 BUILDING SERVICES & ENVIRONMENTAL ENGINEER MARCH 2017
Fan convector speed can be controlled by proportional heat output controls to deliver the right amount of heat for the space required, unlike the control option on a radiator, the TRV. Fan convectors can be fitted with a modulating fan speed and you can even create master/slave controls for up to four fan convectors if they have EC motors on a 0-10 volt signal. Each EC fan convector may also be controlled off a BMS; this control of individual fan convectors can create
uHow a fan convector works via a heat exchanger.
microzones around each heater if that is required. This allows for the temperature of each fan convector to modulate depending on how close to the door they are (for example) or other heat loss factors, and the further they are away from the door (for example) then the fan convectors can work on a slower speed with less output.
This type of scenario is ideal in a large open plan office or in a church that is off grid and has opted for heat pumps and needs a heat emitter up to the task.
Final thoughts
Heat pumps are a viable option for many commercial projects, for new build or for retrofit where the building is off-grid and the client is looking for controlled costs as one of their project outcomes. Selecting the most suitable heat emitter is vital to ensure that all factors, comfort, cost, controls, have been considered. For many projects, fan convectors are the best all-round heating solution because of the versatility and efficiency they provide. We have been involved in a number of heat pump projects over the last few years and time and again we are seeing specifiers making the switch to fan convectors because of all the advantages they provide.
http://www.smithsEP.co.uk VISIT OUR WEBSITE:
www.bsee.co.uk
uThis diagram shows how a standard sized radiator can produce the same amount of heat as a large radiator.
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Heat pumps are
most efficient when the system temperature is as close as possible to the source temperature (air or ground). Fan convectors in all their various forms and guises, from natural convectors, to standard fan convectors to trench heating, all lend themselves very obviously to working with this technology.
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