050 HPM 1012

HPM October 2012

TRAINING/TECHNICAL WWW.HPMMAG.COM Got a story? Ring us on 01732 748041 or e-mail twood@unity-media.com Being in control with compensation HPM’s technical expert, John Love, looks at the methods available to reduce boiler temperature... Htemperature with an 11ºC temperature, required flow temperature, is directlyis cut by reducing the temperature of the waterflowing around the system - the curve is astraight line because the heat loss and, therefore,eating systems in the UK - domestic andcommercial - have traditionally beendesigned to operate at 82ºC flow equivalent to 180ºF with a 20ºF temperature proportional to the outside temperature. drop in 'old money'. Again, the compensating controller senses the The choice of a water flow temperature of outside temperature, but instead of deciding the 82ºC is based on maintaining a good safety length of firing cycles, it calculates the required margin below the boiling point of 100ºC, while flow temperature. It will then either turn the keeping the heat emitter sizes down - the lower boiler on and off, overriding the boiler the flow temperature the larger the heat emitters thermostat so as to keep the flow temperature at have to be to maintain the heat output. the required reduced level, or control a The 11ºC temperature drop stems from the motorised three-port valve, mixing water flowing requirements of old and inefficient high thermal system, for example, pipe runs through an from the boiler with cooler water returning from capacity cast iron heat exchanger boilers. With unheated floor voids. Even if insulated, there will the heating system, to achieve the required flow old cast iron heat exchangers, if you increased still be a heat loss, although it will of course be temperature, which in both cases it detects by a the temperature drop then the thermal considerably less than with uninsulated pipe. sensor in the flow pipe - this is more common expansion, which is proportional to the Compensation is the name given to the with commercial systems. temperature change, would be greater at the flow heating strategy of reducing boiler temperature end of the boiler compared to the return and this in line with the level of demand. Two options MODULATING BURNERS could result in increased thermal stressing and are available: Increasingly, outside compensation is being the risk of failure of the heat exchanger. Load compensation: where the boiler flow provided as an option with modern domestic With oil-fired boilers, the above will apply if temperature is reduced as the room temperature boilers and with the boilers having modulating they have a cast iron heat exchanger, but it is the increases. burners, the control simply reduces the firing rate return temperature which is more critical. Outside (or weather) compensation, where the to keep the boiler 'ticking over' at the required boiler flow temperature is reduced as the outside flow temperature. GAS-FIRED BOILERS air temperature increases. As the outside The use of outside compensation control can Oil contains sulphur and when it is burnt, it temperature varies, so the heat loss from the be advantageous in a larger dwelling, where there produces sulphur dioxide. If there is any water building, and thus the required heat output from may be a problem in finding a suitable location present this will absorb the sulphur dioxide the heating system, varies. Outside temperature for a room thermostat. The outside compensator producing weak sulphuric acid, as well as nitric rises - heat loss reduces and so required heat has overall control of the heating, effectively acid, which is also produced by gas-fired boilers. input reduces, and vice versa. providing the boiler interlock, and thermostatic This can be a serious problem with oil-fired Varying the rate of heat input can be achieved radiator valves (TRVs) can be installed on all the boilers with steel heat exchangers, since, in the in two ways. The first is by pulsing the heat input radiators. Provided the system is correctly same way as with a gas-fired condensing boiler to the building. When the outside temperature is balanced and commissioned, most of the TRVs where we will enter full condensing mode if we at what is called design temperature - the will never need to completely close down, since get the return temperature down to around temperature at which heat losses are calculated - they will just be trimming the heat output if the 50ºC, if the return temperature is this low with the boiler will need to run almost continuously temperature of water selected by the an oil-fired boiler, we will also get considerable in order to keep the building warm. compensator is slightly higher than that required quantities of condensate (water) formed which If the outside temperature rises to a level at in their respective locations. will become acidic and will corrode the steel. which the heat loss is only 50% of that at design The use of compensating control is particularly For this reason, with commercial oil-fired conditions, the boiler need only run for 50% of advantageous with condensing boilers since the non-condensing boilers, we have back end the time to keep the building warm - say ten lower operating temperatures will result in their shunt-pumps, which are designed to get the minutes on and ten minutes off. With pulsing operating in the condensing mode for longer return water temperature above 50ºC as quickly control, a compensating controller senses the periods, but the compensating control must be as possible on start-up, by passing water from the outside temperature and then turns the boiler on overridden whenever there is a demand for hot flow straight to the return. To achieve this we and off for varying length periods so that the water, so that the boiler operates at maximum need to keep the flow temperature as high as amount of heat put into the building balances temperature so as to re-heat the domestic hot possible to limit the duration of a low return the heat being lost to outside. Of course, with a water to the required temperature, around 60ºC, temperature. condensing boiler this is not ideal because it as quickly as possible. With the compensating With traditional domestic heating controls the does not maximise condensing operation. control built into the boiler, this is no problem - boiler flow temperature remains fixed The other method of control is to vary the a switched supply to the boiler signalling a throughout the year, even though this may not temperature of water flowing around the heating domestic water demand will automatically cause be required to meet the heat demands of the system (the image above illustrates the it to revert to maximum temperature operation. building, particularly in mild weather. principle). At design conditions we require water Reducing the boiler's flow temperature will at its highest temperature (usually 82ºC) flowing increase its operating efficiency and also reduce around the heating system. As the outside John K Love CEng, FCIBSE., FIPHE., FIDHE., any standing heat losses from the distribution temperature rises, the heat input to the building MInstR., FConsE 50 OCTOBER 2012 HEATING & PLUMBING MONTHLY


HPM October 2012
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