32 HPM 0813

HPM August 2013

WWW.HPMMAG.COM Got a story? Ring us on 01732 748041 or e-mail twood@unity-media.com TRAINING/TECHNICAL Guaranteeing safe system operation HPM’s technical expert, John Love, looks at the method of excess heat dissipation and how to control it... Aheating system which has two heat sources, one usually being a renewable or low energy heat source, such as a heat pump or solid fuel appliance, and the other an orthodox boiler which could be gas or oil fired, is called a bivalent heating system. Such systems have to be carefully designed so as to maximise the energy saving potential of the renewable or low energy heat source. Last month, I looked at a system design I have just completed which has a gas-fired condensing boiler and a wood burning Rayburn, although it could be a solid fuel stove or boiler. This month, I am going to look at the method of excess heat dissipation and the method of control. As I said last month, you can’t stop the output of a solid fuel appliance at the turn of a switch, as you can a gas or oil fired boiler. So we need to ensure that there is some way of absorbing the heat being given out by the Rayburn. MILDER WEATHER CONDITIONS When the heating system is providing space heating or heating the domestic hot water (DHW), there will be a good load on the system which will take the full output of the Rayburn, and it is likely that the gas boiler will need to top up to meet the requirement. During milder weather conditions, it may be possible to operate the DHW cylinder at a higher than normal temperature (say 70°C) to absorb some of the output of the Rayburn, provided this does not present a risk of scalding and that the temperature is below that at which the cylinder overheat valve 'MV4' will operate. If no more heat can be absorbed by the DHW cylinder then we need to run the towel rail circuit heating pump 'P3' to dissipate the heat. See the diagram (above) for the schematic layout. It is worthwhile noting that although overheat valves are supplied with unvented cylinder, there is no requirement under the Building Regulations to install them when the heating to the cylinder is provided only by a gas or oil fired boiler. In our case it is required because a solid fuel heat source does not have the same energy cut-out controls that a gas or oil fired boiler has. The safe operation of our system is dependent upon the operation of the appropriate pump - one could fail or there could be an electrical supply failure. This is where the heat sink radiator, shown in both of last month's diagrams, comes in. The radiator must be piped top bottom opposite ends and the circulation through it is controlled by a spring return zone valve which is driven to the closed position and opens under the operation of the spring when the motor is de-energised. This is the opposite to normal operation but such valves are readily obtainable. In the event of a power failure, the valve will open and so provide a gravity circulation path to the radiator. The heat sink radiator needs to have an output to match the 'idling' output of the solid fuel appliance, which the appliance manufacturer will advise. In our case, the required output was 1kW, so I allowed 1.5kW to be on the safe side. For efficient gravity circulation, it needs to be positioned as high as possible, even in the roof space, provided there is no risk of freezing. There is an alternative to the heat sink radiator and that is to fit a heat exchange coil which mounts through the immersion heater boss and is provided with a mains cold water supply. In the event of a high temperature situation, a valve on the mains supply opens and cold water runs through the coil to waste, cooling the stored water in the process. This is permissible under the water regulations, since it is classed as an emergency situation, but the discharge must be in a visible location to safeguard against incorrect operation of the valve and wastage of water. So how do you control such a system, which is more sophisticated than a normal installation? The controls for the sealed heating system connected to the gas boiler will be pretty standard but what we need to introduce is the interlink with the Rayburn heating circuit. This requires two single thermostats in the vented cylinder - one control and one high limit. The control thermostat needs to be set to a temperature slightly higher than the main heating system return temperature. The function of this thermostat will be to turn on pump P2 so as to transfer heat from the vented cylinder into the main heating system. As well as starting pump P2, the thermostat will also need to switch on pump P4 if pump P3 is not operating (i.e. no space heating or DHW heating demand) so as to transfer the heat into the towel rail circuit. The high limit thermostat needs to be set at around 85°C to 90°C and this will need to be confirmed by the solid fuel appliance manufacturer/installer. The function of this thermostat will be to de-energise the heat sink radiator spring return valve when the temperature is exceeded, so as to dump the excess heat into the heat sink radiator. The above is a fairly simple control strategy. With a larger system, instead of having a fixed temperature control thermostat, a greater energy saving could be achieved by installing a controller and two detectors, one replacing the control thermostat and the other in the main heating system return. The controller would then be set to run Pump P2 whenever the heating system return temperature is below the vented cylinder temperature. John K Love CEng, FCIBSE., FIPHE., FIDHE., MInstR., 32 AUGUST 2013 HEATING & PLUMBING MONTHLY


HPM August 2013
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