046 HPM 0415

HPM April 2015

Got a story? Ring us on 01732 748041 or e-mail twood@unity-media.com TRAINING/TECHNICAL An important energy efficient measure HPM’s technical expert, John Love, looks at how domestic hot water priority can be achieved... Last month, I looked at the design of a simple control system and highlighted the problem of achieving priority operation with a basic control system. This month, I am looking at how domestic hot water (DHW) priority operation can be with a standard 3-port diverting valve, in order to comply with the requirements of the Building Regulations and give independent timed control of heating and hot water - an important energy efficiency measure. To achieve DHW priority operation, referring to Fig. 1 in last month's issue, we need some way of ensuring that the diverting valve is only energised when the room thermostat is calling for heat which means removing the wire (C) to avoid the feedback to the valve when there is only a DHW demand. But how can the room thermostat then switch the boiler on? By using a relay, as Fig. 1 below. When there is a space heating demand, the diverting valve is energised, as before, and the relay ‘R1' is energised - relay contact 1/1 closes and a supply is put to the boiler so there is no possibility of a feedback. You will notice a number of terminals designated ‘N’ for neutral. These are shown as separate terminals simply to make the wiring diagram easier to follow - in practice they could all be a single or a couple of terminals in the wiring centre. I usually use a plug-in round or flat pin relay and the easiest source is usually Maplins - make sure it has a 240V coil and don’t forget the base - select a DIN rail mounting one. If you have a look on its website, you will see exactly what I mean. Next, you want a small adaptable box with a DIN rail and this will be from the electrical wholesaler - take the relay with you to check for size and the box will double up as your wiring centre. The relay base will snap onto the DIN rail. If you look at the Maplin website you will see quite a variety of relays, but the round base type is the most convenient with its DIN rail mounting base and the round pins are a little more robust than the other types. As an alternative to using a 3-port diverting WWW.HPMMAG.COM valve for DHW priority operation - use a 3-port mid-position valve but operating in only two positions - no mid-position. The grey and white wires are connected to the same terminal that the brown wire (C) in Fig. 2 is connected to, so they are both energised when there is a space heating demand - exactly the same situation as when it is operating as a normal mid-position valve and there is a heating demand when the DHW is satisfied or off. This can be seen in Fig. 2. With larger installations, which may have multiple pumps and boilers, there is another reason for using relays. With a standard domestic system, there is normally a single fused electrical supply to both the controls, boiler and pump. This means that if there is an electrical fault with any component which blows the fuse, you have lost the operation of the complete system. The pump and boiler (fan) motors are the most likely candidates for causing the fuse to blow, as the electrical load of the control system is minimal and there are no electrical motors. Okay, there is one in the control valve, but it is a small synchronous motor, like a clock motor, and with a very minimal electrical loading. ELECTRICAL SUPPLIES With a larger system with multiple pumps/boilers, by segregating the electrical supplies to these components we can avoid a failure taking the whole system down. Instead of having a single 13A fused spur supply, we have a small consumer unit fitted with 6A miniature circuit boards (MCBs). There is no reason that the consumer unit cannot be supplied from a 13A fused spur, although with larger systems the electrician may provide a supply from a 20A way on the main distribution board or consumer unit. This is illustrated in Fig. 2 and you can see that the electrical supplies to the controls, boilers and pumps are all from the individual 6A MCBs. This means that if a pump or boiler trips its MCB, you do not lose the operation of the entire system. 46 APRIL 2015 HEATING & PLUMBING MONTHLY This illustration is based on a system with one boiler doing the DHW heating and the other doing the space heating. The mid-position valve operates as a diverting valve passing water to either the DHW cylinder or space heating, and the first floor heating circuit has a separate zone valve. We still have DHW priority operation which is achieved by relay contact ‘1/1' - you can see it on the supply to the programmer CH switches. When the DHW is ‘ON’ and the thermostat is calling for heat, Relay No.1 is energised. Relay contact ‘1/1', which is normally closed, opens, and so the supply to the space heating thermostats is lost - the 3-port valve will go back to the DHW position on its spring return and the zone valve will close. At the same time, relay contacts ‘1/2’ and ‘1/3' will close, so running No.1 boiler and the main heating pump. In practice, the controls would almost certainly be more complex. You would have one boiler firing for a light load, whether it is DHW or space heating, and the second boiler coming on when the load increases. There could also be a separate underfloor heating circuit and the system could also supply a radiator circuit either the ground floor or the first floor or both. There may also be a separate towel rail circuit with its pump operating whenever a boiler is operating - this would provide limited towel rail heating during the summer, whenever the boiler is heating the DHW cylinder. There may also be wireless programmable thermostats used for each floor, which means that the programmer would be just a single channel one controlling the DHW heating - if there is a secondary DHW circulating pump, then the programmer would be a two-channel one with the second channel controlling the pump, which may also have its own switched electrical supply. John K Love CEng, FCIBSE., FIPHE., FIDHE., MInstR., FConsE Fig.1 Fig.2


HPM April 2015
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