As most houses take an hour or two to warm up when the central heating comes on it is usual practice to set the timer for two hours before you arrive home. This is a reasonably efficient approach but has one drawback. Whether the house actually takes two hours to warm up will depend on the weather. Suppose your thermostat is set to 22c and on a cold winter’s day the temperature before the heating came on was 14c. The central heating came on at 4PM, you arrived home from work at 6PM and it really did take two hours for the central heating to raise the temperature from 14c to 22c. No heat was wasted. But suppose it was a warmer day and the temperature before the heating came on was only 20c. The heating will come on at 4pm and take half an hour to raise the temperature from 20c to 22c. The thermostat is set to 22c so at 4.30PM it will turn off and allow the house to cool to 20c. It will turn on and off every half hour until 6PM. At 6PM when you come home the house will be at the right temperature but the heating has been turning off and on for 2 hours when it didn’t really need to. What would happen if the heating had come on at 5.30PM? It would have taken half an hour for the temperature to rise from 20c to 22c and no heat would have been wasted. But this should only happen on warm days. If the temperature had been 14c and the heating had been running for only half an hour the temperature when you came home would be 16c and you would feel cold.
To use a delayed start stat system you tell the central heating system at what time you want it to check the temperature and at what time you need the house to be at 22c (or the temperature you have set on the thermostat.) You could tell the system to check the temperature at 4PM, 2 hours before you come home. The system would decide, “It’s only 14c today. I’d better turn on now because it’s going to take 2 hours for the house to get to 22c.” On a warm day the system would check the temperature at 4PM and decide, “It’s 21c today. I’m going to turn on at 5.30PM because it’s only going to take half an hour for the house to get to 22c.” A delayed start stat is sometimes called optimum start. A weather compensator is similar to a delayed start stat except that a delayed start stat has a temperature sensor in the room and a weather compensator has a temperature sensor outside.
What do I think of it? As most modern central heating systems have a timer and a thermostat you’re not losing a huge amount by having the heating running when you’re not there. In our example when the temperature was 20c the heating came on for 1.5 hours when it didn’t need to and would have switched off during that time. It would only have run for 1 hour. It’s one of lots of minor modifications you could make to your house that would each save about 10% on the bills. If you want to put a slow flow valve in your shower and get a smart electricity meter have a delayed start stat or a weather compensator as well. Each of these things will save a small amount of money and together they could make a significant difference. But your gas bill won’t be enormous without one. For new build houses, a delayed start stat or a weather compensator is something that is useful for saving you an extra few % on the CO2 emissions if your house is a very borderline pass. It’s worth putting one in then.
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