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Thursday, April 14, 2011

trying to save energy : trying to make sense of the results

So in my previous blog I outlined how it's easy to log your consumption, how to convert everything to the same units (KwH) and a site - imeasure - which makes this a lot easier.

If you do all of the above , or even some of it, you will see how much energy you use per week. Over time you may see trends, probably connected to the seasons. My household gas usage drops in the summer and rises in the winter - pretty predictable stuff.

I'm going to park oil, as I don't have any experience of using heating oil, but much of what I describe will be similar for gas & oil.

Separating the consumption of gas , in my case for central heating (CH) and hot water (HW), from electricty for a moment, it's worth thinking about what determines how much is used.

for gas, it's used by a boiler. the boiler heats water in a heat exchanger which is pumped through the house, and diverted either through a coil in a how water tank (HW) or through radiators in the house. This is referred in the UK as a fully pumped 3-switch central heating system, and I suspect it's pretty common.

Various controls - thermostats and timers - determine when the boiler comes on and for how long. the better insulated the house and tank are the lower the losses and the less gas that needs to be burn in the boiler. Also the better the controls - the more regulation of the amount of gas used.

In my case the HW circuit comes on twice a day determined by a timer/programmer unit. heat is diverted to the HW tank , determined by the tank stat, a small box strapped to the side of the tank , which touches the copper skin of the tank and switches off when the tank is hot enough. 55 degrees is supposedly the best temperature to set the tank to.

the Boiler has an additional thermostat in it, which always seems to be set at 2-thirds by each engineer who has ever serviced it. This sets how hot the boiler can get assuming it is required to deliver full heat to HW, CH or both.

In the case of CH, it's determined by a) the timer/controller i.e.e is this a period of heating and then b) by a room stat in the hall which determines how hot the house is supposed to get.

I'm not a fan of conventional bi-metallic room stats and favour programmable timer/stats which are much smarter and flexible (see my other blog on the CM67 ).

You can further refine the arrangement with more zonal heating. Commonly this is achieved in the UK with thermostatic radiator valves (TRV's for short) which enable a degree of local control on a room by room basis. the ideas is that if some rooms need to be cooler than the main house stat OR if they have some other form of heat (Sunshine coming in, computers/games consoles pumping out hot air) they regulate the radiators nearby to compensate. this in turn means the system requires less heat and this feeds back to the boiler needing to be on for less time.

Personally have never found TRV's to work that well. They rely on mechanical temperature expansion properties of a solid to open/close the valve, which given that they are attached to the principal heat source ,the radiator, is pretty crude and suffers from hysteresis (over swing).

reducing the amount of gas you use is most easily addressed by :

a) reducing losses . Better insulation for the house and the hot water tank. I fit a second jacket on the tank , even if it has a polyurethane foam one pre-bonded to it.

b) fitting good controls. A tank stat will ensure you only heat HW to the right temperature. A good thermostat will set the house to the right temperature. The timer stats also set the temperature based on previous days data , so start heating earlier to achieve a set temperature at a specific time - an optimise feature.

c) setting the controls. Figure out what temperature you need the house at different times. if no-one is in perhaps during the day, the temperature can be allowed to drop, but raise the temperature for the evening when people are at home. Similarly at night it can be cooler.
most systems implement this crudely with a timer which turns on CH at different times of the day, with no consideration of what temperature the house is or may need to be at a future point. My CH is actually left on at the timer all year, even in summer BUT the programmes in the room stat determine if it comes on i.e if the house is at the right temperature at the appropriate time.

d) ensuring the system is performing to spec i.e have the boiler routinely serviced, bleed the radiators.

So by considering and optimising factors a), b) & c) you should be able to see how much impact this has on gas consumption. This is harder to do than electricity. the boiler is essentially a binary device. It is on or off, it's just a matter of how long for. Electricity is more varied depending on the load. It may take months or even a years worth of data to see what impact measure have had, and external factors like weather , will also affect the results. I'd also mention that many electrical appliances generate heat either directly or as a bi-product of some other function, and cutting back on them may mean the CH has to work harder. This is why I favour converting everything to the same units , to try to see what impact this has across fuels i.e lowering electrical use might increase CH demand.

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